I don't believe it is a common practice to apply chromium plating to a duplex stainless steel. Hard chromium is often plated on carbon steels to make them hard and corrosion resistant where excellent wear characteristics are needed. Examples include piston rings, shock absorbers, brake pistons, engine valve stems, aircraft landing gears, dies, and molds. Duplex stainless steels give improved corrosion resistance in chloride containing environments when compared to austenitic stainless steels. From a corrosion resistance standpoint, I am unsure whether the plating is necessary or will improve the corrosion resistance for your particular application.