A review of principles governing alloy oxidation is presented together with the means by which practical oxidation-resistance alloys have been developed. The reactions involved are complex and involve acquiring data from many fields. Wagner developed the basic diffusional models for the formation of oxide scales, but these need to be expanded using detailed physical and metallurgical studies of the alloy phases, oxides and their defects, as well as the mode of scale formation and the structural relationships between alloy and oxide scale. At this time the engineering solutions have outstripped fundamental understanding, but an attempt is made to show how the development of oxidation theory has contributed to this effort by a term now referred to as 'informed empiricism'.

https://iopscience.iop.org/article/10.1088/0034-4885/39/5/001/pdf

The oxidation of alloys

Solid-state electrochemical measurements using various experimental procedures were made with the double cell: 
$$
Ni + NiO|ZrO_2 + Y_2 O_3 |Ni + \underline O |ZrO_2 + Y_2 O_3 |Ni + NiO
$$

 to determine the diffusivity and thermodynamic functions of oxygen dissolved in solid nickel. Non-steady state diffusion of oxygen in the specimen was caused by applying a preselected potential between the reference and specimen electrodes and was monitored by measuring time-dependent potentials and/or currents. The following results were obtained for the diffusivity of oxygen and the solubility of oxygen in nickel in equilibrium with NiO: 
$$D{\text{ = 4}}{\text{.9 }} \times {\text{ 10}}^{{\text{ - 2}}} {\text{ exp}}\left( {{\text{ - }}\frac{{{\text{164 kJ/mole}}}}{{{\text{R}}T}}} \right){\text{cm}}^{\text{2}} /{\text{sec (850 to 1400 }}{}^{\text{o}}{\text{C)}}$$

 
$$C_{\text{O}}^s {\text{ = 8}}{\text{.3 exp}}\left( { - \frac{{55{\text{kJ/mole}}}}{{{\text{R}}T}}} \right){\text{at}}{\text{. pct (800 to 1000 }}{}^{\text{o}}{\text{C)}}$$

 The thermodynamic and transport behaviors of oxygen in solid nickel were fairly well described by a simple quasi-regular model and an interstitial diffusion model, respectively.

The diffusion and solubility of oxygen in solid nickel

X-ray photoelectron spectroscopy was applied to study the composition of the passive film formed on an extremely corrosion resistant amorphous Fe-10at.%Cr-13at.%P-7at.%C alloy in 1 N HCl. The passive film consists mainly of hydrated chromium oxyhydroxide which is a common major constituent of passive films on crystalline stainless steels. The extremely high corrosion resistance of the amorphous alloy can only in part be attributed to the formation of a protective hydrated chromium oxyhydroxide film.

ESCA Study of the Passive Film on an Extremely Corrosion-Resistant Amorphous Iron Alloy

The influence of annealing atmosphere oxidation potential (i.e. p H 2 O/ p H 2 ratio) on the surface condition of an industrial dual-phase steel alloyed with 2 wt.% Mn and 0.3 wt.% Mo has been studied by galvanizing simulation, with particular attention towards alloying element segregation, the occurrence of selective oxidation of the surfaces immediately prior to hot-dip galvanizing and their effect on reactive wetting and coating adherence. The surface chemistry and morphology prior to dipping have been analyzed in detail, along with their evolution during annealing, and the coating quality and chemistry were characterized and interpreted in relation to the surface condition prior to dipping. It was found that Mn segregates to the surface and gives rise to significant selective oxidation for all the experimental atmospheres, with a transition from external to internal oxidation occurring when the oxidation potential of the atmosphere increased. However, in spite of the presence of Mn oxides at the steel surface prior to dipping, samples annealed under all atmospheres exhibited good coating adherence and a well developed inhibition layer. This is attributed to the aluminothermic reduction of the Mn oxides by the Al present in the Zn bath followed by reactive wetting between the substrate and the Zn bath.

Effect of annealing atmosphere on the galvanizing behavior of a dual-phase steel

The embrittlement of three grades of nickel following air exposure at 1000 °C is reported. This embrittlement manifests itself in a dramatic loss in tensile ductility around the 800 °C range, and is accompanied by a transition to intergranular failure. Similar behavior has previously been reported in nickel-base superalloys, but is now shown to be a general property of nickel. Tests in a variety of atmospheres have established that the damaging species is oxygen, and that this can have a profound effect, even when present at very low partial pressures. Grain boundaries are observed to be pinned and recrystallization suppressed after oxygen exposure. This lack of boundary mobility has been used to explain the temperature dependence of the embrittlement observed, and has been related to some of the results seen in superalloys. Finally, a few implications for environment-property interactions are considered.

The embrittlement of nickel following high temperature air exposure

The diffusion constants of oxygen in nickel and α-iron are calculated from experimentally determined internal oxidation rates in dilute silicon-bearing alloys. A single crystal alloy of composition 0.058 wt % silicon in nickel and polycrystalline alloys of composition 0.48 wt % silicon in nickel and 0.072 wt % silicon in iron were oxidised and allowance is made for the absorption of oxygen in the reaction with silicon. For nickel the values of activation energy Q=73.9 kcal/g atom in the range 800 to 1200° C and Q=74.4 kcal/g atom in the range 900 to 1300° C are in good agreement with a previous result. The activation energy required for oxygen diffusion in α-iron was found to be 39.9 kcal/g atom in the range 700 to 850° C.

The determination of the diffusion constants of oxygen in nickel and α-iron by an internal oxidation method

Amorphous thin films of stainless steel

Results are presented of optical, scanning electron and transmission electron microscope observations on some internally oxidised ferromagnetic alloys. The alloys investigated were of nominal composition 0.058 wt % Si in Ni, 0.48 wt % Si in Ni and 0.17 wt % Si in Co 66.2%-Ni 33.7%. The alloys were in polycrystalline form and in addition single crystals of 0.058 wt % Si in Ni were examined. The diffusion and oxidation rate constants are calculated and the state of the oxide product as a function of (i) depth in the alloy, (ii) oxidising temperature and (iii) alloy composition is considered. The possibility of using such systems as device material and as test material for theories of coercivity and approach to magnetic saturation is considered.

An experimental investigation of the structure of some internally oxidised ferromagnetic alloys

An estimate of the diffusion parameters of oxygen and silicon in ?-cobalt from studies of internal oxidation

The results of an experimental investigation into the morphology of silica precipitates in internally oxidised silicon-bearing nickel and cobalt alloys are compared with the predictions of theories that concern the growth of spheroidal and extended precipitates and dendritic phases. Some degree of agreement is obtained between theory and experiment and a fairly detailed dependence of morphology on oxidation conditions and diffusion and concentration factors is outlined. Some observations on the stability of internal oxide precipitates are discussed and some conclusions, which are believed, in the main, to be general and applicable to any internally oxidised system, are made as to the dependence of morphology on the conditions of experiment.

P. J. Grundy

Papers

The determination of the diffusion constants of oxygen in nickel and α-iron by an internal oxidation method

Amorphous thin films of stainless steel

An experimental investigation of the structure of some internally oxidised ferromagnetic alloys

An estimate of the diffusion parameters of oxygen and silicon in ?-cobalt from studies of internal oxidation

The morphology and stability of precipitates in internally oxidised silicon-bearing nickel and cobalt alloys