Etching aluminum for NDT penetrant inspection

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.

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.

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!

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.

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, '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.