Phorgotten Phenomena:Heat Tints on Stainless Steels Can Cause Corrosion Problems

The dark heat tint formed alongside welds during welding of stainless steel (SS)—the heat-affected zone (HAZ)—is a thicker chromiuin oxide scale with a mixture of iron, nickel, and other oxides. When heat tint scale forms, chromium diffuses outward from the base metal in the heated zone. The zone extends as far as elevated temperatures allow chromium diffusion to occur. The diffusion of chromium into the scale leaves a thin chromium-reduced layer just beneath the heat tint scale. This thin layer is lower in chromium, the primary constituent that gives SS its good corrosion resistance. Corrosion that would not occur elsewhere can initiate in the HAZ unless the heat tint scale and the thin chromium-depleted layer just beneath are removed. Removal by rotating fiber brush, pickling, or electropolishing readily restores this area to base-metal resistance.