General Principles of Damage Mechanisms and Failure Analysis
How API 571 frames a mechanism
API 571 is organized around damage mechanisms rather than equipment. For each mechanism the text gives the damage morphology, susceptible materials, critical process variables (temperature, environment and chemistry, stress, velocity), the equipment where it occurs, inspection and monitoring, prevention and mitigation, and related mechanisms. A candidate must work in both directions: from a mechanism name, state where it occurs and what makes it better or worse; from a described failure, name the mechanism and the most suitable inspection method.
Failure analysis discipline
Start with the failure mode - leak, rupture, cracking, deformation, thinning - then determine the mechanism that produced it. Collect service and upset history and process chemistry, preserve fracture surfaces before cleaning, do visual and NDT examinations before destructive testing, and use metallography, hardness, SEM, and deposit analysis to confirm or eliminate candidates. Separate active from inactive damage, and separate rate-based mechanisms (general and localized thinning) from threshold mechanisms (SCC, brittle fracture, high-temperature hydrogen attack); they require different inspection intervals and mitigation.
Traps
Questions frequently describe a symptom and expect the underlying mechanism, mix contributing factors with root cause, or offer a mechanism that needs a material or temperature not stated in the stem. Read material, environment, temperature, and stress first; each usually eliminates one or two options. Vague answers such as "corrosion" are rarely correct when a specific mechanism exists.
Worked example: pinhole leaks on the underside of an insulated carbon steel line operating near 200 deg F in a coastal plant. Material carbon steel, water present, and temperature inside the CUI range (roughly 10 to 350 deg F) point to corrosion under insulation, not process pitting. The inspection response is insulation removal with visual and UT examination, not a process chemistry change.