XRD is usually done with powders....Powder diffraction? You are going to have to powder the material. The powder must be homogeneous.
With XRF you will not have to powder the steel.
If the electroplating is part of the sample matrix....part of the final product...then the electroplating must be done prior to the powdering and XRD analysis. In XRD you must have a representative sample....it must be homogeneous. The prep has to be done in such a way that the tru directionality of the material is preserved. It cannot be biased directionally.
You cn lathe, sand, grind, crush, sieve, ....there are so many techniques