Wire EDM is literaly burning a path thru the metal. This gives you as wide an array of metal oxides as there are elements in the metal. Some oxides are notoriously hard to activate and may take a fluoride bearing etch to do it. A reverse in chromic acid is enough to etch the clean portion of the part but not etch the oxides enough to get adhesion. Most oxides are non conductors which makes the problem worse.
Machine, sand or blast that surface until it is clean, possibly smooth and it will probably plate.
This oxide layer is the “recast” layer or zone. The thickness and roughness is dependent on feeds and amperage. The harder you push it, the rougher and thicker it gets. We were having problems with vacuume furnace brazing on rough EDM parts. When we made it a lot thinner and smoother, the braze strength was actually higher than to the virgin metal. In plating, if you get it thin enough, it will be less of an insulator and just might plate and get some extra adhesion from the surface roughness. I would grit blast it first. Good Luck. Jim EDM Filter
