Stress assisted corrosion (SAC) is one of the major reasons for carbon steel tube failure in industrial and power boilers. The initiation and growth of SAC strongly depends upon the formation and stability of the protective magnetite film, Fe3O4, on the waterside surface of boiler tubes. A recirculation autoclave facility was set up, in which slow strain rate tests (SSRT) and low frequency cyclic tests were carried out in simulated boiler water environment on smooth carbon steel specimens to investigate the role of environment and cyclic stresses on crack initiation and propagation. Preliminary results have shown that SAC crack susceptibility increased with the increase of mean stress, and amplitude respectively, but decreased as the frequency of cyclic loading goes up. Stress amplitude shows stronger influence on the crack growth and less effect on the crack initiation than the mean stress value. The results from SSRT and low-frequency tests indicate that the strain rate also affects SAC initiation and growth in boiler water environments. Results from this study indicate that the SAC crack initiation and propagation mechanisms involve magnetite film breaking and repassivation.
Waterside cracking is one of the major reasons for failure of carbon steel waterwall tubes industrial and power boilers and has been recognized as a major cause for boiler downtime 1, 2, 3. In the utility industry, waterside tube cracking is generally referred to as corrosion fatigue (CF). The typical corrosion fatigue cracks found in utility boiler are generally long and sharp, as shown in Figure 1.a. A significant amount of work has been published on corrosion fatigue crack initiation and propagation 4-12. However, cracks in industrial boilers, are typically found in areas with heavy attachment welds on the outer surface and are typically blunt, with multiple bulbous features indicating a discontinuous growth, as shown in Figure 1.b. These types of tube failures are typically referred to as stress assisted corrosion (SAC). For recovery boilers in the pulp and paper industry, SAC failures are particularly important as any water leak inside the furnace can potentially lead to smelt-water explosion.
Performance of carbon steel tubes in high purity water strongly depends upon the formation and stability of protective magnetite, oxide film on the waterside surface of boiler tubes 8. Chemical or mechanical disruption of the magnetite film may lead to the initiation of local corrosion or cracks. Rupture of the protective film by fatigue-generated strains may also depend upon the film composition and properties along with the magnitude of strain.
Oxygen concentration in utility boiler environments has been shown to have a significant effect on corrosion fatigue cracks in carbon steels 2, 6-7, 13-15. Research work 16-18 carried out in our lab has shown that the crack morphology differs due to the differences in the water chemistry control during boiler shutdown periods. Water chemistry control in utility boilers, during operating and shutdown conditions, may be tightly controlled compared to most of low pressure industrial boilers. During the boiler startup and shutdown, the oxygen may enter the water to form the surface film.
