Why does the surface of stainless steel heat exchange tubes appear black?

A dense oxide layer (chromium oxide layer) is usually formed on the surface of stainless steel, which is the key to its corrosion resistance. If the surface oxide layer is damaged or corroded by chemicals, it may cause the stainless steel to turn black.
Secondly, surface pollutants can also cause black. If the stainless steel surface is contaminated with pollutants such as grease, dust, fingerprints, etc., these substances will form organic acids after oxidation in the air, and react chemically with the stainless steel surface, causing the surface to turn black.
In addition, the impact of the gas environment is also an important factor. In an environment with high temperature, high humidity or chemically corrosive gases, the surface of stainless steel is prone to chemical reactions, causing surface oxidation or other changes, thus showing a black phenomenon.
Finally, improper cleaning can also cause black. Improper chemical cleaning agents or excessive wear during the cleaning process of the stainless steel surface may damage the surface oxide layer and increase the possibility of blackening.