An in-situ method to check phosphate conversion coating underneath fluorocarbon-coated bolts and nuts

Recently, I found that the fluorocarbon-coated bolts and nuts in my store are already rusted before I can use them. I suspect that these bolts and nuts do not have a phosphate layer underneath. How can I check the bolts and nuts are really fluorocarbon coated and also the presence of the phosphate underneath? Is there any simple QC test method that can be used on site to conduct the checks? TQ.

The important characteristic property of zinc phosphate coating used as a pretreatment before the application of paint or other types of organic coatings is its ability to prevent underfilm corrosion. Hence, in order to check your bolts and nuts for the presence of phosphate pretreatment, you can deliberately create a damage before subjecting them to salt spray test. If a phosphate treatment is applied on them, then it would resist underfilm corrosion.

Sankara Narayanan.