The structure, composition and reactivity of clean and ambient-exposed polycrystalline and monocrystalline Mg surfaces

The present article recounts results from studies on the structure, composition and reactivity of polycrystalline and single-crystal magnesium surfaces, previously exposed to the environment and regenerated to pristine conditions in ultrahigh vacuum. Experimental measurements relied on electron spectroscopy, temperature-programmed mass spectrometry and atomic force microscopy. Reaction-chemistry investigations were primarily with gas-phase reagents although selected treatments in aqueous or ethereal electrolyte were also undertaken. The results obtained have relevance in the description of the corrosion characteristics of this industrially important metal. The more significant observations: (i) Air-exposed Mg is invariably encrusted with a surface film composed predominantly of magnesium hydroxide but with a detectable quantity of magnesium bicarbonate. (ii) The exposure of a clean Mg(0001) surface to gradually increased dosages of O₂₍g₎ initially yielded a (1×1) oxygen layer that suffered surface disorder at the incipient stages of metal–oxide formation; eventually, an epitaxial MgO(100)-on-Mg(0001) adlattice was generated. (iii) Exposure of Mg(0001) to H₂O₍g₎, regardless of the dosage applied, resulted in a non-ordered hydroxylated surface. (iv) Upon exposure of a clean Mg(0001) surface to CO₂₍g₎, an ordered carbonaceous oxide film was initially found; at much higher dosages, the pseudomorphic adlayer was transformed into a totally disordered film. (v) Exposure of an oxide-coated Mg(0001) surface to H₂O₍g₎ yielded a disordered hydroxylated surface. (vi) With pertinence to Grignard chemistry, treatment of the oxided metal with either gaseous or (anhydrous) ethereal HCl enforced an acid–base reaction that led to a substantial reduction in the amount of surface oxide and the concomitant accumulation of a metal-chloride film.