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Citric Passivation of 410/420 Stainless

Anyone know the proper way to citric passivate 410/420 CRES stainless? We are using a proprietary citric-acid based solution, but after a while see rust after doing a corrosion test. I'm sure it has to do with how our process is set up. If an alkaline is necessary, any suggestions for an environmentally friendly one?

Thanks.

Ed:

We get excellent results with the correct product using citric acid on these grades. We have customers with these grades who require the use of these products because of 80% reduction in corrosion problems compared to nitric passivation. Best overall results according to some steel manufacturers is AAA method, Alkaline/Citric/Alkaline.

You need to be using the best formulation of citric on these grades and at the correct time, concentration and temperature. Ultrasonics can benefit also. There are a variety of proprietary methods to achieve the best results.

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