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Passive Layer
A passive oxidized film that forms naturally on a stainless steel surface when exposed to air or similar oxidizing environment thus protecting the underlying base metal from corrosion. Welding disturbs the passive layer by reducing the chromium and increasing the iron, thus altering the chromium/iron ratio (measure of corrosion resistance). Upon completion and approval of the weld, the weld surface and adjacent boundary area must be brought back to a passive state. Additionally, normal operating conditions in typical Water For Injection, reverse osmosis, deionized water, clean steam, Clean In Place, and process piping often lead to formation of the most prevalent form of self catalyzing corrosion called "rouge" (French for red), which is a colloidal form of rust containing iron oxide, chromium and nickel in various forms. This problem is further accentuated by high temperature. The rouge layer acts as a passive layer until it becomes so thick that it "sloughs off" into the process or water stream.