Zinc plating rusts in shipment by sea

Hi Deb. We appended your question to an earlier thread on the same subject to give you some hints about possible causes, and to suggest the type of data you would need to provide for readers to be able to help you. Good luck.

Regards,

A. If you plate enough zinc, say .0002", there will be no rust.

If you don't know how thick your zinc is, then all bets are off.

A. The red rusting is the result of low coating thickness and poor handling during shipment through ship.

A white rusting is the result of poor passivation film and that can be improved by applying top coat sealant after passivation.

A. Hi Praveen. Rusting of the steel substrate is called red rust. Corrosion of the zinc plating is called white rust. The chromate conversion coating (passivation) deters white rust. Yes, red rust before white rust is certainly possible depending on the part (blind holes, spot-welded seams, or other areas of little or no plating). But you'll probably get better, more accurate, more enthusiastic, more helpful responses if you take the time to carefully provide the details of your own particular situation rather than keeping it vague and abstract.

Regards,

A. Hi Mustafa. We appended your inquiry to earlier very similar inquiries so you can read of many contributing factors towards rusting. But you have not told us what your plating spec is, what salt spray hours you achieve, the actual rusting situation, etc., so all we can do is fire back a round of buckshot :-)

Regards,

A. Hi Mustafa. It seems to me that this project is being approached backwards. Salt spray testing is a QA test to make sure your plating quality is not deteriorating; it is a poor indicator of real life, especially in the case of zinc plating which corrodes very differently in a salt spray cabinet than in real life. Trial & error on zinc plating, to no particular spec, until you reach some arbitrary salt spray hours, probably isn't the best approach.

If the automobile company has issued no specs for this component, then you have to choose a specification for yourself. I think most auto companies would rather see 13 microns of a zinc alloy like zinc-iron, zinc-cobalt, zinc-nickel, or tin-zinc than anything else. Then you can search this site or other sources for good salt spray hours to use for white and red rust for QA of such coatings. Best of luck.

Regards,

A. Hi Arun. As you can see from the topic we appended your inquiry to, there are a half dozen or more possible causes: insufficient plating thickness, poor chromating practice, parts not fully dry, poor packaging materials, lack of desiccants & VCIs in the packaging, unreasonable environmental conditions during shipment, etc.

But you said you've had no problem in decades, but this time the boxes got wet, so that certainly sounds like the problem. Please get back to us with comments or additional details.

Regards,