Batch Application of Phosphate Coatings

What's involved with applying a phosphate conversion coating for corrosion resistance?

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Q. We run a batch powder coating operation using a blast cleaning operation to remove dirt, oil and old paint from parts before we coat them. Some of our customers have asked us to apply a conversion coating for corrosion resistance. How complicated is it to apply a phosphate coating? What coating options do we have with a batch operation? Can it be done manually or do I need a spray washer? Does a phosphate coating really help prevent corrosion?

A: Conversion coatings create a surface condition that helps promote good adhesion and retard the spread of corrosion. Iron phosphate coatings can be applied by dip or a spray system with a hand-held wand, a batch washer or in-line spray washer. It is somewhat more complicated than a manual blast operation because you must control the chemical solutions. You also need to invest in tanks or a spray wand. It works very well on light gauge parts that need to be cleaned and degreased. Some light gauge parts cannot take the pressure of a blast operation. In general, application of a conversion coating is not that complicated.

Treatment options are somewhat more limited for hand wand systems than in-line washers. Spray wands will usually provide one to three steps of treatment using iron phosphate. In-line washers can go up to 9 steps and include some much better conversion coatings like zirconium oxide or zinc phosphate.

Iron phosphate helps promote coating adhesion but does very little to inhibit corrosion. Some non-chrome seal rinse products can be very helpful to enhance corrosion performance with an iron phosphate.  

For more demanding corrosion resistance requirements, you need zirconium, zinc phosphate or chrome, processes that would be much more costly and complicated—not to mention the environmental concerns associated with these materials. If the powder is applied properly with sufficient thickness and complete coverage, the adhesion over the blasted surface is very good and the corrosion protection from the powder film is very good. If a customer insists that you add corrosion protection with a conversion coating, I suggest you look for a source that can provide iron phosphating for lighter duty applications or zinc phosphating and have them clean and phosphate it for you. Bring it back to your shop and coat it as soon as possible so that it does not have a chance to pick up any moisture or contamination.  If you decide to treat it with a wand system and you need more corrosion protection than what you can get from the iron phosphate, you can talk to a chemical supplier about specialty products that could be applied from a pressure bottle. A trailer manufacturer I once worked with used up to nine steps in manually treating his steel before powder coating.

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