The potentiodynamic polarisation behaviour of a high-alloy Cr-Mn-Ni steel in its as-cast state was investigated in a 0.1 M H2SO4 solution by means of acoustic emission (AE) testing. Simultaneously, the evolution of hydrogen and oxygen gas bubbles was observed by long-distance microscopy. Three regions of acoustic activity were identified during the polarisation process from the cathodic range up to transpassivity. Bubble release at the pores and gas-assisted exfoliation of the passive oxide were identified as the AE sources. An evaluation of the gas stream revealed two types of oxygen bubbles that differed significantly in size. The effect was attributed to different points of origin at the electrode. (C) 2017 The Authors. Published by Elsevier Ltd.