E-coating Over Zinc-Plated Steel?

Q. Our company has been asked to provide a steel stamping. The print calls for the part to be zinc plated before application of a cathodic epoxy E-coating. The customer apparently feels that the zinc plating will provide an additional level of corrosion protection. However, we have sent requests for quote to three E-coaters and they have all elected to no-quote the project because of the zinc plating. Can you help me understand the problem? T.B.

A. One of our client companies, custom E-coaters, recently received a similar request. Our client explained to their customer that they have applied E-coat to steel parts with zinc plating and there were serious quality issues. Their technical assessment was that moisture was absorbed and trapped in the plating. The moisture then out-gassed in the cure oven, which caused craters in the E-coat film. Our client suggested that the zinc phosphate pretreat process they provide as standard will meet a very wide range of automotive performance requirements and questioned the reasoning behind the desire for zinc plating.

I agree with their assessment and I might add one other thought. The zinc material in the plating can be sensitive to the level of heat exposure in the curing oven and there might be some popping of the zinc material itself. I also cannot think of any circumstance where a good zinc phosphate pre-treatment that is commonly used in E-coat systems that furnish parts to the auto industry would not meet performance requirements, and there should be no need for some added feature like a zinc plating. However, I have seen zinc plating used quite effectively under liquid coating systems where a high cure temperature was not a factor and where a good zinc phosphate pretreatment was not available. Perhaps that is the frame of reference your customer is using. It sounds like you need to communicate with your customer and suggest that they clarify their intentions, review and possibly change their specifications.