In taking some surface potential measurements of odds and ends, I noticed that the surface potential of some allegedly ENIG PCBs I have show values corresponding to brass (or thereabouts). My values for SS from the tableware, copper from wire, and lead from fishing tackle show sensible values. But I still suspect something wrong in my procedure or assumptions.
Since you folks are very meticulous, you probably know why I'm getting the brassy value where I expect a goldy value. Would you please explain?
My procedure: Mix table salt with tapwater to make brine, any proportion. Place two metals into a cup of this brine, not touching each other, and with some portion of each sticking out of the water. Connect the red test lead of a voltmeter to one metal. Connect the black to the other metal. The measured value is the difference is surface potential. Look up the measured value on a galvanic series and you can make a good guess about what the metals are. If you know one of them, you can see from the series what the other is, with high certainty. The metal must not have any oxidation or crud on it; use chemical or mechanical aid to clean, but if you do, immerse and wait for the value to settle (removing some of the surface will cause the surface potential to temporarily climb beyond the metal's natural value). Don't abrade ENIG: you will get copper!