Metal + Oxygen in air → Formation of oxide at high temp Water vapor

1. High Temperature Corrosion : Oxidation Metal + Oxygen in air → Formation of oxide at high temp Water vapor CO2, H2S, etc A. Oxidation reactions: 1) xM + ½ (yO2) → MxOy 2) xM + yH2O → MxOy + yH2 3) xM + yCO2 → MxOy + yCO Fig. 1 Film and scale formation during high tempera-ture metal oxidation

2. B. Thermodynamics of oxidation • Each of the reactions 1), 2), and 3) for any metal is characterized thermodynamically by a standard free energy G° which must be negative in order for the reaction to proceed spontaneously from left to right as written. • Ellingham diagram shows the relative thermodynamics stability of the indicated oxides. The lower on the diagram, the more negative the standard free energy of formation and the more stable the oxide. Fig. 2 Standard Gibbs energies of formation of selected oxides as a function of temperature.

3. C. Electrochemical & morphological aspects of oxidation • Oxidation by gaseous oxygen, like aqueous corrosion is an electrochemical process; • M → M2+ + 2e- ········· Metal oxidation at metal/scale interface. • ½ O2 + 2e- → O2- ········· Oxidant reduction at scale/gas interface. • Role of oxide layer • - an ionic conductor (electrolyte). • - an electronic conductor. • - an electrode at which oxygen is reduced. • - diffusion barrier through which electrons pass and • ions must migrate over defect lattice sites. • - Doping of oxide • - Alloying • - Coating Fig. 3 Schematic illustration of electrochemical processes occurring during gaseous oxidation • Since the electronic conductivity of oxides ≫ their ionic conductivity, the diffusion of either cations or oxygen ions controls the oxidation rate. Thus, the oxidation rate is most effectively retarded by reducing the flux of ions diffusing through the scale;

4. D. Oxide structures In general, all oxides are nonstoichiometric compounds, i.e., their compositions are variable and deviate from their ideal molecular formulas. 1) n-type oxides: anion deficient and contains an excess of electrons or metal excess ; ZrO2, ZnO etc. 2) p-type oxides: metal deficient; NiO, CoO. (a) (a) (b) (b) (c) (c) Fig. 4 Idealized lattice structure of zirconium oxide, an n-type semiconductor: (a) pure ZrO; (b) effect of Ca2+ addition; (c) effect of Ta5+ addition. Fig. 5 Idealized lattice structure of nickel oxide, an p-type semiconductor: (a) pure NiO; (b) effect of Li+ addition; (c) effect of Cr3+ addition.

5. E. Oxide film growth processes Fig. 6 Processes occurring in three types of oxide surface scale during high temperature oxidation

6. F. Effects of alloying on oxidation • 1) For n-type oxides (metal excess) : ZrO2, ZnO • Substitution of lower-valence cation into the lattice increases the concentration of interstitial cation or anion vacancies, and decreases the concentration of excess electrons. A diffusion controlled oxidation rate would be increase. • Substitution of higher-valence cations decreases the concentration of interstitial cations or anion vacancies and increases the concentration of excess electrons. A diffusion controlled oxidation rate would be decreased. • 2) For p-type oxides (metal deficient) : NiO, CoO • The incorporation of lower-valency cations decreases the concentration of cation vacancies or interstitial anions and increases the number of electron holes. A diffusion controlled oxidation rate would be decreased. • The addition of higher valency cations increases the cations increases the cation vacancy or interstitial anion concentration and decreases the electron hole concentration. A diffusion controlled oxidation rate would be increase.

7. Classification of electrical conductors: oxides, sulfides, and nitrides Oxidation of zinc and zinc alloys 390 oC, 1atm O2 Oxidation of nickel and nickel alloys- Nickel and chromium-nickel alloys at 1000oC in pure oxygen* Fig. 7 Effects of chromium on the parabolic rate constant, which is proportional to the oxidation rate

8. G. Oxide properties The oxidation rate of an alloy will be minimized if the oxide film has a combination of favorable properties which include: • Good adherence to prevent flaking and spalling. • High melting point. • Low vapor pressure. • High temperature plasticity • Low electrical conductivity or low diffusion coeff. For metal ions and oxygen. • Similar expansion coeff. Between metal and oxide. H. Oxidation kinetics Three kinetic laws – parabolic, linear, logarithmic – describe the oxidation rates for common metals and alloys. Fig. 8 Weight gain versus time for the commonly observed kinetics law metal oxidation.

9. 1) How to measure the oxidation rate a. Thermal microbalances : plot weight gain of oxide as a function of time. b. Ellipsometer : When polarized light is reflected from a metal surface coated with a metal oxide, the plane of polarization is partial rotated. The ellipsometer is an instrument that can measure this rotation shich can then be related to the oxide thickness. c. Metallographic examination : determine the thickness of oxides and the various oxide layers (phases). Fig. 9 Schematic microbalance assembly for continuous recording of weight gain during high-temperature oxidation

10. 2) Linear rate law and breakaway W = kLt ·········where W is weight gain per unit area, and kL is the linear rate constant. A linear rate law result when a reaction at a phase boundary controls. Thus any surface films or scales that may present must be non-protective : this occurs when the oxide is volatile or molten, when the oxide scale spalls off or cracks, and when a porous oxide forms. 3) Logarithmic rate law Often at lower temp., oxidation rate is inversely proportional to time, t: dx/dt = ke/t → x = kelog(at+1) Logarithmic oxidation is usually obeyed for relatively thin films at lower temp. 4) Parabolic oxidation rate law When the rate controlling step in the oxidation process is the diffusion of ions through a compact barrier layer of oxide with the chemical potential gradient as the drivin force, the parabolic rate law is usually observed. Fick’s first law states that steady state flus of reaction ions is equal to D(c/x) where c is the concentration difference across the oxide thickness, x. dx/dt = CD(c/x) → x2 = kpt The parabolic rate law is usually associated with thick, coherent oxides; oxidation of Fe, Co, Ni and Cu.

11. Fig. 10 Period breakdown and deterioration to a linear law during oxidation of zirconium alloy in water at 288 oC. Fig. 12 Arrhenius plot of parabolic rate constants for oxidation of cobalt, taken from Fig. 11. Fig. 11 Parabolic plots of weight gain squared versus time for oxidation of cobalt at 900 ~ 1350 oC.

12. I. Oxidation of alloys Alloy oxidation is generally much more complex as result of some, or all, of the following; a. The metals in the alloy will have different affinities for oxygen reflected by the different free energies of formation of the oxides. b. Ternary and higher oxides may be formed. c. A degree of solid solubility between the oxides may exist. d. The various metal ions will have different mobilities in the oxide phases. e. The various metals will have different diffusivities in the alloy. f. Dissolution of oxygen into the alloy may result in sub-surface precipitation of oxides of one or more alloying elements (internal oxidation). 1) Selective oxidation An alloy is selectively oxidized if one component, usually the most reactive one, is preferentially oxidized. Consider a binary alloy A-B in which A is more noble than B. A + ½ O2 → AO G°AO > G°BO B + ½ O2 → BO

13. [A] BO [B] AO O2 Alloy For the oxide BO, and assuming that BO is insoluble in AO; GBO = BO - B– 1/2 O2 = oBO– (oB + RT ln aB + ½ oO2 + RT/2 ln(pO2/po) GAO = oAO– (oA + RT ln aA + ½ oO2 + RT/2 ln(pO2/po) When the conc. of B is very low, aB = xB < x*B, thus, GBO > GAO ; The oxide AO forms initially. As oxidation proceeds, the unreacted B accumulates at the interface until GBO is comparable with or less than GAO, at which point BO nucleates. Most alloys of this kind show a subscale of more reactive solute. Typical examples are Cu-Be, Cu-Al, Fe-Al, Fe-Cr, Fe-Al and Ni-Cr alloys. xB < x*B

14. O2(g) O2(g) O2(g) CuO BeO Cu2O 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 Cu+ 。 。 。 。 Cu+ h+ h+ 。 。 。 。 。 。 unoxidized Cu-Be alloy 。 。 。 Cu2O + BeO + O2 gas 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 O CuO + BeO unoxidized Cu-Be alloy N(o)Be = 0.015 N(o)Be = 0.066 N(o)Be = 0.126 At higher conc. of B or xB > x*B, the more stable oxide BO forms first and it is solvent A that accumulates at the oxide/metal interface. In such case, the oxide film very often consists only of BO. If the oxide BO is broken up as a result of internal stresses, then the oxide AO grows outward. [A] BO [B] AO O2 Alloy xB > x*B

15. Fig. 16 Effects of Chromium on the oxidation resistance and oxide morphology of iron-chromium alloys at 1000 oC. Fig. 17 Weight gain of heat-resistant alloy compositions after 1000-hour exposure to air.