Stainless steel is widely used in pharmaceutical applications. The surface treatment of this stainless steel is very important, as in the pharmaceutical industry, hygiene aspects are priority. From several studies it shows there is a relation between surface roughness and the ability to clean and the ability of micro organisms to attach to the surface. The lowest surface roughness can be achieved by (electro) polishing. One of the phenomena that occurs on stainless steel, almost exclusively within the pharmaceutical industry, is rouging. Rouging is a reddish brown film of iron oxides and hydroxides and is found in ultra pure water systems. The rouging film mainly contains iron (ferric) oxides but also can contain chromium and nickel compounds which can give different colours to the rouge. It seems AISI 304 (EN 1.4301) stainless steel is more sensitive to rouging than AISI 316 (EN 1.4401). It has been observed that an electropolished surface is less susceptible to this phenomenon than a mechanically polished one.
One of the main properties of stainless steel is its passive chrome oxide film, which protects the underlying steel. Formation of this chrome oxide film is spontaneous with oxygen from the air and stainless steel has therefore the ability to repair itself and form this film after being damaged. In order to form this film, the stainless steel surface has to be uncontaminated as contaminants can disturb the formation of the chrome oxide film.
In an ultra pure water environment this protective chrome oxide film can be attacked. As ultra pure water lacks any ions, the strength to pull ions into the solution is so strong it can dissolve the protective chrome oxide and leave an active and unprotected stainless steel surface. As chrome and nickel ions can dissolve in water at neutral pH, iron ions dissolve at pH three or higher, and is deposited as iron hydroxides on the active stainless steel surface as it repassivates. Iron hydroxides will oxidize to ferric oxide which is red (rouge). This passivation and repassivation process can cycle which results in various different colours.
Formation of rouge can be dangerous for stainless steel. Under deposits, a micro environment can be formed with a total different chemistry than the bulk. When for instance, sulphides (compound of a stainless steel alloy) react with this micro environment, pitting corrosion can be initiated.
Besides the rouging by ultra pure water, rouging can be formed by external compounds in a water environment. One common source is ferrous bicarbonate. This is commonly used to soften hard water (lower calcium concentration). By means of several chemical reactions and with or without chlorine disinfection, iron hydroxides and ferric oxides are formed and deposit as rouge. Dissolved carbon dioxide gas also contributes the formation of rouge in ultra pure water systems.