Chapter 5

Corrosion Protection
Remember
The five essential components of the corrosion cell (or
Requirements for Electrochemical Corrosion )are:
1- Anodic Zones (A)
2- Cathodic Zones (C)
3- Electrical Contact Between Them
4- Ionically Conducting Solution
5- Cathodic Reactant (1+4 cathodic Reactions)

Electronic Path
i
ANODE (A) CATHODE (C)

Cathodic
Where Corrosion Ionic-Current Path Reaction
Occurs!!!!
 Metal or alloy Corrosive Environment

Anode + Cathode +
+ Cathodic Reactant (1+4 !) + = Corrosion
Electronic Current Ionic Current Path (moisture)
Path

The Five Methods For Control Corrosion

 Material Selection
 Design
 Changing the Environment
 Changing the Metal Potential
 Surface Coating (Zn, Sn, Plastic Paints etc)
 1- Corrosion Control by Material selection

(selection of proper material for a particular

corrosive service depend on the environment)

Metallic [choose the proper metal or alloy]

Nonmetallic (use isolating joints and seals ) [rubbers
(natural and synthetic), plastics, ceramics, carbon and
graphite, and wood]
 Metals and Alloys

No Environment Proper material

1 Nitric acid Stainless steels
2 Caustic Nickel and nickel alloys
3 Hydrofluoric acid Monel (Ni-Cu)
4 Hot hydrochloric acid Hastelloys (Ni-Cr-Mo) (Chlorimets)
5 Dilute sulfuric acid Lead
6 Nonstaining atmospheric exposure Aluminum
7 Distilled water Tin
8 Hot strong oxidizing solution Titanium
9 Ultimate resistance Tantalum
10 Concentrated sulfuric acid Steel
2- Control Corrosion by Design Modifications

Corrosion Control must be properly addressed

during the design, installation, and subsequent
operation of systems.
 Design Do’s & Don’ts
• To avoid SCC, you must avoid any excessive stresses.
• To avoid galvanic corrosion, you must (1) avoid any contact between dissimilar
metals, (2) electrically isolate the two metals & (3) use high surface area anode.
• To avoid erosion corrosion, you must avoid any sharp bends in piping systems
especially when high velocities and/or solid in suspension are involved.
• To avoid crevice/pitting corrosion, you must (1) avoid stagnant water/electrolyte (2)
use good welding practice (weld rather than rivet) (3) use properly trimmed seals,
rubbers, and gaskets.
• Avoid sharp corners where paint tends to be thinner and often starts to fail.
• Prevent solutions from drying/concentrating and ensure complete drainage of tanks
to avoid any differential aeration (conc.) corrosion.
• Consider cathodic protections (1) zinc plating (galvanization), (2) use of sacrificial
anodes of Zn or Mg on steel ship hull (3) use of impressed voltage from a battery.
• Design to exclude air – except for active-passive metals and alloys, they require O2
for protective films.
• Most general rule : AVOID ANY HETEROGENEITY !!!
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3- Corrosion Control by Changing the Environment

•Removal of O2 from the system Reactants of the cathodic

reaction accompanying the
•Control of pH (H+ions) corrosion process

•Addition of complexing agents to remove (mask) the aggressive

ions

•Addition of inhibitors to decrease the rate of oxidation and/or

reduction reactions
 4- Control Corrosion by Changing the Metal
Potential

Cathodic Protections Anodic Protection

Shifting the metal potential to a Shift metal potential into a range
more cathodic value. This shift in where passivation occurs. This is
potential can be done by two different done by applying an external DC
methods. current to the anode (prior to its
use) to form a protective passive
(a) Cathodic protection with film.
impressed current
(b) Cathodic protection with sacrificial
anodes
Current density
Active/passive metals
Zn/NaOH

Passive region

Anodic Potential
Epass ipass
 Cathodic Protection Methods

(a) Sacrificial Anode Method (b) Impressed Current Method

Current is supplied by connecting the Here, an external current is applied
object to a more reactive (easily from a DC source (e.g. a battery) and
corroded) metal as a sacrificial anode. i.e. auxiliary anode. i.e. an electrolytic cell
formation of galvanic cell in which the is formed where corrodible or inert
sacrificial anode is consumed (sacrificed). auxiliary anode can be used

cathode anode
 Cathodic Protection Methods
(a) Sacrificial Anode Method

Protection of Steel Structure

By connecting a piece of Zinc, Al or other
anodic material to the steel, in this case,
the zinc or other anodic metal is termed
“Sacrificed Anode“.

Cathodic protection of a
ship hull using a zinc
sacrificial anode
Cathodic Protection Methods
(b) Impressed current Method
 5- Corrosion Control by Surface Coatings
 Normally, it is the first line of defense against corrosion: provides barrier between
a corrosive environment and the structure.
 No coating is perfect and usually contains holidays (voids) often resulting from
improper application and/or damage.
 Coatings may act as sacrificial anode or release substance that inhibit corrosive
attack on substrate.
Coatings

Metallic coatings Non-metallic coatings

Inert Barrier
 Cathodic coatings Inert Barrier
 Anodic coatings Sacrificial Barrier
Inorganic coatings Organic coatings
 Conversion coating (Painting)
 Portland cement coatings
 Vitreous enamels
Metallic coatings:
(a) Cathodic coatings (Inert Barrier) (b) Anodic coatings (Sacrificial Barrier)

Coating metal

Substrate metal

The coating metal is:

more noble than the substrate metal. less noble than the substrate metal.
i.e. the coating metal is cathodic with i.e. the coating metal is anodic with respect to the
respect to the substrate. substrate
e.g. the coating of iron with silver, copper, e.g. the coating of Fe with Zn, Al.
nickel, Co, Sn, Pb on steel.
Exposed substrate will be cathodic & will be
sacrificially protected.
This coating should be free of
Coating metal will form protective passive layer
pores/discontinuity which creates small
anode/cathode area leading to rapid
enhanced attack at the damaged sites.
Non-metallic coatings
Inorganic coatings
 Conversion coating

 In this process, the metal reacts with a compound so that thin insoluble
coatings are formed on the metal surface. The surface is converted into the
coating with a chemical or electro-chemical process.
(A) Phosphating (Chemical Conversion)
Phosphating is usually used on steel
before painting.

(B) Chromating (Chemical Conversion) :

(C) Oxide Coatings

The oxide treatments are done by heat, chemicals, or
electrochemical reactions. e.g. Anodizing (electrochemical
conversion ).
Properties to consider the
quality of coating
 Average thickness of coating
 Porosity or continuity
 Adherence
 Uniformity of thickness
 Inert or passive