When stainless steel is exposed to such heat, a visible colored oxide layer often forms on the surface. Beneath this oxide lies a region where the chromium content has been depleted due to the heat exposure. Since chromium is essential for the corrosion-resistant properties of stainless steel, this depletion significantly compromises the material’s ability to resist corrosion.

To fully restore the corrosion resistance of the affected area, the damaged surface layer—including both the oxide and the chromium-depleted zone—must be removed. While mechanical methods like grinding or abrasive blasting can achieve this, they often come with drawbacks. These methods can embed foreign particles, such as abrasives or iron contaminants, into the stainless steel surface, potentially leading to localized corrosion. In many cases, mechanical removal is also impractical for complex geometries or internal surfaces.

For these reasons, chemical methods are preferred. Pickling, a chemical cleaning process, is commonly used to remove the heat tint, scale, and the chromium-depleted layer. Standard pickling solutions typically consist of a mixture of nitric acid (HNO₃) and hydrofluoric acid (HF). These acids work together to dissolve the oxide layer and the underlying depleted metal, effectively exposing a fresh, fully alloyed stainless steel surface. In addition to removing thermal oxides, pickling solutions eliminate surface contaminants such as ferrous and ferric oxides, which can also contribute to corrosion.