Chromium halides

Chromizing. The other principal method of obtaining a chromium-rich surface on steel is by chromizing (29). The material to be treated is embedded in a mixture of ferrochromium powder, a chromium halide, alumina, and sometimes NH Cl. The chromium is diffused in by a furnace... 

Most modern methods of chromising rely on the reaction of a chromium halide at the surface of the metal to be coated. Three main methods are commercially operated. 

Purely gaseous method Articles are suitably jigged inside a retort chamber and vapours of chromium halide, usually diluted with a reducing gas, are circulated. The temperature is maintained within the range 950-1 200°C for chromising of iron and ferrous alloys (recent Russian work on... 

Semi-gaseous method The articles, surrounded by a solid compound containing chromium metal, are packed in a retort chamber, and vapours of chromium halide or halogen compound are circulated in the chamber, which is maintained at 950-1 200 C. 

These give rise to the same dangerous reactions as chromium halides. Iron dichloride and trichloride detonate in contact with sodium or potassium. There is also a violent combustion of an iron trichloride/calcium carbide mixture, which leads to the formation of melted iron. 

In contrast to the zirconium compounds the chromium halides are less active than pure Cr(allyl)3 Cr (allyl) Cl2 seems to be completely inactive. The only other element known in which halide substitution of the corresponding allyl compound results in activation is hafnium, suggesting the effect may be confined to group IVa. 

The two most common series of chromium halides have the formulas CrX2 and CrX3 (where X = F, Cl, Br, or I). However, CrF6 is also known. Compounds having the formula CrX3 are Lewis acids, and they also form many coordination compounds. For example, CrX3 reacts with liquid ammonia to yield... 

The known halides of chromium are listed in Table 4 with some of their physical properties. The instability of a hexavalent chromium halide is noteworthy. The higher halides are prepared by direct reaction of the metal with the... 

A. Krupp prepared chromium of a high degree of purity by electrolyzing a fused chromium halide using impure chromium as anode. The electrodeposition of chromium has been investigated by R. Appel, C. L. Long and co-workers,... 

Fig. 2 Selected structures of terphenyl chromium halides without H atoms [21,22]...

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