What is the best pre-penetrant aluminum etch that will not destroy the substrate
A discussion started in 2007 but continuing through 20192007
I am a small processing plant for Aerospace Parts and Use Sodium Hydroxide Mixed with water in a 80/20 mixture (20% NaOH, some parts go to penetrant inspection while others will go to Chemical conversion. I have always been taught that you should never pre-etch if the par(s) are just going to chem. conversion but our supplier desires this step!
My question is that the NaOH mixture has been said to be to harsh and the Alodine test panels fail (big surprise here) when subjected to the 168 hour salt spray test in ASTM B117 [link by ed. to spec at TechStreet] .
So what else is out there besides using the mixture that I am. By the way tank temp is 150 degrees F.
Aerospace Plating & finishing - New York
Why wouldn't you etch before chemical conversion coating? We do chem-film here every day and etching is a standard part of the process for us. And we haven't failed a salt spray test.
Accurate Anodizing Inc.
Compton, California, USA
A. I've got a couple of concerns - NaOH etching prior to chromating, particularly on high copper alloys such as 2024 which is mandated to use for mil spec chromating will indeed concentrate copper on the surface following etching resulting in gross salt spray failure. This is what NOT to use; ask a reputable supplier of chromating material which pre-etch works best - while you're at it, let your local sewer authority know so they can sample your discharge. If you use the right material, it's likely you won't be meeting your discharge limits. Also, most NDT penetrant requirements for etching, I believe, require the use of acidic etches. Either way, if you're using alkaline etching, you need a strong acidic material to de-smut prior to NDT. Chromic/sulfuric and/or nitric/hydrofluoric combinations work well for this - if they sound dangerous, they should! Leave chemical processing up to those who know chemical processing.
Milt Stevenson, Jr.
Syracuse, New York
A. There is very much controversy on whether to alkali etch ahead of aluminum chromate. I feel that the decision is based on the condition of the raw panels.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide
But does your client buy the aluminum for their actual components exclusively from this fourth coupon supplier, Robert? :-)
I've never understood this madness of having to search high and low for a supplier of test coupons which will pass a salt spray test when the actual component material is not similarly selected :-)
Some day, somehow we must all get together and fix this ludicrous corrosion test which tells us nothing about the metal the actual components are made of, and very little about the process sequence, but instead indicates how old and how good the test coupons (which will be thrown into the scrap bin) were :-)
Ted Mooney, P.E.
Pine Beach, New Jersey
Aerospace - Yeovil, Somerset, UK
Aerospace has mandatory process procedures. Etch solutions are specified, and test coupons are used to determine etch rates and times.
- Goleta, California
Finishing.com honored Ken for his countless carefully
Micro-etching of aluminumJanuary 24, 2019
Q. We are thinking of installing a micro-etching line for Pre-treatment of FPI process - does anyone have a procedure for running the line?Robert Robert Bramson
Kenilworth Anodizing Corp - Kenilworth New Jersey USA
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