We present a new model for electrochemical proton-transfer reactions on non-catalytic metals, which is similar to the familiar Grotthus mechanism for proton transport in the bulk. The rate-determining step is assumed to be the transfer of a proton from a hydronium ion in the second water layer to a water molecule in the first layer, from where another proton is passed on to the metal surface. Monte Carlo simulations show that a considerable negative surface charge on the metal is required to obtain a configuration in which the proton can be transferred to the metal surface. The transfer rate is governed by the reorientation of the water molecule that accepts the proton from the hydronium ion.