Letter 22035

Need a replacement process for etching aluminum [Florida] 

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We have an aluminum etching process where our operators mix Hydrochloric acid (37%) and cabosil to form a paste. The paste is then brushed over the surface of the aluminum. The paste stays in contact with the aluminum for 8 minutes and then is rinsed off with water. The process is performed in a booth and the operators are required to wear full face air supplied respirators and acid suits. Is there a safer alternative for etching aluminum, that will give a similar degree of etching? Another chemical we can use rather than HCl? Our pieces we work on vary in size from small brackets to large (up to 16') panels of aluminum.

Thanks in advance for your assistance.

Todd Wegenast
- Melbourne, Florida, USA

Most shops would use an immersion in a caustic tank rather than an acid etching slurry. I don't, however, know if it is practical to dip your parts, and you may have to de-smut them after etching.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

We are trying to find an alternative to etching this way as well. The finish we are getting is too dull. In order to get a shiny finish we have had to switch to stainless steel. Is there any way to get an etched look without the white chalky residue?

Laura Burkholder
Nameplate and domed labels - Arlington Hts, IL, USA

Caustic soda (sodium hydroxide) is commonly used in the industry for etching aluminum. As indicated by Mr Ted Mooney, the parts will have to be desmutted/deoxidized. Dependent upon the concentration, temperature, bath age, and metal loading, the etch rate of caustic on aluminum will be affected, but it is normally quite slow and the depth of etch is fairly shallow. The end use, or post process, i.e. chromate conversion, bonding, paint, will determine the type of topography you will need to create on the part/s. Alternate acidic etchants based on ferric or cupric chloride can be used. In the chemical machining industry the etchant most widely used is ferric chloride. The same basic parameters of concentration, temperature, metal loading, etc. will also influence etch rate. Please note however, that the etch rate of ferric versus caustic is many times higher and tests to determine the etch rate for a specific set of parameters should be determined. As with caustic, ferric etching will cause a smut to form on the parts which will have to be removed. A very effective desmut and bright dip solution can be made as follows:

D.I. water - 95% b.v.
Concentrated nitric acid - 2.5% b.v.
Concentrated hydrochloric acid - 1.5% b.v.
Concentrated Hydrofluoric acid - 1.0% b.v.

Temperature - ambient
Dwell time - ~ 1 minute
Please note!!! Please read MSDS's for all acids, paying careful attention to handling, protective gear, storage, and disposal instructions. These acids are nasty but very effective at performing the desmutting and bright dip step of the process.

Ferric chloride is very effective on stainless steel as well. It will require a higher concentration and longer dwell time. The smut that is formed is very tenacious but can be readily removed using a modified version of the acid desmut cemistry as described above. A 1% - 2% addition of hydrogen peroxide needs to be made to the acids. Compensate for this addition by reducing the water concentration accordingly.

Wade Rohland
Techni-Trax - Ramona, CA, USA

Just a word of caution, Hydrofluoric acid (HF) is very dangerous. It is unlike other acids, in that, after exposure, not only is the immediate first aide important but also monitoring of Calcium levels in the blood. Fluoride ions penetrate through the skin and bind up calcium forming an insoluble salt. An exposure of a 50% soln over an 8" x 8" area can be deadly! Keep Calcium Gluconate on hand and by all means read the MSDS!

James Brock
- Newport New, VA, USA

Thanks for the very important note, James. The dangers of HF are in a whole different category than most other acids.

Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

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