Acid resistant brass screws

I tried to turn some brass wood screws a copper colour by putting in a weak acid solution (to eat the zink away) in order to match a copper alloy casting. The weak acid made no progress at all so I upped the strength and left them in over night. Next morning still bright brass colour, so went over them with 000 steel wool in case they had some sort of protective coating and repeated the process --- still nothing. In desperation I took to them with Hcl acid and still nothing. The screw are very old perhaps 40 to 50 years, but I have never come across any brass before that is so acid resistant.

Has anyone ever encounted this type of resistance? Normally you only have to show acid to brass and it turns red in fright.

Bruce


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Acid resistant brass screws