Nickel, zinc, epoxy and parylene, and what each one survives. Coating decides corrosion life, but it also changes the fit and the air gap, so it belongs in the first design pass.
Neodymium Magnet Coating: How to Choose the Right Plating
Why sintered NdFeB needs plating, how zinc, nickel, epoxy, gold, Parylene and Dacromet compare, and how to pick one for humidity, salt spray and heat.