Coating, Corrosion Control, and Lowering-In
Coating systems
Know the plant-applied coating type, the field coating for girth welds and repairs, and the surface preparation each requires. Field coating demands clean, dry, correctly prepared steel, correct mixing ratios, specified thickness, and adequate cure before backfill. Holiday detection (jeeping) is performed at the voltage specified for the coating thickness and material; excessive voltage can damage a sound coating, and too little voltage misses defects.
Cathodic protection
Understand the difference between galvanic (sacrificial anode) systems and impressed-current systems with rectifiers, and know the role of test stations, reference electrodes, close-interval surveys, and interference bonds. Coating and CP work together: coating is the primary barrier and CP protects the holidays. Coating condition, soil resistivity, and AC interference mitigation all affect CP design and effectiveness.
Lowering-in protection
Lowering-in is the highest-risk moment for coating damage. Padding, rock shield, ditch cleanliness, sideboom spacing, and controlled lowering protect the coating; damaged coating is repaired and re-tested before backfill, and CP readings are taken at the appropriate stage with the specified connections.
Traps
Expect questions that imply coating can be applied over moisture or mill scale, that jeeping at any voltage is acceptable, or that CP alone protects a badly coated line. Another trap is recording CP potentials before backfill or before the reference electrode is properly placed.
Worked example
Rule of thumb: any coating damage found during lowering-in is repaired and re-jeeped before backfill - never "noted for later." Once the ditch is closed, the cost and risk of finding that holiday multiply, and the corrosion record starts with a defect you had already seen.