CORROSION OXIDATION CORROSION PREVENTION AGAINST CORROSIONPrinciples and Prevention of CorrosionD.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)Al2O3Cr2O3Ti2OFe2O3MgONiOCu2OAg2OAu2O31576104585374056821714513+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 oxidesK2ONa2OMgOAl2O3NiOCu2OCr2O3Fe2O30.410.580.791.381.601.712.032.16

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 layerOxygen anionsMetal CationsOxidation occurs at air-oxide interfaceOxidation 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 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 DiffusivityDiffusion in Ionic crystalsFrenkel defect Cation (being smaller get displaced to interstitial voids E.g. AgI, CaF2Schottky defect Pair of anion and cation vacancies E.g. Alkali halides

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)Alloying of Fe with CrOther oxidation resistant alloys Nichrome (80%Ni, 20%Cr) excellent oxidation resistance Inconel (76%Ni, 16%Cr, 7%Fe)

CorrosionTHE ELECTRODE POTENTIALWhen 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 electrodeThe solvent becomes +ve and the opposing electrical layers lead to a dynamic equilibrium wherein there is no further (net) dissolution of the electrodeThe potential developed by the electrode in equilibrium is a property of the metal of electrode the electrode potentialThe 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

Standard electrode potential of metalsStandard potential at 25oCIncreasing propensity to dissolve

SystemPotential in VNoble endAu / Au3++1.5Ag / Ag++0.80Cu / Cu2++0.34H2 / H+0.0Pb / Pb2+0.13Ni / Ni2+0.25Fe / Fe2+0.44Cr / Cr3+0.74Zn / Zn2+0.76Al / Al3+ 1.66Active endLi / Li+3.05

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 SeriesMore reactive

EnvironmentCorrosion rate of mild steel (mm / year)Dry0.001Marine 0.02Humid with other agents0.2

Galvanic series in marine waterNoble endActive end18-8 SS PassiveNiCuSnBrass18-8 SS ActiveMSAlZnMg

Galvanic CellAnodeZn(0.76)CathodeCu(+0.34)e flowZn Zn2+ + 2eoxidationCu2+ + 2e CuReductionor2H+ + 2e H2orO2 + 2H2O + 4e 4OHZn will corrode at the expense of Cu

How can galvanic cells form?Anodic/cathodic phases at the microstructural levelDifferences in the concentration of the Metal ionAnodic/cathodic electrodesDifferences in the concentration of oxygenDifference 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 cathodicO2 + 2H2O + 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) PolarizationCurrent (I) Potential (V) VcathodeVcathodeSteady 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 CorrosionBasic goal protect the metal avoid localized corrosionWhen possible chose a nobler metalAvoid 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 / volumeIn case of microstructural level galvanic couple, try to use a course microstructure (where possible) to reduce number of galvanic cells formedModify the base metal by alloying Protect the surface by various meansModify the fluid in contact with the metal Remove a cathodic reactant (e.g. water) Add inhibitors which from a protective layerCathodic protection Use a sacrificial anode (as a coating or in electrical contact) Use an external DC source in connection with a inert/expendable electrode