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Ted Mooney, P.E. RET
Pine Beach, NJ

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Alochrome plating not working

We are trying to Alochrome an aluminum box of 25x50x63.5 mm. The box is milled from aluminum.

Attempt 1) was with three prototype units that had been made locally and one that was made in England. The samples were first etched in hydrochloric acid and then directly Alochromed. The three local units had a poor quality plate on the exterior and next to none on the interior. They are now black with corrosion on the interior. The sample from England had virtually no plating at all, but did not corrode. We researched the problem and came up with the following solution.

Attempt 2) These boxes were cast using Aluminum Alloy 5052 H-16, which we believe can be electro-plated and a large quantity have been produced, so we are committed. The samples had the following process:-

The samples plated reasonable well on the outside, but had very small pits. We are told that is due to the aluminum being cast and not pressure injected. The inside of the box was hardly plated at all. At my request they tried to plate the same samples again with the Alochrome solution agitated with compressed air. This did put a more plating on the inside bit caused sever corrosion in some areas mainly on the inside.

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Attempt 3) Was made with a single box it followed the same procedure from a) to p) but the outer of the box was screened with plastic for the Alochrome process. The plating was then repeated with no screening. The inside of the box plated quite well but the outside had no Alochrome plating, just the nickel remained and there are very small pits in this. There is no corrosion.

Attempt 4) Was made with a single box. It followed steps a) to p). There was no screening and the solution was agitated. The plating in the box was still inadequate, with exposed nickel areas inside and some corrosion.

Attempt 5) The box was cleaned as per a) to d) and then directly Alochromed. The plating took well on the inside and outside of the box. There is not yet any corrosion, but it was only done about a week ago.

Attempt 6) The box was cleaned as per a) to d) and then copper plated under primitive conditions in my back garden. We used copper cyanide and some scrap copper sheet as the anode. Two amps for 15 minutes. The box was then transported to the works and cleaned again then Alochromed. The solution was agitated but there is insufficient plating on the inside with large areas of copper.

The finish on the good parts is acceptable. There are no small pits. The conductivity is good, which is essential for the application. Copper plating is not available locally and a facility will have to be set up if we require it.

We checked the Alochrome solution and cleaning fluid tanks with litmus paper, which is all we have available, and they are all alkaline. We are now under pressure to find a quick answer to the problem. What is the best solution?

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by Robert Probert

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A. Hello Highton
Alochrome is a Henkel trade name for a chromate conversion process for aluminum (it is not usually called 'plating'). You can review MIL-DTL-5541 [on DLA] for a quick introduction, or look at dozens of threads on this site about 'chromate conversion coating', 'chemfilm', 'Alodine', or 'Iridite' (the latter two are also trade names).

Re. attempt no. 1: Etching in hydrochloric acid is not appropriate. Rather you need a non-etching alkaline cleaner, an alkaline etch, a de-ox/de-smut (which will depend on the alloy being processed because it's purpose is to dissolve the non-aluminum content), with rinsing between each process, and then the Alochrome.

Re. attempts no. 2 thru 4: This was hopeless because Alochrome reacts with aluminum, it will not work on a nickel plated surface -- but perhaps the nickel plating without the Alochrome can meet your needs?

Re. attempt no. 5: This is much better but still lacks etching and de-ox/de-smut which are recommended.

Re. attempt no. 6: Again, Alochrome 'converts' aluminum, it will not react properly with copper plating.

The best solution would be to hire a consultant familiar with Alochrome, but lacking that, please follow my advice listed under 'attempt no.1'. Luck & Regards,

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