New Method for Monitoring Hydrogen Evolution of Magnesium Alloys Under Atmospheric Exposure

Abstract

Magnesium (Mg) alloys are attractive lightweight materials for the automotive industry due to their low density and high specific strength [1]. However, widespread use of Mg alloy components is hindered by their corrosion properties. Full immersion conditions are frequently used to study the corrosion behavior, although Mg alloys are typically subjected to atmospheric corrosion environments in most applications. With a limited amount of electrolyte, the corrosion mechanism can be different [2]. For example, corrosion products that partly dissolve in a bulk electrolyte can remain at the surface and block active sites in the case of atmospheric conditions [3]. Effects of micro-galvanic coupling of secondary phases can be reduced with thin electrolyte films [4]. In addition, atmospheric concentrations of CO2 can influence the surface pH and change the composition of the corrosion products [3]. As a result, corrosion rates in atmospheric environments are smaller and cannot be directly compared to results obtained under immersion conditions. The atmospheric corrosion progress is traditionally tracked gravimetrically by determining mass gain or mass loss of the samples after exposure. This work introduces a new method that makes it possible to monitor the real time atmospheric corrosion rate of Mg alloys during exposure. The idea is to measure the amount of hydrogen evolved during atmospheric corrosion. This is realized with an adaption of the gravimetric hydrogen collection method, originally developed by Curioni [5]. The change in buoyancy exerted by the evolved hydrogen gas can be accurately measured. Therefore, it is possible to monitor the instant atmospheric corrosion rate even at the initial stages of corrosion where mass loss measurements are difficult. Different exposure conditions like relative humidity, temperature or CO2 content can be studied. The possible contribution of the oxygen reduction reaction to the cathodic processes will be discussed.