Which description corresponds to Stage 4 corrosion condition?

• A. Metal sound. Light rust or flash rust. (Bubbling of paint)
• B. Metal sound. Heavy Rust with and/or light pitting
• C. Metal unsound. Heavier rust and pitting. (Scaling)
• D. Metal unsound. Extensive rust and heavy pitting with holes in metal throughout.

Answer: (D)

Stage 4 represents the most severe level of corrosion, where the metal has lost its structural integrity. The description that fits this level shows metal that is unsound with extensive rust and heavy pitting, and importantly, holes throughout the metal. When corrosion has progressed to through-wall damage like this, the material can no longer perform safely or reliably, and major repair or replacement is typically required rather than cosmetic coating or minor repairs.

Earlier stages show surface rust or light pitting with sound or minimally affected metal, or heavier rust without through-wall perforation. But only Stage 4 includes holes forming across the metal, signaling critical loss of thickness and strength.