Role of chlorides in hot corrosion of a cast Fe-Cr-Ni alloy. Part 1: Experimental studies

High alloy stainless steels are often used in corrosive, high temperature applications because they form a protective, adherent Cr2O3 scale. When the environment in such applications includes condensed molten salts, especially alkali sulfates and alkali chlorides, these alloys are likely to undergo hot corrosion, even at moderate temperatures compared to their typical maximum application temperature. The chemical (or electrochemical) reactions and transport modes for hot corrosion in a system involving a multi-component alloy and a multicomponent salt are complex, but some insight can be gained with the help of a multi-component thermochemical model to identify major reactions. The present work consists of two parts: (a) experimental measurements of hot corrosion rates and characterization of corrosion products on a commercial, cast super-duplex stainless steel (HH), which result from exposure to thin deposits of a mixture of alkali sulfates and alkali chlorides, and (b) an evaluation of possible corrosion reactions by a multi-component thermochemical model. In laboratory hot corrosion tests alloy coupons experienced rapid oxidation rates, penetration of the oxide scale, scale blistering, and scale spallation (on cooling). Compared to simple air oxidation, alkali sulfate deposits increased the corrosion rate by a factor of about 200, but mixtures of alkali sulfate and alkali chloride increased the rate by about 20,000 times. A principal goal of the study was to identify the role of alkali chlorides in accelerating hot corrosion. (C) 2004 Elsevier Ltd. All rights reserved.