Cleaning Q&A: Improving Corrosion Protection of Zinc Phosphate
How do I meet a 48-hour salt spray requirement?
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.
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.
The processes, chemicals and equipment, plus control and troubleshooting.
The German Association of the Automotive Industry’s VDA Volume 19 is the first comprehensive standardization document for characterizing the cleanliness of products within the automotive industry’s quality chain.
Specific questions about zinc phosphate and pretreatment are answered in one article...