Protection against Corrosion

Corrosion occurs when metals come in contact with a corrosive

atmosphere. Thus, corrosion can be prevented by the following

• Design improvement (Geometrical Design)

• Change of metals
• Change of metal electrode potential
• Change of environment
• Use of coatings for separating metal from the environment
Corrosion occurs when metals come in contact with a corrosive
atmosphere. Thus, corrosion can be prevented by the following
• Design improvement (Geometrical Design)

• Change of metals
(1)Change
• Change of composition
of metal electrode(e.g., refining or alloying)
potential
(2)Change
• Change of structure (e.g., heat treatment)
of environment
(3)Elimination of for
• Use of coatings tensile residual
separating stresses
metal (e.g.
from the annealing)
environment
(4)Introduction of surface compressive stresses (e.g., penning)
Corrosion occurs when metals come in contact with a corrosive
atmosphere. Thus, corrosion can be prevented by the following

• Design improvement (Geometrical Design)

• Change of metals
• Change of metal electrode potential
• Change protection:
1. Cathodic of environmentLower the potential of the metal in the

negative
• Use (anodic)
of coatingsdirection in the
for separating domain
metal ofenvironment
from the immunity.

2. Anodic Protection: Increasing the potential of the metal in the

positive direction in the domain of passivity.
Corrosion occurs when metals come in contact with a corrosive
atmosphere. Thus, corrosion can be prevented by the following

• Design improvement (Geometrical Design)

• Change of metals
• Change of metal electrode potential
• Change of environment
• (1)
UseRemoval
of coatingsof
forcorrosion elements
separating metal from the environment
(2) Use of inhibitors
(3) Change of operating variables
Corrosion occurs when metals come in contact with a corrosive
atmosphere. Thus, corrosion can be prevented by the following

• Design improvement (Geometrical Design)

• Change of metals
• Change of metal electrode potential
• Change of environment
• Use of coatings for separating metal from the environment
1. Metallic Coatings
2. Non-metallic Coatings
Geometrical Design

A design satisfy all the basic mechanical requirement, but it

may incorporate features that tend to promote the corrosion.
This will results either failure or expensive maintenance
throughout the life of the components.

So, these feature can not be incorporated in the final design,

extra protection should be provided against corrosion failure.
 1. Simplify forms of the components:

Structure having simpler forms can be protected easily and

efficiently. A complicated shaped having more angle, corners,
edges and internal surfaces, will have a larger surface area
exposed to the corrosive environment and will be difficult to
protect by painting or other surface treatment.
 2. Avoid the crevices in the components:
Crevices allow moisture to be trapped which result in increased
corrosion. If crevices are either present in the structure or can not
be avoided, that should be filed by welding or by using a filler.

Avoiding Crevices
 3. Avoid the storage Moisture:
the design should be protect the structure from retained moisture. In
closed constructions drainage holes should be placed that the
contraction receives proper ventilation. Condensation should be
reduced by allowing free circulation of air
4. Avoid sharp corners:

Sharp corners must be avoided and rounded corners should

be used. The sharp corners result in thin coating at corners,
which are not protected effectively, rounding corners result
in an smooth coating.
5. Avoid contact with absorbent Materials:

Metals should not be exposed in contact with absorbent

materials. The absorbent materials may absorb moisture and
may be corrosive by themselves.
6. Avoid Galvanic Corrosion:
To avoid the possibility of galvanic corrosion, different metals and
alloys should not be jointed and particularly when they are situated far
from each other in the galvanic series.
If this is unavoidable, galvanic corrosion can be prevented by the
following methods: • Use an electrical insulators
• Introduce an easily exchangeable corroding pieces
• Use paints
• Exclude oxygen from the environment
• Adjust fluid flow direction
 7. Avoid sharp bends
Sharp bends and other areas where the fluid direction is changed rapidly
can promote erosion corrosion and should avoid the sharp bends.
8. Avoid the mechanical stress on the components:

Mechanical stress should be minimized due to stress corrosion

cracking occurred in the materials. So dimension changed due to
corrosion of metals and the coating may cause failure of a part on
account of mechanical failure or stress corrosion cracking.
9. Avoid the protruding parts

A stud should not be protruding inside the wall of a pipe or

components, this may promote the deposit or erosion corrosion.
 Change of Metal

The corrosion resistance of a metal can be improved by the following:

(1)Change of composition (e.g., refining or alloying)
(2)Change of structure (e.g., heat treatment)
(3)Elimination of tensile residual stresses (e.g. annealing)
(4)Introduction of surface compressive stresses
(e.g. penning, rolling etc)
(1) Change of composition

The resistance of metallic materials can be improved by changing its

composition by either refining or alloying. For refining, lowering the
contents of C, S and P in steel, Fe, Cu and Si in aluminum etc.

However, pure metals are used in special cases only because they are
very expensive and relatively soft and weak.
Alloy addition are often used to improve the corrosion
resistance of metal. Various alloying additions can be classified
on the basis of their function as follows:

•Oxide Film Forming: Al and Mg additions to Copper improve its

oxidation; Cr, Al and Si addition to heat resistance steels.
Oxide Film Improving: Ni and Ti addition to Zr+2.5% Nb alloy; Li in
nickel; Al in Zinc.
Neutralizing:
. Mn and Cu for neutralizing sulphur in steel; Mg and Mn
for neutralizing (Fe and Si) in aluminium; Ti, Nb and Ta as carbon
neutralizing in austenitic stainless steels.
Passivating: Cr (>11 wt%), Ni and Mo in steels.
Cathodic: Pt, Ag or Cu in acid resistant steel, Ni in aluminium, Pt or
Ag in Zirconium, Pt in titanium
(2) Change of Microstructure

Change of microstructure of metal by heat treatment or by working

can improve its corrosion resistance.
For examples, sensitization effect of austenitic stainless steel are
minimised by cold working, quenching and tempering.
(3) Elimination of tensile residual stresses

Elimination of tensile residual stresses by stress-relief annealing

treatments can often improve the resistance of material to several
failures, i.e. stress corrosion creaking, hydrogen embrittlement, etc.
(4) Introduction of surface compressive stresses

The resistance of a material to stress corrosion cracking and

corrosion fatigue can be improved by the creation of surface
compressive stresses by penning or rolling.
Change the metal potential
1. Cathodic protection: Lower the potential of the metal in the
negative (anodic) direction in the domain of immunity.

2. Anodic Protection: Increasing the potential of the metal in the

positive direction in the domain of passivity.
 Pourbaix diagram for Fe-H2O
system at 25oC

Pourbaix diagram for Fe-H2O

system showing domains of
corrosion behaviours
Cathodic protection is the most widely applied electrochemical corrosion
control technique. This is accomplished by applying a direct current to
the structure which causes the structure potential to change from the
corrosion potential (Ecorr) to a protective potential in the immunity
region.
 E – log(i) diagram

There are two method of applying of cathodic protection to metallic

structure
(1)Galvanic or sacrificial anode method
(2)Impressed current method
Galvanic or sacrificial anode method: metallic structure is made
cathode by connecting with galvanic anodes, which is more
electronegative than the metallic structure to be protected.

Back fill: consisting of coke breeze, gypsum, which improves electric

contact between the anode and surrounding soil.
Cathodic protection to get the required anodic current. The anodic
current is a function of the close circuit potential difference.
In Galvanic method, the following factors must be taken into
consideration:

(i) Anode materials: Factors to be considered in choosing anodes

include driving voltage, capacity and cost.
Mg and Mg alloy(6% Al, 3 Zn, 0.25Mn)
(a)Zn and Zn alloy(5%Al , 3%Si)
(b)Al and Al alloy(4.5%Mg, 3%Zn)

(ii) Geometry of anode: the geometric design of an anode depends upon

the application and current life in any particular electrolyte. The anode
can be cylindrical, plate type.

(iii) Number and spacing of anodes: Number and spacing of anode

depends upon the system to be protected. The ideal arrangement
would be to have a number of anodes in such a position that all the
steel work is equidistant, so that the protective current spreads
uniformly.
Advantages of galvanic method:
1. No external power supply necessary, this method can be used
in remote and difficult to reach areas.
2. Low installation cost
3. Minimum maintenance cost.

Limitations of galvanic method:

(1)Limited driving potential and current output
(2)Soil resistivity limitation
(3)Not applicable for large diameter bar or poorly coated pipe.
(4)Mutual interference in multiple or parallel installation.
Municipal water main protected from corrosion using coating
and sacrificial magnesium anodes.
Impressed current method:
This method the metallic structure is made cathode the use of impressed
current by connecting the negative terminal of the external power supply
source to the metallic structure to be protected and positive to an inert
anode.

Inert anode can be made of

scrap steel, Al, graphite and Pt
group metals.

Back fill: consisting of coke breeze,

gypsum, which improves electric
contact between the anode and
surrounding soil.
Cathodic protection
Advantages of impressed current method
• Larger voltage

•Larger flexibility of control

•Applicable to large objects
•Uncoated parts can be protected

Limitations of impressed current method

•Larger installation cost
•Higher maintenance cost
•Interference problem with parallel currents
Limitations of Cathodic Protection:

1. If the cathodic polarization to high, some metals such as Pb, sn etc

are attacked by gasification with formation of gaseous hydrides

which can lead to weakening and consequently disconnection form
the articles.
2. An applied cathodic current may lead to reducing passive alloy,
such as stainless steel, which can destroy the passivity or least
hinder its conservation.
3. If polarization is too weak, materials remain exposed to the
corrosive medium and thus remain unprotected.
4. In some cases cathodic protection might accelerate failure due to
hydrogen embrittlement
5. When neighbouring structure are not in direct electrical connection

with the cathodic protection but located between the ground anode and
the structure, stray current may cause localized corrosion of unprotected
structure.
This can be minimized either by suitable layout of structure and anodes
or by providing a alternative current path by sort circuiting.

Stray Current Corrosion Protection by short circuiting