Paint Grip Electro-Galvanized

Q. I work for a company that manufactures welded steel chain. Over the past year or so we have had requests from several customers to have the chain powder coated. We have been sending the chain to an outside processor to do the powder coating and have had great success until just recently. We normally send them chain that has been either zinc electro-galvanized (which we do in house) or we send them hot dip galvanized chain, which we have done at an outside processor. We recently sent them some larger chain (0.520-inch wire diameter) to have it matte black powder coated. When they returned the chain, we immediately noticed the powder coat was chipping off (see photo). The photo is the worst of what was returned; the majority was not chipping quite this badly but was still chipping, and we could not send the chain to our customer in this condition. We contacted our supplier about the problem. Below is their verbatim response.

“Dwight has examined the photos. He agrees that in some places the coating does look light. The wash was the same wash process our vendor always used. He’s not really sure why the chain is chipping but what he did say was that a “paint-grip” galvanized must be used (I just learned that fact this morning) to enable successful coating over the galvanized surface. The other thing is the extreme weight of this chain compared to most of the chain we receive from you. I’m not sure if we have ever coated 1/2 -inch chain in this matte black before or not. We do not have much call to use this powder and therefore are not as experienced with it as with our staple colors.

“Dwight said if you want to return this 300 ft of chain for us to recoat you are welcome to do so, he will recoat at no charge. Hopefully, that would turn out better; however, due to all the various factors listed above, he cannot guarantee there will be no chipping even then. We’ve not had this problem before, so it is new territory for us. But again, we will attempt to make it right for you. Let us know your decision.”

My question is, what is “paint-grip” galvanized? I have been electro-galvanizing chain for years and I contacted our chemical supplier and they never heard of “paint-grip” galvanized either. Can you enlighten me?

In my opinion, I think they just did a poor job of cleaning/activating the surface prior to the powder coating, but I know nothing about the powder coat process. A. M.

A. “Paint Grip” galvanizing applies to a pretreated sheet metal product that is galvanized using methods that prepare the surface for paint. Obviously, this does not apply to chain. However, the point your powder coater is making is still important: the galvanizing must be prepared to accept paint. Using hot-dip galvanizing can be a problem if zinc oxide is allowed to form on the surface before powder coating. Proper cleaning and pretreatment will remove this oxide and allow for good powder adhesion. Proper cleaning for this substrate must include a deoxidizer to chemically etch the surface and remove any oxides. Your coater must have the capability of doing this if you want to have the coating stay put on the surface.

Now for what I think is the root cause of your problem. By your own admission, this is the heaviest chain you have had coated to date. Being heavier, there is more mass to heat for proper powder coating cure. This means more time and temperature in the cure oven. Without proper and full cure, the powder coating will chip readily and perform poorly.

Also, since the chain is much heavier, the impact forces on the coating are also much higher. Just think how heavy each link is and as this chain is coiled into a box, bucket or even the floor for that matter, how much force is imparted on the next link. The bottom links are further stressed by the weight of all the chain piled on top of it. All this leads to higher impact forces on the coating. You may have been using a coating that had marginal impact resistance in the past but because the chain was lighter, the impact forces were within acceptable margins. This coating does not stand a chance with the heavier chain, as the impact forces exceed the capability of the coating to resist them. Couple this fact with the possibility of under-curing the coating, and it is no wonder you had problems.

Let’s recap the solution:

Prepare the galvanized surface by removing all organic (oils) and inorganic (oxides) soils.
Choose a powder coating with excellent impact resistance (? 160 inch-lb).
Provide enough energy to fully cure the powder coating on this heavier product.
Take care with packaging and shipping the coated product.