Coating copper alloy (Bronze or Brass) can be used for breathing Oxygen
When I worked for a rocket company, we routinely piped LOX and hi-pressure gas oxygen.  We avoided any kind of plating at all costs - the risk is that a flake of plating could dislodge, be carried thru a sonic orifice and impinge on a downstream surface at high velocity -- the resulting impact then sparking a fire.
I seem to remember that Monel was recommended as a Cu alloy for LOX.  There is a NASA-GSFC or possibly -MSFC document somewhere with detailed recommendations for oxygen piping.
I managed to find MIL-DTL-2/41 specifically dealing with Oxygen valves for medical use. The requirement there is that the Hydraulic valve should be chromium plated.

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