Unit 4: Corrosion and its Control

What we are going to learn in this unit ?

 What is corrosion ?
 Why its happened ?
 What is the mechanism ?
 Types of corrosion ?
 What are the factors influencing corrosion ?
 How to control corrosion ?
 Corrosion ??

Reason ?

Stability of the metals

Where do they come from ?

What is the most stable from ?
 Corrosion
Most of the metals exist in nature in combined form as their oxides,
carbonates, sulphides etc. in their ores. Metals are extracted from their
ores and for that high amount of energy is required.

Therefore the metals can be regarded as excited state than their ores. So they
have a tendency to revert back to combined state when exposed to environment
and the destruction and deterioration of the metal starts at surface.

Any process of deterioration (or destruction) and consequent loss of a solid

metallic material, through an unwanted (or unintentional) chemical or
electrochemical attack by its environment, starting at its surface is called
Corrosion
E.g. rusting of iron (Fe3O4), green film on copper surface (CuCO3 + Cu(OH) 2)
 Types of Corrosion ??
Majorly classified based on the way corrosion takes place due to the surrounding

Dry corrosion

Wet corrosion
 Dry or Chemical corrosion
Direct chemical action of environment/atmospheric gases such as oxygen,
halogen, hydrogen sulphide, sulphur dioxide or anhydrous inorganic liquid
with metal surfaces in immediate proximity.

Oxidation corrosion
By direct action of oxygen at low or high temperatures on metals, usually in absence
of moisture. Alkali and alkaline-earth metals can be oxidized at relatively low
temperature. At high temperatures, almost all metals got oxidized (except Ag, Pt
etc.)

Mechanism- Oxidation corrosion

Mechanism
Oxidation occur first at the surface of the metal and the resulting metal oxide scale
forms a barrier

For oxidation to continue, either the metal must diffuse outwards or oxygen must
diffuse inwards through the scale to the underlying metal

Both transfer can occur but the outward diffusion of metal is more rapid than
the inward diffusion of oxygen, since the metal ions are usually smaller than
the oxide ion and hence have higher mobility
When the oxidation starts, a thin layer of oxide is formed on the
metal surface and the nature of this film decides further action

If the film is
Stable
A stable layer is fine-grained in structure and can get adhered tightly to the
parent metal surface. So it can be of impervious nature (inhibits penetration
of attacking oxygen to the underlying metal) and can behaves as protective
coating. E.g. Oxide films on Al, Sn, Pb, etc.
stable

protective coating

If the film is
Unstable
Oxide layer formed, decomposes back into the metal and oxygen
Consequently, oxidation corrosion is not possible in this case (e.g. Ag, Au,
Pt etc.)
Volatile
Oxide layer volatilizes as soon as it is formed, thereby leaving the
underlying metal surface exposed for further attack.

This causes rapid, continuous and excessive corrosion (eg. MoO3)

Porous
Having pores or cracks. In such cases, the atmospheric oxygen have
access to the underlying surface of the metals and thereby
corrosion continues unobstructed, till the entire metal is completely
converted to its oxide
 Types of Corrosion
Majorly classified based on the way corrosion takes place due to the surrounding

Dry corrosion

Wet corrosion
 Wet corrosion

OR
Electrochemical corrosion

Electrochemical set up
 Wet Corrosion
It occurs:
 Where a conducting liquid is in contact with metal or
 When two dissimilar metals or alloys are either immersed or
dipped partially in a solution

This corrosion occurs due to the of separate anodic or cathodic

areas/parts, between which current flows through the conducting
solution

At anodic area, oxidation (liberation of electron) occurs, so anodic metal is

destroyed by either dissolving or assuming combined state (such as oxide
etc.). Hence corrosion always occurs at anodic areas. On the other hand,
at cathodic areas, reduction takes place: dissolved constituents in the
conducting medium accepts the electrons and form some ions like OH-, O2-

The metallic ions (at anodic part) and non-metallic ions (formed at
cathodic part) diffuse towards each other through conducting medium
and form a corrosion product. The electrons set free at anode flow
through the metal and finally consumed in the cathodic reaction.
 Mechanism of Corrosion:
An Electrochemical Phenomenon
Flow of electron between anodic and cathodic areas

At Anodic area:
Dissolution of metal with liberation of electron (oxidation)

At Cathodic area:
Consumption of electron (reduction) either by
Evolution of hydrogen or
Absorption of oxygen

Evolution of hydrogen: Occurs in acidic environment

Displacement of hydrogen ions from acidic solution. All metals above
hydrogen in electrochemical series can undergo this type of corrosion. In
this type of corrosion, anodes usually have very large areas where cathodes
are of small areas.

Absorption of oxygen
Eg. Rusting of iron in neutral aqueous solution of electrolyte (NaCl solution) in
presence of atmospheric oxygen
Surface of iron is coated with a thin film of iron oxide. If some cracks
developed, then anodic areas are created on the surface. Anodic areas are
small surface parts; while nearly the rest of the surface of the metal forms
large cathode

Fe2+ and OH- ions will diffuse and then combine to form the precipitate of
ferrous hydroxide. Smaller Fe2+ will diffuse more rapidly than larger OH-
ions. So corrosion occurs at the anode and rust deposited at or near the
cathode.
If enough oxygen is present, ferrous hydroxide will be oxidized to form
ferric hydroxide, which will form yellow rust

If supply of oxygen is limited, the corrosion product will be black

anhydrous magnetite (Fe3O4)
Effect of increasing oxygen content
Forces the cathodic reaction producing more OH- ions
As it removes more electrons, so accelerates the corrosion at anode
Combination of these reactions, will cause more corrosion and rust-
formation
 Galvanic Corrosion

Galvanic or Bimetallic Corrosion

When two dissimilar metals are connected and exposed to an electrolyte,
the metal higher in electrochemical series undergoes corrosion.

Zinc will dissolve at anodic areas and oxygen will take up electrons
at cathodic areas to form OH- ions.
Concentration Cell Corrosion
This type of corrosion is due to electrochemical attack on the metal surface,
exposed to an electrolyte of varying concentration or of varying aeration.
Differential aeration corrosion
(most common type of concentration cell corrosion)
When one part of metal is exposed to a different air concentration from the other

Corrosion of metals partially immersed in

a solution of a neutral salt just below
the waterline
The parts above and closely adjacent to
the waterline are most strongly aerated
(due to easy access of oxygen) and hence
become Cathodic
Generally poor oxygenated parts are
anodic. Therefore parts immersed to
greater depth (less access of oxygen)
become anodic. So a difference of
potential is created, which causes a flow
of electron between two differentially
aerated areas of same metal
Iron corrodes under drops of water (or salt solution)
Areas covered by droplets having less access of oxygen, become
anodic with respect to other areas, which are freely exposed to air.

Oxygen concentration cell increases corrosion, but it occurs where

the oxygen concentration is lower. Corrosion may be accelerated in
apparently inaccessible places, because oxygen deficient areas
serve as anodes and therefore cracks or crevices serves as foci for
corrosion

Corrosion is accelerated under accumulation of dirt, sand, scale or

other contaminations: Because accumulation of rust, scale etc
restricts the access of oxygen and establishes an anode and
undergo localized corrosion.

Metals exposed to aqueous media corrode under blocks of wood or

glass: Screen some portion of the metal from oxygen access;
proceed to localized attack and thereby corrosion
Passivity
Metal or any alloy exhibit a much higher corrosion resistance than
expected from its position in the electrochemical series. This is due
to the formation of a highly protective but very thin film on the
surface of metal or an alloy.

The film is insoluble, non-porous and of “self-healing” nature (if

broken, it will repair itself on re-exposure to oxidizing conditions)
E.g. Ti, Al, Cr, stainless steel (Containing Cr): corrosion resistance
(i.e. passivation) in oxidizing environments, but in reducing
environment they become chemically active. In oxidizing
environment the protective oxide films will be automatically
repaired whenever any damage occurs

Al containers can store a concentrated solution of HNO3

Pitting Corrosion
Localized accelerated attack results in formation of pinholes, pits and
cavities in metal

This is due to the breakdown or cracking of the protective film on a

metal at specific points. This gives rise to the formation of small
anodic and large cathodic areas.

Metals owing their corrosion resistance to their passive state,

show a marked pitting under the condition leading to the
destruction of their passivity, i.e. those will be the starting points
of pitting corrosion. For e.g. stainless steel and Al show characteristic
pitting in chloride solution
Intergranular Corrosion
Occurs along grain boundaries

Selective attack at only the grain boundaries, leaving the grain

interior untouched or only slightly attacked. This is due to the fact
that the grain boundaries contain such materials which shows
electrode potential more anodic than that of the grain centre in a
particular corroding medium.
For e.g, during welding of stainless steel (an
alloy of Fe, C and Cr), chromium carbide is
precipitated at the grain boundaries,
thereby the region just adjacent to grain
boundaries will have lower amount of Cr
composition and is more anodic w.r.t. the
solid solution within the grain (which is
richer in Cr)

Solution of this problem: Heat treatment

method, which dissolves the chromium
carbide precipitated during welding
This intergranular corrosion occurs in microscopic scale, without any
apparent external signs. But sudden failure of the material (without
any pre-warning) occurs due to loss of cohesion between grains
Waterline corrosion
When water is stored in steel tank, it is generally found that the
maximum amount of corrosion takes place along a line just beneath
the level of water meniscus. The area above the waterline (highly-
oxygenated) acts as the cathodic will be unaffected by corrosion.

In case of ships, this type of corrosion is accelerated by marine

plants attached to the sides of the ship
Crevice Corrosion
Crevice between different metallic objects e.g. bolts, nuts, rivets,
in contact with liquids

Crevice area has lack of oxygen (thus become anodic region and
corrosion takes there). The exposed area acts as cathode.

e.g. at the junction of two metals exposed to a corrosive

environment
Underground or Soil Corrosion
In soil, presence of moisture, bacteria micro-organisms and
electrolyte etc are responsible for corrosion which is further
promoted by differential aeration

Eg. Buried pipelines passing from one type of soil to another suffer
corrosion due to differential aeration: like pipelines passing through
clay and then through sand. Since clay is less aerated than sand
hence corrosion starts.

Microbiological Corrosion
Due to metabolic activity under aerobic or anaerobic conditions

Direct chemical action of sulfuric acid formed by the oxidation of

sulfur or sulfide by microorganism

Generation of local electrochemical cells due to change in pH,

concentration and oxidation potentials

Removal of protective coatings or corrosion inhibitors

Stress Corrosion or Stress cracking
Combined effect of static tensile stresses and the corrosive
environment on a metal. Here highly localized attack is occurring, when
overall corrosion is negligible
The corrosive agents are highly specific and selective such as
Caustic alkalis and strong nitrate solution for mild steel
Traces of ammonia fro brass
Acid chloride solution for stainless steel

This type of corrosion is seen in fabricated articles of certain alloys

due to the presence of stresses caused by heavy working like rolling,
drawing or insufficient annealing. This localized electrochemical
corrosion occurs along narrow paths, forming anodic areas w.r.t. more
cathodic areas at the metal surface.
Presence of stress also produces strain, which result in localized zone
of higher electrode potential. This becomes so chemically active that
they are attacked, even by mild corrosive environment and finally
results crack which will propagate further.

Examples of stress corrosions

Season cracking
Stress corrosion of copper alloys (containing small amount of alloying
elements like P, As, Sb etc), whereas the pure metal is resistant to
stress corrosion. Intergranular cracking occurs in an atmosphere
containing traces of ammonia or amines. The attack occurs along the
grain boundaries which become more anodic w.r.t. grain themselves.

Caustic Embrittlement
Stress corrosion occurs in mild steels exposed to alkaline solution at
high temperature and stress (like in steam boilers)
Caustic embrittlement
Caustic embrittlement is the phenomenon in which the material of
a boiler becomes brittle due to the accumulation of caustic substances.

As water evaporates in the boiler, the concentration of sodium

carbonate increases in the boiler. In high pressure boilers, sodium carbonate
is used in softening of water by lime soda process, due to this some sodium
carbonate maybe left behind in the water. As the concentration of sodium
carbonate increases, it undergoes hydrolysis to form sodium hydroxide.
Na2CO3 + H2O → 2NaOH + CO2
The presence of sodium hydroxide makes the water alkaline. This alkaline
water enters minute cracks present in the inner walls of the boiler by capillary
action. Inside the cracks, the water evaporates and the amount of hydroxide
keeps increasing progressively. The concentrated area with high stress works
as anode and diluted area works as cathode. At anode , sodium hydroxide
attacks the surrounding material and then dissolves the iron of the boiler
as sodium ferrate forming rust. This causes embrittlement of boiler parts like
rivets, bends and joints, which are under stress.
Factors influencing corrosion
Nature of the metal
Position in Galvanic Series
When two metals or alloys are in electrical contact, in presence of an
electrolyte, the more active metal (higher in Galvanic series) suffers
corrosion.

The rate depends on their difference in position and more the

difference, faster the corrosion of the metal

Relative areas of the anodic and cathodic parts

When two dissimilar metals or alloys are in contact, the corrosion of
the anodic part is directly proportional to the ratio of the areas
of the cathodic part and the anodic part

So corrosion is more severe and highly localized if the anodic area is

small : (e.g small steel pipe in copper tank)
Purity of metal
Impurity in metals form minute/tiny electrochemical cells and the
anodic part gets corroded.

E.g. Zn metal containing impurity like Fe, Pb, undergoes corrosion of

Zn due to formation of local electrochemical cells. The rate
increase with increasing exposure and extent of impurity.
Physical State of the metal: Such as grain size, orientation of
crystals, stress etc.
Smaller the grain-size of the metal or alloy, greater will be its
solubility and hence greater the corrosion

Nature of surface film

Get covered with a thin film of metal oxide on surface. The ratio of
the volume of the metal oxide to the metal is known as a “specific
volume ratio”. Greater the specific volume ratio, lesser the oxidation
corrosion rate.
Passivity of the metals like Ti, Al, Cr etc. shows much higher
corrosion resistance than expected from their position in Galvanic
series due to formation of highly protective film. Moreover the film
is of “self-healing” nature, if broken repairs itself.

Solubility of corrosion products

In electrochemical corrosion, if the corrosion product is soluble in
the corroding medium, then the corrosion proceeds faster rate. On
contrary, corrosion of Pb in H2SO4 is suppressed due to formation of
PbSO4.
Volatility of corrosion product
If it volatilizes as soon as possible, leaving the underlying metal
exposed for further attack

Nature of the corroding environment

Temperature
On increasing temperature, the reaction as well as diffusion rate
will increase, thereby corrosion rate will be enhanced.

Humidity of air
Critical humidity: The relative humidity above which the
atmospheric corrosion rate of metal increases sharply. The value of
critical humidity depends on the physical characteristics of the
metal as well as the nature of the corrosion products.

Reason of corrosion in humid environment

Humid atmosphere furnish water to the electrolyte, essential for
setting up an electrochemical corrosion cell
Oxide film on metal surface can also absorb moisture and thus
electrochemical type corrosion can occur
Presence of impurities in atmosphere
In industrial areas, corrosive gases like CO2, H2S, SO2 and fumes of
HCl, H2SO4. In presence of these gases, acidity adjacent to metal
surface increases ad its electrical conductivity will also increase.
This increase the corrosion current flowing in local electrochemical
cells on the exposed metal surfaces.
In marine atmosphere, presence of NaCl

Influence of pH: Generally acidic media is more corrosive than

alkaline and neutral media.

Nature of ions present:

Presence of anions like silicate in the medium leads to the formation
of insoluble reaction products (e.g. silica gel) which inhibits further
corrosion. On the other hand, presence of Cl- destroy the
protective surface film, thereby exposing the metal surface for
further corrosion.
Presence of even traces of Cu in mine water, accelerates the
corrosion of iron pipes
Corrosion Control
1. Proper designing
The design of the material should be such that corrosion even if it
occurs, it should be uniform and does not result in intense and
localized corrosion.
Avoid the contact of dissimilar metals in presence of corroding
solution.
When two dissimilar metals are to be in contact, the anodic area
should be as large area as possible; whereas the cathodic metal
should have as much smaller area as possible.
If two dissimilar metals in contact, they should be as close as
possible to each other in Galvanic series.
When the direct joining of dissimilar metals is unavoidable, an
insulating fitting may be applied in between them to avoid direct
metal-metal electrical contact.
Corrosion Control
2. Using pure metal
Corrosion resistance of a given metal may be improved by increasing
its purity

3. Using metal alloys

Corrosion resistance may be increased by alloying with suitable
elements, but alloy should be completely homogeneous. Chromium is a
good alloying element for iron, because its film is self-healing.
4. Cathodic Protection
Force the metal (which has to be protected) to behave like a
cathode, so corrosion will not occur.

4.1 Sacrificial anodic protection method

The metal (which has to be protected) is connected by a wire to a
more anodic metal, so that all the corrosion will be concentrated at
this more active metal.
The more active metal gets corroded slowly, while the parent
structure (cathodic is protected). The more active metal is called
“Sacrificial Anode”. The corroded sacrificial anode (Mg, Zn, Al or
their alloys) will be replaced by a fresh one, when consumed
completely.
4.2 Impressed current Cathodic Protection
An impressed current is applied in opposite
direction to nullify the corrosion current, and
convert the corroding metal from anode to
cathode.
The impressed current is derived from a direct
current source with an insoluble anode (graphite,
scrap iron etc.). Usually a sufficient d.c. current is
applied to an insoluble anode, buried in soil (or
immersed in corroding medium) and connected to
the metallic structure to be protected.
E.g. Useful for large structures for long-term
operations.

5. Modifying the environment

Corrosive nature of the environment can be reduced either by:---
Removal of harmful constituents
Addition of specific substances, which neutralize the effect of
corrosive constituents of the environment
5.1 Deaeration
Removal of oxygen from aqueous environment reduces metal
corrosion by
(a) adjustment of temperature
(b) Mechanical agitation

5.2 Deactivation
Addition of chemicals capable of combining rapidly with oxygen in
aqueous solution (e.g. Na2SO3, hydrazine hydrate)

5.3 Dehumidification
Reduces the moisture content of air to such an extent that the
amount of water condensed on metal is too small to cause corrosion
(e.g. alumina, silica gel)

5.4 Alkaline Neutralization

Neutralization of acidic character of corrosive environment (due to
the presence of ). Alkaline neutralizers like NH3, NaOH are
generally injected in vapor or liquid form to the corroding system.
6. Corrosion Inhibitors
On addition of the inhibitor in small quantities to the aqueous
corrosive environment, effectively decreases the corrosion of a
metal.
Two types of inhibitors:
6.1 Anodic inhibitors
Ions of transition elements like chromate, phosphate etc, with a high
oxygen content, suppress the corrosion reaction (at anode) by forming
a sparingly soluble compound by reacting with a newly produced metal
ion. These compounds will then be adsorbed on metal surface and
form a protective film and thereby also reducing corrosion rate.

6.2 Cathodic Inhibitors

(a) In Acidic solution, main cathodic reaction is evolution of hydrogen

Therefore corrosion can be reduced by:

Slowing down the diffusion of hydrated H+ ions to the cathode and/or
Increasing the overvoltage of hydrogen evolution
Increasing the overvoltage of hydrogen evolution

Hydrogen overvoltage is the potential difference that can be found between an electrode
and a reversible hydrogen electrode within a single solution. This is where hydrogen
(H2) undergoes formation from ions of hydrogen.

Overvoltage can also be referred to as overpotential when there is a difference that

exists between cells under current passage and cell potential under the same
experimental situations without electrolysis. The term overvoltage is used when the
difference is expressed or measured in volts, which are an absolute value. Volts serve as
the measure of rates of processes related to an electrode reaction.

In terms of hydrogen overvoltage, there is a difference in potential between the

gas hydrogen or metal and an electrode in two conditions:
•When placed in a certain solution and corrosion does not occur
•When corrosion begins

The key here is to elevate the hydrogen overvoltage in order to slow down or halt the
corrosion reaction. For instance, there are corrosion inhibitors that can make hydrogen
overvoltage increase and at the same time make dissolution current decrease as it
inhibits both cathodic and anodic processes.
The diffusion of H+ ions can be decreased by organic inhibitors
(amines, heterocyclic nitrogen compounds, substituted urea etc.),
which are capable of being adsorbed on the metal surface.

Arsenic or antimony oxides are used as inhibitors, because they deposit

adherent film of metallic arsenic or antimony at cathodic areas,
thereby increasing considerably the hydrogen overvoltage
(b) in Neutral solution:
The corrosion can be controlled either by:
Eliminating oxygen from the corroding medium (by using reducing
agents like Na2SO3 or by deaeration)
By retarding the diffusion to the cathodic areas (by adding Mg, Zn or
Ni salts, which react with OH- ions at cathode, and thereby producing
insoluble hydroxides which will be deposited on the cathodes making
self-barriers)

7. Protective Coatings
Coatings should prevent the penetration of corroding environment
to the material, which need to be protected
Coating must be chemically inert to the corroding environment
7.1 Anodic Coating
Coating metals are anodic w.r.t. the base metal (i.e. the metal to be
protected).
Eg. Coating of Zn on Fe

Under corrosive environment, if pores, breaks or discontinuities occur

in such anodic coating, a galvanic cell is formed between the coating
metal and the exposed part of base metal
Then the coating metal will be attacked leaving the base metal
protected.
So this is basically “Sacrificial Coating”
7.2 Cathodic coating
Coating of a more noble metal than the base metal. Protection is
due to higher corrosion resistance of that metal than base metal.
This coating provides effective protection to the base metal only
when the protection is completely continuous and free from pores,
breaks and discontinuities.
If such coating breaks, much more corrosion damage can occur

Eg. Coating of tin on iron sheet and the protection will be as long as
the surface of iron is completely covered. If the coating punctured,
then tin will act as cathode and exposed iron will act as anode. A
galvanic cell will be set up and an intense localized attack at the small
exposed part will occur, resulting severe pitting of the base metal.
Methods of application of metal coating
Hot dipping
For coating of low melting metals like Zn, Sn, Pb, Al etc on iron,
steel, copper etc., which have relatively higher melting points.
Immerse the base metal in a bath of molten coating metal
For good adhesion, the base metal surface must be very clean,
otherwise it cannot be properly wetted by the molten metal.

Two most widely applied hot dipping method:

(i) Galvanizing
(ii) Tinning
(i) Galvanizing:
Process of coating iron or steel sheets with a thin coat of Zn to
prevent rusting
Procedure:
First clean the iron article by dilute H2SO4
Then washed and dried
Dip in the bath of molten Zn. The surface of the bath is covered by
a flux (ammonium chloride) to prevent oxide formation of molten
coating metal
Remove excess Zn and produce a thin film of uniform thickness by
hot rollers.
(ii) Tinning
Process of coating iron or steel articles with a thin coat of Sn

Procedure:
First clean the iron article by dilute H2SO4
Passed through a bath of zinc chloride flux and then pass through a
tank of molten tin and then through a series of rollers and finally
through palm oil, which protect the hot tin coated surface against
oxidation.
Metal Cladding
Dense, homogeneous layer of coating metal is bonded firmly and
permanently to the base metal on one or both sides.
Corrosion resisting metals (like Ni, Cu, Pb, Ag, Pt etc.) and alloys (like
stainless steel etc.) can be used as cladding materials.
Procedure
Thin sheets of coating metal and base metal are arranged in form of
sandwich
That sandwich then passed through rollers, under the action of heat
and pressure
Eg. Plate of duralumin is sandwiched between two layers of pure Al

a covering or coating on a structure or

material.