Materials, Corrosion, Mechanical Integrity and Asset Management
Scope
This domain tests material selection, corrosion mechanisms, inspection strategy, and fitness-for-service. The exam wants evidence that you can predict the failure mode and set an inspection interval, not just name a stainless steel.
Core rules
- Corrosion types include uniform, galvanic, crevice, pitting, intergranular, stress corrosion cracking, erosion-corrosion, and hydrogen damage. Each has a different control: material, coating, cathodic protection, inhibitor, drainage, or stress relief.
- Material selection considers environment, temperature, chloride level, pH, velocity, and contaminants. Type 304 stainless steel is not a generic solution; chloride stress corrosion cracking and pitting are real risks above ambient temperature.
- Corrosion allowance is consumed over time. Remaining life = (current thickness - retirement thickness) / corrosion rate. Inspection interval is often half the remaining life, adjusted by risk.
- Mechanical integrity covers pressure vessel codes, non-destructive testing such as UT, RT, PT, and MT, hydrotest, relief device inspection, and management of change. Risk-based inspection ranks equipment by probability and consequence.
- Corrosion under insulation and dead legs are classic hidden threats.
Worked example
A carbon-steel line has current wall thickness 8.0 mm, retirement thickness 4.0 mm, and measured corrosion rate 0.2 mm/yr. Remaining life = (8.0 - 4.0) / 0.2 = 20 years. A common rule sets the next inspection at half-life, 10 years, but a risk-based plan may shorten it if the line is corrosive and inaccessible. The calculation is linear and conservative only if the rate is stable. If the rate is accelerating, or if pitting is local, a thickness average hides the failure. Use UT mapping and a fitness-for-service assessment.
Traps
- Using nominal thickness rather than measured minimum thickness.
- Ignoring the difference between general corrosion and localised pitting.
- Specifying a material from a generic corrosion chart without checking temperature and chloride.
- Treating a hydrotest as a substitute for inspection and corrosion monitoring.
Asset management links inspection findings to repair, rerating, or replacement. A thickness reading without a decision is just data.