Powder Coating Over Finished Chrome Parts

Question:

We have a four-stage washer with a first-stage iron phosphate wash heated to about 130°F, two freshwater rinses, and a final seal rinse. Sometimes we powder coat over new parts that are finished in chrome.

Is there something special we should do to prep these parts before we coat them, should the chrome finish be removed, if so what is the best way to do so? Normally we wash them and blow off the water before our dry off oven to prevent any water spots, which show through as defects and we have to re-coat them.

We have tried sandblasting the parts to give some type of profile for better powder adhesion. Is this worth it? None of our customers have called to say the powder is falling off or chipping, but we would like to make sure we give the best possible finish we can. 

Answer:

I guess the first question that comes into my mind is, why are you painting over parts that have a chrome finish (I assume chrome plating). The cost of chrome plating something is not cheap, so I think you first need to understand exactly what the finish is and why your customer is getting these parts plated to begin with. Not everything that looks like “chrome” is chromium plated. For instance, many parts that are nickel plated have a microdiscontinuous (or microcracked) chromium coating over them. But the bright, shiny part that most people call chrome is the nickel underneath. A straight chromium plating is generally duller and is used for abrasion resistance (sometimes with tooling).

The next step is to find out why these chrome plated parts are being painted. Are they attempting to provide an extremely long functional corrosion life? I can’t think of any other reason for taking such a “belt and suspenders” approach to specifying a finish like this in their design. You will need to dig a little deeper than your customer’s purchasing agent in order to find out what they really intend for this part. Are they after abrasion resistance, aesthetics, corrosion resistance or something else? In the case of the first two reasons, you would be defeating the purpose of the plating by painting over it. Again, I would encourage you to go back to your customer to find out their final expectation for the parts that are plated. You may have uncovered a problem in their system that they have yet to figure out. This could be an opportunity for you to gain some recognition with them for not accepting the status quo and adding value to their business. Anyway, to answer your original question, I am not sure that I would start to add additional steps to your process that are not necessary (or not paid for by the customer) unless you find them necessary. For the reasons mentioned above, you generally do not hear about painting over chrome plating, so I cannot comment directly on that. However, other types of plating, such as zinc, are often used for fasteners and can be painted over fairly easily.

I would suggest that before you invest much more time into blasting the customer’s pieces that you benchmark all aspects of your current process to find out where you are today. There are some relatively simple tests all paint shops should be capable of performing such as adhesion, hardness, and a simple solvent rub for cure. The adhesion test you would be interested in is ASTM D 3359, Standard Method for Measuring Adhesion by Tape Test. This standard can be obtained by contacting ASTM through their web site at www.astm.org. You would use a straight edge knife with a ruler or a cross-cut tool to make a right angle cross-hatch pattern in the paint. Then apply the tape to the surface and pull off. The painted surface can then be compared to standards shown in the specification to evaluate the quality of the paint adhesion. I would again work with your customer to let them know you intend to do this on one or more parts (possibly at the beginning and end of a production run). They should again see this extra effort you put forth as a sign of a good supplier that will increase your value as a high-quality supplier.