The Principle and Elimination of Thermal Coloring of Stainless Steel Plates After Welding

Thermal coloring is the thickening of the naturally existing oxide layer on the surface of the stainless steel plate, and the formed color is similar to toning, which is the effect of light interference. This is due to the difference in optical path length between the light reflected from the surface of the oxide film and the light reflected from the oxide/metal interface, which results in a range of colors depending on the thickness of the oxide layer.

Regarding when the heat tint should be removed, if the intended application is dependent on the heat resistance or oxidation resistance of the stainless steel then it is not important to remove the heat tint as depending on the temperature of use oxidative heat tint will occur and mix into any localized welds coloring.
As a result of the heat tint that develops on stainless steel, chromium is drawn from below the surface of the metal to form a chromium-rich oxide surface layer. This leaves less chromium below the surface of the metal.
In the case of applications involving "aqueous" i.e. low temperature corrosion hazardous environments, a local reduction of surface chromium can affect the corrosion resistance of the steel. Although some sources indicate that heat tint removal is not always necessary, it is critical to remove welding heat tint in order to restore full corrosion resistance to the finished product after welding. This will help avoid unnecessary service corrosion problems in the fabrication weld area.
For optimal corrosion performance of stainless steel welded joints, crevice features and contamination all weld heat tints darker than buff should be removed by mechanical dressing followed by pickling. So, in general, if a tint to the surface due to heat can be seen, this must be removed for potable water applications and good post-weld cleaning practices should be employed for any stainless steel welded joints.
Brush paste or gel, spray or dip pickling, or electrolytic methods generally perform well in removing heat stain from stainless steel. The nitric acid used in these treatments also leaves the steel surface in an oxidized state.
A combination of finishing techniques may be required, especially if nitric acid treatment alone cannot be relied upon to remove sufficient metal from the surface. This may include mechanical treatment (grinding or lapping) followed by nitric acid cleaning (passivation).
Where a better surface finish is required, for aesthetics or to further optimize corrosion resistance, a final electrolytic polish can be used. These finishing techniques will also improve the overall appearance and presentation of the finished product. It is equally or more important to consider the internal weld faces of the fabrication. Although these areas may not be visible, they are intended for use in direct contact with stainless steel. Although a weld backing gas system may have been used, removal of weld heat tint may be more important in these areas than on the exterior face.