Can't remove blue-black from maraging steel for E-nickel plating

We have problem with heat treated maraging steel in pretreatment section of electroless nickel plating. because the blue-black layer does not disappear in any acidic and alkaline solutions . please help me

The blue black is normally an oxide layer. The darker it is, the thicker it is. Consider this layer of metal oxide or ceramic coating.
It probably will take a nitric and fluoride pickle to remove it.
Wet or dirty parts are contributing factors as well as heat treatment done in an air environment, too long a time and possibly too high a temperature all contribute to the problem.

It will take a vacuum furnace or a reducing atmosphere furnace to eliminate the causes (as well as totally clean parts).

Another option could be a sand blast if you have the means and the parts and cost allow it.

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