THE EFFECT OF HEAT TREATMENT ON CORROSION AND STRESS CORROSION CRACKING OF CARBON STEEL IN AGRO-FLUIDS

Abstract

The effect of heat treatment, extraneous ions addition, cathodic and anodic polarisation on corrosion and stress corrosion cracking (SCC) of medium carbon steel in cassava fluid, tomato and pepper juices has been investigated. The general corrosion performance of the steel in various environment was evaluated by measuring electrochemical potential of steel samples in the environment with respect to Ag/AgCl reference system, and by determining weight loss of some steel samples owing to corrosive action of the environment. In the SCC tests, the steel samples were subject d to conjoint action of stress and corrosion after which, the samples were stressed to failure. The ductility of tested samples gave measure of their SCC susceptibility. The rate of general corrosion attack of heat 'treated samples was found to increase as the rate of cooling from austenitising temperature was increased. Quench hardened steel showed minimum susceptibility to corrosion attack when tempered at 300°C. Results of extraneous ions addition, and polarisation effect have gjven the mechanistic model of stress corrosion crack propagation. Whereas, mechanism of cracking of quench hardened steel was hydrogen embrittlement in cassava fluid, the same steel cracked due to anodic dissolution in tomato and pepper juices. It was also established that, while steel with martensitic structure failed due to hydrogen embrittlement in cassava fluid, steel with pearlitic structure failed by anodic dissolution in the same fluid. The rate of cracking of medium carbon steel in tomato juice was found to increase steadily as applied anodic potential was increased. The steel showed marked susceptibility in cassava fluid and pepper juices when the applied anodic potential was between 12 and 15V