corrosion and its types Methods to avoid corrosion

1. CORROSION
CHEMISTRY

2. Corrosion is defined as the destruction and consequent loss of metals
through chemical or electrochemical attack by the environment
Metal Oxides, Metal Sulfides,
or Metal Hydroxides

3. Dry corrosion
(Chemical
corrosion)
Wet Corrosion
(Electrochemical
corrosions)
oxygen, hydrogen sulfide, halogens and Sulphur
dioxide in the absence of moisture to form metal
oxide layer
occurs in presence of an aqueous solution of
electrolyte and atmospheric oxygen by setting up
of tiny galvanic cells on the metal surfaces.
CORROSION

5. Electrochemical theory of corrosion / WET Corrosion
1) Formation of Galvanic cells - Anodic and cathodic areas are formed resulting in a large
number of minute galvanic cells.
2) Anodic Reaction: At the anodic area metal undergoes oxidation with the release of
electrons
Fe → Fe2+ + 2 e-

6. 3) Cathodic Reaction - The electrons flow from the anodic to cathodic area and cause reduction. There are 3 possible
ways in which reduction can take place.
•In acidic medium and absence of oxygen: hydrogen ions are reduced to hydrogen gas.
•In neutral and in the absence of oxygen: If the solution is neutral and in the absence of oxygen, water is
reduced to H2 and OH-
•In neutral and aerated medium: when the solution is neutral and aerated, hydroxyl ions are formed as follows.
• Formation of corrosion product:
The hydroxyl ions migrate towards anode and react with metal ions (Mn+ ions) and forms corrosion
product. In the case of iron OH- reacts with Fe2+ ions and forms an insoluble hydrated ferric oxide known
as brown rust.

7. Types of corrosion
1)Differential metal corrosion
2)Differential aeration corrosion
3)Stress Corrosion
4)Grain Boundary corrosion

8. 1) Differential metal corrosion :
Differential metal corrosion arise when
two dissimilar metals are in contact
with each other
Higher the difference in potential,
higher will be the rate of
corrosion.
– 0.44 V 0.34 V
At anode : Fe → Fe2+ + 2e-
At cathode (either hydrogen evolution or
oxygen absorption)
2 H+ + 2e- → H2↑
O2+ 2 H2O + 4 e- → 4 OH-

9. 2) Differential aeration corrosion :
 It occurs when a metal surface is
exposed to differential air (or)
oxygen concentration.
The difference in oxygen concentration
produces a potential difference and causes
corrosion current to flow from cathode to
anode
Part of the metal below the water level acts as anode
undergoes corrosion and the part above the water
level acts as cathode and remain unaffected.

10. Waterline corrosion is a case of differential aeration corrosion.
Water line corrosion
 A distinct brown line is formed just below
the water line due to the deposition of rust.

11. Corrosion control
Corrosion control methods are broadly classified into three types
1. Anodizing
2. Cathodic protection
3. Metal coating

12. 1)Anodizing
Process of anodizing of Aluminum
 Anodizing is generally produced on non-ferrous metals like Al, Zn, Mg and their alloys.
 Aluminium when made cathode allows the passage of electrons but cease to
conduct when it is made anode in aqueous solution of chromic acid (or)
H2SO4.This is because of the formation of a thin layer of Al2O3 over the metal.
 Anode: Aluminum
 Cathode: Cu / Steel/Lead
Electrolyte: 5-10 % chromic acid (or) 10 %
H2SO4
Temperature: 35oC
Voltage: 40 V
Current density: 100 A /m2

13. At anode
(Oxidation): 2Al + 3 H2O → Al2O3 + 6 H++6 e-
At cathode
(Reduction): 6 H+ + 6 e- → 3 H2
Overall reaction: 2 Al + 3H2O → Al2O3 + 3H2
A current density of 100 A/m2 is applied which
oxidizes the outer layer of Al to Al2O3

14. Advantages
• Increases the corrosion resistance
• Increases in surface hardness and abrasion resistance
• Provides better adhesion for paint primers
• Electrical and thermal insulation
• Porous layer allows for the coloring and sealing of the coating
Application
Anodized articles are used as soap boxes, tiffin carriers, window frames,
nameplates decorative object.

15. 2) Cathodic protection
In cathodic protection electrons are provided from an external source so that the
metal or alloy remains as cathode.
It is broadly divided into two:
Sacrificial anodic protection: SAP method
Impressed current cathodic protection: ICCP method

16. Sacrificial anodic protection: SAP method
 Metals like Mg, Al and Zn are more active
and hence are used as anodes.
 These metals being more active acts as anode
undergo corrosion and supply electrons to
the specimen.
 Since anodic metals are scarified to protect
the metal structure this technique is called
sacrificial anode method.

18. Benefits
• No power source is required
• Simple to install, operate and maintain
• Additions easily installed
Example: Mg block connected to buried oil storage tanks,
Zn wire fixed to the sides of ocean-going ships

19. 3) Metal coating
Galvanization
The process of depositing a protective layer of zinc on steel or iron
is called galvanization.
 Zinc coatings are generally obtained by
hot dipping of the base metal in a
molten zinc bath and the process is
called galvanization
Why Zn ?
• Low cost of zinc
• easy application of the coating
• efficient anodic protection afforded.

20. Hot dip galvanization
Base metal surface is
washed with organic
solvents
remove organic impurities
such as oil and grease
water wash
remove excess organic
solvents
Washed with dilute
sulphuric acid
(pickling)
remove rust and other
inorganic deposits.
water wash
remove excess acids
Steam zone process
(Drying process)
Treating with Flux
mixture of aqueous solution ZnCl2
and NH4Cl which acts as flux
flux treatment enhances the adsorption
process of base metal
Dry
Dipped in
molten zinc
4500 C.
Passing the metal
through rollers
Excess zinc is released
Annealing
(calcination)
enhances the adsorption and
coating efficacy

22. Advantages
Since the coating is anodic in nature the exposure of base metal will not cause
corrosion.
Disadvantages
Since Zn salts are poisonous , food materials cannot be stored in container.
Uses/Applications
• To make galvanized articles
• Galvanized Iron objects are used as roofing sheets, wires, pipes
• To manufacture automobile parts, bolts & nuts, nails etc

23. Corrosion Penetration Rate (CPR)
thickness loss per unit of time.
Several pieces of data must be collected to calculate the corrosion penetration rate
for any given metal:
 The weight lost (the decrease in weight of the metal during the period of
reference).
 The density of the metal.
 The total surface area initially present.
 The time taken for the metal to corrode.

24. CPR = (k x W) / (D x A x T)
where k = a constant
W = total weight lost
T = time taken for the loss of metal
A = the surface area of the exposed metal
D = the metal density in g/cm³

25. CPR in mpy (mil per year)
k = 534
W = mg
D = g/cm3
A = inch2
T = hours
CPR in mmpy (millimetre
per year)
k = 87.6
W = mg
D = g/cm3
A = cm2
T = hours
SI units for CPR
 For most application a corrosion penetration rate
less than 20 mpy (0.5 mm/yr) is acceptable.

26. Numerical problem:
Q1) A piece of corroded metal alloy plate was found in a submersed
ocean vessel ,it was estimated that the original area of plate was 800
cm2 and that approximately 7.6 Kg had corroded a way during the
submersion , assuming a corrosion penetration rate of 4 mm/yr for this
alloy in seawater , estimate the time of submersion in years , the
density of alloy is 4.5 g/cm3.

27. Q2: A steel of area 100 inch2 is exposed to air near the seashore. After 1 year it was
found that the steel sheet has lost 485 g due to corrosion. What is the value of CPR in
mils/year and in mm/year ? Can such steel sheet be applicable for the construction
purpose where the steel sheet is exposed?