Adhesion to Electroless Nickel

Q. I am a materials and process engineer for a company producing aerospace products. We are experiencing paint-adhesion issues on some of our parts plated with electroless, high-phosphorus nickel. Our plating process requires greater than 10.6 percent phosphorous. We know that the higher the phosphorus content, the more amorphous the surface, which could lead to reduced paint adhesion. Is there a process to improve paint adhesion to a high-phosphate, nickel-plated surface? Our current process involves machining aluminum prior to nickel plating and then applying a DOD-P-15328 wash primer, MIL-23377 primer and MIL-22750 topcoat.    C.G.

A. DOD-P-15328 is a primer that’s also listed as a pretreatment. In fact, its primary use is as a metal pretreatment instead of phosphatizing and chromating metals to enhance corrosion resistance. It must be applied directly to bare metal, where it reacts with the surface. It will not react with nor form a good bond to pretreated metal surfaces. The presence of the relatively great amount of phosphorous on the surface of the greater-than-10.6-percent phosphorous-nickel plating acts as a phosphate pretreatment. To solve the problem, you could apply the Mil-P-23377 directly to the EN surface, where it will have better adhesion than the wash primer. However, remember that, if this is a DOD job, you must get permission before making any changes in the finishing process.