Plating on Aluminum

Question:

Our company manufactures parts out of various aluminum alloys. One of our customers now requires that the parts be plated with a decorative nickel/chr-omium finish. We do not do our own electroplating but instead rely on a local electroplating company to do this for us. The problem is this: Some of the parts come back from the plating operation and are fine, no blisters or flaking on the surfaces. Others are returned and start to blister and flake in a short period of time. The plating company has told us that the aluminum alloy used to make the part causes this. Can you give us any advice to help us solve this problem? I.W.

Answer:

Your plating supplier is partially right but he is also at fault because he probably is trying to use the same plating cycle for all of the aluminum parts that come to his delivery dock. In other words, one plating sequence does not work for all aluminum alloys.

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More than likely, the parts that are successfully plated are one type alloy while the failed parts are another. Let’s start at the beginning.

When you send your parts out for plating you must specify to the electroplating shop the alloy or alloys the parts are manufactured from. “Aluminum” is not an adequate description! Parts should be segregated according to alloy type.

In turn, your electroplater should use different processes for different alloys. There are a number of different process sequences that can be used for nickel/chromium decorative plating. As an example, one such process sequence is as follows:

1. Degrease
2. Soak clean
3. Rinse
4. Acid dip
   a. Silicon containing alloys (hydroflu- oric, 10 – 12 ml; water, 1 liter)
   b. Copper containing alloys (nitric acid, 400 – 450 ml; sodium bifluoride, 40 g; water, 1 liter)
   c. Tin containing alloys (sulfuric acid, 475 – 500 ml; chromic acid, 70 g; water, 1 liter)
5. Rinse
6. Zincate
7. Rinse
8. Strip zincate
9. Rinse
10. Zincate
11. Rinse
12. Acid dip to neutralize alkalinity from zincating step
13. Copper strike
14. Rinse
15. Bright nickel plate
16. Rinse
17. Chromium plate
18. Rinse

Note that step 4 lists three different acid dips with others being available. If your electroplating shop is using only one acid dip for all alloys you will have problems.

An alternative process that seems to work well for many aluminum alloys is to substitute mid-phos electroless nickel for the copper strike bath.

The bottom line is this: If your plating vendor is aware of the nuances of plating on different aluminums, there should be no problems. If he is not, then you may want to find a new vendor.