Transcript CORROSION  OXIDATION  CORROSION  PREVENTION AGAINST CORROSION Principles and Prevention of Corrosion D.A.

CORROSION
 OXIDATION
 CORROSION
 PREVENTION AGAINST CORROSION
Principles and Prevention of Corrosion
D.A. Jones
Prentice-Hall, Englewood-Cliffs (1996)
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
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 25oC (KJ/mole)
Al2O3
Cr2O3
Ti2O
Fe2O3
MgO
NiO
Cu2O
Ag2O
Au2O3
1576
1045
853
740
568
217
145
13
+163
 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
0.41
0.58
0.79
1.38
1.60
1.71
Cr2O3 Fe2O3
2.03
2.16
 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
Progress of oxidation after forming the oxide layer: diffusion controlled
 activation energy for oxidation is activation energy for diffusion through
the oxide layer
Oxygen anions
Oxidation occurs
at metal-oxide
interface
Oxide
Metal Cations
Oxidation occurs
at air-oxide
interface
Metal
• 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
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
Diffusion in Ionic crystals
 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
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
Alloying of Fe with Cr
 A protective Cr2O3 layer forms on the surface of Fe
(Cr2O3) = 0.001 (Fe2O3)
 Upto 10 % Cr alloyed steel is used in oil refinery components
 Cr > 12%  stainless steels  oxidation resistance upto 1000oC
 turbine blades, furnace parts, valves for IC engines
 Cr > 17%  oxidation resistance above 1000oC
 18-8 stainless steel (18%Cr, 8%Ni)  excellent corrosion resistance
 Kanthal (24% Cr, 5.5%Al, 2%Co)  furnace windings (1300oC)
Other oxidation resistant alloys
 Nichrome (80%Ni, 20%Cr)  excellent oxidation resistance
 Inconel (76%Ni, 16%Cr, 7%Fe)
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
-ve ions
+ve
Standard electrode potential of metals
System
Potential in V
Noble end
Au / Au3+
+1.5
Increasing propensity to dissolve
Standard potential at 25oC
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
Li / Li+
3.05
Active end
 Alloys used in service are complex and so are the electrolytes (difficult to
define in terms of M+) (the environment provides the electrolyte
Environment
Corrosion rate of mild steel (mm / year)
Dry
0.001
Marine
0.02
Humid with other agents
0.2
 Metals and alloys are arranged in a qualitative scale which gives a measure
of the tendency to corrode  The Galvanic Series
Galvanic series
Galvanic series in marine water
Noble end
18-8 SS
Passive
Active end
More reactive
Ni
Cu
Sn
Brass
18-8
SS
Active
MS
Al
Zn
Mg
e flow
Galvanic Cell
Anode
Zn
(0.76)
Zn  Zn2+ + 2e
oxidation
Cathode
Cu
(+0.34)
Cu2+ + 2e  Cu
Reduction
Zn will corrode at the expense of Cu
or
2H+ + 2e  H2
or
O2 + 2H2O + 4e  4OH
Anodic/cathodic electrodes
Anodic/cathodic phases at the
microstructural level
How can galvanic cells form?
Differences in the concentration of the
Metal ion
Differences in the concentration of
oxygen
Difference in the residual stress levels
 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
O2 + 2H2O + 4e  4OH
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
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
Potential (V) →
Vcathode
IR drop through the electrolyte
Vcathode
Steady state current
Current (I) →
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
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