Protective Coatings
Protective coatings are the primary barrier against corrosion for most structures, and CP is often a backup. A coating system typically includes surface preparation, a primer, and one or more topcoats. Surface preparation is the single most important variable: SSPC/NACE surface cleanliness (e.g., near-white or white metal blast) and anchor profile determine adhesion and performance. Contaminants such as salts, oil, and mill scale cause premature failure.
Know coating types and their mechanisms: barrier coatings (epoxies, polyurethanes), zinc-rich primers that provide galvanic protection, and sacrificial or inhibitive pigments. Understand the difference between adhesion failure modes (cohesive vs adhesive) and common defects: blistering (osmotic or solvent), undercutting, and cathodic disbondment when a coating is used with CP.
Traps: assuming a thicker coat is always better (over-application can cause solvent entrapment and cracking), ignoring environmental conditions (dew point, humidity, substrate temperature), and confusing dry film thickness (DFT) with wet film thickness (WFT). Recoating windows and intercoat adhesion are frequent question topics.
Worked example: A specification requires 250 µm DFT. If the coating is 60% volume solids, WFT = DFT / volume solids fraction = 250 / 0.60 ≈ 417 µm. If measured WFT is 300 µm, expected DFT ≈ 180 µm, below spec, so the coat is too thin or the solids content assumption is wrong.
Understand inspection: DFT gauges, holiday detection (low vs high voltage), adhesion tests (pull-off, cross-cut), and the role of a coating inspector.