Atmospheric corrosion of the MgAl alloy AM50 in rheocast and high pressure diecast conditions - Influence of Microstructure, CO2 and NaCl

Licentiate thesis, 2014

The NaCl-induced atmospheric corrosion of the high pressure diecast (HPDC) MgAl alloy AM50 was investigated. Commercially pure (CP) Mg was used as the reference. Two levels of salt; i.e. 14 and 70µg/cm² NaCl were added to the polished samples before exposure. Specimens without salt were used as references. The exposures were carried out in air with 0 or 400ppm CO2 to investigate the influence of carbon dioxide on corrosion. Gravimetric data was collected after the exposures. Alloy AM50 corroded less severely compared to CP Mg. The passive film and the corrosion products were analyzed qualitatively by means of X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Brucite was the main corrosion product in the absence of CO2. Three magnesium hydroxy carbonates were the major corrosion products in the presence of CO2. The microstructure of the as-cast material and the morphology of the corroded samples were studied by scanning electron microscopy (SEM) and the elemental composition of the different alloy constituents was investigated by energy dispersive X-ray spectroscopy (EDX). The corrosion behavior of HPDC AM50 was compared to that of the alloy manufactured by rheocasting (RC). The average corrosion rate was lower for the RC alloy compared to the HPDC form in the presence of CO2. Larger primary α-Mg grains were observed in the RC alloy. Also, the RC alloy had a larger area fraction of β particles compared to the HPDC alloy.