Pitting corrosion is first noticeable as what color deposit on a metal surface?

• A. Blue-green oxide layer
• B. Red rust
• C. Black smut
• D. White or gray powdery deposit

Answer: (D)

Pitting corrosion is localized attack, so the first thing you notice is the corrosion products building up at the tiny pit. On zinc-coated or galvanized metal, these products form as zinc hydroxide/zinc oxide and appear as a white or gray, powdery coating—often called white rust. This specific white, powdery deposit is a telltale early sign of pitting because it reflects the initial reaction products from the localized attack under moisture. The other colors correspond to different forms of corrosion: blue-green oxide is typical of copper-based alloys forming patina, red rust is the common iron oxide from general steel corrosion, and black smut isn’t the usual early indicator of pitting.