Cr was oxidized in 1 aim O2 at 980, 1090, and 1200°C. ElectropolishedCr and some orientations of etched Cr oxidize rapidly and develop compressive stress in the growing Cr2O3; other orientations oxidize slowly, apparently free of stress. SEM examination of fracture sections shows that the thick oxide is polycrystalline whereas the thin oxide on etched Cr is monocrystalline. It is deduced that the monocrystalline oxide grows by lattice diffusion of cations outward, and the polycrystalline layer by the two-way transport of cation diffusion outward and anion diffusion inward along oxide grain boundaries. The consequent formation of oxide within the body of the polycrystalline layer generates compressive stress and leads to wrinkling by plastic deformation. The activation energy for oxidation of Cr by cation lattice transport is 58 kcal/mole. Polycrystalline Cr2O3 forms on Fe-26Cr alloy, whether electropolished or etched; oxidation is accordingly rapid and accompanied by compressive stress.

Effect of oxide grain structure on the high-temperature oxidation of Cr

At high temperatures most alloys rely on the protective effect of oxide scales formed by the reaction between oxygen from the environment and components of the alloy. The protective effect of these scales may, however, be impaired if stresses lead to cracking or spalling. Therefore, the mechanical properties of the scales play a vital part in protection, in particular under service conditions of the materials where the presence of stresses cannot be excluded. The paper provides a survey of the existing models that describe the mechanical properties of scales (the emphasis being on mechanical scale failure), the measuring techniques and the data. It will be demonstrated that, in the case of tensile conditions, the situation is relatively well understood and that a fair amount of data exists. With regard to compressive conditions scale failure consists of a combination of different failure steps, which can each be described by a quantitative model, but there is still clearly a need for experimental verification. While the measuring techniques for the properties under tensile stresses are fairly well developed, those for the behavior under compressive stresses, in particular those characterizing scale adhesion, are still under discussion and relatively few experimentally determined data are available in this case. Nevertheless, existing knowledge can be put to good use in assessing scale behavior under the effect of stresses, based on the models and data given in the paper.

Mechanical properties of oxide scales

This paper discusses the role of thermal stresses on the mechanical integrity of oxide layers. It is shown that the processes of cracking and spalling of oxides differ depending on the stress state (tensile or compressive) and on the relative strengths of oxide and oxide-metal interface. Under tensile conditions, through-thickness cracks develop from pre-existing defects in the oxide layer and these generate shear stresses along the interface which may result in decohesion. Under compressive conditions, spallation may result either from the growth of a tensile, wedge crack along the interface or by buckling and cracking of the oxide layer. Initial results of a finite element analysis are provided for both mechanisms.

Cracking and spalling of protective oxide layers

The oxidation mechanisms of sputtered microcrystalline Co−30Cr−5Al coatings were investigated by an acoustic emission technique, scatch test, transmission electron microscope (TEM), which was compared with CoCrAlY alloy. The results indicated that the beneficial effects of microcrystallization on the scale adhesion of Co−30Cr−5Al alloy are as follows: (1) The sputtered CoCrAl coating possesses a columnar structure, and oxidation along the columnar grains may form many micropegs which can anchor the scale to the metallic substrate, enhancing bonding of the scale. (2) The grain size of the sputtered coating is several orders of magnitude smaller than that of the cast alloy, and the grain size of oxide scales formed on the former is finer than that of the latter. The finer oxide scale may relieve the growth stresses during isothermal oxidation and partial thermal stresses during cooling by plastic deformation through grain sliding. The microcrystalline coating is more plastic than the cast alloy, which may relieve a certain amount of thermal stresses of the oxide scales. On the basis of oxide adhesion and plasticity, microcrystallization is more beneficial than the addition of reactive elements.

The mechanism of scale adhesion on sputtered microcrystallized CoCrAl films

The acoustic-emission (AE) technique has been applied to study scale-damage processes during thermal cycling of a tube, preferentially between 600 and 300°C in air, oxygen, and air + 0.5% SO2. The AE measurements were accompanied by optical and electron-optical investigations on tube rings exposed to the same cycling conditions. During the first period of cycling, a scale rich in hematite is formed. It suffers compressive stresses during cooling. The result is a buckled multilayered scale with separated lamellae. The scaling rate is lower than under isothermal conditions. AE signals start after 175°C cooling. After longer exposure times, the scale contains an increasing amount of magnetite and becomes more compact. The scaling rate increases and is comparable to that under isothermal conditions. AE signals are already observed after 50°C cooling and are correlated with crack formation in the magnetite caused by tensile stresses there. The addition of SO2 to air enhances the crack-healing process due to higher Fe diffusion in FeS. The scale is more compact.

https://link.springer.com/content/pdf/10.1007/BF00665485.pdf

Behavior of oxide scales on 2.25Cr-1Mo steel during thermal cycling. I. Scales formed in oxygen and air

When metallic components operate at high temperatures in aggressive environments their useful life is governed by the presence of a protective surface film which acts as a barrier to reactants. Considerable effort has been directed toward understanding the mechanisms by which ions diffuse through these surface films, but the mechanical maintenance of the film is often a greater prerequisite for many engineering applications.

P. Hancock

Papers

The Mechanical Properties and Breakdown of Surface Oxide Films at Elevated Temperatures