Under-deposit corrosion is a common mode of failure in boiler tubes. Mechanisms include phosphate attack, caustic gouging, hydrogen damage, and wall thinning due to metal loss. Many aspects of these mechanisms and their mitigation are well-understood, while certain specific details remain uncertain or subject to controversy.
Stress assisted corrosion (SAC) is a major concern for kraft recovery boilers in the pulp and paper industry. A water leak can cause a smelt-water explosion in the boiler. 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.
The unavailability of coal fired electrical generating capacity is often attributable to corrosion in steam generators. Electricity demand grows faster than new plants come online, forcing greater use of old plants. Identifying such problems requires proper inspection tools and techniques to take the guesswork out of whether boiler tubes should be repaired or replaced.