No registration or passwords; no pop-up ads -- just aloha, fun, & answers.
(as an eBay Partner & Amazon Affiliate earns from qualifying purchases).
Home /
T.O.C.
Fun
FAQs
Good
Books
Ref.
Libr.
Advertise
 
Help
Wanted
Current
Q&A's
Site 🔍
Search
pub  Where the
world gathers for metal finishing
Q&As since 1989



-----

Etching aluminum for NDT penetrant inspection




Q. I AM IN CHARGE OF A PLATING SHOP. WE ETCHED 2024 T-8 ALUMINUM BELIEVING IT TO BE 2024 T-3. WE FIRST ETCHED A 2024 T-3 COUPON TO DETERMINE THE ETCH RATE IN ALUMINUX 1000, 6 OZ./GAL. FOR 5 1/2 MINUTES AT 160 F. WE THEN RAN THE 2024 ALUMINUM PARTS UNDER THESE SAME PARAMETERS AND THE PARTS PITTED BADLY. CAN ANYONE EXPLAIN WHY 2024 T-8 ALUMINUM PITS UNDER THE ABOVE CONDITIONS?

Jerry Wahlin
plating shop - Compton, California
2000


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 pullizi signature
Tom Pullizzi
Falls Township, Pennsylvania
2000


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
2001




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 [affil link] .
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
2007



simultaneous replies

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
Compton, California, USA
2007


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.
Syracuse, New York
2007


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
probertbanner
2007


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, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007



simultaneous replies 2007

Bob,

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 [affil link], '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 [deceased]
- Goleta, California

contributor of the year Finishing.com 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, and we at finishing.com, continue to benefit from.

2007




Micro-etching of aluminum

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?

Bob Bramson
B&M Finishers / Prismatic Stainless Steel
supporting advertiser
Kenilworth, New Jersey
b&m finishers banner
January 24, 2019




Aluminum finish and Liquid Penetrant (NDT) Testing

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 2, 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 [on eBay or Amazon] 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 5, 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

dan keenan [returning]
- denver, North Carolina, usa
December 8, 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 [affil link or DLA] , Type 2 (non-hexavalent chromium conversion coatings), applied in accordance with MIL-DTL-5541 [affil link or DLA], Type 2. This is a very thin coating, which will not mask defects from liquid penetrant.

Brian Terry
Aerospace - Yeovil, Somerset, UK
December 10, 2019


none
finishing.com is made possible by ...
this text gets replaced with bannerText
spacer gets replaced with bannerImages



(No "dead threads" here! If this page isn't currently on the Hotline your Q, A, or Comment will restore it)

Q, A, or Comment on THIS thread -or- Start a NEW Thread

Disclaimer: It's not possible to fully diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations might be harmful.

If you are seeking a product or service related to metal finishing, please check these Directories:

 
Jobshops
Capital
Equipment
Chemicals &
Consumables
Consult'g, Train'g
& Software


About/Contact  -  Privacy Policy  -  ©1995-2024 finishing.com, Pine Beach, New Jersey, USA  -  about "affil links"