Phase changes, carburization

Table I. Principal Carburization Reactions of Iron and Phase Changes...

High-temperature corrosion is considered to be electrochemical in nature, with the high-temperature scale formed acting as an electrolyte. Corrosion is usually uniform in nature. The predominant effects are oxidation and carburization/decarburization. Changes in mechanical properties, specifically a loss of ductility due to phase changes, also take place. 

In the case of cobalt, unstable cubic cobalt was identified as the product of the reduction of standard cobalt catalysts, while hexagonal cobalt was found as a product of the hydrogenation of cobalt carbide. Used cobalt catalysts show no carbide by x-ray examination. Bulk phase carbide decreases the activity of cobalt catalysts. Surface area measurements show no appreciable change when the cobalt of cobalt catalysts was converted to cobalt carbide. Carburization at conditions where free carbon is formed increases the area considerably. 

The changes in the ferromagnetic phases during the 72-hour carburization of catalyst B-6 (Figure 5) were measured by TMA at the times indicated in Figure 1. As the a-Fe phase decreased, the Fe2C (Hagg) phase increased, and the FesC (cementite) phase exhibited a transient existence, reached a maximum at about five hours. 

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