The high temperature corrosion of a low alloy steel in aqueous sodium chloride

The corrosion behavior of a low alloy steel (Fe-2.25 Cr-1 Mo) was investigated in 1 m NaCl over a range of high temperatures with various levels of contamination with CuCl₂. The net weight loss of the alloy and the weight of the corrosion product film on the alloy surface were measured over exposure times up to 480 h, and a temperature range of 75-250°C. The corroded surfaces and the corrosion products were characterized using stereoscopic examination, SEM and X-ray diffraction (XRD). Sigmoidal shaped relations were obtained between the film weight and exposure time at several temperatures, suggesting a mechanism of lateral growth of the corrosion product film. This was confirmed by metallographic observations. Under some conditions, parabolic film growth was observed indicating diffusion control on the rate of corrosion. In all cases, a clear inflection was observed in the film weight-time relation at a certain transition time, the magnitude of which decreases with the temperature of the test. The alloy surface underwent pitting corrosion, to an extent dependent on the temperature and the concentration of CuCl₂. The pits exhibited a lace-like morphology similar to that reported previously at ambient temperatures. The presence of CuCl₂ in the corrosive medium leads to extensive detachment of the corrosion product off the corroding surface. Under these conditions, the spalled corrosion product was much more enriched with metallic copper than the corroding surface. This points to the serious effects of copper ions on the integrity and protection efficiency of the resulting corrosion product film.