During stainless steel welding, an oxide film forms in the high-temperature zone, with colors ranging from silver-white, pale yellow, rainbow, deep blue to black, representing progressively more severe oxidation.

General acceptance criteria: silver-white or pale gold is good; rainbow colors indicate insufficient shielding and require pickling and passivation; deep blue to black means chromium has been heavily depleted and corrosion resistance has noticeably declined, usually requiring removal and re-passivation.

For demanding applications such as food, pharmaceutical, and chemical processing, it is recommended to specify weld color grades in the technical agreement and to use the blue spot test or rust test as the final determination.