The document discusses corrosion and oxidation of metals, including the principles behind corrosion and methods for preventing corrosion. It describes how metals oxidize when exposed to oxygen and how the formation and properties of metal oxides, such as their adherence and volume, influence corrosion rates. Factors that affect oxidation rates and the development of protective oxide layers are covered. Methods for improving corrosion resistance through alloying and the use of coatings or inhibitors are also summarized.

1. CORROSION
 OXIDATION
 CORROSION
 PREVENTION AGAINST CORROSION
Principles and Prevention of Corrosion
D.A. Jones
Prentice-Hall, Englewood-Cliffs (1996)

2. Attack of Environment on Materials
 Metals get oxidized
 Polymers react with oxygen and degrade
 Ceramic refractories may dissolved in contact with molten materials
 Materials may undergo irradiation damage

3. Oxidation
 Oxide is the more stable than the metal (for most metals)
 Oxidation rate becomes significant usually only at high temperatures
 The nature of the oxide determines the rate of oxidation
Free energy of formation for some metal oxides at 25o
C (KJ/mole)
Al2O3 Cr2O3 Ti2O Fe2O3 MgO NiO Cu2O Ag2O Au2O3
−1576 −1045 −853 −740 −568 −217 −145 −13 +163

4.  For good oxidation resistance the oxide should be adherent to the
surface
 Adherence of the oxide
= f(the volume of the oxide formed :
the volume of metal consumed in the oxidation)
= f(Pilling-Bedworth ratio)
 PB < 1 ⇒ tensile stresses in oxide film → brittle oxide cracks
 PB > 1 ⇒ compressive stresses in oxide film → uniformly cover metal
surface and is protective
 PB >> 1 ⇒ too much compressive stresses in oxide film → oxide cracks
Pilling-Bedworth ratio for some oxides
K2O Na2O MgO Al2O3 NiO Cu2O Cr2O3 Fe2O3
0.41 0.58 0.79 1.38 1.60 1.71 2.03 2.16

5.  If the metal is subjected to alternate heating and cooling cycles →
the relative thermal expansion of the oxide vs metal determines the
stability of the oxide layer
 Oxides are prone to thermal spalling and can crack on rapid heating or
cooling
 If the oxide layer is volatile (e.g. Mo and W at high temperatures) ⇒
no protection

6. Progress of oxidation after forming the oxide layer: diffusion controlled
⇒ activation energy for oxidation is activation energy for diffusion through
the oxide layer
Oxide
Metal
Oxygen anions Metal Cations
Oxidation occurs
at air-oxide
interface
Oxidation occurs
at metal-oxide
interface
• Diffusivity = f(nature of the oxide layer, defect structure of the oxide)
• If PB >> 1 and reaction occurs at the M-O interface → expansion cannot
be accommodated

7. Oxidation resistant materials
 As oxidation of most metals cannot be avoided the key is to form a
protective oxide layer on the surface
 The oxide layer should offer a high resistance to the diffusion of the species
controlling the oxidation
 The electrical conductivity of the oxide is a measure of the diffusivity of the
ions (a stoichiometric oxide will have a low diffusivity)
 Alloying the base metal can improve the oxidation resistance
 E.g. the oxidation resistance of Fe can be improved by alloying with
Cr, Al, Ni
 Al, Ti have a protective oxide film and usually do not need any alloying

8.  Schottky and Frenkel defects (defects in thermal equilibrium) assist the
diffusion process
 If Frenkel defects dominate → the cation interstitial of the Frenkel defect
carries the diffusion flux
 If Schottky defects dominate → the cation vacancy carries the diffusion
flux
 Other defects in ionic crystals → impurities and off-stoichiometry
• Cd2+
in NaCl crystal generates a cation vacancy → ↑s diffusivity
• Non-stoichiometric ZnO → Excess Zn2+
→ ↑ diffusivity of Zn2+
• Non-stoichiometric FeO → cation vacancies → ↑ diffusivity of Fe2+
 Electrical conductivity ∝ Diffusivity
Diffusion in Ionic crystals
Frenkel defect
 Cation (being smaller get
displaced to interstitial voids
 E.g. AgI, CaF2
Schottky defect
 Pair of anion and cation vacancies
 E.g. Alkali halides

9.  A protective Cr2O3 layer forms on the surface of Fe
σ(Cr2O3) = 0.001 σ(Fe2O3)
 Upto 10 % Cr alloyed steel is used in oil refinery components
 Cr > 12% → stainless steels → oxidation resistance upto 1000o
C
→ turbine blades, furnace parts, valves for IC engines
 Cr > 17% → oxidation resistance above 1000o
C
 18-8 stainless steel (18%Cr, 8%Ni) → excellent corrosion resistance
 Kanthal (24% Cr, 5.5%Al, 2%Co) → furnace windings (1300o
C)
Alloying of Fe with Cr
Other oxidation resistant alloys
 Nichrome (80%Ni, 20%Cr) → excellent oxidation resistance
 Inconel (76%Ni, 16%Cr, 7%Fe)

10. Corrosion
THE ELECTRODE POTENTIAL
 When an electrode (e.g. Fe) is immersed in a solvent (e.g. H2O) some metal ions
leave the electrode and –ve charge builds up in the electrode
 The solvent becomes +ve and the opposing electrical layers lead to a dynamic
equilibrium wherein there is no further (net) dissolution of the electrode
 The potential developed by the electrode in equilibrium is a property of the
metal of electrode → the electrode potential
 The electrode potential is measured with the electrode in contact with a solution
containing an unit concentration of the ions of the same metal with the standard
hydrogen electrode as the counter electrode (whose potential is taken to be zero)
Metal
ions-ve
+ve

11. System Potential in V
Noble end Au / Au3+
+1.5
Ag / Ag+
+0.80
Cu / Cu2+
+0.34
H2 / H+
0.0
Pb / Pb2+
−0.13
Ni / Ni2+
−0.25
Fe / Fe2+
−0.44
Cr / Cr3+
−0.74
Zn / Zn2+
−0.76
Al / Al3+
− 1.66
Active end Li / Li+
−3.05
Standard electrode potential of metals
Standard potential at 25o
C
Increasingprope

12. Galvanic series
 Alloys used in service are complex and so are the electrolytes (difficult to
define in terms of M+
) (the environment provides the electrolyte
 Metals and alloys are arranged in a qualitative scale which gives a measure
of the tendency to corrode → The Galvanic Series
Environment Corrosion rate of mild steel (mm / year)
Dry 0.001
Marine 0.02
Humid with other agents 0.2
Galvanic series in marine water
Noble end Active end
18-8 SS
Passive
Ni Cu Sn Brass 18-8
SS
Active
MS Al Zn Mg
More reactive

13. Galvanic Cell
Anode
Zn
(−0.76)
Cathode
Cu
(+0.34)
e−
flow
Zn → Zn2+
+ 2e−
oxidation
Cu2+
+ 2e−
→ Cu
Reduction
or
2H+
+ 2e−
→ H2
or
O2 + 2H2O + 4e−
→ 4OH−
Zn will corrode at the expense of Cu

14. How can galvanic cells form?
Anodic/cathodic phases at the
microstructural level
Differences in the concentration of the
Metal ion
Anodic/cathodic electrodes
Differences in the concentration of
oxygen
Difference in the residual stress levels

15.  Different phases (even of the same metal) can form a galvanic couple at the
microstructural level (In steel Cementite is noble as compared to Ferrite)
 Galvanic cell may be set up due to concentration differences of the metal ion in the
electrolyte → A concentration cell
Metal ion deficient → anodic
Metal ion excess → cathodic
 A concentration cell can form due to differences in oxygen concentration
Oxygen deficient region → anodic
Oxygen rich region → cathodic
 A galvanic cell can form due to different residual stresses in the same metal
Stressed region more active → anodic
Stress free region → cathodic
O2 + 2H2O + 4e−
→ 4OH−

16. Polarization
 Anodic and Cathodic reactions lead to concentration differences near the
electrodes
 This leads to variation in cathode and anode potentials (towards each other)
→ Polarization
Current (I) →
Potential(V)→
Vcathode
Vcathode Steady state current
IR drop through the electrolyte

17. Passivation
 Iron dissolves in dilute nitric acid, but not in concentrated nitric acid
 The concentrated acid oxidizes the surface of iron and produces a thin protective
oxide layer (dilute acid is not able to do so)
 ↑ potential of a metal electrode → ↑ in current density (I/A)
 On current density reaching a critical value → fall in current density
(then remains constant) → Passivation

18. Prevention of Corrosion
Basic goal → • protect the metal • avoid localized corrosion
 When possible chose a nobler metal
 Avoid electrical / physical contact between metals with very different electrode
potentials (avoid formation of a galvanic couple)
 If dissimilar metals are in contact make sure that the anodic metal has a larger
surface area / volume
 In case of microstructural level galvanic couple, try to use a course
microstructure (where possible) to reduce number of galvanic cells formed
 Modify the base metal by alloying
 Protect the surface by various means
 Modify the fluid in contact with the metal
• Remove a cathodic reactant (e.g. water)
• Add inhibitors which from a protective layer
 Cathodic protection
• Use a sacrificial anode (as a coating or in electrical contact)
• Use an external DC source in connection with a inert/expendable electrode