Rouge, a reddish-brown oxide film, is commonly classified into three types. This article specifically addresses the formation of rouge within steam autoclave mahcines and explores effective derouging methods to eliminate it.
Can often be removed by wiping. It is thought that this class originates from iron contamination somewhere within the system – and it migrates throughout – often being found on gaskets and filter meshes.
Is often seen as a wide-spread “stain” that doesn’t wipe off. It is usually the result of a damaged passive layer in which the chromium-oxide matrix is partially replaced with an iron-oxide matrix (mostly haematite Fe2O3), consisting of various oxides and hydroxides of iron in its different oxidation states.
Is purple/black and very tightly adhered. It is predominantly magnetite (Fe3O4) and incredibly difficult to remove. More often than not, it can only be removed by mechanical polishing. Aggressive derouging chemicals (such as pickling liquid) can also be effective, but run the risk of irreparably tarnishing the underlying surface finish). This class of rouge is usually found in high temperature steam systems.
The harsh conditions in autoclaves (vacuum, over-pressure, water-vapour saturated atmosphere, high temperature) will often result in class 2 rouge. This can be removed with mechanical polishing, electropolishing or chemical cleaning (derouging).
In most cases – and certainly in a clean-room environment – of the three, chemical derouging is the only viable option.
Traditionally, derouging has been performed with strong, corrosive acids. Because of the complex (and varying) nature of the rouge / iron-oxide matrix, finding an effective acid has proven difficult. Too strong or corrosive and you will damage the underlying base material. Too weak, and you won’t be able to fully breakdown and dissolve class 2 rouge. This inevitably led to unpredictable results. What achieved good results on one job, wouldn’t necessarily work on the next.
INOX Passivation use a pH neutral derouging chemical which works on a different principle. It converts the oxidation state of the rouge into a more soluble form (reduction of Fe3 to Fe2) – then binds it into the chemical solution to be eventually flushed out. This process is highly selective towards iron-oxides and will not corrode the base material – causing no material loss from your equipment. It is also much quicker than conventional (acid) formulas, and in some cases, can fully remove the rouge within 30 minutes.
Rouge is a naturally occurring process that is virtually impossible to prevent in ultra-pure water systems and steam autoclaves. Periodic derouging will remove the discolouration, and restore the surface to its original condition – minimising the risk of transferring rouge and contaminants to your product. You can contact our team for more information on derouging your Steam autoclave machines.