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.
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 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 colors.
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.
Cleaning and removal of rouging
Rouging will contribute to iron release into the ultra pure water. Even small amounts of other compounds as chrome and nickel might dissolve. The common practice is to prevent or minimize rouge and perform a chemical cleaning to de-rouge and passivate the stainless surface.
Procedure for chemical cleaning
There are various ways in which rouging can be removed from an affected system. Conventional pickling will dissolve the passive layer and strip back the rouge, leaving a clean surface. The negative side is that this process will etch the surface of the steel, roughening and dulling the finish. Ra critical components will inevitably be ruined or at the very least, require further polishing to achieve the correct surface smoothness. Surfaces that have been pickled will subsequently become rougher and therefore more susceptible to further cases of rouging.
Where possible, (i.e., in vessels, etc) mechanical polishing can also remove this contamination. This is however an extremely expensive and time consuming process which is very dirty and will require extensive cleaning operations afterwards. It is also impossible to perform this process on the more commonly affected parts (pumps, pipe work, valves, etc).
Vecom have developed a chemical formula that will remove the affected passive layer and remove the rouging with no detrimental effect.
Our sulphuric / phosphoric based solution can be easily pumped through a pharmaceutical system with minimum disruption to the plant, leaving no mess, totally removing all traces of rouging and most importantly, not affecting the Ra finish of the parts.
After this cleaning operation (or any other), the system should be fully repassivated. Vecom are able to offer this service using either a nitric acid based passivating solution, or a more environmentally / operator friendly citric acid based one.