Corrosion behaviour of Mg–Al alloys with Al thermal spray coatings in humid and saline environments

Abstract

The corrosion behaviour of thermal spray Al coatings deposited on AZ31, AZ80 and AZ91D Mg–Al alloys was evaluated in salt water (3.5 wt.% NaCl), neutral salt fog (ASTM B 117) and high relative humidity (98% RH, 50 ºC) environments. The changes in the morphology and corrosion behaviour of the Al coatings induced by cold–pressing post–treatment were also examined. In chloride–containing environments, the presence of interconnected pores in the as–sprayed Al coatings facilitated the galvanic acceleration of the corrosion of the magnesium substrates. The cold–pressed Al coatings revealed improved corrosion behaviour associated with a lower level of porosity. In high humidity environment, 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 Al coating with formation of bayerite (β–Al2O3·3H2O) in the case of the cold–pressed specimens