Electrocoat over Zinc-Plated Steel—Revisited

#plating #automotive


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Q. Dear Mr. Gilbert, please note that our company successfully E-coats (both anodic and cathodic) zinc and zinc alloy plated material every day.

Automotive Engineering Specifications for zinc as the base are specified to provide extended corrosion protection. It also provides self healing capability in case of damage to the E-coat.

I wonder why you think the automotive community should be questioned for specifying a zinc or zinc alloy prior to E-coat. Performance testing done by all the major OEMs will confirm better corrosion performance with zinc or zinc alloy than with a phosphate base. I think you should revisit your response. Thank you. A.B.


A. This is a follow-up to a topic addressed in our last Electrocoat Clinic (June 2009, page 62). Briefly, the writer of that question was charged with purchasing E-coat services for a stamping company and was having trouble finding a custom coater who would apply E-coat over zinc plated steel.
The writer wanted to know if I could shed some light on the reasons behind the reluctance of these sources to accept this application. I recalled having some problems with that process during my earlier days with custom E-coat operations. However, it has been more than 15 years since I have been personally involved directly in that application. So I consulted with a couple of our clients who are custom E-coaters. These clients had also experienced problems with the application of E-coat over zinc plated steel. They offered their opinions about why they thought they were having problems. I related those thoughts and comments in my answer to the writer.

The clients also expressed questions about the reasoning or justification for the added step of applying the zinc plating on certain functional parts. The common suggestion was that today’s zinc phosphate chemistry does a very good job and when combined with the superior properties of the E-coat the process should satisfy about any automotive application. That sounded logical to me based on clients’ comments and my earlier experiences, so I tended to agree.

Apparently this was a very good topic for the Clinic, because it has generated several responses and comments from our readers—some in agreement, some not. The above response certainly points out that more research on this topic would have been in order. I have looked into the subject in more depth and would like to share some of my latest findings with you.

There were other responses from custom E-coaters who are successfully applying E-coat over zinc plated steel, which highlights the fact that all custom E-coaters are not the same. For instance, some E-coaters use a square transfer system design and others use the monorail design. So any buyer who is reading this and has a requirement for E-coat over zinc plated steel should continue searching, because there are apparently several custom E-coaters operating systems that can accommodate this process and coat this substrate successfully. For references, I suggest you visit the Web site for The Electrocoat Association at electrocoat.org.

Now, why can some custom coaters apply E-coat over zinc plated steel and others cannot? Well, after talking to several people about this, including a technical representative for an electrocoating manufacturer, here is what I now understand about the situation.

After the parts exit the last permeate and rinse phase of the E-coat process, the film needs to experience a certain amount of dehydration. If the dehydration process is not sufficient the problems many have experienced may occur. This seems to be largely a function of process equipment design. For example, if the parts are vertically transported directly upward and into an oven that is, let’s say, over 400°F, the moisture in the film and plating is shocked and the dehydration process is too abrupt. Apparently, this is often the case with square transfer systems that are designed with relatively short oven systems where the temperature must be relatively high to ensure cure.
On the other hand, let’s say the parts are transferred over a longer path to the oven and enter the oven with horizontal travel, where there is some drying time. In this case the temperature increase and the dehydration process will be more gradual, more complete, and the coating film is more able to adapt to the more gradual increase in temperature. We would expect to find this process more often with monorail E-coat systems.

Now for the question of why the zinc plating is specified. Thinking back to my days as a design engineer, I would say that I might justify the specification of a zinc-plated steel substrate when the absolute minimum possibility of red rust being exposed is very important. This might be for a part that is located on a vehicle where it is subject to abrasion damage to the coating and red rust might create either a structural integrity problem or an undesirable appearance issue. Zinc does have the sacrificial function that dramatically slows the oxidation process. So the damage-exposed substrate will oxidize, but it is mostly white rust which will be less damaging structurally and less undesirable in appearance.