Pitting on electroless nickel

Q: My electroless nickel bath is pitting on my cast aluminum parts, how can I fix this? 

 A: Pitting on aluminum is caused by pores in the substrate or bad bath operation. Die cast aluminum, unlike machined aluminum, is extremely porous. These pores can be enlarged by an aggressive pre-plate process and look like pits when the nickel is applied. Minimizing the pore size in the pre-plate is important to the final aesthetic. However, even a good surface treatment can be ruined by a bad plating bath. Remember these three tips
to alleviate the issue:

1. Use an acidic etch. Alkaline etchants are more aggressive and more likely to attack the surface of the parts, eating through the smooth outer layer and exposing the rough underlayer.
2. Avoid rework at all cost. Sure, this is always the goal, but it is especially important with cast aluminum. Every time you expose cast aluminum to the aggressive pre-treatment you decrease the integrity of the outer layer and expose more pores from underneath.
3. Pay attention to the agitation of your plating process. Electroless nickel is a relatively inefficient process and produces a great deal of hydrogen gas. This gas is trapped within the interface geometry. Vibratory agitation is the best method to shake off all that hydrogen gas. If this isn’t possible use sufficient air agitation, making sure the air is evenly dispersed across the parts. If you use mechanical agitation ensure that the spargers are pointed in such a way as to continually circulate chemistry across the plated surface.