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Appearance issue on zinc-nickel plated parts (heat treated parts)

November 21, 2008

Dear All,

Nowadays, I have an issue regarding the appearance of the parts, after we apply the heat treatment process before plating in order to increase the hardness.

The heat treatment process is done by barrel because the parts are very small. In the end of the process, we remark that many parts (about 50%) are stick each other which make the surface not in a good condition. From the explanation from my plating supplier, I might conclude that this problem happen only for those affected parts. I've asked the heat treatment supplier to reduce the quantity of the parts inside the barrel into a half from the normal quantity. But the result is still the same.

So, right now, I am thinking, what can be done on the plating side to eliminate this bad surface. Here is my proposition :

Soak cleaning (55-65 deg C for 10-20 minutes) - water rinse 3x - hydrochloric acid pickling (1-3 minutes at room temperature) - water rinse 3x - anodic electrocleaning ( 45-55 deg C for 3-7 minutes) - water rinse 3x - hydrochloric acid pickling (1-3 minutes at room temperature) - water rinse 3x - Zinc plating - water rinse 2x - activation by nitric acid - water rinse 2x - de-hydrogen treatment - water rinse 2x - activation by nitric acid - water rinse 2x - chromatation (Surtec 680 LC) - water rinse 2x - oven baking.

My question right now is whether the duration for acid pickling (20-30% HCl) is enough or not to eliminate those bad surface (after heat treatment) as you may see above? Is it necessary to apply the pickling with 5% sulfuric acid + inhibitor for the second pickling for 30s-1 minute, if the two picklings with hydrochloric acid is not enough? May I use the combination of Hydrochloric acid (30%) + sulfuric acid (5%) + inhibitor (2%), so it would be only one stage of pickling and it's done before the anodic electrocleaning.

Is the nitric acid is better to be done before and after the de-hydrogen treatment? For how long and which concentration is the best? Do this nitric acid may also help on eliminating the bad appearance?

Thank you very much for your kind help regarding this issue.

Reynald SUKIN
Customer - Batam, Indonesia