Prepaint Treatments

Question:

We are currently plating zinc and yellow chromate for a customer. Our customer, because of shop soils and fingerprints, precleans all the zinc/yellow chromated parts we have plated prior to painting. Their cleaning process includes:

1. Spray cleaner station (152F, pH 10.6)
2. Spray rinse station
3. Spray iron phosphate station (132F, pH 4.9)
4. Spray rinse station
5. Spray sealant station

The customer is seeing slough-off of the yellow chromate after the spray cleaner stage but only in sporadic spots. We have tested the surface and no chromate remains. If the parts are baked at 250F prior to cleaning, they are fine. This tells me that the chromate film is not fully cured. We have addressed that by setting a 48 hr cure before shipping and made new chromates, but the spotting keeps returning. I would like to get your opinion on the customer’s pretreatment process on our plating. From all my inquiries, this is not a normal process to be done, and I’ve never heard of it being done. All our customers that paint parts that have been zinc plated and yellow chromated, paint without cleaning. Seems to me that our customer set up this line to pretreat steel prior to painting and that it was convenient to run zinc plated parts through there also. I would appreciate any help you can give us. Thank you. K.H.

Answer:

I agree with your assessment of the situation—he finds it convenient to run the parts through his phosphate pretreatment in an attempt to remove shop and finger oils. Although it may be well intentioned, it is causing more harm than good. I would never process pretreated parts through another pretreatment because of the effect of the process chemicals on the pretreatment. It’s no wonder that you and your other customers have never heard of it being done.

I have no problem with cleaning parts that have been handled before painting. Your customer is also aware that he must remove oily soils before he paints. However, this could be done by degreasing using solvents, detergent solutions and even the alkaline cleaning and first rinse stage of his pretreatment system.