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Overview of Stress Assisted Corrosion

Stress Assisted Corrosion is a type of corrosion that can occur in metals due to elevated static tensile stress levels. Specifically, it is a stress attack directed at locations where applied or residual stresses are imparted to the metal. Stress assisted corrosion is also sometimes known as "stress enhanced corrosion" or "stress influenced corrosion."

Stress assisted corrosion is primarily known to cause failures in the waterside tubes of water tube type carbon steel boilers. Residual stresses tend to be higher where the tubes have been rolled into the drum walls. The pits or linear grooves affiliated with this damage mechanism are more likely to occur in regions of residual stresses, such as in the rolled tube ends at the interface with the drum, the bent tube sections, or parts of the tubes with externally welded attachments. Once they have become "crack-like," they are more likely to propagate during periods of high stress, such as start up or shut down, rather than during normal operation.

The main factors that contribute to the initiation and continued development of stress assisted corrosion are water chemistry, stress conditions, and the metallurgical properties of the material itself. The key to preventing stress assisted corrosion lies within maintaining certain water chemistry parameters; these include maintaining a pH between 9 and 11, and a dissolved oxygen (DO) concentration less than 5 ppb. As long as this water chemistry is maintained, stress assisted corrosion should not propagate.

Stress assisted corrosion can usually be identified through a simple visual inspection. It can be distinguished from corrosion fatigue because stress assisted corrosion tends to have blunt, rounded tip cracks, while corrosion fatigue has sharper-tipped cracks. Boroscoping can also be used in examinations; although eddy current testing has also proven to be very effective.

 

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