What signs might indicate corrosion in steel bridge components behind protective coatings?

• A. Blistering paint only.
• B. Rust streaks and pitting only.
• C. Delamination of coating and visible corrosion products.
• D. Blistering paint, rust streaks, pitting, delamination of coating, visible corrosion products.

Answer: (D)

Corrosion under protective coatings often shows a combination of problems because the coating failure allows moisture and ions to reach the steel. Blistering paint indicates moisture trapped beneath the coating pushing it away from the metal. Rust streaks reveal oxidation products migrating to the surface, often through defects or microcracks in the coating. Pitting is a sign of localized attack that can occur where moisture concentrates or where tiny coating imperfections expose the steel. Delamination of the coating means the coating has lost adhesion and is separating from the metal, creating pathways for corrosion to continue. Visible corrosion products confirm that rust or other oxidation is present beneath or at the coating interface. Because corrosion behind coatings can manifest in several ways at once, the most complete finding includes all of these indicators.