Bonding to electroless nickel

Unfortunately, I have never been asked to paint over electroless nickel. However, when I have questions relating to paint adhesion, I have found the tech service department of our paint vendor very helpful. You may want to contact a large paint corporation that staff these type of experts, such as Dupont or Sherwin Williams. I have personally used Dupont for these resources.

Unfortunately electroless nickel surfaces can be as diverse as the adhesive or coating that is intended to be bonded to it.

For instance has the nickel coating been baked.  Heat treatment changes the nickel structure to form nickel phosphite which gives the coating its hardness and desired engineering properties.  In the process the coating will shrink by around 5% which causes internal stress and therefore cracking.  The cracking will inprove the mechincal bond to anything bonded to it.

In addition the heat treatment will promote the formation of surface oxides will reduce the adhesion.  Adhesion can be improved in this instance by the use of a chloride based cleaner or hydrochloric acid prior to bonding.

As Skelton points out the phosphorous content of the nickel coating also plays a part.  Lower phosphorous coatings tend to give better adhesion to their substrate due to intermetalic alloys formed at the interface, but afford lower adhesion to any non metallic coating applied to them (higher phosphate coatings are more crystalline and tend to have a rougher surface)

Unless the electroless nickel plating process is well controlled two coatings from the same supplier are unlikely to be identical as, nickel content, pH, nickel : hypophophite ratio and themerature all affect the phosphorous content in the coating.

Unfortunately this does not go very far towards answering your questions but it would be wrong to assume that all electroless nickel coatings are the same, so finding hard and fast adhesion figures may be difficult.

An alternative approach to improving the bonding onto an EN coating would be to make it a composite EN coating.  Including fine particles within the EN would provide a uniform array of particles protruding from the surface of the coating, and therefore dramatically increasing the amount of surface area onto which subsequent materials can be bonded.  Let me know if you would like further guidance on details of possible composite options to serve this purpose.