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304 Stainless is "Splotchy" After Passivate
(-----) January 18, 2008
We are having a problem with 304 stainless ending up "splotchy" after passivating in a 30% nitric acid solution for 30 minutes. Pretreatment included 5 minutes in an alkaline soak, 5 minutes in an alkaline electroclean, 5 minutes in 25% HCl (parts are welded and welds needed to be scrubbed while in HCL). Of the 10 parts we passivated only 3 of them came out flawlessly. Upon blowing the other
7 dry the whitish splotches/streaks became visible.
Any suggestions - can these be re-passivated? What could cause the splotching/streaking?
Plating Shop Emloyee - Atlanta, GA, USA
February 13, 2008
To start, you should never put stainless steel in HCl solution. This is asking for future corrosion. Re-passivating may help you get rid of this, but do not put them in HCl, even if it is inhibited. White blotches can be several things. I assume that they are not water soluble. Is the surface etched in those areas?
There are a number of ways to clean the welds without putting them in HCl. You need to change the process.
Stellar Solutions, Inc.