CWMC

Basics of Corrosion
Ashwini K Sinha
Principal Consultant, CWMC
(Ex-Additional General Manager (NETRA), NTPC)
Core Member, CII-Avantha Corrosion Management Committee,
Member NACE International, Life Fellow Member SAEST
ashwiniksinhacwmc@gmail.com
www.cwmcindia.com

Module C1

Corrosion and Water Management Consultants

“Improving Plant Performance, Availability & Reliability by Chemical Interventions”
CWMC

Basics of Corrosion - Outline

 Introduction to Corrosion

 Cost of Corrosion

 Basics of Corrosion

 Forms of Corrosion

 Corrosion Electrochemistry

 Corrosion Assessment

 Basics of Corrosion Control

2
CWMC

Basics of Corrosion - Outline

 Introduction to Corrosion

 Cost of Corrosion

 Basics of Corrosion

 Forms of Corrosion

 Corrosion Electrochemistry

 Corrosion Assessment

3
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Corrosion – Some Examples

4
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Corrosion – Some Examples

5
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Corrosion – Some Examples

6
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Corrosion – Some Examples

Piping Rupture Caused by Flow Accelerated Corrosion

(FAC):

A piping rupture likely caused by flow accelerated corrosion

and/or cavitation-erosion occurred at Mihama-3 at 3:28pm on
August 9, 2004, killing four and injuring seven. One of the
injured men later died, bringing the total to five fatalities.
The rupture was in the condensate system, upstream of the
feedwater pumps, similar to the Surry and Loviisa locations.
The AP reports that sections of the failed line were examined
in 1996, recommended for additional inspections in 2003, and
scheduled for inspection August 14 (five days after the
rupture). This story was published Wednesday, August 11th,
2004 By James Brooke, New York Times News Service

On Monday, four days before the scheduled shutdown,

superheated steam blew a 2-foot-wide hole in the pipe, fatally
scalding four workmen and injuring five others seriously. The
steam that escaped had not been in contact with the nuclear
reactor, and no nuclear contamination has been reported.

The rupture was 560 mm in size. The pipe wall at the

rupture location had thinned from 10mm (394 mils) to
1.5mm. 7
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Corrosion – Some Examples
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Corrosion – Some Examples
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Corrosion – Some Examples
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Corrosion – Some Examples
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Corrosion – Some Examples
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Corrosion – Some Examples
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Corrosion – Some Examples
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Corrosion – Some Examples
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Corrosion – Some Examples
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Corrosion – Some Examples

Iron Pillar, Delhi

The Iron Pillar located in Delhi, India,

is a 7 m (23 ft) column in the Qutub
complex, notable for the rust-resistant
composition of the metals used in its
construction for more than 1600 years.

The pillar has attracted the attention

of archaeologists and metallurgists and
has been called "a testament to the skill
of ancient Indian blacksmiths" because
of its high resistance to corrosion. The
corrosion resistance results from an
even layer of crystalline iron hydrogen
phosphate forming on the high
phosphorus content iron, which serves
to protect it from the effects of the local
Delhi climate
CWMC
Effects of Corrosion in
Power Plants

Perhaps most dangerous of all is corrosion that occurs in

major industrial plants, such as electrical power plants or
chemical processing plants. Plant shutdowns can and do
occur as a result of corrosion. This is just one of its many
direct and indirect consequences. Some consequences are
economic, and cause the following:

 Replacement of corroded equipment

 Overdesign to allow for corrosion
 Preventive maintenance, for example, painting
 Shutdown of equipment due to corrosion failure
 Contamination of a product
 Loss of efficiency—such as when overdesign and
corrosion products decrease the heat-transfer rate in
heat exchangers
CWMC
Effects of Corrosion in
Power Plants
 Loss of valuable product, for example, from a container that
has corroded through
 Inability to use otherwise desirable materials
 Damage of equipment adjacent to that in which corrosion
failure occurs
Still other consequences are social. These can involve the
following issues:
 Safety, for example, sudden failure can cause fire, explosion,
release of toxic product, and construction collapse
 Health, for example, pollution due to escaping product from
corroded equipment or due to a corrosion product itself
 Depletion of natural resources, including metals and the
fuels used to manufacture them
 Appearance as when corroded material is unpleasing to the
eye
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Effects of Corrosion in
Power Plants

• Reduced life of components

• Reduced efficiency of equipment
• Reduced availability of plant equipment
• Reduced reliability of equipment & structures
• Endanger to life of people around
• Enhanced maintenance
• Contaminations in process fluids
• Secondary failures in other associated equipment
• Higher costs of generation
CWMC
Losses Due to Corrosion

Losses due to Corrosion4

4Uhlig, H.H. and R.W. Revie, Corrosion and Corrosion Control. 3rd ed. 1985, New York: John Wiley & Sons.
CWMC
Losses Due to Corrosion

Low-temperature corrosion problems that occur in the following power plant

systems and components:
 Raw water and pretreatment systems
 Cooling water systems,
 Cooling towers,
 Service water systems,
 Auxiliary heat exchangers,
 Fire protection systems,
 Condensers,
 Feedwater piping systems,
 Low-pressure feedwater heaters,
 Deaerators,
 Low-pressure steam turbines,
 Electric generators,
 Air heater and ducts,
 Flue gas desulfurization systems
 Flue gas ducts,
 Stacks.
CWMC

Basics of Corrosion - Outline

 Introduction to Corrosion

 Cost of Corrosion

 Basics of Corrosion

 Forms of Corrosion

 Corrosion Electrochemistry

 Corrosion Assessment

23
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Cost of Corrosion &

Control Measures
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COSTS OF CORROSION

25
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COSTS OF CORROSION
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COSTS OF CORROSION

Estimated Corrosion Costs in India:

Direct Cost 4% of GDP

GDP in 2011 – 101640 Billion Rs.

4% of GDP – 40656 Billion Rs.

Indirect Cost – Same as Direct Cost

= 40656 Billion Rs

Total Costs of Corrosion = 82000 Billion Rs.

CWMC

COSTS OF CORROSION

“EPRI in its research report on the "Cost of Corrosion in the

Electric Power Industry" estimated that the cost of corrosion
in Electrical Industry of USA was of the order of US $ 34.5
Billion per annum in 2003. Based on the studies various
corrosion problems in the Fossil power plants were
identified. Around US $ 11 billion was due to boiler tube
failures followed by US $ 6 billion due to corrosion problems
in turbines”

At present no such studies have been conducted for Indian

power sector.
CWMC
Opportunities in Corrosion Control

The massive costs of corrosion provide many opportunities to users,

manufacturers, and suppliers. Opportunities exist to reduce corrosion costs
and the risks of failure, and to develop new, expanded markets. Examples
of these opportunities and the means to implement a program to capitalize
on the opportunities are presented in Table below.
The costs of corrosion vary considerably from industry to industry; however,
substantial savings are achievable in most industries. The first step in any
cost-reduction program is to identify and quantify the present costs of
corrosion. Based on this analysis and a review of the present status of
corrosion control in the industry, priorities can be determined and the most
rewarding cost-reduction projects pursued.
Risk of corrosion failure can be lowered in the producer’s facility and in its
products. Both process and products can be analyzed to identify the areas
where corrosion failures can occur. Once identified, the risk of failure can
be evaluated from the perspectives of impact on safety, product liability,
avoidance of regulation, and loss of goodwill. Where risks are too great,
technological changes can be implemented to reduce the risk.
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Opportunities in Corrosion Control
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Basics of Corrosion
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WHAT IS CORROSION

CORROSION is a natural process. Just like water flows to the lowest

level, all natural processes tend toward the lowest possible energy
states. Thus, for example, iron and steel have a natural tendency to
combine with other chemical elements to return to their lowest energy
states. In order to return to lower energy states, iron and steel frequently
combine with oxygen and water, both of which are present in most
natural environments, to form hydrated iron oxides (rust), similar in
chemical composition to the original iron ore.
CWMC
WHAT IS CORROSION
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CORROSION
Corrosion is a natural process and
is a result of the inherent tendency
of metals to revert to their more
stable compounds, usually oxides.
Most metals are found in nature in
the form of various chemical
compounds called ores. In the
refining process, energy is added to
the ore, to produce the metal. It is this
General Corrosion
same energy that provides the driving
force causing the metal to revert back Pitting Corrosion
to the more stable compound.
corrosion can also be defined as a
chemical or electrochemical
reaction between a material,
usually a metal, and its Under deposit Corrosion
environment that produces a
deterioration of the material and
its properties.
CWMC
CORROSION
The environment consists of the entire surrounding in contact with the
material. The primary factors to describe the environment are the following:

(a) physical state—gas, liquid, or solid; (b) chemical composition—

constituents and concentrations; and (c) temperature. Other factors can
be important in specific cases. Examples of these factors are the
relative velocity of a solution (because of flow or agitation) and
mechanical loads on the material, including residual stress within the
material.

Reference to marine corrosion of a pier piling means that the steel piling
corrodes because of its reaction with the marine environment. The
environment is air saturated seawater. The environment can be further
described by specifying the chemical analysis of the seawater and the
temperature and velocity of the seawater at the piling surface.

For power plants important parameters are high purity water, dissolved
gases, flue gases, ash and coal slurry, cooling water, coal quality, steam
impurities, etc are some of the environment that need to be considered.
CWMC

WHAT IS CORROSION

CORROSION IS A NATURAL PROCESS BY VIRTUE OF

WHICH THE METALS TEND TO ACHIEVE THE
LEAST ENERGY STATE – I.E. COMBINED STATE

M M2+ + 2e-
ANODIC REACTION
N 2- + 2e N
MIC CATHODIC REACTION

Dezincification
CWMC
ELECTROCHEMICAL NATURE
OF CORROSION

Electrotype:- solution capable

of conducting electricity e.g.
fresh/salt water, moisture,
alkali’s, acids

Cathode is the metal

Anode is the metal which is not Corroding
which is Corroding
CWMC
Corrosion Basics

• Corrosion requires:

– Oxygen & Water

– Rusting takes place in

presence of Air & Water

– No rusting will occur if either

water or air is removed
CWMC
Corrosion Basics

• Corrosion is electrochemical
– Anode (Oxidizing – losing electrons) Electrode
– Cathode (Reducing – gaining electrons) Electrode
– Need ―Short circuit‖ for electrons between terminals
– And need a medium for ion transport

• Electricity and chemicals are main drivers

• Influenced by other factors

CWMC
Corrosion Basics

• Usual Textbook Equations

– Chemical: Zn + 2HCl = ZnCl2 + 2H
– Electrical: Zn Zn+2 + 2e- (anode)
– 2H+ + 2e- 2H (cathode)

• Note: hydrogen is atomic, not diatomic

– This can come back to get you

• Generally, the electrical part is not shown

• Oxygen reaction can be inserted as well

CWMC
Corrosion Basics

Note that atomic hydrogen forms on

surface and becomes diatomic in solution.

Hydrogen atoms can be absorbed into

materials (Ti and carbon steel couple)

Figure – Electrochemical reactions occurring during the corrosion of zinc in air-free

hydrochloric acid.
CWMC
Corrosion Basics- Other Factors

 Corrosion rates are almost initially very high

 Polarization – something to slow down reactions

 Cathodic and anodic surface polarization
 Film thickness of corrosion product
 Rate of hydrogen or oxygen diffusion to and from surfaces
 Rate of corrodant ion diffusion away

 Areas of reaction (anode to cathode)

 Oxygen Content (cathodic depolarizer)

 Temperature – every 10°C = 2 x corrosion rate

 Velocity effects – moving species to & fro

 CWMC

• Oxidation: the loss of electrons by a species, leading to an increase in

oxidation number of one or more atoms. loss of electron(s) by a species; increase
in oxidation number; increase in oxygen.
• Reduction: the gain of electrons by a species, leading to an decrease in
oxidation number of one or more atoms. Gain of electron(s); decrease in
oxidation number; decrease in oxygen; increase in hydrogen.
•Oxidizing agents: the species that is reduced in a redox reaction (or)Electron
acceptor;
•Reducing agents: the species that is oxidized in a redox reaction (or) Electron
donor;
 CWMC

corrosion
The process of decay of metal by environmental attack is called corrosion.
The basic reason for this attack is most of the metals (except Pt, Au, Ag) exist in nature in the form
of their ores as oxides, chlorides, silicates, carbonates etc.,
Metals have the natural tendency to go back to their combined states, as a result when metal is
exposed environmental conditions forms stable compounds of metals, known as corrosion.
The process of corrosion is reverse of metal extraction.
 CWMC

Effects of the corrosion process

1. The valuable metallic properties like conductivity , malleability ,

ductility etc., are lost due to corrosion.

2. The process of corrosion is very harmful and is responsible for the

enormous wastage of metal in the form of its compounds.

3. Life span on the metallic parts of the machineries is reduced.

4. The failure of machinery takes place due to lose of useful properties

of metals.
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CWMC

CHEMICAL (OR)DRY CORROSION

THE DIRECT CHEMICAL ATTACK OF THE ATMOSPHERIC GASES
LIKE O2, HALOGENS, H2S, SO2 ,ANHYDROUS INORGANIC LIQUID
METALS ON METAL SURFACES IN THE ABSENCE OF MOISTURE.

1. OXIDATION CORROSION: DIRECT ACTION OF OXYGEN AT HIGH(OR)LOW

TEMPARETURES ON METAL SURFACE.

2. CORROSION BY OTHER GASES: ATTACK OF GASES LIKE SO2,CO2,Cl2,H2S,F etc ON

METAL SURFACE.

3. LIQUID METAL CORROSION: ATTACK OF INORGANIC LIQUID METALS ON SOLID

METALLIC SURFACE
CWMC

OXIDATION CORROSION
2M 2 M2+ 2 e- (Oxidation by loss of electrons)
O2 2 e- 2 O2- (Reduction by gain of electrons)

Total reaction 2M O2 2 M2+ 2 O2- 2 MO

Atmospheric O2
CWMC

If the formed metal oxide is stable further corrosion of metal is

prevented by the formed metal oxide.
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If the formed metal oxide is unstable corrosion

WILL not occur.
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If the formed metal oxide is volatile ,fresh metal

surface is rapidly exposed and converted into
metal oxide and evaporated.
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If the formed metal oxide layer is porous , under laying

metal is attacked and converted in to metal oxide. the
total metal is converted into metal oxide form.
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Pilling-Bedworth rule

specific ratio= volume of metal oxide/volume of metal

Three types of oxides may

form, depending on the
volume ratio between the
metal and the oxide:

(a) magnesium produces a

porous oxide film,

b) aluminum forms a
protective, adherent,
nonporous oxide film, and

(c) iron forms an oxide film

that spills off the surface and
provides poor protection.
53
 CWMC

Corrosion by other gases

The gases such as SO2, CO2, H2S, Cl2,F2 etc., when come in
direct contact with metal surface corrosion is occurs.

•The extent of corrosion depends on the chemical affinity

between the metal and the gas concerned.

•The prevention of metal corrosion can be known from the

nature of corrosion product ., i.e.

whether the layer of corrosion product is protecting or non protecting in nature .

 CWMC

1. If the formed corrosion product is protecting (or) non

porous metal is prevented.

Ex: AgCl layer on metallic silver by the action of Cl2 gas.

2. If the formed corrosion product is non protecting (or)

porous , the corrosion of metals occurs non stop.

Ex: H2S gas attacks on steel at high temperature forming

FeS , a corrosion product which is porous.
 CWMC

Liquid metal corrosion

The chemical action of the flowing liquid meta at high temperature , on
a solid metal or alloy produces liquid metal corrosion.

There are two reasons for this corrosion

1. Dissolution of the solid metal by liquid metal

2. Internal penetration of the liquid metal into the solid phase,

weakening the solid metal.

Ex: liquid Na or liquid Nitrogen used as a coolant in a nuclear plants,

these causes cadmium corrosion.
CWMC

wet corrosion (or) Electro chemical corrosion

Chemically non uniform surface of metal in

the presence of conducting medium behaves
as electro chemical cell and electron flow is
occur due to the electrons which are
released from oxidation of metal, known as
electro chemical corrosion.
Mechanism :
1. Anodic reaction : Oxidation of metal

2. Cathodic reaction : consumption of electrons

CWMC

Mechanism of wet corrosion

2 H+(aq) + 2e-  H2
M(s)  M2+(aq) + 2e- (Acidic medium)
(Dissolution or ½ O2(g) + H2O (aq) +2 e- OH-
corrosion of metal (Neutral medium)
takes place)
 CWMC

Wet corrosion takes by the following two ways based the medium:

1. Evolution of H2 :
Anode: Fe(s)  Fe2+(aq) + 2e-
Cathode: 2 H+(aq) + 2e-  H2
CWMC

Absorption of O2 :
Anode: Fe(s)  Fe2+(aq) + 2e-
Cathode: ½ O2(g) + H2O (aq) +2 e-  2OH-
CWMC

Salt speeds up process by increasing

conductivity
Water
Fe2+ Rust

e-
Iron Dissolves- O2 + 2H2O +4e-  4OH-
Fe  Fe+2

Fe2+ + O2 + 2H2O  Fe2O3 + 8 H+

CWMC

Dry or chemical corrosion Wet or electrochemical corrosion

 This occurs at dry conditions  This occurs at wet conditions

(electrolytic medium)
 Corrosion is uniform  Corrosion Is not uniform
 It is a slow process  It is a rapid process
 It involves direct chemical  It involves formation of
attack electro chemical cells
 Explained by absorption  Explained by mechanism of
mechanism electro chemical reactions
CWMC

Galvanic corrosion

Galvanic corrosion is an electrochemical

corrosion. It is due to a potential difference
between two different metals connected through a
circuit for current flow to occur from more active
metal (Anode) to the more noble metal (Cathode)

 Galvanic coupling is a galvanic cell in which the anode

is the less corrosion resistant metal than the cathode
CWMC

Anode (oxidation): Zn (s)  Zn2+ + 2e-

Cathode (reduction): ½ O2 +H2O +2 e-  2OH-

Zn2+ + 2OH -  Zn(OH)2

(Corrosion product)
CWMC

HCl
Fe+2
CWMC
Galvanic Cells

anode cathode
oxidation reduction

- +

spontaneous
redox reaction

19.2
 CWMC

Concentration cell corrosion

 Because of differential aeration the concentration of O2 is

varied and caused for Oxidation of metal .

oxidation : Zn (s)  Zn2+ + 2e-

Reduction : ½ O2 +H2O +2 e-  2OH- High conc.O2

Zn2+ + 2OH -  Zn(OH)2

(Corrosion product)
e-
This is two types :
Low conc.O2

1. Pitting corrosion

2. Water line corrosion

 CWMC
1.Pitting corrosion:
Pitting Corrosion is an extremely localized corrosion mechanism that
causes destructive pits.
 CWMC

2.Water line corrosion :

This type of corrosion is occurred by differential aeration by presence
of water line. This is caused for formation of concentration cell.
CWMC

Factors affecting the rate of corrosion

1. Nature of metal :
 Position in galvanic series

Over voltage

Relative areas of cathodic and anodic parts

 Purity of metal

 Physical state of metal

 Nature of surface film

 Passive character of metal

 Volatility of corrosion product

 Solubility of corrosion product

CWMC

2. Nature of corroding environment :

 Temperature

 Humidity of air

 Presence of impurities in atmosphere

 Nature of ions present in environment

 Conductance of corroding medium

 Amount of oxygen in atmosphere

 Velocity of ions which flow in the medium

 PH value of the medium

 Suspended impurities
CWMC

Basics of Corrosion - Outline

 Introduction to Corrosion

 Cost of Corrosion

 Basics of Corrosion

 Forms of Corrosion

 Corrosion Electrochemistry

 Corrosion Assessment

72
CWMC

The Many Forms of Corrosion

Corrosion occurs in several widely differing forms. Classification is usually

based on one of three factors:

 Nature of the corrodent: Corrosion can be classified as ―wet‖ or ―dry.‖ A

liquid or moisture is necessary for the former, and dry corrosion usually
involves reaction with high-temperature gases.

 Mechanism of corrosion: This involves either electrochemical or direct

chemical reactions.

 Appearance of the corroded metal: Corrosion is either uniform and the

metal corrodes at the same rate over the entire surface, or it is
localized, in which case only small areas are affected.
CWMC

The Many Forms of Corrosion

Eight forms of wet (or aqueous) corrosion can be identified based on

appearance of the corroded metal. These are:
 Uniform or general corrosion
 Pitting corrosion
 Crevice corrosion, including corrosion under tubercles or deposits,
 filiform corrosion, and poultice corrosion
 Galvanic corrosion
 Erosion-corrosion, including cavitation erosion and fretting corrosion
 Intergranular corrosion, including sensitization and exfoliation
 Dealloying, including dezincification and graphitic corrosion
 Environmentally assisted cracking, including stress-corrosion cracking,
 corrosion fatigue, and hydrogen damage

In theory, the eight forms of corrosion are clearly distinct; in practice

however, there are corrosion cases that fit in more than one category. Other
corrosion cases do not appear to fit well in any of the eight categories.
Nevertheless, this classification system is quite helpful in the study.
CWMC

The Many Forms of Corrosion

CWMC

The Many Forms of Corrosion

CWMC

HERE‟S HOW YOU CAN SPOT THE MANY

COMMON VARIETIES OF CORROSION

It can show up in a host of ways and forms. And

many of the most common types of corrosion
conditions overlap each other
electrolyte pit (onode)

corrosion corrosion
product product
(cathode)

metal corrosion current

onode metal
Pitting corrosion
Uniform attack
CWMC

HERE‟S HOW YOU CAN SPOT THE MANY

COMMON VARIETIES OF CORROSION

It can show up in a host of ways and forms. And

many of the most common types of corrosion
conditions overlap each other
intercrystalline crack stress-corrosion
cracks

load

Intergranular corrosion Stress corrosion cracking

CWMC

HERE‟S HOW YOU CAN SPOT THE MANY

COMMON VARIETIES OF CORROSION

It can show up in a host of ways and forms. And many

of the most common types of corrosion conditions
overlap each other
plug-type
dezincification
cyclic
loading
layer-type
dezincification

fatigue
crocks
Selective attack
Corrosion fatigue
CWMC

HERE‟S HOW YOU CAN SPOT THE MANY

COMMON VARIETIES OF CORROSION

It can show up in a host of ways and forms. And many of

the most common types of corrosion conditions overlap
each other
flow fretting at fight fits
corrosion film subject to vibration

impinging stream

Fretting corrosion
Impingement attack
CWMC
Uniform Corrosion

Formerly a ship

81
CWMC
Uniform Corrosion

What general corrosion

might look like!
CWMC
Corrosion Top Mechanisms - Pitting

• Most common form of localized attack

• Break down of protective scale
• Localized attack in break
• Pit sets up its own environment
• Draws in chlorides and sulfates
• Can form caps over pits
• Low corrosion rates are deceitful
CWMC
Corrosion Top Mechanisms - Pitting

Pitting corrosion – small and large

CWMC
Pitting

Pitting is a localized form of corrosive

attack. Pitting corrosion is typified by the
formation of holes or pits on the metal
surface. Pitting can cause failure, yet the total
corrosion, as measured by weight loss, may be
minimal.

304 stainless
steel / acid
chloride
solution

5th Century sword

Boiler tube
CWMC
PITTING CORROSION
CWMC
Corrosion Top Mechanisms - Crevice

• Much like a large area pit.

• Occurs in cracks or crevices
• Think of flanged connections such as
– Piping flanges
– Column body flanges
– Trays on tray rings
– Car or truck doors
• It will also set up its own environment
CWMC
Corrosion Top Mechanisms - Crevice

Crevice attack on titanium from

fluorinated o-ring

Severe crevice attack as well as general

CWMC
Crevice Corrosion

Narrow and confined spaces.

CWMC
Corrosion Top Mechanism - Underdeposit

• Very similar to crevice corrosion but a

larger
• Usually an unplanned occurrence
– Tools left on floor
– River water silt buildup in bottoms
• Sometimes called poultice corrosion
• Sometimes called oxygen concentration
cell
CWMC
Corrosion Top Mechanism - Dealloying

• Copper alloys
– Brasses with >30% zinc (Dezincification)
– Copper nickel alloys (nickel removed)
• Cast iron (graphitization)
• Almost any alloy can have the problem
• Two Theories
– One element is ―leached‖ from solution
– Both elements corroded but more noble
plates back.
CWMC Corrosion Top Mechanism - Dealloying

Brass River Water Impellor suffering

from dealloying and cavitation
CWMC
Corrosion Mechanisms – Galvanic

• Think dry cell battery

– Carbon center cathode
– Zinc jacket anode
– MnOH (manganese hydroxide paste)
– Switch short circuit provided by your flashlight
• Galvanized water pipe to your house
• Powerhouse soot blower of SS nozzle and steel pipe
• Over the road trailers with Al sides and steel rivets
• Your water heater with aluminum sacrificial anode
CWMC
Corrosion Top Mechanism - Dealloying
CSTL Pipe
SS Nozzle

Soot blower metallographic sample

CWMC
Corrosion Top Mechanism - Dealloying

 Dissimilar metals are physically joined

in the presence of an electrolyte.
 The more anodic metal corrodes.

Bilge pump - Magnesium

shell cast around a steel
core.
CWMC Selective Leaching

Preferred corrosion of
one element/constituent
[e.g., Zn from brass (Cu-Zn)].
Dezincification.
CWMC
Stress Corrosion Cracking (SCC)

(SCC) is the cracking induced from the combined influence of tensile stress
and a corrosive environment. The impact of SCC on a material usually falls
between dry cracking and the fatigue threshold of that material. The required
tensile stresses may be in the form of directly applied stresses or in the form of
residual stresses. The problem itself can be quite complex. The situation with buried
pipelines is a good example of such complexity. The impact is most commonly
catastrophic

Cold deformation and forming, welding, heat treatment, machining and grinding can
introduce residual stresses. The magnitude and importance of such stresses is
often underestimated. The residual stresses set up as a result of welding operations
97
tend to approach the yield strength.
CWMC
Stress Corrosion Cracking (SCC)
The build-up of corrosion products in confined spaces can also generate
significant stresses and should not be overlooked. SCC usually occurs in certain
specific alloy-environment-stress combinations.

Usually, most of the surface remains unattacked, but with fine cracks
penetrating into the material. In the microstructure, these cracks can have
an intergranular or a transgranular morphology. Macroscopically, SCC fractures
have a brittle appearance. SCC is classified as a catastrophic form of corrosion,
as the detection of such fine cracks can be very difficult and the damage not
easily predicted. Experimental SCC data is notorious for a wide range of scatter.
A disastrous failure may occur unexpectedly, with minimal overall material loss.
The micrograph on the right (X500) illustrates intergranular SCC of an Inconel
heat exchanger tube with the crack following the grain boundaries

The micrograph on the left (X300) illustrates SCC in a 316 stainless steel
chemical processing piping system.Chloride stress corrosion cracking in
austenitic stainless steel is characterized by the multi-branched "lightning bolt"
transgranular crack pattern.

98
CWMC
Stress Corrosion Cracking (SCC)

The most effective means of preventing SCC are: 1) properly with the right
materials; 2) reduce stresses; 3) remove critical environmental species such as
hydroxides, chlorides, and oxygen; 4) and avoid stagnant areas and crevices in
heat exchangers where chloride and hydroxide might become concentrated.
Low alloy steels are less susceptible than high alloy steels, but they are subject
99
to SCC in water containing chloride ions.
CWMC
Stress Corrosion Cracking (SCC)

Chloride SCC

One of the most important forms of stress corrosion that concerns the nuclear
industry is chloride stress corrosion. Chloride stress corrosion is a type of
intergranular corrosion and occurs in austenitic stainless steel under tensile
stress in the presence of oxygen, chloride ions, and high temperature. It is
thought to start with chromium carbide deposits along grain boundaries that
leave the metal open to corrosion. This form of corrosion is controlled by
maintaining low chloride ion and oxygen content in the environment and use of
low carbon steels.

Caustic SCC

Despite the extensive qualification of Inconel for specific applications, a number

of corrosion problems have arisen with Inconel tubing. Improved resistance to
caustic stress corrosion cracking can be given to Inconel by heat treating it at
620oC to 705oC, depending upon prior solution treating temperature. Other
problems that have been observed with Inconel include wastage, tube denting,
pitting, and intergranular attack.
100
CWMC
Stress Corrosion Cracking (SCC)
Environments and Stress Corrosion Cracking

The specificity of environments that will promote Stress Corrosion Cracking is

significant. It is important to realize that not all corrosive environments promote the
formation of stress corrosion cracks. Those that do will usually be those that do not
promote widespread corrosion in the sense of the attack being spread fairly
uniformly over all exposed surfaces, since, if for no other reason, this is not likely to
lead to the geometry of a crack, which requires that the crack sides remain relatively
inactive whilst the tip remains active to maintain propagation into the metal.

Consequently those environments, such as sea water, that normally promote

general corrosion of mild steel, are not likely to promote stress corrosion, whilst
those chemicals sometimes used to control corrosion by addition to an otherwise
corrosive environment may result in a borderline condition, between general
corrosion and no corrosion, wherein the attack can be localized. Thus, the addition
of caustic soda to boiler feed waters to reduce the corrosiveness of the latter
towards mild steel can result in the form of stress corrosion frequently referred to
as „caustic cracking‟. The important general point is that those environments that
cause stress corrosion are frequently highly specific to the particular alloy involved
and a list of some environments that have been shown to promote stress corrosion
in various materials is given in the following Table. 101
CWMC
Stress Corrosion Cracking (SCC)

Combinations of some alloys and environments that have been shown to

promote stress corrosion cracking.

102
CWMC
Stress Corrosion Cracking (SCC)

Environments and Stress Corrosion Cracking

Although this list of environments that have been shown to promote stress corrosion
cracking may appear extensive it is by no means exhaustive. For a given alloy
however there are many more environments that do not cause stress corrosion than
those that so act. It is possible, by appropriate electrochemical measurements or by
laboratory stress corrosion tests properly conducted, to identify potent environments
for a given material, although failures continue to occur in circumstances that may
not reasonably have been expected.

103
CWMC
Stress Corrosion Cracking (SCC)
Stress Corrosion Cracking Definitions

Stress: The intensity of the internally distributed forces or components of forces

that resist a change in the volume or shape of a material that is or has been
subjected to external forces. Stress is expressed in force per unit area and is
calculated on the basis of the original dimensions of the cross section of the
specimen. Stress can be either direct (tension or compression) or shear. See
also residual stress.

Stress concentration factor (Kt): A multiplying factor for applied stress that
allows for the presence of a structural discontinuity such as a notch or hole; Kt,
equals the ratio of the greatest stress in the region of the discontinuity to the
nominal stress for the entire section. Also called theoretical stress concentration
factor.

Stress-corrosion cracking (SCC): A cracking process that requires the

simultaneous action of a corrodent and sustained tensile stress. This excludes
corrosion-reduced sections that fail by fast fracture. It also excludes
intercrystalline or transcrystalline corrosion, which can disintegrate an alloy
without applied or residual stress. Stress-corrosion cracking may occur in
combination with hydrogen embrittlement.
104
CWMC
Corrosion Top Mechanisms – Environmental
Cracking

• Stress Corrosion Cracking

– Chlorides (aluminum, 300 series SS)
– Caustic (cstl, 300 series SS, nickel alloys)
– Ammonia (brass drain)
• Hydrogen Embrittlement
• Liquid Metal Embrittlement
– Copper on stainless steel pipe
– Zinc on stainless steel pipe
CWMC
Corrosion Top Mechanisms – Environmental
Cracking
Weld metal

Knife line attack

Transgranular chloride SCC in 316 stainless steel

CWMC
Corrosion Top Mechanisms – Environmental
Cracking

Intergranular caustic SCC in

304L stainless steel finned
tube.
CWMC
Stress Corrosion Cracking, SCC

• A structure that has SCC sensitivity, if

subjected to stresses and then exposed
to a corrosive environment, may initiate
cracks and crack growth well below the
yield strength of the metal.
• Consequently, no corrosion products are
visible, making it difficult to detect or
prevent; fine cracks can penetrate deeply
into the part.
CWMC
Intergranular
Corrosion along
grain boundaries,
often where precipitate
particles form.
CWMC
INTERGRANULAR CORROSION
 CWMC ENVIRONMENT/ALLOY SYSTEMS SUBJECT
TO STRESS CORROSION CRACKING
ALLOY ENVIRONMENT

Aluminium Base • Air

• Seawater
• Salt & Chemical Combinations
• Nitric Acid
Magnesium Base • Caustic
• HF Solution
• Salts
• Coastal Atmospheres
• Primarily Ammonia & Ammonium
Hydroxide
Copper Base • Amines
• Mercury
• Caustic
Carbon Steel • Anhydrous Ammonia
• Nitrate Solutions
Martensitic & Precipitation Hardening Stainless • Seawater
Steels • Chlorides
• H2S Solutions
• Chlorides Inorganic & Organic
Austenitic Stainless Steels • Sulfurous & Polythionic Acids
• Caustic Solutions
• Caustic Above 600 F (315 C)
• Fused Caustic
Nickel Base • Hydrofluoric Acid
• Seawater
• Salt Atmospheres
Titanium • Fused Salt
CWMC
Corrosion Top Mechanism – Corrosion Fatigue

• Starts with an alternating stress state

• Protective oxide breaks open
• Corrosive species attack and form
products
• Next cycle repeats:
– crack growth
– more corrosion product
– accelerated fatigue failure
• Seen in rotating shafts
CWMC
Corrosion Top Mechanism – Corrosion Fatigue

Corrosion fatigue, cracks can be oriented the other direction depending on

stress state of shaft.
CWMC
Corrosion Top Mechanisms – Cavitation

• Mostly found at
• Pump impellor tips
• Boat propellers
• Constriction in fast fluids
• Caused by formation of low pressure bubble
• Bubble is a vacuum
• Collapse of bubble slams the metal
• Breaking protective oxide
• Causing great mechanical damage
CWMC
Corrosion Top Mechanisms – Cavitation

Piece of pump impellor with tip

cavitation

Valve trim diffuser with

cavitation

Centrifuge feed nozzle

CWMC
Corrosion Top Mechanism - Erosion

• Can be from
– Gaseous vapor (steam cuts on flanges)
– Liquid
– Solids (Coal slurry)
• Removes the protective oxide layer faster
than it can heal
CWMC
Corrosion Top Mechanism - Erosion

Look for “comet tails”! Water was flowing

from right to left in copper water pipe.
CWMC
Erosion-corrosion

Combined chemical attack and

mechanical wear (e.g., pipe
elbows).

Brass water pump

CWMC
CORROSION INDEX

Chloride + Sulphate (epm)

C.I. =
M-Alkalinity (epm)

Where
 epm cl = ppm cl- x 0.0282

 epm SO4 = ppm SO4-2 x 0.208

 epm M - Alk = ppm (M - Alk) x 0.02 Ca as CO3

Application
 Generally in untreated water

 If pH 7.0 - 8.0, C.I. < 0.1, water free from corrosion

 Bicarbonate ions mildly inhibits, corrosion of steel

 Chloride and Sulphate ions helps in corrosion of steel

CWMC

304.8

254
(In Mils per year)

203.2
Corrosion rate

152.4

101.6

50.8

0 1 2 3 4 5 6 7 8 9 10

Oxygen (PPM)

EFFECT OF OXYGEN CONCENTRATION ON CORROSION AT

DIFFERENT TEMPERATURES
CWMC
Tafel Plots
CWMC
ELECTROCHEMICAL NATURE
OF CORROSION

-550mV
-600mV

-575mV

Potential Differences on Steel Surface

CWMC
ELECTROCHEMICAL NATURE
OF CORROSION

1) ANODE
2) CATHODE
3) ELECTROLYTE Anode
Cathode
-600mV
4) ELECTRICAL -550mV
CONNECTION

-575mV
CWMC
Standard EMF Series

• EMF series • Metal with smaller

o
Vmetal o corrodes.
metal V metal
Au +1.420 V
• Ex: Cd-Ni cell
Cu +0.340
V <o V oCd corrodes
Pb - 0.126 Cd Ni
more cathodic

Sn - 0.136 - +
Ni - 0.250
Co - 0.277 DV o=
Cd - 0.403 0.153V
Fe - 0.440
Cr - 0.744 Cd 25°C Ni
Zn - 0.763
more anodic

Al - 1.662
1.0 M 1.0 M
Mg - 2.363
Cd 2 + solution Ni 2+ solution
Na - 2.714
K - 2.924
124
CWMC
Driving force

• A driving force is necessary for electrons to flow between the

anodes and the cathodes.
• The driving force is the difference in potential between the anodic
and cathodic sites.
• This difference exists because each oxidation or reduction
reaction has associated with it a potential determined by the
tendency for the reaction to take place spontaneously. The
potential is a measure of this tendency.

125
CWMC

PRACTICAL GALVANIC SERIES

Material Potential*
Pure Magnesium -1.75
Magnesium Alloy -1.60
Zinc -1.10
Aluminum Alloy -1.00
Cadmium -0.80
Mild Steel (New) -0.70
Mild Steel (Old) -0.50
Cast Iron -0.50
Stainless Steel -0.50 to + 0.10
Copper, Brass, Bronze -0.20
Titanium -0.20
Gold +0.20
Carbon, Graphite, Coke +0.30
* Potentials With Respect to Saturated Cu-CuSO4 Electrode
CWMC
Anodic Polarization Curve -1

• This curve is usually

scanned from 20 mV below
Types of Tests the Eoc (open circuit
potential) upward.
ANODIC POLARIZATION CURVE
•The curve can be used to
•this curve is usually scanned from 20mV below the
identify theEoc upwards
following
•by scanning at a slow rate (.2mV/s) this
corrosion regions:
curve can be used to identify
several corrosion mechanisms shown below

ip - passive current density

Epp - primary passivation potential

icrit - critical current density

Etrans - transpassive potential

CWMC

Basics of Corrosion - Outline

 Introduction to Corrosion

 Cost of Corrosion

 Basics of Corrosion

 Forms of Corrosion

 Corrosion Electrochemistry

 Corrosion Assessment

128
CWMC
Corrosion Electrochemistry

The 6 Electrochemical Reactions Involved in Corrosion

Corrosion electrochemistry is a crucial aspect of truly understanding - and

preventing - corrosion.

Corrosion is an electrochemical method by which materials are

deteriorated. In many cases - and especially when liquids are present - it
involves chemistry. During corrosion, electrons from distinct areas of a
metal surface flow to alternative areas through an atmosphere capable of
conducting ions. That's the simple chemistry of corrosion, but the details
are anything but. The same goes for the impact.

In fact, the economic impact of corrosion is much bigger than many

realize. According to a 2001 report by CC Technologies Laboratories Inc.,
the cost of corrosion within the U.S. alone was $276 billion annually. Of
this, $121 billion was spent to manage corrosion, while the remaining
$155 billion was incurred as a net loss to the economy.
CWMC
Corrosion Electrochemistry

The 6 Electrochemical Reactions Involved in Corrosion

Utilities, particularly water and sewer systems, suffer the biggest
economic impact, with motorized vehicles and transportation coming a
close second.
Because metallic corrosion is an ongoing electrochemical process, it's
crucial to know the essential nature of electrochemical reactions to
properly inhibit corrosion and reduce its impact on structures. In this
article, we'll discuss the mechanisms of corrosion by covering the details
of:
 Eectrochemical reactions
 The Daniell cell
 The anodic method
 Faraday's law
 The cathodic method
 Surface area impact
CWMC
Corrosion Electrochemistry

What Is Corrosion Electrochemistry?

Corrosion in an aqueous environment and in an atmospheric setting is an

electrochemical method in which electrons are transferred between a
metal surface and a liquid electrolyte solution resulting in deterioration of
the substrate. Corrosion occurs because of the great tendency of metals
to react electrochemically with oxygen, water and alternative substances
within the atmosphere. In this context, the term anode is employed to
explain the portion of the metal surface that's really corroding, whereas
the term cathode is employed to explain the metal surface that consumes
the electrons created by the corrosion reaction. Ulick R. Evans, an early
pioneer in explaining corrosion as an electrochemical process, said that it
could be described as destruction by electrochemical or chemical
agencies. Corrosion electrochemistry, therefore, is simply an
electrochemical method through which we can perceive the mechanisms
of corrosion.
CWMC
Corrosion Electrochemistry

Electrochemical Reactions

An electrochemical reaction is outlined as a reaction involving the transfer

of electrons. It's also a reaction that involves oxidation and reduction. The
very fact that corrosion consists of a minimum of one chemical reaction
and one reduction reaction isn't entirely obvious because both reactions
are usually combined in one piece of metal (e.g., Zn) as illustrated
schematically below.
CWMC
Corrosion Electrochemistry

Figure 1: Electrochemical reactions during the corrosion of Zn in air-free HCL.

CWMC
Corrosion Electrochemistry

Electrochemical Reactions
In Figure 1, a bit of Zinc immersed in acid solution is undergoing
corrosion. At some point on the surface, Zn is transformed to Zn ions
losing electrons. These electrons go through the solid conducting metal to
alternative sites on the metal surface, wherever hydrogen (H) ions are
reduced to hydrogen gas consistent with the following equation:

These equations illustrate the character of an electrochemical reaction in

Zinc. Throughout such a reaction, electrons are transferred or, to view it in
a different way, an oxidation reaction happens in conjunction with a
reduction reaction.

Therefore in electrochemistry, anodic and cathodic reactions are occurring

simultaneously and at equivalent rates. However, corrosion happens
solely in the areas that function as anodes.
CWMC
Corrosion Electrochemistry

The Daniell Cell and Electrochemical Corrosion

The doctrine of electrochemical reactions is employed in a Daniell cell

during which copper and zinc metals are immersed in solutions of their
individual sulfates. The Daniell cell was the primary sensible and reliable
battery that supported several 19th-century electrical innovations, such as
the telegraph.

In a Daniell cell, electrons are transferred from the corroding zinc to the
copper through an electrically conducting path as an electric current. Zinc
loses electrons more readily than copper, which means that putting zinc
and copper metal in solutions of their salts will cause electrons to flow
through an external wire that leads from the zinc to the copper as per the
following reactions:
CWMC
Corrosion Electrochemistry

The Daniell Cell and Electrochemical Corrosion

The difference in corrosion potential between the two metals will usually
cause a scenario that's referred to as galvanic corrosion, which was
named in honor of its discoverer Luigi Galvani.
This situation is common in natural corrosion cells wherever the setting is
the electrolyte that completes the corrosion cell. The conduction of a liquid
atmosphere like soil, concrete, or water has usually been associated with
its corrosivity.

The short-hand description within the following equation is valid for each
Daniell cell configuration.

This equation identifies the zinc electrode as the anode because it is

negative in the case of a spontaneous reaction, while the copper electrode
is the cathode due to its positive charge.
CWMC
Corrosion Electrochemistry

The Anodic Method and Corrosion

Now we move into more detail about what takes place at the anode once
corrosion occurs. For example, the corrosion reaction for Fe (iron), which
involves the reduction of hydrogen ions to hydrogen gas, is consistent with
the electrochemical reaction of Zn in hydrogen chloride (HCl). This
hydrogen evolution reaction happens in a variety of metals and acids, and
may involve hydrochloric, sulfuric, perchloric, hydrofluoric, formic and
alternative acids. The individual anodic reactions for iron, nickel and
aluminum are listed as follows:

We can write the general anodic reaction occurring throughout corrosion

by examining the above equations as:
CWMC
Corrosion Electrochemistry

The Anodic Method and Corrosion

That is, the corrosion of metal "M" leads to the chemical reaction of metal
"M" to an ion with a valence charge of n+ and therefore the release of "n"
electrons. The worth of n, of course, depends totally on the character of
the metal. Some metals, like silver, are univalent, whereas multivalent
iron, titanium and uranium possess positive charges as high as +6. This
equation is a general one and it applies to any and all corrosion reactions.

Faraday's Law and Corrosion Electrochemistry

If the current generated by one of the anodic reactions expressed earlier

was familiar, it'd be attainable to convert this current to a similar mass loss
or corrosion penetration rate using a helpful relation discovered by
Michael Faraday. Faraday's empirical laws of electrolysis relate the
current of an electrochemical reaction to the quantity of moles of the
element being reacted. Supposing that the charge needed for such a
reaction was one electron per molecule, as is the case for the plating or
the corrosion attack of silver can be shown as:
CWMC
Corrosion Electrochemistry

Faraday's Law and Corrosion Electrochemistry

According to Faraday’s law, the reaction with one mole of silver would
need one mole of electrons, or one Avogadro's number of electrons (6.022
x 1023). The charge carried by one mole of electrons is known as one
Faraday (F). One Faraday equals 96,485 C/(mole of electrons). Uniting
Faraday’s main beliefs with specific electrochemical reactions of
acknowledged stoichiometry gives us the following equation:

Where, N is the number of moles and ΔN is the change in that quantity

n is the number of electrons per molecule of the species being reacted
I is the total current in amperes (A)
t is the period of the electrochemical method in seconds (s)
CWMC
Corrosion Electrochemistry

The Cathodic Method

When hydrogen (H) ions are reduced to their atomic type, they typically
mix, as shown earlier, to provide H gas through a reaction with electrons
at a cathodic surface. This reduction of H ions at a cathodic surface can
disturb the balance between the acidic hydrogen (H+) ions and the base-
forming hydroxyl (OH-) ions, making the solution less acidic, or more
alkaline or basic in this region.

In neutral water, the anodic corrosion of some metals, like Al, Zn or Mg,
creates enough energy to separate water directly, as illustrated within the
following equation and figure:
CWMC
Corrosion Electrochemistry

The Cathodic Method

Figure 2: Electrochemical reactions of Mg during corrosion in neutral

water.

The change in the concentration of H ions, or the increase in hydroxyl

(OH) ions, may be shown by testing pH levels to find surfaces on which
cathodic reactions are taking place. There can be many cathodic reactions
encountered throughout the corrosion process. They include the following:
CWMC
Corrosion Electrochemistry

The Cathodic Method

Oxygen Reduction

:
CWMC
Corrosion Electrochemistry

Oxygen Reduction

Oxygen reduction is a common cathodic reaction because oxygen exists

within the atmosphere and in solutions exposed to the environment.
Although not frequent, metal ion reduction and metal deposition will cause
severe corrosion problems, for instance, the plating of copper ions, which
are created upstream in a water circuit, on on the inner aluminum surface
of a radiator. Therefore, the use of a copper conduit in a water-based
circuit where aluminum is also present should generally be avoided.

All corrosion reactions are merely combinations of one or many of the

above cathodic reactions in conjunction with an anodic reaction. Thus,
each case of liquid corrosion may be reduced to those equations in most
cases, either on an individual basis or in combination. Take into account
the corrosion of Zn (zinc) by water or wet air. By multiplying the Zn
oxidization reaction by 2 and summing this with the oxygen reduction
reaction, one obtains the following equation:
CWMC
Corrosion Electrochemistry

Oxygen Reduction

The products of this reaction are Zn2+ and OH-, which at once react to
make insoluble Zn(OH)2. Likewise, the corrosion of Zn by copper sulphate
represented within the following equation is simply the summation of the
oxidization reaction for Zn and the metal deposition reaction involving
copper (II) ions:
CWMC
Corrosion Electrochemistry

Oxygen Reduction

The products of this reaction are Zn2+ and OH-, which at once react to
make insoluble Zn(OH)2. Likewise, the corrosion of Zn by copper sulphate
represented within the following equation is simply the summation of the
oxidization reaction for Zn and the metal deposition reaction involving
copper (II) ions:

During corrosion, more than one oxidation and one reduction reaction
might take place. In the corrosion of Zn in a concentrated HCL solution
containing dissolved oxygen, for example, two cathodic reactions are
possible. One is an evolution of H, while the other is the reduction of
oxygen.
CWMC
Corrosion Electrochemistry

Oxygen Reduction

Because there are two cathodic reactions or methods that consume

electrons, the general corrosion rate of zinc is overstated. Thus, it is
typically more corrosive than air-free acids, and removing oxygen from
acid solutions can typically make these solutions less corrosive. This is a
typical method for reducing corrosivity in many settings. Oxygen can be
removed by either chemical or mechanical means.

Surface Area Impact

In corroding a piece of metal, the electrons created at anodic areas flow

through the metal to react at cathodic areas that are equally exposed to
the environment where they restore the electrical balance of the system.
The very fact that there is no net accumulation of charges on a corrosion
surface is kind of vital for understanding most corrosion processes and
ways to mitigate them. However, the equality between the anodic and
cathodic currents expressed within the following equation doesn't mean
that the current densities for these currents are equal:
CWMC
Corrosion Electrochemistry

Surface Area Impact

By taking the relative anodic (Sa) and cathodic (Sc) surface areas (and
their associated current densities ia and ic expressed in units of mA/cm2),
this equation can be expressed in terms of current densities.

The importance of the surface area ratio (Sc/Sa) of the above equation is
notably vital when it comes to several varieties of localized corrosion,
such as pitting and stress corrosion cracking. It has great implications for
the occurrence of corrosion in dissimilar metals as well.
CWMC
Corrosion Electrochemistry

Surface Area Impact

It is simple to know that the result of a particular quantity of anodic current

focused on a small area of a metal surface will be much bigger than when
the same quantity of current is dissipated over a much larger area. This
issue is an important amplifying issue of the anodic current when Sc/Sa is
greater than one and a stifling factor when it is less than one.

The cause of current density may be seen when two dissimilar metals are
joined (here it's Cu and Fe), which are shown diagrammatically in Figure 3
below.
CWMC
Corrosion Electrochemistry

Surface Area Impact

Figure 3: The areas affected by dissimilar metals, where "a" shows rivets of steel on
copper plates, while "b" shows rivets of copper on steel plates.
CWMC
Corrosion Electrochemistry

Surface Area Impact

When steel rivets are part of copper plates, the corrosion of the cathodic copper
plates will be low, while the corrosion of the small anodal steel rivets will be high. On
the other hand, if copper rivets are joining steel plates, corrosion on the copper will
be high, while corrosion of the steel plates will hardly be noticeable.
CWMC

Basics of Corrosion - Outline

 Introduction to Corrosion

 Cost of Corrosion

 Basics of Corrosion

 Forms of Corrosion

 Corrosion Electrochemistry

 Corrosion Assessment

151
CWMC
Corrosion Assessment

Without corrosion assessment, mitigating or eliminating corrosion in any

industry is almost impossible.
Corrosion testing is one of a corrosion engineer's most important responsibilities. In
fact, without corrosion assessment, mitigating or eliminating corrosion in any industry
is almost impossible.

There are several reasons for corrosion examination. Sometimes, in a materials

selection process for an industrial application, an evaluation of different kinds of
materials in a specific environment is required. The assessment of a new type of
alloy in different types of environments to compare with conventional commercial
alloys; an estimation of inhibitors' efficiency in reducing the corrosion rate of metals;
and understanding the mechanism of corrosion are the other reasons.

Corrosion tests are usually divided into two main categories: laboratory tests and
field tests, each of which has its merits and demerits. For example, the
environmental conditions present in real-world applications are different to those in
laboratory situations. Therefore, it is difficult to extrapolate the results of laboratory
tests to industry settings. On the other hand, in laboratory tests, it is possible to
accelerate the corrosivity of the environment to obtain results more rapidly,
something that is impossible in field testing.
CWMC
Corrosion Assessment
Laboratory Corrosion Tests

Immersion Testing
One of the most common and simplest methods in laboratory tests is the immersion
test. In this kind of test, whose procedure is clarified by ASTM and NACE, the
weights of dried test specimens are measured by an analytical balance before and
after being exposed to a corrosive environment for a specific period of time. Before
and after samples are weighed, specific preparation should be carried out to remove
any corrosion product or organic contaminants. The corrosion resistance of the
samples is generally calculated as the corrosion rate in terms of weight loss or
thickness loss in mils (0.001 inch) per year (mpy) or millimeters per year (mm/yr).
The results depend on the type of metal (specific weight) being tested, the exposed
surface area, and test duration factors.

Visual Examination
Some visual examination is also suggested to evaluate localized corrosion like
pitting or exfoliation. Furthermore, optical or scanning electron microscopes;
elemental and compositional analyses such as energy dispersive X-ray
spectroscopy (EDX); X-ray diffraction (XRD); and energy dispersive X-ray
spectroscopy (XPS) are useful techniques to evaluate the corroded surface and
corrosion product more precisely.
CWMC
Corrosion Assessment
Laboratory Corrosion Tests

Visual Observations

Immersion Tests
CWMC
Corrosion Assessment
Laboratory Corrosion Tests

Visual Examination
There are several ways to evaluate the pitting corrosion of tested samples.
Determining the density of pits (number of pits in a specific surface area) or pitting
factor (ratio of depth of deepest pit divided by the value of thickness loss due to
uniform corrosion) are two important methods to evaluate pitting corrosion. There are
different types of practical tools to measure pit depth. A contour gage can be used to
achieve a profile of pit depth when it is impossible to use pit gages.

Salt Spray / Fog Testing

Some test samples and procedures are designed to assess specific kinds of
corrosion, such as crevice corrosion, stress-corrosion cracking, and erosion
corrosion. The atmospheric corrosion of coated samples can be examined by salt
spray or fog testing. Here, a 5% NaCl solution is atomized in a chamber at a
temperature adjusted to 95°F (35°C). The time that samples can resist against
corrosion is the criterion used to understand test sample durability. Although the
environment in a salt spray test is a kind of accelerated marine atmosphere, it is
accepted that the salt spray results could extrapolate to other atmospheric
environments.
CWMC
Corrosion Assessment

Salt Spray Test Chamber Weathering Test Chamber

CWMC
Corrosion Assessment
Laboratory Corrosion Tests

The Weathering Test

In another atmospheric method called the weathering test, the durability of organic,
paint-coated samples is examined by exposing them to UV light and cyclic cooling-
heating along with a corrosive environment.

Electrochemical Testing
Electrochemical tests are the other category of laboratory tests that can provide
valuable information about corrosion electrochemical reactions and the mechanisms
behind them. Apotentiostat instrument is usually used to perform this sort of test. A
three-electrode setup, including working electrode, reference electrode, and counter
(auxiliary) electrode, are usually used. Potential, current, and time are three
important parameters in electrochemical tests. In these tests, an applied potential
generally scans in a certain range and the current is measured.

There are various types of electrochemical corrosion tests. Each of them is used for
a particular purpose.
CWMC
Corrosion Assessment

Electrochemical Tests using

Potentiostat/Galvanostats
CWMC
Corrosion Assessment
Laboratory Corrosion Tests

Electrochemical Testing

Linear Polarization Resistance (LPR):

The simplest electrochemical corrosion test islinear polarization resistance, in which
current is measured when the applied potential scans in a narrow range (~20 mV)
from lower to higher than corrosion potential (Ecorr). The slope of the current versus
the potential curve shows the polarization resistance, which is inversely related to
the corrosion rate. This test is very fast and straightforward, and is usually accepted
as a kind of non-destructive test. Moreover, this method is very useful to measure
extremely low corrosion rates. This is important in some industrial systems such as
food processing, nuclear, and pharmaceutical equipment
.
CWMC
Corrosion Assessment
Laboratory Corrosion Tests

Electrochemical Testing

Linear Polarization Resistance (LPR):

.

This image shows the LPR curve. The slope of the line shows polarization
resistance(Rp).
CWMC
Corrosion Assessment
Laboratory Corrosion Tests

Electrochemical Testing

Potentiodynamic Polarization Tests: The passivation behavior of active-passive

metals, like stainless steels, can be evaluated by potentiodynamic polarization tests.
In this method, the potential scans in a wide range. Critical current density, passive
potential, and passive current density can be extracted from this test.

Cyclic Polarization Method: This is the other kind of test that is used to determine
the tendency of active-passive metals to localized crevice or pitting corrosion. In this
test, the sweeping direction of applied potential is reversed at some potential in the
transpassive region. The intersection between forward and backward scans shows
the tendency and intensity of localized corrosion.
CWMC
Corrosion Assessment
Laboratory Corrosion Tests

Electrochemical Testing

This image shows the cyclic polarization curve, which is used to evaluate pitting
corrosion. Less ER and a bigger metastable pitting loop shows more susceptibility to
pitting corrosion.
CWMC
Corrosion Assessment
Laboratory Corrosion Tests

Electrochemical Testing

The Electrochemical Potentiodynamic Reactivation (EPR) Test:

This is the other test that has been suggested to predict the tendency of stainless
steels to sensitization or intergranular corrosion. Electrochemical potentiodynamic
reactivation is very simple and fast in comparison to other conventional intergranular
corrosion tests that are suggested by ASTM A-262, such as Huey or Streicher.

The abovementioned electrochemical tests are conducted under DC conditions.

However, understanding the Helmholtz double layer (which can act as a
capacitance) or adsorption of inhibitors on the metallic surface (which can act as an
inductance) requires alternative currents. This type of test is called Electrochemical
Impedance Spectroscopy (EIS), and can reveal valuable information about corrosion
mechanisms. Moreover, this technique is very useful when the overall electrical
resistance in the electrochemical system is very high, such as when a sample is
covered by thick organic coatings or is immersed in organic solutions.
CWMC
Corrosion Assessment
Field Corrosion Tests

Corrosion Coupons
Installing corrosion coupons is a very simple and common method for monitoring
corrosion in pipelines, heat exchangers and storage tanks. The coupons are inserted
into a plant or equipment with a coupon holder for a period of time. Although many
factors can influence the location of coupon installation, the coupons are usually
placed in locations where severe corrosion is expected. The change in weight and
size or visual inspection will be considered after retrieving the coupons. The
drawback to this method is that it is impossible to accelerate the environmental
conditions to achieve faster results.

Ultrasonic Thickness Monitoring

An ultrasonic thickness (UT) gage is one of the instruments used to monitor the
internal corrosion of pipelines or storage tanks. The ultrasonic sound wave, which is
produced by an ultrasonic transducer, traverses to the back wall and reflects back to
the source, making it possible to calculate the thickness of metal by measuring the
reflection time and considering the velocity of sound waves in tested material. The
UT gage test is useful when there is no access to both sides of a test specimen.
CWMC
Corrosion Assessment

Corrosion rate meter

UT Testing for Corrosion

Corrosion Coupons
CWMC
Corrosion Assessment
Field Corrosion Tests

Electrical Resistance Testing

Electrical resistance (ER) probes are used to measure the corrosion rate of coupons,
especially when the on-line corrosion rate in required. When a metal corrodes in an
environment, its electrical resistance will increase due to a reduction in the thickness
or surface area of a cross-section. By measuring the change in electrical resistance
of metal over time, the rate of metal dissolution can be determined and the corrosion
rate can be calculated in mpy or mm/yr. The ER probes can be used in any kind of
environment, including aqueous solutions, oil (hydrocarbons), soil, gas, and
atmosphere. The probe can be produced in various geometries depending on the
type of metals, system, and environment being tested.

There are many other tests that are used to monitor the structures protected against
corrosion by cathodic protection. Most of these tests are based on the measurement
of the electrochemical potential of structure versus environment.
CWMC
Corrosion Assessment

Pipe to soil potential measurement

Electrical Resistance Probes

CWMC
Corrosion Monitoring

Introduction to Corrosion Monitoring

What is Corrosion Monitoring?

The field of corrosion measurement, control, and prevention covers a very

broad spectrum of technical activities. Within the sphere of corrosion control
and prevention, there are technical options such as cathodic and anodic
protection, materials selection, chemical dosing and the application of
internal and external coatings. Corrosion measurement employs a variety of
techniques to determine how corrosive the environment is and at what rate
metal loss is being experienced. Corrosion measurement is the quantitative
method by which the effectiveness of corrosion control and prevention
techniques can be evaluated and provides the feedback to enable corrosion
control and prevention methods to be optimized.
CWMC
Corrosion Monitoring
A wide variety of corrosion measurement techniques exists, including:

Non Destructive Testing Analytical Chemistry

• Ultrasonic testing • pH measurement
• Radiography • Dissolved gas (O2, CO2, H2S)
• Thermography • Metal ion count (Fe2+, Fe3+)
• Eddy current/magnetic flux • Microbiological analysis
• Intelligent pigs

Operational Data Fluid Electrochemistry

• pH • Potential measurement
• Flow rate (velocity) • Potentiostatic measurements
• Pressure • Potentiodynamic measurements
• Temperature • A.C. impedance

Corrosion Monitoring
• Weight loss coupons
• Electrical resistance
• Linear polarization
• Hydrogen penetration
• Galvanic current
CWMC
Corrosion Monitoring
Some corrosion measurement techniques can be used on-line, constantly
exposed to the process stream, while others provide off-line measurement,
such as that determined in a laboratory analysis. Some techniques give a direct
measure of metal loss or corrosion rate, while others are used to infer that a
corrosive environment may exist.

Corrosion monitoring is the practice of measuring the corrosivity of process

stream conditions by the use of “probes” which are inserted into the process
stream and which are continuously exposed to the process stream condition.
Corrosion monitoring “probes” can be mechanical, electrical, or electrochemical
devices.

Corrosion monitoring techniques alone provide direct and online measurement

of metal loss/corrosion rate in industrial process systems.

Typically, a corrosion measurement, inspection and maintenance program used

in any industrial facility will incorporate the measurement elements provided by
the four combinations of on-line/off-line, direct/indirect measurements.
CWMC
Corrosion Monitoring
• Corrosion Monitoring Direct, On-line
• Non Destructive Testing Direct, Off-line
• Analytical Chemistry Indirect, Off-line
• Operational Data Indirect, On-line
CWMC
Corrosion Monitoring
The Need for Corrosion Monitoring

The rate of corrosion dictates how long any process equipment can be usefully
and safely operated. The measurement of corrosion and the action to remedy high
corrosion rates permits the most cost effective plant operation to be achieved
while reducing the life-cycle costs associated with the operation.

Corrosion monitoring techniques can help in several ways:

(1) by providing an early warning that damaging process conditions exist which
may result in a corrosion induced failure.
(2) by studying the correlation of changes in process parameters and their effect
on system corrosivity.
(3) by diagnosing a particular corrosion problem, identifying its cause and the rate
controlling parameters, such as pressure, temperature, pH, flow rate, etc.
(4) by evaluating the effectiveness of a corrosion control/prevention technique
such as chemical inhibition and the determination of optimal applications.
(5) by providing management information relating to the maintenance
requirements and ongoing condition of plant.
CWMC
Corrosion Monitoring
Corrosion Monitoring Techniques

A large number of corrosion monitoring techniques exist. The following list

details the most common techniques which are used in industrial applications:

• Corrosion Coupons (weight loss measurements)

• Electrical Resistance (ER)

• Linear Polarization Resistance (LPR)

• Galvanic (ZRA)

• Hydrogen Penetration

• Microbial

• Sand/Erosion
CWMC

Corrosion Control Basics

CWMC
Corrosion Control
Introduction

Corrosion of metals causes damages worth millions, of Rupees both in terms of

direct losses and indirect losses due to outage in the industry. Many instances
corrosion could lead to severe disasters, which could cause loss of precious lives.
The basic aim of all corrosion studies is to minimize losses due to corrosion both
to human life and to costly equipment. Corrosion control or prevention methods
are employed in various industries to achieve this objective.

The important means of controlling or preventing corrosion are discussed below.

The choice of corrosion control method to be employed depends on (not
necessarily in order of preference)

 Nature of corrosion
 Criticality of corrosion problem
 Economics of controlling
 Technology available
 Effect on other material/equipment in the system/industry
 Nature of effluents likely to be discharged.
 Effect of control measure on the process etc.
CWMC
Corrosion Control

Selection of Alloy/Material:
The most common method of preventing corrosion is the selection of the proper
metal or alloy for a particular corrosive service. One of the most popular mis-
conceptions to those not familiar with metallurgy or corrosion engineering,
concerns the uses and characteristics of stainless steel. Stainless steel is the
generic name for a series of different alloys containing 11.5 to 30% chromium and
0 to 22% nickel together with other alloy additions. Though highly corrosion
resistant in most of the common environments in service, they are more
susceptible to localized corrosion such as intergranular corrosion attack than the
ordinary steels.

In alloy selection, there are several "natural" metal corrosive combinations. These
combinations of metal and corrosives usually represent the maximum amount of
corrosion resistance for the least amount of money..
There are some general but usually accurate rules, which may be applied to the
resistance of metals and alloys. For reducing or non-oxidizing environments such
as air-free acids and aqueous solutions, nickels, copper and their alloys are
frequently employed. For oxidizing condition, chromium-containing alloys are
used, for extremely powerful oxidizing conditions, titanium and its alloys have
shown superior resistance.
CWMC
Corrosion Control
Alteration of Environments:
Changing Mediums
Altering the environment provides a versatile means for reducing corrosion.
Typical changes in the medium that are often employed are (1) Lowering
temperature (2) decreasing velocity (3) removing oxygen or oxidizers and (4)
changing concentration. In many cases, these changes can significantly reduce
corrosion, but they must be done with care.
Lowering temperature
This usually causes a pronounced decrease in corrosion rate. However, under
some conditions, temperature changes have little effect on corrosion rate. In other
cases, increasing temperature decreases attack. This phenomenon occurs as hot
fresh or salt water is raised to the boiling point and is the result of the decrease in
oxygen solubility with temperature.
Decreasing Velocity

This is often used as practical method of corrosion control. Velocity generally

increases corrosive attack, although there are some important exceptions. Metals
and alloys that passivate such as stainless steels generally have better resistance
to flowing mediums than stagnant solutions. Very high velocities should be always
avoided where possible, because of erosion-corrosion effects.
CWMC

Corrosion Control
Alteration of Environments:

Removing Oxygen or oxidizers:

This is a very old corrosion-control technique. Boiler feed water was de-aerated
by passing it through a large mass of scrap steel. In modern practice this is
accomplished by vacuum treatment, inert gas purging, or through the use of
oxygen scavengers. Hydrochloric acid containing iron (Fe Cl3), as impurity is
highly corrosive to nickel molybdenum alloys (hastalloy B, Chlorimet 2) which is
otherwise highly corrosion resistant to pure hydrochloric acid. Although Dearation
finds vide spread application, it is not recommended for active-passive metals or
alloys. These materials require oxidizers to form and maintain their protective films
and usually possess poor resistance to reducing or non- oxidizing environments.

Changing concentration:

Decreasing corrosive concentration is usually effective. In many processes, the

presence of corrosive is an accidental. Many acids, such as sulphuric and
phosphoric are virtually inert at high concentrations at moderate temperatures. In
these cases, increasing acid concentration can reduce corrosion.
CWMC
Corrosion Control
Alteration of Environments:

Inhibitors:

An inhibitor is a substance which, when added in small concentration to an

environment, decreases the corrosion rate. In a sense, an inhibitor can be considered
as a retarding catalyst. There are numerous inhibitor types and compositions. The
broad classification of inhibitors is as follows:

Adsorption type inhibitors :

These represent the largest class of inhibiting substances. In general, these are
organic compounds that adsorb on the metal surface and suppress metal dissolution
and reduction reactions. In most cases it appears that adsorption inhibitors affect
both the anodic and cathodic processes, although in many cases the effect is
unequal. Typical examples of this type are the organic amines.

Hydrogen-evolution reaction :

These substances such as arsenic and antimony ions specifically retard the hydrogen
evaluation reaction. As a consequence, these substances are very effective in acid
solutions but are in effective in environments where other reduction processes such
as oxygen reduction are the controlling cathodic reactions.
CWMC
Corrosion Control

Alteration of Environments:

Scavengers:

These substance act by removing corrosive reagents from solution. Example of

this type of inhibitor are sodium sulfite and hydrazine which remove dissolved
oxygen from aqueous solutions as indicated in following equations:

2 Na2 SO3 + O2 2 Na2 SO4

N2 H4 + O2 N2 + 2H2 O

It is apparent that such inhibitors will work very effectively in solutions where
oxygen reduction is the controlling corrosion cathodic reaction but will not be
effective in strong acid solutions.

Oxidizers:

Such substances as chromate, nitrate and ferric salts also act as inhibitors in
many systems. In general, they are primarily used to inhibit the corrosion of
metals and alloys, which demonstrate active-passive transitions. Such as iron and
its alloys and stainless steels.
CWMC
Corrosion Control
Alteration of Environments:
Vapour Phase Inhibitors:
These are very similar to the organic adsorption type inhibitors and possess a
very high vapour pressure. As a consequence, these materials can be used to
inhibit atmospheric corrosion of metals without being placed in direct contact with
the metal surface. In use, such inhibitors are placed in the vicinity of the metal to
be protected, and sublimation and condensation transfer them to the metal
surface. The vapour phase inhibitors are usually only effective if used in closed
spaces such as inside packages or on the interior of machinery during shipment.
These are also known as temporary rust preventives, though there are other
substances also which act as temporary rust preventives to metals and alloys
under shipment and storage.
Design:
The design of a structure is frequently as important as the choice of materials of
construction. Design should consider mechanical and strength requirements
together with an allowance for corrosion. In all cases, the mechanical design of a
component should be based on the material of construction. This is important to
recognize, since materials of construction used for corrosion resistance very in
their mechanical characteristics.
CWMC
Corrosion Control
Design:
Wall thickness:
Since corrosion is a penetrating action, it is necessary to make allowances for this
reduction in thickness in designing pipes tanks and other components. In general,
wall thickness is usually made twice the thickness that would give the desired life.
Such a design factor allows for some variation in the depth of penetration during
uniform corrosion. Which in most cases is not completely uniform.
Design rules:
Some important design rules for best corrosion resistance are as follows:
 Weld rather than rivet tanks and other containers riveted joints provide sites for
crevice corrosion,
 Design tanks and other containers for easy draining and easy cleaning. Tank
bottom should be sloped toward drain holes so that liquids cannot collect after the
tank is emptied.
 Design systems for the easy replacement of components that are expected to
fail rapidly in service.
 Avoid excessive mechanical stresses and stress concentrations in components
exposed to corrosive mediums. Mechanical or residual stresses are one of the
requirements for stress corrosion cracking.
CWMC
Corrosion Control
Design:

Design rules:

 Avoid electric contact between dissimilar metals to prevent galvanic corrosion. If

possible, use similar materials through out the entire structure, or insulate
difference materials from one another
 Avoid sharp bends in piping systems. Sharp bends and other areas where fluid
direction changes rapidly tend to promote erosion-corrosion. This is particularly
important in systems susceptible to erosion-corrosion such as lead, copper and
their alloys.
 Avoid hot spots during heat transfer operations, uneven temperatures
distribution leads to local heating and high corrosion rates. Further, hot spots tend
to produce stresses that may produce stress corrosion cracking failures.
 Design to exclude air. Oxygen reduction is one of the most common cathodic
reactions during corrosion and if oxygen is eliminated, corrosion can often be
reduced or prevented, and active passive metals are exception.
 The most general rule for design is to avoid heterogeneity. Dissimilar metals,
Vapour spaces, uneven heat and stress distributions, and other differences
between point in the system lead to corrosion damage. Hence in design attempt
to make all conditions as uniform as possible throughout the entire system.
CWMC
Corrosion Control
Electrochemical Protection:
Cathodic Protection:
Cathodic protection is achieved by supplying electrons to the metal structure to be
protected. The principles of cathodic protection may be explained by considering the
corrosion of a typical metal M in an acid environment. Electro-chemical reactions occurring
are the dissolution of the metal and the evolution of hydrogen gas as per the following
equations:
M = M+n Na
2 H+ + e = H2
Considering these equations it can be seen that addition of electrons to the structure will
tend to suppress metal dissolution and increase rate of hydrogen evolution. If current is
considered to flow(+) to (-) as in conventional electrical theory, than a structure is protected
if current enters it from the electrolyte. Conversely, accelerated corrosion occurs if current
passes from metal tot he electrolyte. This current convention has been adopted in cathodic
protection technology.
There are two ways to cathodically protect a structure (1) by an external power supply
(Impressed current) or (2) by appropriate galvanic coupling (sacrificial system). In the
impressed current system the negative terminal of the DC power supply is connected to
system to be protected and positive terminal is connected to an inert or insoluble anode (S)
such as graphite.
CWMC
Corrosion Control
Electrochemical Protection:
Cathodic Protection:
Electrolyte such as water or moist soil serves as a medium for transfer of electrons to the
structure to be protected. In the sacrificial systems and anode of a metal which is more
active then the metal to be protected, such as zinc or Aluminium for iron, is electrically
connected to the structure to be protected. In such cases the anode gets dissolved in the
electrolyte thereby generates electrons, which protect the structure in question.
One of the major precaution in case of cathodic protection is that, protecting current
should be so selected, that neither the system is under protected (in which case severe
localized corrosion can take place at places where current density of the protecting
current is lower, nor is over protected (in which case there is a danger of Hydrogen
embrittlement damage occurring to the system). The design of cathodic protection
requires knowledge of the following:
 Areas of the structure to be protected
 Size & shape of the structure to be protected.
 Composition of the electrolyte under which the system is to be protected.
 Susceptibility of the structure to hydrogen embrittlement
 Type of coating provided
 Other material in close proximity of the structure to be protected.
CWMC

Corrosion Control
Electrochemical Protection:

Cathodic Protection:

In many instances an anode such as zinc in pure form gets passivated in the corrosive
medium (such as fresh water and in such cases it leads to accelerated corrosion of the
structure in question. To avoid such situations certain elements are added to anode
metal, which accelerate corrosion of sacrificial anode. Sometime the shape of the
structure is quite complex and simple anodes may not provide full protection due to
insufficient potential distribution at all points. In such cases it becomes difficult to attain
uniform potential and by placing normal anodes will lead to accelerated corrosion of the
places where the current distribution is very low (such as crevices, joints etc.). To over
come this sometimes use of an inert electrode, which matches the shape of the structure
to be protected, is made by connecting it to the main anode. These inert electrodes are
known as auxiliary anodes.

Use of organic coatings over the structure reduces the requirements of protection
current. Many times cathodic protection provided to underground pipeline results in
generation of stray current which causes accelerated corrosion of other & burried
pipeline or underground structure.
CWMC
Corrosion Control
Electrochemical Protection:
Anodic Protection:
Anodic protection is based on the principle of formation of a protective film on metals by
externally applied-anodic currents. Considering equations given under cathodic
protection, it appears that the application of anodic current to a structure should tend to
increase the dissolution rate of a metal and decrease the rate of hydrogen evolution. This
usually does occur except for metals with active-passive transitions such as nickel, iron
chromium titanium and their alloys. If carefully controlled. Anodic currents are applied to
these materials, they are passivated and the rate of metal dissolution is decreased. To
anodically protect a structure, a device called a Potentiostat is required. A Potentiostat is
an electronic device, which maintains a metal at a constant potential with respect to a
reference electrode. The Potentiostat has three terminals, one is connected to the
structure to be protected, another to an auxiliary cathode (a platinum or platinum clad
electrode) and the third to a reference electrode (e.g. saturated calomel electrode). In
operation, the Potentiostat maintains a constant potential between the structure and the
reference electrode. The optimum potential for protection is determined by
electrochemical measurements.
Although anodic protection is limited to passive metals and alloys, most structural
materials contain these elements. The primary advantages of anodic protection are its
applicability in extremely corrosive environments and its low current requirements.
CWMC
Corrosion Control
COATINGS:

Surface preparation:

Surface preparation is a critical part of the coatings operation and must provide a surface that is
compatible with the coating material to be applied. The major considerations concern the cleanliness
of the surface required and the surface area to be obtained.

Material Selection:

A coating combination must be specified after testing that will provide proper stability during the life
of the project. The number of coats to be applied, the compatibility of the various coats, and the
requirements for their maintenance must be considered.

Application:

The coating material must have application characteristics that would allow its proper application
under all conditions existing during the coating process. Proper identification of the physical or other
properties of the film expected from the application must be specified.

Economics:
Economics of different alternative methods should be well considered before selecting a particular
coating.
CWMC

Corrosion Control
COATINGS:

Metallic Coatings:

Metal coatings are applied by Electro-deposition, flame spraying, cladding, hot dipping
and vapour deposition.

Electrodeposition:

This process, also called electroplating, consists of immersing a part to be coated in a

solution of the metal to be plated and passing a direct current between the part and
another electrode. The character of the deposit depends on many factors including
temperature, current density, time and composition of the both. These variables can be
adjusted to produce coatings that are thick or thin, dull or bright, soft or hard and ductile
or brittle. Hard platings are utilized to combat erosion-corrosion. The electroplate can be
a single metal, layers of several metals or even an alloy composition.
CWMC

Corrosion Control
COATINGS:

Flame Spraying:

This process, also called Metallizing, consists of feeding metal wire or powder through a
melting flame so the metal, in finally divided liquid particles, is blown on to the surface to
be protected. Oxygen and acetylene or propylene is commonly used for the melting
flame. The coatings are usually porous and are not protective under severe wet corrosive
conditions. Generally the porosity decreases with the melting point of the metal. The
surface to be sprayed must be roughened (sand blasted) to obtain a mechanical bond.
Sometimes paint coating is applied over the sprayed metal to fill the voids and provide a
better barrier.
High melting metals may be deposited by plasma jet spraying. These are used for special
applications such as for coating composite materials, alloys etc. for Erosion, Corrosion
resistance. Examples are Alumina, Zirconia, Stainless Steels, Gas turbine coatings,
cavitation resistant coatings etc.
CWMC

Corrosion Control
COATINGS:

Vapour Deposition:

This is accomplished in a high vacuum chamber. Heating it electrically, and the vapour
deposits on the parts to be coated vaporize the coating metal. This method is more
expensive than others and generally limited to "critical" parts such as high strength parts
for missiles and rockets. More recently Chromium deposition over steel is done for making
automobile headlight reflectors.

Diffusion:

Diffusion coatings involve heat treatment to cause alloy formation by diffusion of one
metal into the other. For this reason the process is also termed “surface alloying”. Parts to
be coated are packed in solid materials or exposed to gaseous environments, which
contain the metal that forms the coating. Sherardizing (zinc coating), chromizing
(chromium) and anodizing (Aluminium) are examples. Calorizing and chromizing are
utilized mainly for resistance to high temperature oxidation.
CWMC

Corrosion Control
COATINGS:

Chemical Conversion:

“Corroding” the metal surface to form an adherent and protective corrosion product
produces coatings from chemical conversion. Anodizing consists of anodic oxidation in an
acid both to build up oxide layer. Aluminium is one such example. Sufficient corrosion
resistance is not obtained, so anodized Aluminium should not be used where untreated
Aluminium would show rapid attack. The surface layer is porous and provides good
adherence for paints. The anodized surface can be “sealed” by exposing to boiling water.

Additional example are bondorizing and parkerizing (phosphatizing) in a phosphoric acid

both), chromatizing (exposure to chromic acid and di-3chromates) and oxide or heat
coatings for steel. Phosphating provides a good base for painting and also provides for
some corrosion resistance to the steel before rusting can take place in the event of failure
of the paint finish.
CWMC

Corrosion Control
COATINGS:

Organic Coatings:

These involve a relatively thin barrier between substrate material and the environments.
Paints, varnishes lacquers and similar coatings protect more metal on a tonnage basis
than any other method for combating corrosion. Extension surfaces are most familiar, but
inner coatings or linings are also widely utilized. As a general rule, these coatings should
not be used where the environment would rapidly attack the substrate material.

Aside from proper application, the three main factors to be considered for organic
coatings are (1) surface preparation (2) selection of primer or priming coat and (3)
selection of coat. If the metal surface is not properly prepared, the paint may peel off
because of poor bonding. If the primer does not have good adherence or is not
compatible with the topcoat, early failure occurs. Poor performance is, in most cases due
to poor application and surface preparation.

Surface preparation involves surface roughening to obtain mechanical bonding as well as

removal of dirt, rust mill scale, oil, grease, welding flux, crayon marks, and other
impurities. In other words a clean rough surface is needed.
CWMC

Corrosion Control
COATINGS:

Organic Coatings:

Best method is to grit blast or sand blast the steel surface. Other methods are pickling
and other types of chemical treatments, scrapping wire brushing, flame cleaning,
chiseling and clipping. In addition to economic considerations, the selection of surface
preparation method depends upon the metal to be painted the shape, size and
accessibility of the structure, the coating system and the service conditions.

Primers can contain rust-inhibitive pigments such as zinc chromate and zinc dust and
there by provide another function in addition to acting as barriers. Wet-ability is needed
so that crevices and other surface defects will be filled rather than bridged short drying
times, are advantageous to preclude contamination before top coats are applied,
particularly in field applications.

Topcoat selection is important. Many times paint is applied primarily for appearance.
Good appearance and good corrosion resistance in severe atmospheres can be obtained
at a justifiable cost by selecting a good topcoat material.
CWMC

Corrosion Control
COATINGS:

Organic Coatings:

The coating thickness must be such that bare metal is not exposed. It is almost
impossible, to apply one coat of paint and have it completely free of fine holes or other
defects. Thickness is important also because paint deteriorates or weathers with time.

A large number of paints are available. Asphalt and bituminous paints are often used on
pipelines. Sometimes a cloth wrapping is used with the coating for reinforcement. Alkyds,
glyptals, concrete, red lead, iron oxide, phenolics, lithopanes, titanium dioxide paints and
chlorinated rubber are just a few examples. Vinyl and epoxy paints have been widely
adopted for corrosion applications.

Many methods of applying a coating are available. The familiar techniques involve
brushing, rolling, dipping, and painting of coatings of acrylic, acrylic latex or varnish
bases. The important techniques are:
CWMC

Corrosion Control
COATINGS:

Organic Coatings:

(1) Brushing and rolling (2) palming (3) dipping (4) air spraying (5) airless spraying (6)
electrostatic spraying (7) electro-phoratic painting (8) fluidized bed (9) Powder spraying
(10) flame spraying (11) Trowelling (12) vacuum deposition (13) calandered or chat
lining.

These are broad indications of different Corrosion Control Techniques that are used in
Industries. More details of these Control techniques would be discussed in subsequent
presentations.
CWMC
Corrosion prevention
CWMC

Material Selection

198
CWMC

Materials Selection
• Basic Groups • Balance of “+”s and “-”s
– Metals – Corrosion Resistance
– Plastics – Availability
– Ceramics – Mechanical Properties
– Elastomers – Cost
– Coatings – Code Compliance
– Linings – Fabricability
– Repair options
CWMC

Materials Selection – Metals

• Carbon steels
• Aluminum (3000, 5000 and 6000 series)
• Coppers, brasses, and bronzes
• Stainless steels
– Austenitic (200 and 300 series)
– Martensitic (400 series)
– Precipitation hardening (17-4 PH)
– Duplex (2101, 2205, 255, and 2507)
• Nickel Alloys (600 series, C, B, X, Inconel®,
Hastelloy®)
• Titanium alloys (common grades 2, 5, 7, and 11)
• Zirconium
CWMC

Materials Selection - Plastics

• HDPE and Polypropylene (low end)

• PVC and CPVC
• Resins – epoxy, vinyl esters
• Fluoropolymers
– PTFE, PFA, FEP, ETFE, PVDF
• PEEK (high end)
• Can be used as monolithic or composite
pieces in equipment
CWMC

Material Selection - Coatings

• Coatings (thin or thick films)

– Many different technologies
– Always have holidays
– With or without reinforcement?
• Linings (how to anchor)
– What’s your permeation rate?
– Differences in thermal expansion rates
– How do you clean?
CWMC

Materials Selection -
Ceramics
• Concrete
• Acid proof bricks and mortar
• Refractory
• Glass lined steel
• Alumina
• Silicon carbide
• Silicon nitride
CWMC

Materials Selection -Elastomers

• Natural Rubbers • Compatibility
• Nitriles • Temperature limit
• Neoprenes • Mechanical properties
• Polyurethanes • Availability
• EPDM • Supply Chain
• Silicones • Identification
• Viton® • Specifications
• Kalrez® or Chemraz®
CWMC

Gaskets & Sealing

• Gasketing has many options

• CNA Fiber sheet gaskets (250°F limit)
• Rubber sheet goods (250°F to 350°F limits)
• PTFE sheets and composites (350°F limit)
• Expanded PTFE products (600°F limit)
• Graphite gaskets (600°F limit)
• Spiral wounds (rings, windings and fillers)
• Ring joints (for high T & P)
• Specialty materials (Thermiculite®, Cogebi®)
CWMC

Know Your Service

• Know your process conditions?

• What are the upset conditions?
• What are your projected lives for process?
• What external/environmental factors?
• How are you going to clean your
equipment?
CWMC

Last quote that sums it up for

Corrosion!

• ―…..until you return to the ground from

which you were taken, for you are dirt and
to dirt you shall return‖ (Genesis 3:19)
CWMC

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