Causes of Deterioration and Failure in Piping Systems
Organize damage by family
Candidates should group mechanisms rather than memorize a list: uniform thinning; localized loss (pitting, crevice, galvanic, under-deposit); velocity-related damage (erosion, erosion-corrosion, cavitation, flashing); environmental cracking (chloride SCC of austenitic stainless steel above roughly 60 C/140 F, caustic SCC, amine cracking, wet H2S cracking); high-temperature attack (sulfidation, naphthenic acid, oxidation, carburization, creep above roughly 800 F for carbon steel); hydrogen damage; metallurgical embrittlement (885 F/sigma, temper embrittlement, brittle fracture, thermal shock); and mechanical damage (water hammer, vibration fatigue at small-bore branch connections, thermal cycling fatigue).
Where questions go wrong
- CUI is an external mechanism under insulation, commonly in the 10 F to 350 F band for carbon steel systems and aggravated by steam tracing and cyclic service. It is invisible to process-side monitoring, so insulation windows are the detection method.
- HIC and SSC are both wet-H2S damage but differ: HIC is internal hydrogen-induced cracking and blistering driven by corrosion, while SSC is stress-driven cracking of hard welds and HAZ. Both are mitigated by hardness control and PWHT.
- Corrosion rate belongs to the metal-environment couple, not the alloy alone: dry hydrocarbon is benign, wet hydrocarbon is not.
- Deadlegs, low-flow lines, and injection points deteriorate from the process side and often fail at the mixing zone rather than mid-run.
Rule of thumb: any place where water can accumulate, condense, or stagnate, namely low points, deadlegs, bottoms of horizontal lines, soil-to-air interfaces, and under insulation, should be treated as a probable damage site. API 571 is the reference for confirming the mechanism and its inspection options.