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topic 43963

Etching aluminum for NDT penetrant inspection

A discussion started in 2001 but continuing through 2019



Jerry Wahlin
plating shop - Compton, California


A. No. Did a coupon of T-8 material also pit? I think your processing solution is past its useful life.

tom pullizzi monitor
Tom Pullizzi
Falls Township, Pennsylvania 


A. I know this comes a bit late and perhaps you might already know why your 2024 parts pitted and the coupon didn't. But why in the heck would you be etching 2024 at all under those conditions? You ween't prepping to plate for sure. And the amount of aluminum you removed would be significant. The heat treat designation from you coupon to your part is different. I have coupons also but they are not heat treated as the parts are. I also use them to set up a etch rate . I use 1 minute and determine from there. I think it has a lot to do with the difference in heat treat designation. But also, WOW, thats a long etch time

Todd Huehn
- Minneapolis, Minnesota


Q. Hi,

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.

Bob Pitts
Aerospace Plating & finishing - New York

simultaneous 2007

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.

Admitted, we do use a purchased blend for our etch, but I don't see why you should have problems.

Contact the supplier of your conversion chemicals - they may be able to help you with any process parameters that are a bit off.

Jim Gorsich
Accurate Anodizing Inc.
supporting advertiser
Compton, California, USA
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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.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser 
Syracuse, New York

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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.

The Ex-Allied Kelite (before being Macdermidized), who worked with the Navy in writing the amendments to 5541, said they got better results with NO alkali etch.

Many cases in the field that I have personally serviced, have found that alkali etching enhances the possibilities of passing salt spray.

The single most important variable in passing salt spray is the quality of the panel material that you start with.

On a recent service case, my client failed with three different panel sources, - then passed with the fourth!

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina


thumbs up sign 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, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

simultaneous 2007


I am very surprised that an Aerospace company is allowing you to use such an aggressive etch solution. A straight caustic etch at 20% will have a huge effect on dimensional tolerances.

If I were you I would have a word with some of the proprietary chemical suppliers such as Chemetall or Atotech and see what they have to offer in the acid based inhibited systems. These etches will give you a more controlled etch rate, most are self-desmutting which then cuts out another stage in your operation.

As for salt spray failure, there are 101 reasons why Chromate Conversion fails such as corrosion in the panels before you start, drying at too hot a temperature, incomplete removal of smut, etc.

Brian Terry
Aerospace - Yeovil, Somerset, UK


Aerospace has mandatory process procedures. Etch solutions are specified, and test coupons are used to determine etch rates and times.

A 20 vol% solution of NaOH (50 wt%) gives an NaOH concentration of 150 g/L (20 oz/gal). If 20 wt% NaOH is meant, even worse. Anodizers typically maintain ~5 oz/gal of free NaOH (determined by titration) and etch as necessary to give a desired surface finish. For chem film, etch only if specified to hide machining lines, etc.

Per ASTM E1417 [link by ed. to spec at TechStreet], 'Standard Practice for Liquid Penetrant Testing,' paragraph 7.1.5, etch only as necessary. "Etching processes shall be developed and controlled to prevent damage to the component under test." Aerospace companies have already developed the procedures. Caustic etching, especially at excessive concentration, is clearly damaging as demonstrated by salt spray testing. NaOH etching leaves a roughened surface high in constituents that chem film poorly or not at all. Caustic etching isn't in any Alodine® process bulletin (non-etching cleaners are even specified), so I cannot understand "our supplier desires this step!"

There seems to be a missing link in the information pipeline. If you actually need to come up with an etch, try the universal deoxidizer: 50 vol% nitric, 25 vol% sulfuric and 25 vol% water, plus 60 g/L ammonium bifluoride. Good luck in straightening things out.

Ken Vlach
- Goleta, California

contributor of the year honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.

Micro-etching of aluminum

January 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

Aluminum finish and Liquid Penetrant (NDT) Testing

December 2, 2019 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello, I am working on a project that involves machined aluminum that needs a protective coating, but still needs to be inspected via Liquid Penetrant (NDT) Testing. I have used anodizing in the past, but this new project does not allow anodizing due to material fatigue concerns. Is there a coating that will not affect the mechanical properties of the aluminum and still allow Liquid Penetrant (NDT) Testing inspection during service life?

Dan Keenan
- Denver, North Carolina, USA

December 5, 2019

A. Hi Dan,

Chromic acid anodizing is considered fatigue neutral, so may be a way to go. The coating is only approximately 2-5 microns thick, so will not affect your dimensions, unless you are operating to very tight limits. You can NDT after this type of anodizing.

Another couple of possibilities for what are considered fatigue neutral anodizing, which are less hazardous to do are sulfuric-boric acid anodizing and sulfuric-tartaric acid anodizing. I've not played with these, so don't know if you can NDT afterwards, although I can think of no reason why you couldn't.

Brian Terry
Aerospace - Yeovil, Somerset, UK

December 8, 2019

thumbs up sign  Brian, Thank you for the input. The customer has specified no anodizing, so regardless of type of anodizing, it is not acceptable.

dan keenan [returning]
- denver, North Carolina, usa

December 10, 2019

A. Hi Dan,

You could consider a conversion coating. This will not give you as much corrosion protection as anodizing, but you might be able to live with that, dependent on end-use. Look at coatings to Mil-DTL-81706 [link is to free spec at Defense Logistics Agency,], Type 2 (non-hexavalent chromium conversion coatings), applied in accordance with Mil-DTL-5541 [link is to free spec at Defense Logistics Agency,], Type 2. This is a very thin coating, which will not mask defects from liquid penetrant.

Brian Terry
Aerospace - Yeovil, Somerset, UK

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