When a metal reacts with an acid, say HCl, it reacts with the H+ atoms of the acid to produce H2 gas and chloride ions. Since pH is the negative log of H+ concentration, then it would sensibly decrease because it was used up. The metal oxide would form a coating over the metal. However, as the reaction proceeds, the acid corrodes the metal and exposes a purer surface removing the coating. Thus increasing the concentration of H+.
