A stainless steel may be discoloured during use

• if it has a rough surface finish that provides a foothold for corrosive substances and corrosion products from the surroundings.
• if fasteners from ordinary steel are used for securing the material, or if the material comes in direct contact with adjacent components made of plain carbon steel in wet or humid conditions.
• if the design of the structure is inappropriate, with pockets and narrow gaps
• if the surface is contaminated by iron particles, for example swarf etc. from tools used in the installation works.
• if it is exposed to a more aggressive environment than that for which that particular grade of steel is intended, e.g. highly polluted air, salt solutions or residuals of cleaning agents.

The risk of corrosion is grater for the lower-alloy stainless steel grades and can be reduced substantially right from the start by specifying molybdenum alloyed stainless grades (such as 1.4401). In the case of surface contamination, it is the rust from carbon steel that causes discolouration.