plating, anodizing, & finishing Q&As since 1989
Passivation for type 420SS stainless
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Q. We are currently passivating 303 with a Nitric acid line. But we are having trouble passivating 420. FYI the parts are heat treated.
Should we be using citric acid for 420? Or is the nitric acid okay to use?
October 12, 2023
420 stainless is assigned to Nitric 1 in ASTM A967 [affil link] , aka Type 2 in AMS2700 [affil link], aka Type II in QQ-P-35C [link is to free spec at Defense Logistics Agency, dla.mil] C. This is the same recommended nitric recipe as 303.
420 is definitely more troublesome, as far as grades of stainless go it barely qualifies. A well-passivated 420 part is nowhere near as corrosion resistant as other grades, especially compared to 300 series grades.
Maybe give citric a try, you could very well have better results than you are now, and ultimately replace all your nitric passivation with citric.
(Adv.) Let us know if we can help you with this.
Stellar Solutions, Inc.
October 13, 2023
If 420 parts regularly goes through sterilization or other cleaning procedures, then you will be required to conduct such passivation regularly. We did ongoing testing on 420 ss passivation.
The results shows that passivation layer is pretty stable in a few of our electrolytes. We provide consultation to the finishing shops interested
As an alternative we have developed super thick passivation method by electropolishing briefly in our Universal electrolyte. Such passivation stands even the impact of diluted HCl. Also withstands sterilization. Tested on parts in salt water environment.
Down this conversation you will find my old posting back in 2002 when we used high voltage polishing to protect ss 400 series. Since then the Universal technology has been developed, and there is no need to use high power.
Contact me for more information.
October 15, 2023
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Q. I have questions about passivation for 420SS. We are going to build up a new passivation process for precision medical tool made of 420SS. I need to get information about the acid (type, concentration, vendor), tank (size, material, vendor), temp (for passivation acid), time (how long is good for passivation), prepassivation treatment, postpassivation treatment, and the defects may caused by passivation.
A. You need to get a copy of ASTM A967 [affil link] . This will answer many of your questions.
In the past I have had problems passivating some grades of 400 series stainless steel (in particular 416) and so have many other people. I strongly suggest that you run some lab experiments using the ASTM procedures prior to committing to a particular system for passivating. The solutions are relatively easy but require great care and extremely good technique if you are to avoid ruined batches.
Good luck,John Holroyd
- Elkhorn, Wisconsin
A. So far the best possible results we have obtained on passivation of medical instruments 420SS is through high-voltage special process for few minutes is electrolyte based on water solution of salts. Passivation stands even acid test. The only drawback is that steel becomes a little yellow.
"I need to get information about the acid (type, concentration, vendor)" - The best acid to use for this is a citric acid based formulation which will give you reasonable results if you do it right. You must know that 420ss is not a very good grade and is hard to passivate, but it is possible if you do it right. Proprietary citric formulations give you the best result without "burning" the parts.
"tank (size, material, vendor)"-- Tank size depends on size of your parts and quantity per unit time. Material should be either austenitic stainless steel or a material like polypropylene. If you want to use ultrasonics, stainless steel is best, but this depends on the type of ultrasonic device used. There are many manufacturers of tanks- contact us if you need references.
"temp (for passivation acid)" Best results can usually be attained at 140-160F.
"time (how long is good for passivation)" This depends on the amount of free iron on the surface, and the pretreatment. Normally, 10-15 minutes is good.
"prepassivation treatment"-- It is important for 420ss to pretreat in an alkaline material.
"postpassivation treatment" Rinsing good is all that is necessary with citric formulations, but alkaline soak will not hurt.
"the defects may caused by passivation"-- It is always possible to "burn" the surface of 420ss if it is not hard enough, or if it comes in contact with other metals in the bath. If proper precautions are taken, this should not happen.
adv. Contact us if we can help.
Stellar Solutions, Inc.
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