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Passivation of Stainless Steel for Increased Adhesion / Better Gluing

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<- Ed. note: No abstract
    questions please!
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Q. Can a part be passivated if it has a plasma spray tungsten carbide coating on it?

Kevin Duncan
- San Fernando, California
August 24, 2022

A. Hi Kevin. Huh? Yes the tungsten carbide can be "passivated". The problem here though is the word "passivated" ... it can mean very different things and very different processes. There are ways of passivating tungsten carbide, probably involving treatments with oxygen gas, but you can't passivate the stainless steel underlying an existing tungsten carbide coating by dipping the coated part in nitric acid or citric acid if that's what you're talking about.

Why are you interested in this process? What are you looking to achieve?

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. Hello Ted,

I have some 440C parts that have a plasma spray tungsten carbide coating on some of the areas of the part and they want the part to be passivated with Type 8 solution to AMS 2700. I'm hesitant to do 440c and now the also have the coating on them.


Kevin Duncan [returning]
- San Fernando, California

A. Hi again. The thing to do, if practical, is to coat some test panels or rejected parts with tungsten carbide so you can actually see what happens.

Lacking that, my guess is nitric acid passivation will destroy the tungsten carbide coating but citric acid will not.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Closely related historical posts, oldest first ...

Q. Would a passivated surface on stainless steel improve or inhibit the adhesion of a powder coat finish?

Lloyd Robert Morris
gauges - Dallas, Texas


A. Hi, Lloyd.

To my knowledge it isn't common to powder coat passivated stainless steel (with the possible exception of clear coating outdoor fixtures), and there are several different passivating solutions which could have slightly different effects, so I can't reference any particular study or personal experience--yet I doubt that it has measurable effect either way since all it does is remove small amounts of iron contamination and chromium enrich the surface. I suspect that the surface profile "key" before passivation has a lot more effect on the adhesion than the passivation does. If it's possible, please try to offer us some more detail on that. Thanks.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


Q. We are building a tasting bar for a sparkling wine company, the counter is 16g 304 stainless 2b finish that is glued to a plywood sub-structure. The architects have called out a random orbital ( DA ) finish. My question is after we sand ( DA ) the surface do we need to passify it? If so what would be the best way to passivate the counter given that it has a plywood sub-structure.

Thank you,

Chris French
owner - Oakland, California


A. Chris:

The easiest and most environmentally friendly answer is to use a gelled citric acid based passivation process. There are commercial suppliers of citric acid based passivation agents who advertise herein. Citric acid based passivation systems performed at ambient conditions are greatly improved by the use of the gelling agent and physical application by brushing the surface.

This application can be localized to the stainless steel surface and masked from the wood surfaces.

Good luck,

Daryl L. Roll
Costa Mesa, California, USA

Need increased adhesion of stainless to thermoplastic

March 14, 2012

Q. Our process involves bonding a thermoplastic (see below) to stainless steel. This adhesion needs to be moderately heat resistant (~200 °F), highly resistant to water with "wetting agents" added (mostly glycols) and mineral spirits. From previous testing it has been determined that the best way to improve this adhesion is by increasing the chromate to iron composition ratio on the surface of the metal, which is why we have been using a chromate conversion.
In the past, the stainless we used was a type 304 with a 1CBA (CA3 was the old designation) or "#1 course bright annealed" finish; it has a dull grey appearance and a good surface roughness (Ra=35). For various reasons we have recently switched to a new metal supplier that gives us type 304 with a 2R or "bright annealed" finish, and the same chromate conversion as before. This new metal is very shiny and smooth (Ra=5) and we get good "dry" bonds to the thermoplastic, but when the bond is exposed to water+glycols @167 °F for a few days, the bond fails.
We have been experimenting with roughening the surface, which helped greatly, but will require a substantial capital investment. Someone recently suggested passivation (30 min HCl @RT + 30 min 20% citric acid @ 120 °F), and initial testing looks really good. Now we are looking at bringing in various passivating chemicals in house (nitric acid, citric acid, sodium dichromate, and HCl) and the plan is to experiment with different concentrations/soak times.
My questions to you guys:
- How is passivation different from our current chromate conversion?
- What chemicals/concentrations/soak times will achieve the greatest Cr/Fe surface ratio?
- Is there anything else out there that might increase adhesion?

A quick word about the thermoplastic:

It is an extruded sheet of a linear low-density polyethylene with maleic anhydride bonding agent grafted into the backbone. I think a lot of the bonding issues stems from the maleic anhydride bonding agent, as it is very susceptible to hydration, forming maleic acid.

P.S. Our stainless is very thin (~8 thousandths of an inch)

Sorry about the long post and thanks in advance for the help!

Zach Shupe
product engineering - Arden, North Carolina, USA

April 4, 2012

A. Zach,
You should only need a good citric acid bath there. At the most I could see you running a comparison between citric and nitric. The nitric + dichromate baths are NOT used for 304, and HCl should NEVER be used on stainless steel.

I'm not a chromate conversion coating guy myself, but suffice it to say that process involves creating a passive coating of chromium on the parts, while passivation exploits the chromium that already exists in the stainless steel, and forms a passive surface layer that is more pure in chromium by taking away the iron.

Surface adhesion is a interesting topic, obviously it depends a lot on the chemistry of the thing being adhered to the stainless. I am often asked if passivation increases adhesion, my general assumption is that surface roughness influences that more than passivation does. However if you have specifically found in your situation that more chromium and less iron increases your adhesion, passivation is certainly the easiest, best thing for that.

As for good passivation process parameters, please refer to the industry standards (ASTM A967 / A967M [affil. link to spec at Techstreet] AMS2700 [affil. link to spec at Techstreet], or the defunct but free AMS-QQ-P-35 [affil. link to spec at Techstreet]) ...

or contact us for assistance. (Nutshell version though, 4 weight percent or greater citric acid, at room temperature 15-20 minutes should be more than enough, less if heated.)

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner

February 25, 2015

Q. Hi

I am doing some research around passivation of surgical stainless steel UHB AEB-L. Current method is using Citric acid, and am exploring the effects of Nitric passivation. One important aspect that I wanted to ask, would Nitric passivation affect the adhesion of this SS to glue differently than using Citric passivation.

Mark Kobain
- Galway, Ireland

Ed. note --
Gentle Readers: If you can contribute any insights into this adhesion issue, please do!
But we have no less than 6 very long and highly informative, but sometimes contentious, threads on the generalities of citric vs. nitric acid passivation. No need to start another general debate :-)

March 4, 2015

Again, I am often asked if passivation increases adhesion, my general assumption is that surface roughness influences that much more than passivation does. The cleanliness of the surface is probably also a major factor, perhaps the cleaning that parts receive during the passivation run is more significant here than the passivation itself. However, I think it greatly depends on what the coating or glue actually is. With some things you need to activate the stainless surface (opposite of passivate) to get the coating to go on properly.

I think rather than us trying to give a general answer here, readers would be best advised to consult the manufacturer of the coating/glue/etc. they are using and/or run some test experiments themselves.

I wouldn't expect any inherent difference between citric and nitric passivation as far as adhesion goes, if in fact it's for something where passivation even matters. If surface finish is a factor, nitric may or may not affect that, while citric almost never does.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner

March 5, 2015

A. Passivation may not help in improving adhesion. But plasma cleaning does help. It could be either a low pressure or atmospheric pressure plasma treatment depending on the situation. In fact, plasma cleaning helps in improving the adhesion of most of the cleaned solid surfaces.

H.R. Prabhakara - Consultant
bangaloreplasmatek.com - Bangalore Karnataka India

March 2015

Hi Mr. Prabhakara. How does plasma cleaning fit into the process sequence though? Would you do it before or after passivation?


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

March 6, 2015

If adhesion is the goal, plasma treatment would be the last step before adhesion. The effect of plasma would have a finite life depending on the surface. Generally bonding is carried out within an hour of plasma cleaning. However, I have seen the effect of plasma cleaning to last longer.

H.R. Prabhakara - Consultant
bangaloreplasmatek.com - Bangalore Karnataka India

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