Mechanism of Stress Assisted Corrosion on Carbon Steel Boiler Tubes

摘要

Stress assisted corrosion (SAC) on water-touched surfaces of carbon steel boiler tubes can lead to smelt-water explosion in kraft recovery boilers and industrial boilers. The initiation and growth of SAC strongly depends upon the formation and stability of the protective magnetite film, Fe_3O_4, on the waterside surface of boiler tubes. Tests were carried out to simulate SAC in laboratory using a recirculation autoclave. Coupon exposure tests have shown that the magnetite film characteristics are affected by the cathode to anode area ratios where the large cathode to anode ratios favors non-protective porous film. Crack initiation and propagation growth rates were measured using slow strain rate tests (SSRT) method in simulated boiler water environments. Results have shown that the concentration of dissolved oxygen in the boiler water is one of the major factors in the initiation and growth of SAC of carbon steels. Crack velocity and crack density increased with temperature. Interrupted slow strain rate tests (ISSRT) were conducted to simulate boiler shutdown conditions. Typical blunt SAC cracks with bulbous morphology were simulated in the laboratory by introducing oxygen in the water during boiler shut down period of ISSRT. Based on our results a mechanism of stress assisted corrosion of carbon steel boilers under industrial boiler water environmental conditions was proposed.