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.

Passivation and potential fluctuation of Mg alloy AZ31B in alkaline environments

“…The corrosion product layer on top of the intermetallics is likely Mg(OH) 2 ( Table 5). The formation of similar corrosion product domes has been observed on Mg-Al alloys exposed to NaCl solutions, and was related to local cathodic activities [18,47]. Fig.…”

“…Table 4 suggests that Al was completely dissolved from the Al-Mn intermetallics and there is significant reduction of Mn and Fe content. The dissolution of Al-Mn intermetallics may be explained by selective leaching of various element(s) from the particles [18,51]. According to the equilibrium E-pH diagrams [22], Al exhibits the strongest amphoteric characteristics among all the major alloying elements (Zn, Mn and Fe) and higher Al content possibly leads to more severe dealloying ( Table 2).…”

“…Besides Mg x Al y phase, the Al x Mn y intermetallics are another phase that affects the corrosion of Mg-Al alloys. The addition of Mn to the Mg alloys can limit the distribution of the detrimental elements Fe and Cu and therefore improve overall corrosion performance [17,18]. However, the formation of Al x Mn y intermetallics might cause micro-galvanic corrosion of the α-Mg matrix by acting as strong cathodes, especially when they are contaminated with Fe [19].…”

“…However, the formation of Al x Mn y intermetallics might cause micro-galvanic corrosion of the α-Mg matrix by acting as strong cathodes, especially when they are contaminated with Fe [19]. Cathodic reaction (i.e., hydrogen evolution reaction) occurring on these particles can significantly increase the local pH and alter the particle morphology at the interface [18,20]. The corrosion behavior of eutectic α is somewhat controversial for Mg-Al alloys [21].…”

Efecto de las nanopartículas de ZrO₂ y L-Cys como agentes dopantes de recubrimientos sol-gel de sílice mesoporosa para la protección anticorrosiva de la aleación de magnesio AZ61

“…Manganese is one of the common elements added to most commercial magnesium alloys, essential for enhancing the corrosion resistance and these intermetallic particles are usually present in most of commercial Mg-Al based alloys (Han and Liu, 2016). The simulation of the micro-galvanic cathodic behavior of Al-Mn intermetallic particles has confirmed their cathodic behavior and the deposition of the corrosion products occurres in the vicinity with the active anodic α-Mg grains, in the presence of chloride ions (Asmussen et al, 2016). This fact explains the persistence of these particles on the alloy surface after the removal of the corrosion layer.…”