Coatings and Protective Systems
What you must understand
Coatings are the primary barrier on offshore structures. Candidates should know generic types - epoxies (including high-build and glass-flake), polyurethanes, zinc-rich primers, thermally sprayed aluminum (TSA), and elastomeric or neoprene sheathing - and their roles: barrier, inhibitive, and sacrificial (galvanic) protection. Understand surface preparation (abrasive blast to Sa 2½, cleanliness, chlorides, dew point), application variables (temperature, humidity, DFT, stripe coats at welds and edges), and the importance of the coating specification and inspection regime.
Know how coatings interact with CP: a good coating reduces current demand but causes current shielding if it disbonds; holiday and damage detection matters. Coating degradation modes include blistering (osmotic), undercutting, chalking, cracking, and disbondment. TSA provides a conductive, sacrificial layer that works with CP and resists splash zone attack.
Traps in questions
A common trap is assuming a coating defect is cosmetic - disbonded coating can shield CP and cause severe localized corrosion. Another is confusing DFT (dry film thickness) with WFT (wet film thickness) or ignoring stripe coats on edges and welds where coatings thin out.
Worked example / rule of thumb
If a coating is damaged in the submerged zone, the exposed steel becomes a small anode relative to a large coated cathode - rapid localized corrosion unless CP is adequate. Rule of thumb: specify stripe coats on welds, edges, and cutouts; these are where coatings fail first.