Hmmm, I guess your right, I never thought of it that way, probably because in electronic class, I was taught that AC = alternating current, which is + and - wave. (sine wave that is).
A constant wave of +5v for example, going from +5v to 0v, is just a pulse train. (A square wave).
In terms of preventing corrosion due to the galvanic process, you would need + and - voltage.
I guess I should brace myself, for the onslaught of replies now.
Who knows, maybe I will learn something.