Materials, Corrosion and Mechanical Integrity
This domain tests material selection, corrosion mechanisms, inspection planning, and fitness-for-service. The exam expects you to connect environment, alloy, temperature, and stress to credible damage modes.
Core rules
- Identify the corrosion mechanism before choosing a material: uniform, galvanic, pitting, crevice, erosion-corrosion, stress-corrosion cracking, or hydrogen damage.
- Carbon steel is economical for many dry, neutral services but is vulnerable to acids and caustic stress-corrosion cracking at high temperature. Austenitic stainless steels resist many acids but are susceptible to chloride pitting and chloride stress-corrosion cracking.
- Corrosion allowance adds thickness for expected uniform loss. Remaining life = (current thickness - retirement thickness)/corrosion rate.
- Mechanical integrity programs use inspection, NDT, and remaining-life calculations. A code-compliant thickness can still be unacceptable if cracking or localized loss is present.
Worked example
A vessel starts at 10.0 mm. After 2 years it measures 9.4 mm, so the uniform corrosion rate is (10.0 - 9.4)/2 = 0.3 mm/yr. If the retirement thickness is 6.0 mm, remaining life is (9.4 - 6.0)/0.3 = 11.3 years. The reasoning: use the measured loss rate, not the original allowance, and compare current thickness with the retirement limit. Localized pitting could shorten this life and would require separate assessment.
Traps
Assuming a linear corrosion rate when pitting or cracking dominates, ignoring chloride concentration and temperature for stainless steel, and treating a hydrotest as a substitute for inspection.