Influence of pH and chloride ion concentration on the corrosion of Mg alloy ZE41

The influence of pH and chloride ion concentration on the corrosion behaviour of ZE41 was studied using immersion tests and electrochemical measurements. A shorter incubation period to the onset of corrosion; a more negative corrosion potential; and a higher corrosion rate correlated with a higher chloride ion concentration at each pH value and correlated with a lower pH value for each chloride ion concentration. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by a partially protective surface film, with the corrosion reactions occurring predominantly at the breaks or imperfections of the partially protective film. The implication is that the fraction of film free surface increases with decreasing bulk pH and with increasing chloride ion concentration. This is consistent with the known tendency of chloride ions to cause film breakdown and the known instability of Mg(OH)2 in solutions with pH less than 10.5. The electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.