Cleaning Q&A: Improving Corrosion Protection of Zinc Phosphate

Q. How can I make my zinc phosphate conversion coating process meet a 48-hour salt spray requirement?

A. You do not mention if the zinc phosphate coating you are applying is considered a “heavy” phosphate that will be coated with oil for additional corrosion protection or a traditional phosphate that will be painted. In general, all phosphating processes can be applied with a fairly wide range of coating weights. With all other factors constant, a higher coating weight will produce a greater corrosion resistance than a lower one.

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The heavy zinc phosphating process can be applied in the range of 1,000–2,000 mg/ft2. This will typically produce a large surface crystal structure that is a good base for retention of oil as a rust preventative. A heavy zinc phosphate with oil will easily satisfy your 48-hour salt spray requirement.

A traditional zinc phosphate is typically applied in the range of 100–500 mg/ft2, but is often modified with an additive to produce a microcrystalline structure. Additional steps for this type of pretreatment can be an “activating” rinse that will nucleate a higher number of crystal sites and a sealing rinse that will provide additional corrosion protection. This type of zinc phosphate is rarely used alone (without the addition of a paint topcoat). However, if it is done as part of a quality control regimen, then the microcrystalline zinc phosphate at the higher end of the coating range along with a seal rinse should be capable of meeting a 48- to 72-hour salt spray requirement.

Originally published in the October 2015 issue.