Plating for overseas shipment 3

[yanish]

I have a question that I hope will be easy for you metal guys. I am having problems with rust on a part that is shipped overseas to Europe. It is mainly a plastic assembly, but there are a couple of metal components on it. Originally, the metal was finished in bright zinc. We saw some problems with rust and have changed to a clear zinc. This seems to have helped, but what I am wondering is, is there a better finish for this application. Cost is of course a consideration, but does anyone know of a better option that is at least similar in price to these options. Thanks for any help you can give me.

 

[TVP]

In case you are not totally familiar with zinc plating, what you are describing by "clear zinc" is actually two different coatings. First is an electrodeposited zinc layer, usually 5-12 micrometers in thickness. Subsequent to the electroplating process is a dip process-- chromate conversion coating. Depending on process conditions, including bath chemicals, immersion time, etc., the chromate conversion coating can be:

a) very thin and essentially colorless, which is your "clear zinc" finish

or

b) much thicker and colored. The chromate film changes from clear, to a faint yellow, and then transitions to a more iridescent bronze, then into dark colors of brown, olive drab, and maybe black. As the chromate film becomes thicker, and assuming that the bath chemistry is of the hexavalent chromium type, it is enriched in hexavalent chromium, and offers more corrosion protection to the zinc layer.

So, one way to improve the corrosion resistance is to improve the chromate coating. This delays the onset of corrosion of the zinc layer, often called "white rust". A thicker zinc layer will alloy for a longer delay until basis metal corrosion, often called red rust.

I would recommend that you try zinc plating to ~ 9 micrometers with an iridescent bronze chromate. This provides approximately 96 hours of salt spray resistance, compared to ~ 0-8 hours for the clear chromate. ASTM B 633 is an industry standard for specifying zinc plating with chromate conversion coatings. The thicker the zinc layer and the more protective the chromate (bronze to olive drab, etc.) the more expensive the coating. Good luck.