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Etching 316 stainless as preparation for good bond to epoxy

September 29, 2009

It was very difficult to get a bottle of Nitric acid for this application (it seems to be a controlled substance nowadays, what with dangerous lunatics loose in the world), and then it didn't work as advertized by the consultant (immersed in 40% Nitric Acid heated to 200F or higher for 30 min.). Besides, it was VERY dangerous to work with since we aren't really equipped for handling powerful heated acids!
I need to etch the inside of 316 stainless T-fittings, so methods that work on a flat surface are out. I can cap two ends of the "T" (to protect threads and ferrule surfaces at those ends) and fill it with an active liquid, leaving the open end pointed up during treatment, but I will still need to be careful to not expose the inner ferrule surface and outer threads of the un-capped upper end to etching. Less desirable: I can completely immerse the fitting with the two end caps secured if masking of the surfaces described above is possible. I need a good toothy etch, as the bonded assembly is subject to very high static pressure, modest pressure differentials, and abrupt plunges to very low temperatures, all at the same time. And now it's urgent, since I spent all my time testing variations of the acid method.

David Martin
Small equipment manufacturer - Santa Barbara, California
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October 1, 2009

David, I would recommend using a blast media such as Stainless cut wire shot, 304 or 316 alloy or even glass beads to rough up the inside of the Tee fiiting. The outside threads can be maked with a vinyl sleeve. I have seen both used prior to painting stainless steel. Tim

Tim Deakin
Pellets, LLC
  
North Tonawanda, New York



October 2, 2009

Stainless is not the most predictable when it comes to surface preparation prior to bonding with epoxy. You may need to try a couple of methods before you pick the right one.

Henkel have the following reference

www.henkelna.com/us/content_data/14258_LT4536_TT_Aerospace_Surface_Preparation_Guide.pdf

ASTM D2651 also will provide you with some options.

Ciaron Murphy
Aerospace - South Wales, UK


First of two simultaneous responses -- October 4, 2009

Thanks for your response Tim. Interesting you propose mechanical methods, since I have been investigating this approach since our disappointing acid etch tests. Do you know if a mechanically abraded surface is as good as a chemically etched one?
These parts are really small and the surface deep, so getting in there with a blast nozzle is a problem. I have used both a diamond point bit and a small grinding stone but have not tested the bond in application yet. Other thoughts or ideas?

David Martin
- Santa Barbara, California


Second of two simultaneous responses -- October 6, 2009

The problem with blasting is that you get poor bond durability.

The only successful methods I have used have involved hazardous chemicals unfortunately.

Probably the most successful method I have come across is 30% sulphuric acid at 60 °C for 2-5 minutes, followed by a desmut of 10% sulphuric acid/2% sodium dichromate at 55-60 °C for about 10 minutes.


I have tinkered with ferric chloride solutions with varying degrees of success, the best probably be 30% ferric chloride at 70 °C.

After that we are getting into the really nasty stuff such as hydrofluoric acid containing etches.

Just my personal experience. Hope it helps.

Brian Terry
Aerospace - Yeovil, Somerset, UK


October 6, 2009

David, granted, most of our applications have been for large Stainless Steel components where a customer has specified a epoxy coating. How small are you part ID's? 1/4" , 1/8" or smaller? How deep ? Can the part be fixtured to hold in place? What quantity of parts need to be epoxy painted?
The stainless as cut wire shot is readily available in .020", .017", .014". The stainless cut wire shot has a minimum hardness of Rc 45 ,but is typically Rc 50-55. If these size are suitable to use, they should be able to rough up the 316 alloy. What hardness is the 316 alloy fitting?
You would have to test blast some parts to see if this process yields acceptable epoxy adhesion.

Tim Deakin
Pellets, LLC
North Tonawanda, New York


October 7, 2009

In reponse to Ciaron-
Thank you for the Henkel document reference. It's very detailed,and certainly adequate for our purposes. It is interesting to me that Nitric acid is not even one of the constituants in any of the three baths reccommended by this Spec.

Additional response to Tim-
Subsequent production units will all be larger, but our smallest current parts have a 1/8"ID, and also 1/4"ID. At this stage the quantity is small: probably not more than about 30 pieces. The depth of these holes extends to about 1", so the angle of attack for the blast media would be very shallow, especially at max. depth and may not allow enough surface roughness. These are stock and special cast Swagelok parts, but I don't know the hardness. Thanks for your help.

In response to Brian-
Thank you for your procedure. It is a simple one which doesn't involve the least desirable acids. Very helpful.

David Martin
- Santa Barbara, California

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