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.
This paper reviews the concerns of applying excessive levels of cathodic protection current to pipelines and the need for establishing an upper potential limit. Coating disbondment, hydrogen induced stress cracks, stress corrosion cracking, hard spots and the problems associated with measurement of a true polarized pipe-to-electrolyte potential are addressed.