A first quantitative XPS study of the surface films formed, by exposure to water, on Mg and on the Mg–Al intermetallics: Al3Mg2 and Mg17Al12

An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water, on mechanically ground Mg and the two Mg–Al intermetallic compounds: Al3Mg2 and Mg17Al12. The mechanically ground Mg surface had a film of MgO at the Mg metal surface covered by a Mg(OH)2 layer, formed by the reaction of the MgO with water vapour in the air. Upon immersion in ultrapure water, this film converted to a duplex film with an inner MgO layer next to the Mg metal and an external porous hydroxide layer. For both intermetallics, the XPS data is consistent with (i) preferential dissolution of Mg and (ii) a 10 nm thick film on the surface after immersion in ultrapure water; the film composition on Al3Mg2 was AlMg1.4O0.2(OH)5.4AlMg1.4O0.2(OH)5.4 whilst on Mg17Al12 the composition was AlMg2.5(OH)8AlMg2.5(OH)8.