The influence of tempering on the corrosion resistance of newly developed steels

Master Thesis


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University of Cape Town

This thesis deals with the effect of heat treatment on the localized corrosion resistance of the low carbon, chromium containing steels, designated 825, 102A and 122 which recently have been developed. The potentiodynamic polarization technique has been employed to determine the corrosion characteristics of the three steels. The results do not accurately reflect the effect heat treatment has on corrosion rates but scanning electron microscopy of corroded surfaces does allow a characterization. Both tempering temperature and time at temperature have a significant influence on the corrosion behaviour of chromium steels because the type, size and morphology of carbide precipitates are determined by the temperature and time of temperi ng. Localized pitting corrosion predominates for specimens tempered at temperatures below 450°C. Intergranular corrosion together with general corrosion occur after tempering at temperatures. in excess of 450°C. The resul ts of hardness tests show that secondary hardeni ng occurs after tempering between 450°C and 600°C. Secondary hardening suggests the presence of chromium carbides which deplete the surrounding matrix of chromium leaving it susceptible to active general corrosion (within the grains) and intergranular corrosion (at grain boundaries). A model showing the effect that 12% chromium, in comparison to 8% chromium, has on the corrosion resistance, is proposed. The significance of these results with regard to the application of the steels is discussed.

Bibliography: pages 105-113.