Oxidation Under Paint

Question:

We clean our steel parts and use a phosphating process for pretreatment. After this we use chemical to neutralize this. Some weeks later we can find that there is oxidation below the paint and peels off the paint. H.D

Answer:

Without knowing more about the paint, I can’t comment on that, but you would expect even a relatively poor paint with moderate storage conditions to perform better than this. I would first focus on the chemical neutralization step that you mention.

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Overall, most phosphating processes start with a cleaning step. That is necessary to remove grease, oils, particulate, and anything else that could prevent the effective formation of a good phosphate coating. Adequate rinsing should follow this step (assuming it is done in an aqueous solution).

Next would be the iron phosphating process step. A zinc phosphate system may have a nucleating rinse bath prior to the phosphating that provides more sites to initiate the phosphating process (providing a smaller and a more dense phosphate coating). Again, following that would be adequate rinsing.

Most phosphating systems (iron or zinc) have a final seal process that is mildly acidic. The rinse(s) following the phosphate step should be adequate to halt the phosphate reaction. There should not be a need for a chemical neutralization step. Depending on the chemical, pH, time and temperature, it is possible that the neutralization step could be doing harm to the phosphate coating. Even if it is not doing harm to the coating, it could be leaving an alkaline residue that is not compatible with most paints. If anything, the surface of the part should be slightly acidic to be more compatible with the paint application.
Presumably you have a chemical supplier for this process line. I would ask them for technical support and recommendations looking especially at the final neutralization step you are performing.