In semiconductor manufacture the corrosive fluid delivery systems are subjected to the same scrutiny as the chemicals and gases used, to prevent any contribution to contamination, corrosion, or particle generation. Electropolished 316L stainless steel has become the industry standard due to its superior corrosion resistance and the passive oxide layer formed by the electropolishing. However, welding of the tubing often leads to discoloration in the heat-affected zone (HAZ) leading to corrosion. In this study, specimens from various heat lots were welded under identical parameters, with varying concentrations of oxygen leaked into the argon purge gas during the welding. The chemical composition and thickness of the discoloration in the HAZ and a clean reference area were analyzed by Auger electron spectroscopy. The cause of the discoloration was due to iron diffusing through the chromium oxide layer in the HAZ in the sensitizing temperature range and forming an iron oxide layer, the thickness of which depended upon the concentration of oxygen available in the purge gas. The composition of the original steel was found to be only a minor contributor to the extent of the discoloration.