Initiation and propagation of stress-assisted corrosion (SAC) cracks in carbon steel boiler tubes

Industrial boilers experience bulbous cracks in carbon steel water-wall tubes and other water-touched surfaces. Because these cracks are blunt and different from sharp fatigue cracks, they are generally referred to as stress-assisted corrosion (SAC) cracks. The performance of carbon steels in industrial boilers strongly depends on the formation and stability of the magnetite film on the waterside surface. To understand the mechanism for SAC crack initiation and propagation, slow strain rate tests were conducted in a recirculation autoclave under industrial boiler water conditions. The dissolved oxygen in the water was maintained from a negligible amount (5 ppb) to 3 ppm. The SAC crack initiation and propagation mechanism involves magnetite film damage and requires the presence of dissolved oxygen in the water Increasing the test temperature accelerates the process. A mechanism for SAC cracking is proposed, and interrupted slow strain rate tests were carried out to validate this mechanism. Temperature and dissolved oxygen in boiler water are important factors in initiation and propagation of stress assisted corrosion cracks. SAC in boilers can be controlled by controlling the dissolved oxygen levels around 5 ppb.