Steel Pretreatment Prior to Powder Coat

Question:

What pretreatments would you suggest for hot-rolled, cold-rolled, and satin finish steel prior to powder coating? S.B.

Answer:

The answer is partially dependent on the condition of the incoming material and the requirements of the final product. I am also not sure why you are trying to treat such a variety of substrates. Is it possible to make them all from cold-rolled steel?

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In general, cleaning hot-rolled material can be much more difficult and is subject to much more variability as it comes in the door. It may require a good acid or electrolytic cleaning for the removal of mill scale and/or smut prior to a conventional pretreatment and paint line. This may cost more, but could be offset by changing the satin finish steel to cold-rolled. I am not sure of the benefit of a satin finish when it will be painted.

The typical pretreatment for steel prior to powder coating would be some sort of phosphating. The type and size of the phosphate coating depends on floor space, available capital, current and potential future customer requirements, etc. The minimum you would want to do would be a three-stage phosphate system where the first tank is a cleaner/coater. It contains surfactants to degrease the material, and when clean, an iron phosphate coating is deposited. This system would not likely be capable of reliably coating the hot-rolled steel. In general, the steel cannot be very dirty as it enters this stage, since the first stage is meant to do two steps. It would be followed by rinse and seal steps.
The more common phosphate line is a five-stage system: clean, rinse, iron phosphate, rinse and seal, providing fairly good paint adhesion and moderate corrosion resistance.

To obtain increased corrosion resistance, a zinc phosphate line would be necessary. Although there are variations, it typically consists of the following steps: clean, rinse, activating rinse, zinc phosphate, rinse, and seal. Some seals also may require one or more DI water rinses afterwards.