Materials of construction for equipment and piping in pharmaceutical processing plants must be resistant to corrosion from the high-purity water, the buffer solutions used in preparation of the products, and the cleaning solutions used to maintain the purity of the product. The primary water used in pharmaceutical production is water for injection (WFI). This article presents the steps for preparing WFI and discusses the effect of chlorides on stainless steel. It provides information on passive layer of stainless steels and chromium-containing nickel alloys. The article illustrates the surface finish of pharmaceutical equipment. Rouge is iron oxide, and its different colors result from different valences and degrees of hydration. The article discusses the classification of rouge and the characteristics of cast type 316L stainless steel. It explains how and when to perform cleaning and repassivation process on classes of rouge.