The stability of aluminum‐manganese intermetallic phases under the microgalvanic coupling conditions anticipated in magnesium alloys

Abstract

The electrochemical behaviour of two Al‐Mn materials (Al‐ 5.5 at % Mn and Al‐ 13.5 at % Mn) has been studied in 0.275 M NaCl and 0.138 M MgCl2 solutions to simulate the cathodic environment of Al‐Mn particles during the corrosion of a Mg alloy. Upon polarization in NaCl solution to a potential in the range expected on a corroding Mg alloy, the Al‐5.5 at % Mn alloy proved unstable undergoing de‐alloying (loss of Al) and delamination of layers of the Al(OH)3 formed. This leads to a steady increase H2O reduction current. When polarized in MgCl2 solution the surface was partially protected from de‐alloying and the current for H2O reduction suppressed by the deposition of Mg(OH)2. The Al‐13.5 at % Mn alloy was considerably more stable when cathodically polarized. This increased stability was attributed to the higher density of Mn‐enriched areas in the alloy surface. This simulation of the microgalvanic cathodic behaviour of Al‐Mn intermetallic particles confirms that the appearance of corrosion product domes on the Al‐Mn intermetallic particles during the corrosion of Mg alloys as an indication of their cathodic behaviour and that Al‐Mn intermetallic particles are efficient, yet unstable cathodes.