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• -----"Stains after Stainless Steel Passivation, Rinsing Requirements"
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Current question and answers:
November 3, 2021Q. I work as an engineer for a small company and we use some machined parts in our assemblies. The parts quite small and intricate (~3x3x1 cm with both threaded and thru holes) and are made of 416 SS and undergo passivation at a local metal shop that we use regularly. These parts are then sonicated in our facility in a citric acid detergent solution and recently they have come out with extreme staining. We sent the parts back to the shop to see if they could remove the staining and they were unsuccessful. I tried blasting some of the pieces with aluminum oxide and while it did get rid of the surface staining, it also destroyed the brushed finished that these parts need. I plan on also trying glass beads.
Does anyone have any suggestions on if these parts can be cleaned? We would like to try salvaging them as they are quite expensive. We haven't seen extreme staining on these parts before and do not experience any staining on the other SS we send to be passivated.
- Santa Ana, California
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November 9, 2021
A. Lucy,
"Stains" tend to be one of two different things: residue, or etching. Residue is the kind that will come off, even if you have to physically scratch at it. Etching is when the surface has been eaten into, so it can only be "removed" by redoing the surface finish (which of course also negates any passivation treatment that has been done).
Residue tends to be a factor when there is poor rinsing or dirty rinse water. Etching tends to be a factor when a chemical treatment attacks the surface, such as an acid bath that is a bit too strong for what the material can tolerate.
Do you know the concentration and pH of your citric detergent solution? 416 is a lower-end grade of stainless as far as resistance to corrosion and chemical attack goes. It's entirely possible for a low-pH citric acid bath to etch it.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
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November 9, 2021
Q. Hi Ray
We use Alconox Citranox concentrated cleaner at less than or equal to 1% concentration. The pH of the cleaner is 2.5.
- Santa Ana, California
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November 9, 2021
A. adv. It is possible to just briefly electropolish parts in our Universal Electrolyte to remove stains without impacting dimensions and simultaneously having them passivated. Please contact me directly.
Anna Berkovich
Russamer Lab
Pittsburgh, Pennsylvania
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Previous closely related Q&A's, oldest first:
August 3, 2011Q. I am currently working on Chemical cleaning services at Saudi Arabia. We observed staining of passivating solution on pipelines/vessels after passivation using ammoniated citric acid with sodium nitrite (as oxidizer to form gamma iron oxide layer). Here are my questions:
1. Is it possible to rinse after passivation ? (Our worry that passivation may strip-off after rinsing).
2. What is the life of iron oxide and other passivation chemistry when it is in process water line? when it is in oxygen line?
Chemical Engineer - Saudi Arabia
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August 16, 2011
A. Dom,
Assuming you are talking about stainless steel passivation here, since pipelines are generally stainless. Yes, you can rinse, in fact, you NEED to rinse. The passivation treatment with acid removes free iron from the surface, but most or all of the formation of the passive chromium oxide layer takes place when the iron-depleted surface comes into contact with air. Any stainless surface being passivated must be rinsed, dried or allow to dry, and left in contact with air for at least a few hours if possible before entering service.
The passivation will not strip off via rinsing because stainless passivation is a treatment, not a coating. The passive layer is formed from the metal itself.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
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August 28, 2011
Q. Thanks a lot Mr. Ray. Is this standard practice in industry? or you tested in your lab? How about if it is citric based passivated carbon steel, is there any issue if we will do the post rinsing?
Dom Marcos [returning]Chemical cleaning - Saudi Arabia
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September 2, 2011
A. Is rinsing after acid treatment standard practice in industry for stainless steel passivation? It's a required step, if you look at the passivation standards (ASTM A967 / A967M [affil. link to spec at Techstreet]
, AMS2700 [affil. link to spec at Techstreet], even the now defunct QQ-P-35C [link is to free spec at Defense Logistics Agency, dla.mil]).
Carbon steel cannot be passivated by acid treatment. If you use citric acid on carbon steel you had better rinse it off or you may not have much steel left.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
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July 2, 2019
Q. Hi I'm currently working on a passivation set up and I encounter myself with a question regarding post treatment and neutralization processes after passivation of stainless steel in accordance with AMS2700 [affil. link to spec at Techstreet] F.
my question is: how necessary is neutralization needed after the parts have been passivated?
- Anaheim California USA
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July 16, 2019
A. Hassan,
With citric acid, a water rinse is all you need. With nitric acid, a water rinse should be sufficient in most cases, but many people go with an alkaline neutralizing bath first just to make sure, especially if the parts have blind holes or other types of nooks and crannies where acid residue may become trapped.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
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July 17, 2019
A. We use a Ion exchange system for our final rinse. a two tank counterflow.
If you check your pH of the last tank often you should have no issue with a 3 to 5 minute rinse.
Edwards Interiors - Rincon, Georgia
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December 4, 2020
Q. Hello,
When passivating with Citric acid, do you rinse immediately after the bath and then let 24 hour dry with air, or do you let it dry first just after the bath, and rinse 24 hours after?
Thank you
- Grasse, France
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December 4, 2020
You should thoroughly rinse your parts immediately after the passivation step before the chemistry dries on the parts.  Jon Hickman Best Technology Inc. Plymouth, Minnesota  ^ December 4, 2020 You generally want to follow the citric passivation with a RO water or DI water rinse right after the citric solution, which 3 to 5 minutes is generally enough for time and you can use a conductivity meter for verification that parts are full rinsed.  Mark Battles Best Technology Inc. Plymouth, Minnesota ^ |
February 4, 2021
Rinse first, dry after.
Jeffrey Holmes, CEF
Spartanburg, South Carolina
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