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"Rouging of 316L stainless steel in non-aqueous solutions"


Rouging of polished 316L stainless steel in steam and aqueous solutions in bioprocessing and pharmaceutical industries is quite well known. I am seeking further information on the formation of a grey smut (a dark grey rouging effect) on 2B finish 316L stainless steel equipment occurring in organic solvents (polyols) with organic active agents for an animal remedy formula (batch process). There is no high purity water in the system. The rouge is dark grey (it swabs off) and forms on the 316 SS surfaces (2B finish) after some time in service. The welds are highly polished and also have a grey rouge present.
Would anyone have any suggestions as to the cause and remedial action required to remove the grey rouge off the SS surfaces. Nitric acid passivation seems a likely start.

Les H Boulton
Stainless steel technologist - Auckland, New Zealand



You should have a wipe sample analysed with ICP-MS to accurately identify the metals in the rouge material. It is very likely ferrous oxide. Do any of the solvents used contain chlorides? Even very low ppm levels of water in solvents can lead to corrosion of stainless steel in organic solutions, especially if chloride levels above 50 ppm are present.

Testing of a wipe to confirm the presence of iron oxide would be suggested. Cleaning, derouging and passivation would be recommended, if it is iron oxide. Austenitic stainless steel prefers oxidizing environments. Solvent systems with low levels of water and chlorides content can provide a corrosive environment.

Daryl L. Roll
Costa Mesa, California, USA

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