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Failed Passivation Stainless Steel (PH Grade): Pickling/Acid Descale Preparation
January 22, 2013
We typically experience difficulty with 15-5PH stainless steels after passivation from AMS2700. We are alkaline cleaning, then using a nitric acid (~20-25%) and sodium dichromate (2-3 wt %). The failures occur during corrosion resistance testing (salt spray or copper sulfate sol'n).
The steel is in the aged condition and is passivated after heat treatment. We have found that areas which are machined after heat treatment always pass the corrosion resistance testing, while areas which do not have any material removal are always a trouble spot and fail the lot test. This leads to the assumption that there is an oxide scale after heat treatment which is may or may not be easily removed by our passivation bath. We've tried multiple cleaning techniques, making new baths, new rinses, etc all with mixed results.
We also have mixed results with mechanical blasting due to the small sizes and large quantities of our parts.
AMS2700 does refer back to ASTM A 380 for oxide removal. We're looking into HF/HNO3 mixes and I've been reading that the specification for these solutions with HF is for the annealed heat treat condition. There is another section for pickling that again is for the annealed condition but does allow for use in other HT conditions provided we have past experience to show our results.
I am wondering if anyone has any solutions for the following questions:
1) Are they alternatives that would be compliant to AMS2700 for removal of oxide scale?
2) What are the dangers of the HF/HNO3 dip for an aged PH steel? I am prepared for a 24 hour bake (ref ASTM A 380) but am worried about stress corrosion cracking. Is there a test for SCC I can run before full scale production?
3) Based on the description: any advice on what is out in the industry for surface preparation for PH steels prior to passivation?
- Downers Grove, Illinois, USA
That is an interesting problem. I've seen cases with stainless steel castings where the machined surface passivates fine but the raw surface does not, but this is the first I've heard of that effect coming from a heat treat.
I'm a little curious to see if using a citric acid based passivation bath would yield different results. 15-5 isn't normally much of a problem.
Stellar Solutions, Inc.