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Letter 35096
Rework of etched 303 stainless
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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, New York
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.
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Jeffrey Holmes,
CEF
- Spartanburg, SC, USA
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+++++
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.
+++++
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 spec at TechStreet] was a SUGGESTION
ONLY, folks.
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Lee Gearhart
metallurgist
East Aurora, NY
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