Troubleshooting the Pretreatment and Paint Process

Q. We had finished parts sectioned, mounted, polished and examined under a microscope with the hopes of identifying deficiencies within our coating system that cause premature failure. One of the items brought up are oxides trapped within scratches and scale texture under the e-coat. The substrate is hot-rolled-steel (recycled steel) bar that has been steel shot blasted then put through the standard cleaning process prior to e-coating. My question is, have you ever heard of or seen steel shot blasting driving oxides into small crevices or texturing natural to hot-rolled steel? We cannot seem to figure out why these oxides are still present after shot blast and pickling. A.W.

A. You should be commended for the thorough investigation you are performing to establish the root cause of your coating issue. Not many go to the lengths you have to diagnose the issue. I believe you have performed all the correct analytical steps, but the interpretation for root cause identification and corrective action needs to be revisited.

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First, I would recommend reconsidering the process sequence used to pretreat the parts in question. The shot-blast process may remove some of the embedded oxides, but could just as easily “fold” some material over the oxide or drive it further in, effectively sealing it into a more inaccessible crevice. I would suggest trying the cleaning process prior to the shot blasting to see if that improves your results.

Second, in addition to the shot-blast process possibly embedding some oxide further into the material, it is possible that some of the steel shot could be embedded into the steel surface. Depending on the aggressiveness of your cleaning process, it is possible that little to none of it will be removed. A heated acid soak may be necessary to remove both the embedded steel shot and remaining oxide from the hot-rolled surface.

In the end, I would reconsider your choice of hot-rolled steel as a substrate to receive an e-coat paint. The investment in e-coat is significant and would suggest you have demanding requirements from your customers regarding their expected paint finish. The incoming quality of hot-rolled steel is not very controlled or repeatable, especially as a service center moves from one supplier to another. The ability or results of pickling and descaling vary significantly from one mill source to another. Considering the investment you have in your system, I would strongly urge you to review your substrate options at some point in the future. The increased cost of the cold-rolled material could be at least partially justified in time and quality savings by having a more consistent material. Also, I would expect that you could recoup cost in the elimination of the shot-cleaning process, since it would no longer be required.