Effects of surface micro–galvanic corrosion and corrosive film on the corrosion resistance of AZ91–xNd alloys

Abstract

The effects of surface micro–galvanic corrosion and corrosive film on the corrosion resistance of AZ91–xNd alloys was evaluated by scanning electron microscopy (SEM), scanning Kelvin probe force microscopy (SKPFM), and electrochemical measurements. The microstructural morphologies showed that the formations of Al2Nd and Al–Mn–Nd phases were observed in AZ91–Nd alloy. According to the results of SKPFM, the Nd–rich phases were slightly noble than the β–Mg17Al12 phase but less than the Al–Mn phase. Consequently, AZ91–1.0Nd alloy with dispersed Nd–rich phases showed the largest polarization resistance values of 5129 Ω·cm2, about 3.9 times than that of AZ91 (1313 Ω·cm2). The main factors affecting the corrosion resistance of AZ91–xNd alloys were as follows: (i) the suppression of surface micro–galvanic couple corrosion, since the Volta potential difference (secondary phases/α–Mg) was reduced after Nd addition, the effect of surface micro–galvanic corrosion was weakened, thereby enhancing the corrosion resistance of the tested alloy (AZ91); (ii) corrosive film, the uniform corrosive film containing Nd2O3 isolated the specimens and the electrolyte, which suppressed the penetration of harmful anions (OH–, CO32-) and inhibited the occurrence of surface micro–galvanic corrosion of AZ91–Nd alloys.