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Brown film on passivated stainless steel castings

We are a steel foundry and have some austenitic stainless steel castings (CF8, CF8M, CG8M, & CN7M) passivated as required by customer specification, by an outside vendor. The castings have a brown film left on them after the second rinse. All castings pass a Copper -sulphate test. In the past our vendor was able to wash the film off by spraying with a garden hose. Recently, the film will not wash off. However, it is easily removed by wiping, but this is not practical. I noticed The castings are initially cleaned by blasting with steel shot. This has always been the practice here and has never been a problem except it requires removing more iron from the surface. Our vendor uses a Nitric acid solution IAW ASTM A967, section 6. He thinks it is a cleaning problem but our investigation cannot detect anything in our process that has changed. Any suggestions will be appreciated. Thank you.

A shock to me, I would not have thought that you could get a part shotpeened with steel shot to ever pass a salt spray test. I would suspect that you have a new lot of shot or that the shot was also used on some other type of metal part.

Look at all of the things that could have changed in all of your processes from the time of no problem to the problem starting.

Bob, What type of steel shot has been used? Is it a cast steel , cut wire or other? This can be a large factor. Also, as James Watts suggested, check into what other types of substrates this current shot has been used on. The steel shot may just need to be resized to remove small fines and dust. Is your separator in shot blast working properly? Some types of shot break down faster than others.

Your problem could be in the rinse water, might need ionized water, a lot of heavy metals in city water.

If the colouration is uniform all around then it could be a problem associated with iron ferrous/heavy metals contaminated rinse water,or any other cleaning means being used by workmen may be unknowingly.

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