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.
An unexpected failure of 316L Stainless Steel instrument tubing occurred in a high pressure Hydroprocessing unit resulting in a shutdown of the unit. The tubing system consisted of a compression type fitting commonly used in instrument systems and had only been in service for 3 years when the failure occurred. The failed tubing samples were removed for metallurgical analysis and determination of damage mechanism.
Metallurgical analysis and finite element analysis of the tubing identified excessive cold working leading to hydrogen embrittlement as the primary mode of failure. This paper details the investigation into the failure to arrive at the root cause and the preventive measures adopted to assess the installed population of tubing in similar service.