Stress assisted corrosion (SAC) of carbon steel boiler tubes is one of the major causes of waterside failure in industrial boilers. SAC is a major concern for kraft recovery boilers in the pulp and paper industry as any water leak into the furnace can cause a smelt-water explosion in the boiler. Failed carbon steel boiler tubes from different kraft recovery boilers were examined to understand the role of carbon steel microstructure on crack initiation and SAC crack morphology. A number of carbon steel tubes showed a deep decarburized layer on the inner surface (water-touched) and also an unusually large grain size at the inner tube surface. SAC cracks were found to initiate in these areas with large-graineddecarburized microstructure. Tubes without such microstructure were also found to have SAC cracks. It was found that the decarburization and large grained microstructure may facilitate initiation and growth but is not necessary for SAC of carbon steel boiler tubes.
Failures of carbon steel boiler tubes and other water-touched surfaces in utility boilers and industrial boilers have been reported since the 1960s. In the utility industry, waterside tube cracking, generally identified as corrosion fatigue (CF), has been recognized as a major cause for boiler downtime.1-2 It was found that the CF occurs on the waterside of tubes where attachments are welded on these tubes from the cold-side.1 An EPRI report in 1992 noted that these boiler tube failures had often been incorrectly identified as weld defects or stress-corrosion cracks. The typical CF cracks found in utility boilers are generally un-branched, long and sharp. However, blunt and bulbous cracks were found in a number of industrial boilers. These cracks were called stress assisted corrosion (SAC) rather than corrosion fatigue. Typically, utility boilers operate at higher temperatures and pressures than industrial boilers. A significant amount of work has been published on properties of the surface film formed on the carbon steel as well as on crack initiation and propagation in high temperature water environments of both type of boilers.1-30 There are differences in water chemistry and frequency of possible upsets in the operating conditions between high-pressure power boilers and lower pressure industrial boilers. It has been speculated that the water chemistry control in utility boilers, during operation as well as during shutdown, may be more tightly controlled as compared to most low-pressure industrial boilers. Boiler tube leaks in utility boilers may lead to lower energy efficiency for the boiler but may not always force the boiler to shutdown immediately. However, if water leaks into the furnace some kraft recovery boilers in the pulp and paper industry may experience a smelt/water explosion. In recovery boilers, where SAC is expected or is detected, boiler pressures have to be reduced to avoid failures or the boiler has to be shut down to remove and replace affected tubes.
A number of papers on SAC problems in industrial boilers have been published in recent years 3-5, 15-31. Sharp 25 has reviewed the SAC problem in the pulp and paper industry.