Iron Phosphate—Rinsing Required?


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Q. My question is must iron phosphate be rinsed before painting. What problems might I have if not rinsed? R.K.


A. It is generally considered good practice to rinse residual chemicals from a conventional iron phosphate bath from your substrate prior to painting. The chemicals are acidic (typically in a pH range of 4–5), so they could interfere with some paint systems. Additionally, if operated in a lower range, it could cause some amount of flash rust before the parts are even coated.

That said, some paint chemistries tend to tolerate a slightly acidic surface for painting. That is why most steps in a five-stage iron phosphate are a slightly acidic final seal rinse. This is typically much less acidic (typically pH ~6.0) than the iron phosphate stage and contains much less dissolved solids (i.e. the phosphating salts in the tank). The residual salts could also cause you problems with paint adhesion.

Due to environmental concerns, manufacturing footprint or other reasons, rinsing following the iron phosphate stage may be impossible. If that is the case, you should approach the situation cautiously. I would suggest first documenting the performance of your current system, prior to making any changes. This would include paint tests, particularly adhesion and salt spray testing. You could then perform an initial trial to determine what problems you may encounter when going with a no-rinse system. I would suggest running the phosphating tank on the dilute end of your supplier’s recommended control range. The pH of this system will be slightly higher and will minimize the amount of residue left on the surface. Again, you would need to follow up with paint testing. I would expect adhesion and salt spray to be the two test most affected by this change in your process.

If this arrangement does not appear to give you the results desired or undermines the quality of your final product, you should talk to your supplier or others regarding no-rinse phosphating systems. There are some that are formulated to be used with no rinse following a cleaning and coating step. Although advantageous in economizing your process, these pretreatments typically do not provide the performance that a multi-stage system can give you. The PRODUCTS FINISHING web site (www.pfonline.com) can help you find a supplier if your current one cannot provide the product you want. Follow the Supplier link to get to “Cleaning and Pretreatment.”