TABLE OF CONTENTS Oxidation of metals and alloys has been discussed in detail in Chapter 4. In industrial processes, materials are exposed not only to oxygen but also to other environmental constituents in the form of gases such as CO 2 and SO 2, fused or molten salts like alkali and alkaline earth sulfates, chlorides, and solid particles in the form of sand and fly ashes. Some of the constituents are the exhaust products of the industrial processes, whereas others are ingested from extraneous sources. The solid particles may melt during their transit through the system or remain in particulate form before exiting the system. The interaction of these environmental constituents with the materials of construction results in corrosion and erosion. Corrosion consists of parasitic chemical conversion reactions, whereas erosion involves mechanical impact and associated material loss. The latter may, however, accelerate environmental interactions by removing protective oxide scales.
Corrosion of materials including Ni and Co base superalloys induced by molten salts in an oxidizing gas at elevated temperatures is termed "hot corrosion" (Pettit and Giggins, 1987; Goward, 1986; Rapp and Zhang, 1994; Sidhu et al., 2006) to distinguish it from the traditional low-temperature corrosion. It is the result of accelerated oxidation at temperatures typically between 700 C (1300 F) and 925 C (1700 F) when metals and alloys become covered with contaminant salt films. The salts in the vapor phase are generally benign. The temperature range within which hot corrosion occurs strongly depends on the salt chemistry and gas constituents,...
