Letter 35096

Rework of etched 303 stainless [New York] 

+++++

We have an inhouse passivation shop. We passivate 303 material that sometimes etches. In one batch of about 10 parts, we could get 3-8 etch and the others do not. What could cause this situation? Once this situation occurs, what options do we have for salvaging the parts? The etching isn't deep into the base, it mostly just puts a frost on the part. Sometimes we scrap the parts, sometimes we blast them and repassivate, what is the best practice?

Debra Burm
Chemical Process shop - Phelps, NY, USA

Ed. note: Please start with our FAQ on this subject, Debra. Some people assert that the formation of aggressive organic acids from tiny amounts of oil is the cause of this common sporadic problem, and that ludicrously long cleaning times before passivation will solve it.

Random etching of 303 is a common problem. Most likely what you're working with is 303Se. Can the mfr change to another alloy? - 304, for example will give no problems.

We are at the point of not bidding on passivation of 303. It's just too risky, and impossible (for us anyway) to predict which parts will etch and which will not.

I've seen reports which claim that the problem can be solved by extended soak cleaning on hot alkaline cleaner (several hours) before passivation. Maybe worth a try.

Jeffrey Holmes, CEF - Spartanburg, SC, USA

303, 303S and 303Se are all major problems in passivation with nitric acid formulations unless you know how to handle them. Organics on the surface are always a problem with nitric acid, but the high sulfur in these grades is what causes the problems. Pretreating with HOT NaOH solutions or cleaners with a pH greater than 12 (with possible sequestering agents) will eliminate the problem.

You can also eliminate the problem by using good citric acid products that are on the market, with the precleaning in high pH solutions.

Lee Kremer Stellar Solutions, Inc. Algonquin, IL, USA

Don't use type II. Use type VIII. We switched and our etching problems went away.

The table in the back of QQ-P-35 [link is to info about spec at TechStreet] was a SUGGESTION ONLY, folks.

Lee Gearhart
- East Aurora, NY

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