The role of hydrogen evolution during corrosion of light alloys.Wednesday (06.11.2019) 11:05 - 11:25 Part of:
High performance aluminium and magnesium alloys are obtained by addition of alloying elements and thermomechanical processing, in order to generate a microstructure providing the desired mechanical properties. Such alloys, however, might suffer from increased corrosion susceptibility, since the microstructural heterogeneities introduce galvanic coupling at different length scales or locally reduce protective properties of the oxide/hydroxide film.
In most cases, corrosion takes place at specific locations, rather than homogeneously over the entire surface. Localized corrosion requires the presence of a region supporting the anodic reaction, a region supporting the cathodic reaction, sufficient electrolyte to ensure ions exchange between the two regions and a metallic conduction path for electrons.
Due to the requirement of charge neutrality, the rate of the anodic and of the cathodic reactions must be equal; consequently, the slower reaction determines the corrosion rate. The anodic reaction is the oxidation of metal, resulting in corrosion, but the cathodic reaction can be either reduction of molecular oxygen dissolved in the electrolyte, water reduction resulting in the formation of gaseous hydrogen or a combination of the two, depending on the environmental conditions. Hydrogen formation can be much faster than oxygen reduction and might be able to sustain high rates of metal oxidation on the anodic regions. For example, on AA2024T3, hydrogen bubbles have been observed to form in correspondence of major corrosion sites, and can be considered an indicator of the locations of the most severe attack. Furthermore, hydrogen often evolves abundantly from within crevices, resulting in accelerated attack.
This work discusses the details of the process of hydrogen evolution during corrosion of light alloys, by coupling real-time imaging with real-time hydrogen evolution measurement and electrochemical measurements. It is found that, when the naturally formed oxide film is disrupted and regardless of the conditions that induce such film disruption, hydrogen evolution is triggered. Such behaviour is common to aluminium and magnesium. The presence of noble alloying elements stimulates hydrogen evolution processes, and promotes their persistency. Unlike magnesium, on aluminium, hydrogen evolution can be triggered at cathodically active regions. The practical implications of the findings on light alloy corrosion are discussed.