Corrosion Behavior of Mg-Al Alloys with Aluminum Thermal Spray Coatings in Humid and Saline Environments

The corrosion behavior of thermal spray aluminum coatings deposited on AZ31 (UNS M11311), AZ80 (UNS M11800), and AZ91D (UNS M11916) Mg-Al alloys was evaluated in salt water (3.5 wt% sodium chloride [NaCl]), neutral salt fog (ASTM B117), and high relative humidity (98% RH, 50°C) environments. The changes in the morphology and corrosion behavior of the aluminum coatings induced by cold-pressing posttreatment were also examined. In chloride-containing environments, the presence of interconnected pores in the as-sprayed aluminum coatings facilitated the galvanic acceleration of the corrosion of the magnesium substrates. The cold-pressed aluminum coatings revealed improved corrosion behavior associated with a lower level of porosity. In high-humidity environments, even though the corrosion rates for all the examined materials were lower than in the case of salt fog, similar corrosion features were observed; formation of hydromagnesite on the surface of untreated materials, galvanic corrosion in the Al-TS specimens, and almost negligible corrosion attack of the aluminum coating with formation of bayerite (b-Al2O3·3H2O) in the case of the cold-pressed specimens.