Determination of the corrosion resistance of low alloyed steels by magnetic method

Abstract

An innovative method of measuring and control of the steel corrosion rate by changing magnetic characteristics is developed. The impact of heat treatment on the corrosion rate of the samples of high-carbon steels (maximum corrosion loss is observed at a tempering temperature of 400°C) is attributed to the appearance of micro-galvanic pairs (MGP) between the phase components of the material. MGP undergo redistribution under heat treatment thus changing conditions of the galvanic current flow. The structural phase composition, in turn, determines the magnetic properties of steel and correlation between the magnetic properties and corrosiveness. The goal of the study is demonstration of the possibility and expediency of using the magnetic parameters of steel for determination of the steel corrosion rate. A close correlation dependence is observed between the coercive force and the electrochemical potential (relative to the silver chloride electrode) which are direct indicators of the corrosiveness. Case study of a pipeline made of 09G2S steel along which change in the coercive force attained 25% revealed rather high risk of developing micro-galvanic pairs. A rapid method of scanning magnetic parameters is proposed to detect potentially corrosive zones. A multi-parameter approach can be used to solve the problem of the ambiguity of the relationship between the corrosiveness and magnetic parameters. Harmonic decomposition of magnetic hysteresis loops of 45Kh steel samples is used to obtain a number of odd harmonics. Some of them weakly correlate with the corrosion loss, whereas complexes of several harmonics correlate to a greater extent. The results can be used in technical diagnostics and prediction of the corrosion activity of steel structures before their operation. The results can be used in technical diagnostics and forecasting of the corrosiveness of steel structures prior to their operation.