Failure Analysis, Troubleshooting, and Remediation
What the exam expects
Failure analysis questions present a symptom and ask for the most probable cause and the appropriate remediation. Know the common failure modes: blistering (osmotic, solvent, or gas), undercutting, cathodic disbondment, chalking, cracking and mudcracking, sags and runs, pinholing, dry spray, delamination, and rust staining/spot rusting. Match each to a mechanism - surface contamination, application error, environmental exposure, or material incompatibility.
Understand a systematic investigation: document conditions, sample the failed area, test for soluble salts, examine the interface, and compare against the specification. Remediation follows from the cause - cleaning and re-blasting for salt contamination, changing application conditions for solvent blistering, or revising the coating system for a chemical incompatibility.
Traps
- Jumping to a material defect when contamination or application conditions are the real cause.
- Confusing osmotic blistering (soluble salts at the interface) with solvent blistering (trapped solvent from recoating too soon).
- Assuming spot repair is adequate when the failure is widespread and systemic.
- Missing that a failure at edges and welds often indicates inadequate strip coating or profile.
Rule of thumb
Blister fluid that is clear and the blister appears over a salt-contaminated area points to osmotic blistering; blisters appearing soon after application with solvent odor point to solvent entrapment.
Worked example
A tank lining blisters within weeks of service, with clear fluid under the film and chloride detected at the interface. Cause: soluble salt contamination not removed during preparation. Remediation: remove the lining, re-blast, verify salts are below threshold, and reapply.