Kinetics of high temperature corrosion of a low Cr-Mo steel in aqueous NaCl Solution

The kinetics of corrosion of a low Cr-Mo steel alloy were studied over the temperature range of 75-250 degrees C in 1 m NaCl in absence and in presence of various levels of contamination with CuCl2. Corrosion rates, weights of corrosion product (magnetite) film and total (integral) weight loss of the alloy over exposure times from I up to 480 h were measured. The corrosion rate decreases rapidly with time, before it levels off at longer time, indicating the formation of a protective corrosion product film. The ability of the alloy to retain an adherent corrosion product (magnetite) film was expressed in terms of a retention coefficient. This was found to increase with temperature and exposure time and to decrease with the level of contamination with CuCl2. This effect of temperature was attributed to the improvement of the crystallinity of the corrosion product. On the other hand, the effect of CuCl2 was attributed to the electrodeposition of Cu and its impregnation within the corrosion product, which becomes less adherent. The free corrosion potential was found to be affected by the presence of the CuCl2, in a fashion compatible with the Wagner-Traud theory of mixed potential.