My company produces and powder coats parts with an epoxy hybrid powder. These parts are used exclusively indoors. In an attempt to lower costs, I decided to test our parts without using phosphate. With our salt spray requirements being only 96 hours and 3mm infiltration from the scribe, we are far exceeding our requirements without the use of phosphate. What could be the long-term effects? Should we expect a gradual loss of adhesion over time? Or should our salt spay results indicate that long-term performance will not be compromised? Any advice you could give would be appreciated. M. J.
Salt spray testing is an accelerated test that is used to determine the corrosion resistance of a coated product. The trick has always been to select the correct amount of hours in the accelerated test to mimic the product’s life cycle in the fielded conditions. This is not as easy as it sounds. I have tried to determine product life versus salt spray hours for years, and at best it comes down to an educated guess. Using anecdotal information of similar life cycle and salt spray hours comparisons help. For instance, 5,000 salt spray hours can relate to 20 years of outdoor exposure, while less than 100 hours can relate to strictly indoor use in relatively benign conditions (with little, if any, moisture). For this reason, I must caution you to first verify that 96 hours of salt spray is equivalent to your product life in its fielded conditions.
Another condition for using iron phosphate on carbon steel products (beyond corrosion resistance) is increased adhesion. Iron phosphate creates a microcrystalline structure on the steel surface that looks like hills and valleys. It is said that iron phosphate actually increases the surface area (because of the hills and valleys) by more than 70%. More surface area equals more contact between the coating and the surface. This means increased adhesion of the coating to the surface. That means a carbon steel surface with an iron phosphate will have better adhesion to the coating than a similar surface without iron phosphate. This fact may change your mind on eliminating iron phosphate from your product’s surface, especially since the cost of the pretreatment is relatively low.