Changing from Zinc Phosphate to Iron Phosphate
David S. Peterson
We have been coating for the last 12 years (supply to a major automotive manufacturer). Since then we have been zinc phosphating the steel products. Would a shift to iron phosphate warrant the same qualities in withstanding salt spray tests?
ron phosphate can compete with zinc phosphate in most performance categories except for corrosion resistance. The iron phosphate provides a very good paint base on steel which results in very good adhesion properties and generally acceptable salt spray resistance. The zinc phosphate provides superior corrosion resistance (therefore better salt spray resistance). By nature of the process, it is usually applied at a significantly higher coating weight than the iron phosphate. This increased coating weight cannot be achieved with an iron phosphate and would make it more difficult to get to the base steel to start corrosion when compared to the iron phosphate. Additionally, the zinc is sacrificial to the steel.
However, if your current process significantly exceeds the customer requirements for salt spray and corrosion resistance using the zinc phosphate, it may be possible to convert to the iron phosphate and still pass all applicable specifications. This could be a cost savings to your bottom line since the iron phosphate generally requires fewer process steps, requires less maintenance and does not generate the amount of sludge that the zinc phosphate process does.
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