Where does the voltage come from? Good question! My first thought was that it came from the ongoing galvanic corrosion. The voltage you got was because the dissimilar metals were forming a nice battery effect, and the effect became more pronounced as more charge carriers became available. But, I'm not sure of that. Wouldn't the electrical potential be there even if the solution had no charge carriers?
BTW: the chelating agent should trap the charge carriers. That's what it is for.
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