Chromium oxide phase diagrams

The chromium-carbon phase diagram shows the existence of three carbide phases Ct2T,C(, CryC3 and Cr3C2. Only Cr3C2 has industrial importance and it is manufactured by the carburization of chromium(IIl) oxide under a hydrogen atmosphere. 

The production of corrosion-resistant materials hy alloying is well established, hut the mechanisms are noi lull) understood. It is known, of course, that elements like chromium, mckcl. titanium, and aluminum depend for their corrosion resistance upon a tenacious surface oxide layer (passive film). Alloying elements added for the purpose of passivation must be in solid solution. The potential of ion implantation is promising because restrictions deriving from equilibrium phase diagrams frequently do not applv li e., concentrations of elements beyond tile limits of equilibrium solid solubility might he incorporated). This can lead to heretofore unknown alloyed surfact-s which are very corrosion resistant... 

The phase diagram of the aluminium oxide (AI2O3) and chromium oxide (Ct203) system is given in Figure 4.23. An equilibrium... 

As before, it is convenient to consider this system in order of increasing Cr content. At low Cr contents, no internal oxidation zone is seen. This is because the rate at which the external scale is formed is so rapid that the thickness of the internal oxidation zone is negligibly small. The system is best described in terms of the Fe-Cr-0 phase diagram shown schematically in Figure 5.18. The rhombohedral oxides FeiOs and Cr203 show a continuous series of solid solutions. The iron- and chromium-oxides react to form spinels which form solid solutions with Fc304. 

Low-pressure Processes. Three processes for the polymerization of ethylene have recently been developed. The commercial process of the Phillips Petroleum Company for the polymerization of ethylene is carried out at relatively low pressures (100-500 psi) in either fixed-bed or slurry-type operations. The catalyst consists of 2-3 weight per cent chromium as oxide on silica alumina, and the reaction temperature varies from 90— 180°C. In fixed-bed operation, purified ethylene and hydrocarbon solvent streams are passed downflow, liquid phase over the catalsrst bpd. Solvent and polymer are collected, and the solvent is flashed overhead. Unreacted gases are removed from the solvent, taken overhead, and metered the solvent is recycled to the reactor. The solvent and polymer in the first receiver are cooled to room temperature to precipitate the polymer, which is then filtered and dried in a vacuum oven. In the slurry-type operation (indicated in Fig. 15-33 by a proposed flow diagram), solvent and a small... 

Rapp explained the expected stability of the protective oxide Cr203 with respect to dissolution either as acidic solutes such as 2(804)3 or as basic solutes such as Na2Cr04 or NaCr02. The phase stability diagram for the Cr-8-0 system can be superimposed on that for Na-S-O, as shown in Fig. 7.4. The two abscissa scales at the bottom and top of the figure provide alternative parameters for melt basicity (or acidity). Under no conditions does the metal chromium remains stable in contact with Na2804 at 1200 K (927°C). 

---