Zamak 5 parts do not pass humidity tests

A. That idea might work but only if it is done properly. Done improperly, the part might look good after plating but will fail the testing miserably. Bright acid copper, duplex nickel and tight control of the Cr deposit should be maintained. If you have a small pit or two caused by careless plating or incorrect process the coating will probably fail quickly.

I see you have threaded parts there. No matter what coating you try you might want to go thick. For example, if you try to hone the existing process you could perhaps go much thicker. If you go with Cu-Ni-Cr, a thick duplex Ni would be good.

Before you switch to Cu-Ni-Cr, try plating some pure zinc over that junk alloy to cover up the metals on the surface that do not "take" the chromate, THEN, apply a good zinc chromate over the pure zinc.