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.