You can safely apply another layer on Ni-P layer, for example, hard chrome. Such an application is typical when corrosion and wear protection are required at the same time (underlayer Ni-P protects against corrosion and top coat hard chrome - from wear).

However, a underlayer made of galvanic nickel is cheaper with better quality (hard chrome adheres somewhat better to galvanic nickel than to Ni-P).

You need to clean surface with a cathodic degreaser, then 20% HCl for 10 seconds and repeat 30 seconds in a cathodic degreaser. If you have a High Phos EN it works well, if it is Mid Phos you can risk to oxidize the NiP layer with lack of adhesion of the second layer. The alternative is to plate with a cyanurate alkaline copper bath and then coat with whatever you want.

Dear Eleonora,

If you mean that you want to have a clear steel surface without coating, the best option is grinding with emery paper with the following polishing.

If you mean you want to activate Ni-P layer, then try to clean it with absolute ethanol first and then anodically activate it in Ni-P bath for 20-30 sec.

Giacomo's idea with HCl should work too

Giacomo Bordiga thank you. I forgot to mention that an electroless method would be better. Also, we cannot use anything containing cyanide.

Vadim Verlotski chrome is a bit nasty to use. Also, I forgot to mention that an electroless method would be better. We cannot apply current to the item we want to coat.