The Formation of Oxide Films On Chromium and 18 Cr-8 Ni Steels

The formation of oxide films on simple chromium steels and 18-8 stainless steels oxidized in air at temperatures of 300 C to 700 C has been studied by electron microscopy, electron diffraction and X-ray diffraction methods. A thin uniform film forms in the first few minutes of oxidation and grows very slowly thereafter. As oxidation proceeds the protectiveness of the base film fails at randomly distributed positions, permitting the rapid growth of localized polycrystalline nodules of oxide. These nodules continue to grow and new ones are formed until the base film is completely covered. The rate at which the process proceeds depends on alloy composition and temperature in the same way as does the scaling rate at higher temperatures.

There is no evidence that the failure is due to cracking of the base film. There seems to be no detrimental effect produced by the usual amounts of minor constituents in the alloys. Local failure may be due to blistering or recrystallization.

The principal component of the base film is a solid solution of ferric and chromic oxides preferentially oriented with the (111) plane parallel to the surface. The degree of the orientation and the chromic oxide content of the solution increase with time of oxidation. A small amount of spinel may be present also. In the case of the 18Cr-8Ni alloy the spinel contains some Ni. The nodules are principally αFe₂O₃ or a low chrome ferric-chromic oxide solution and contain somewhat more spinel than does the base film. 3.2.3