Industrial boilers like kraft recovery boilers experience stress assisted corrosion (SAC) cracks in their carbon steel tubes and other water touched surfaces. The performance of carbon steel in industrial boilers strongly depends upon the formation and stability of the protective magnetite film, Fe3O4, on the waterside surface of boiler tubes. Tests were carried out in a recirculation autoclave under industrial boiling water conditions. Boiler water chemistry was controlled during a series of tests to keep the dissolved oxygen in the range of 5 ppb - 10 ppm. Initial tests were conducted to develop the magnetite film on carbon steel tube samples under different test conditions. Results have indicated that the water chemistry and ratios of anode/cathode have an effect on the magnetite film morphology. Film characterization by atomic force microscopy (AFM) and scanning electron microscopy (SEM) has shown that the magnetite film changes from an irregular-grained compact and protective film to a fine-grained porous (non-protective) film with tetrahedral crystals at the surface when the anode to cathode area ratio decreases. Corrosion fatigue crack initiation and growth mechanisms involved in boiler water environments include magnetite film damage as an important step. Slow strain rate tests were carried out in simulated boiler water environments, using smooth carbon steel samples, to investigate the role of temperature and water chemistry on crack initiation. The mechanism of stress assisted corrosion was proposed here, and an interrupted slow strain rate test was designed and carried out in lab to validate the proposed SAC mechanism.
Failures of carbon steel boiler tubes from the waterside have been reported in the utility boilers and industrial boilers for a long time. In the utility industry, waterside tube cracking, generally referred to as corrosion fatigue (CF), has been recognized as a major cause for boiler downtime. The typical corrosion fatigue crack that found in utility boiler is generally long and sharp, as shown in Figure 1 a. Cracks in industrial boilers are typically found in areas with heavy attachment welds on the outer surface. These cracks are typically blunt, with multiple bulbous features indicating a discontinuous growth, as shown in Figure 1b. These types of tube failures are typically referred to as stress assisted corrosion (SAC). For recovery boilers in the pulp and paper industry, these failures are particularly important as any water leak inside the furnace can potentially lead to smelt-water explosion. Utility boilers also have waterside cracking in carbon steel tubes but most of the cracks are sharp and are referred to as corrosion fatigue cracks. A significant amount of work has been published on corrosion fatigue crack initiation and propagation.
It is clear from the previous published literature, why utility boilers may have sharp corrosion fatigue cracks whereas majority of industrial boilers have blunt and bulbous cracks. Typically, utility boilers operate at higher temperatures and pressures compared to the industrial boilers. It has been speculated that the water chemistry control in utility boilers, during operating and shutdown conditions, may be tightly controlled compared to most low pressure industrial boilers. Some researchers have reported the effect of temperature on corrosion fatigue .
Previous work in utility boiler environments has shown that the oxygen concentration has a significant effect on corrosion fatigue cracks in carbon steels. For carbon steel boiler tubes, Dooley reported that the corrosion fatigue is strongly influenced by water chemistry with oxygen concentration being the major factor. Results from ph