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Chromate conversion works in SS 316L fixtures but not PP barrels

We've recently attempted to introduce a polypropylene plating barrel and have found that the chromate conversion does not occur. We use a nitric/hf deoxidizer and DI rinses. As a result of this failure to convert, we made a SS 316L wire (hand held) fixture and manually dipped it through the process and it was successful. So, we made a PP hand held fixture and ran it along side the SS 316L. The PP fixtured part did not convert and the SS fixtured part did convert.
Has anyone seen a similar situation?

David Mays
- Elgin, Illinois, USA


It's not clear to me if you are talking about zinc plating with chromate conversion coating, since you mention plating barrels, or conversion coating of aluminum because you mention 'deoxidizer'. Although I'm certainly aware of some shops who do their chromating in baskets after unloading the zinc plated parts from the plating barrels, this is less common than doing the chromating while the parts are still in the polypropylene plating barrels.

Your story is a curious one and I don't know if it relates to the particular style of the plating barrels, the particular chromate, or the particular parts you are processing. Correct me if I'm wrong, platers, but most shops retain 10-20 percent of their chromate bath when they dump it because new baths can be slow to start. It could be that your fresh chromate bath lacks that special something and will soon start performing well. But I'm very curious on this one, and will eagerly read what people have to say.

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


First the preferred material for a basket to do conversion coating of zinc or cadmium plated parts is polypropylene. An electric current can be induced at the point of contact with ss resulting in a gap in the chromate film. If you are doing a dark color (od, black) these are easily evident.

What I think you may be doing, expecially if you are using barrels, is that you are rotating the barrels for too long a time and are removing the chromate film.

A little more information about your process cycle would be helpful

Gene Packman
process supplier - Great Neck, New York


I have the same in my zinc barrel plating plant.
Our company has been running the best quality in South Africa.
We have a totally manual plant and we always passivate in ss baskets. We did try in pp or pvc baskets because of the ss cost but found the same problem. I once spoke to a old plater of 50 years before he died and he told me that it had something to do with some kind of current created.
I did not really understand him as I was too hasty and young but it just seems to work that in ss and not pp.
We are now putting and semi-automatic barrel line in and we will still use ss barrels in the pass line.
If it works in ss , use ss.

gary joseph
Gary Joseph
South Africa

Dear Sir,
I would like to reiterate that PP barrels in the automatic line and PP/ LDPE buckets in the manual plants are successfully in use for many years. If its a freshly prepared bath without aged solution, one would experience "no plating" or "passivation rub off" from the components. In fact all supply houses recommend the above material for holding the components for processing.

- Mumbai, India

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