Plating of chemicals means the accumulation of metals (in general) on the plates because of chemical reactions occuring due to the passage of electrons through the salt. D.C flows in one direction so if you use D.C for a longer time, more substance will accumulate on the plates. This porcess also inceases with the increase in current. If you use A.C, current will change direction continuously. One reaction due to the flow of electrons in one direction will be reversed by the flow of electrons in the opposite direction. So the net result will be no accumulation of substance on the plates and no change in the chemistry of the solution.