Passivation of Stainless Steel for Increased Adhesion / Better Gluing

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,

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,

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.

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,

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 A967AMS2700, or the defunct but free AMSQQP35 [canceled]) ...

adv.
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.)

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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 :-)

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.

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.

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

Regards,

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.