Tham khảo tài liệu 'modern physical metallurgy and materials engineering part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Corrosion and surface engineering 381 is a compound oxide of both the solute and solvent metals. The best-known examples are the spinels with cubic structure . and . It is probable that the spinel formation is temperaturedependent with Cr2 O3 forming at low temperatures and the spinel at higher ones. Stainless steels ferritic austenitic or martensitic are among the best oxidation-resistant alloys and are based on Fe-Cr. When iron is heated above about 570 C the oxide scale which forms consists of wustite FeO a p-type semiconductor next to the metal magnetite Fe3O4 a p-type semiconductor next and haematite Fe2O3 an n-type semiconductor on the outside. When Cr is added at low concentrations the Cr forms a spinel with the wustite and later with the other two oxides. However a minimum Cr addition of 12 is required before the inner layer is replaced by Cr2 O3 below a thin outer layer of Fe2O3. Heat-resistant steels for service at temperatures above 1000 C usually contain 18 Cr ormore and austenitic stainless steels 18 Cr 8 Ni. The growth of Cr2O3 on austenitic stainless steels containing up to 20 Cr appears to be rate-controlled by chromium diffusion. Kinetic factors determine whether Cr2O3 or a duplex spinel oxide form the nucleation of Cr2O3 is favoured by higher Cr levels higher temperatures and by surface treatments . deformation which increase the diffusivity. Surface treatments which deplete the surface of Cr promote the formation of spinel oxide. Once Cr2O3 is formed if this film is removed or disrupted then spinel oxidation is favoured because of the local lowering of Cr. When chromium-bearing alloys such as austenitic stainless steels are exposed to the hot combustion products of fossil fuels the outer layer of chromium oxide which forms is often associated with an underlying sulphide phase Figure . This duplex structure can be explained by using phase stability diagrams and the concept of reaction paths . Previously in
