The dark heat tint formed alongside welds during welding of stainless steel is a Cr oxide scale with a mixture of Fe, Ni, and other oxides.
When heat tint scale forms, Cr diffuses outward from the base metal in the heated zone leaving a thin layer of Cr-reduced just beneath the heat tint scale.
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.
For some mildly corrosive environments, it is not harmful and can be left in place. However, in environments involving chlorides, or in water systems that are not sterile, preferential local corrosion (pitting, in particular) tends to occur in heat-tinted areas.
Heat tint occurs when the metal is heated in the presence of oxygen. One way to eliminate heat tint is to make oxygen unavailable while the metal is hot.
Heat Tint may have a straw yellow, deep golden, rose, blue or gray tone, depending upon the thickness of the oxide.
Several methods of removing oxides from stainless steels are described in ASTM A380
1- Heat Tints on SS - Ni Institute - https://www.nickelinstitute.org/media/1696/heattintsonstainlesssteelscancausecorrosionproblems_14050_.pdf
3- ASTM A380
By: Baher Elsheikh