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 25 o C (KJ/mole) Al 2 O 3 Cr 2 O 3 Ti 2 OFe 2 O 3 MgONiOCu 2 OAg 2 OAu 2 O 3  1576  1045  853  740  568  217  145 

 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 K2OK2ONa 2 OMgOAl 2 O 3 NiOCu 2 OCr 2 O 3 Fe 2 O

 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 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

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

 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  Cd 2+ in NaCl crystal generates a cation vacancy   s diffusivity  Non-stoichiometric ZnO  Excess Zn 2+   diffusivity of Zn 2+  Non-stoichiometric FeO  cation vacancies   diffusivity of Fe 2+  Electrical conductivity  Diffusivity Diffusion in Ionic crystals Frenkel defect  Cation (being smaller get displaced to interstitial voids  E.g. AgI, CaF 2 Schottky defect  Pair of anion and cation vacancies  E.g. Alkali halides

 A protective Cr 2 O 3 layer forms on the surface of Fe  (Cr 2 O 3 ) =  (Fe 2 O 3 )  Upto 10 % Cr alloyed steel is used in oil refinery components  Cr > 12%  stainless steels  oxidation resistance upto 1000 o C  turbine blades, furnace parts, valves for IC engines  Cr > 17%  oxidation resistance above 1000 o C  18-8 stainless steel (18%Cr, 8%Ni)  excellent corrosion resistance  Kanthal (24% Cr, 5.5%Al, 2%Co)  furnace windings (1300 o 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)

Corrosion THE ELECTRODE POTENTIAL  When an electrode (e.g. Fe) is immersed in a solvent (e.g. H 2 O) 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

SystemPotential in V Noble endAu / Au Ag / Ag Cu / Cu H 2 / H Pb / Pb 2+  0.13 Ni / Ni 2+  0.25 Fe / Fe 2+  0.44 Cr / Cr 3+  0.74 Zn / Zn 2+  0.76 Al / Al 3+  1.66 Active endLi / Li +  3.05 Standard electrode potential of metals Standard potential at 25 o C Increasing propensity to dissolve

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 EnvironmentCorrosion rate of mild steel (mm / year) Dry0.001 Marine0.02 Humid with other agents0.2 Galvanic series in marine water Noble endActive end 18-8 SS Passive NiCuSnBrass18-8 SS Active MSAlZnMg More reactive

Galvanic Cell Anode Zn (  0.76) Cathode Cu (+0.34) e  flow Zn  Zn e  oxidation Cu e   Cu Reduction or 2H + + 2e   H 2 or O 2 + 2H 2 O + 4e   4OH  Zn will corrode at the expense of Cu

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

 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 O 2 + 2H 2 O + 4e   4OH 

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) → V cathode Steady state current IR drop through the electrolyte

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