Dr. Norman R. Roobol

Industrial Painting Consultant

Handling Pretreated Parts Before Coating

I’ve followed your column in Industrial Paint & Powder for a couple of years and have found it very helpful. Do you think a mild steel part (or any substrate for that matter) that has been through a seven-stage pretreatment system (cleaner 1, cleaner 2, rinse, rinse, zinc phosphate, seal, DI rinse) and dry-off oven can be manually moved from the pretreatment line to a different powder coating line without creating problems in the finished coating? I’ve come across a situation where I’d like to pretreat densely racked parts on one line and then rehang the parts by hand on a separate powder coating line.

If the people handling the parts wear proper gloves, will that be enough to keep the conversion coating from being damaged? Unfortunately I’m not in a position to try it in the field just yet. This approach (handling pretreated parts) is not a long-term plan. But if it will work in the short term, it will be quite helpful to my operation. Do you have any thoughts?

What you propose should be perfectly okay, especially since it is a temporary solution. The phosphate coating is not readily damaged by manually handling unless the parts are allowed to hit against each other. Be certain that handlers do wear gloves and that they change them when they become soiled.

Preventing Corrosion on Aluminum Parts

Why do some aluminum parts seem to corrode while others stand up fine? We clean and phosphate our assemblies before coating with 2 to 2.5 mils of epoxy. We check film thickness all over, and it is almost equal in every area of the assembly, yet one of the assembly components consistently shows white rust corrosion while none of the other parts, which are also aluminum, ever have any corrosion. We’ve scratched our heads for years over this problem but still have no answers. We’d appreciate hearing any ideas you might have as to what is causing this and how we can avoid it.

I don’t think galvanic corrosion is the culprit here. Notice (from the pictures sent) that the problem part is a casting, but the other parts are extrusions or formed sheet aluminum. Could it be that the casting is an aluminum alloy with a higher copper content than other components? If so, that may be the cause of earlier corrosion onset. Is there any reason why you cannot change to a lower percent of copper in the alloy for this casting? That may solve the problem. If an alloy change is not possible, let me recommend working with your pretreatment supplier on a new or additional pretreatment step to enhance the resistance of the casting to white rust formation. The latter added step will be a bit awkward, so changing the alloy would be preferable.

Textured Powder Coating Problems

It was very nice to go through your Web site; it is very helpful and educational. I’m working for a powder coating R&D based in India. For our texture-based powder coatings, we use troy, which is post-blended into the formulation. This is not compatible with other nontextured powders and thus leads to pinholes if present even in traces in the charging gun. To overcome this, we tried some acrylic resins and also cellulose acetate butyrate (CAB) as structure additives. This gives a very good pattern and it is compatible with other powders, but it shows some contamination on the panels (black spots, though there is no black pigment used.) It seems like these acrylic resins are not compatible with the system.

What base resin is used for the textured powder coating? I have no idea what “troy” might be, so I can’t comment on why it causes pinholes if not totally cleaned out of the powder system. Can you use another material for this purpose? I’m not fond of post-blending because of potential segregation difficulties. What is a “structure” additive? CAB should give better flowout, and whatever acrylic resin you added may do the same. However, acrylics often cause subsequent problems if introduced into powder systems because they are not compatible with most epoxy and polyester powders and cause film blemishes. But I’ve never found them to produce black contamination. That may be a reaction among the nonresin powder components. Give me more detailed formulation information and I may be able to advise you further.