Formation of manganese-rich layers during anodizing of Al–Mn alloys

Abstract

The behaviour of manganese during formation of barrier-type anodic films on non-equilibrium Al–2.5 at.% Mn and Al–16 at.% Mn alloys in ammonium pentaborate electrolyte (pH 8.2), and, for the latter alloy, also in sodium hydroxide (pH 12.0) and sulphuric acid (pH 0.2) electrolytes, has been investigated by transmission electron microscopy, Rutherford backscattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS). For both alloys, a manganese-free, amorphous alumina film develops initially, as manganese is enriched in an alloy layer, of about 2 nm thickness, located immediately beneath the growing film. At a critical level of enrichment, oxidation of manganese commences and the alumina film thereafter contains manganese species, while the enrichment of the alloy remains approximately constant. For the Al–2.5 at.% Mn alloy, the critical enrichment corresponds to approximately 2.6×10 19 Mn atoms m −2, equivalent to about 22 at.% Mn in the enriched alloy layer; a similar level of enrichment is probably attained for the Al–16 at.% Mn alloy, although this could not be quantified by the RBS analyses. The manganese species migrate outward in the anodic films about three times faster than Al 3+ ions and eventually reach the film surface where a thin manganese-rich layer forms during anodizing in pentaborate and hydroxide electrolytes, but not in sulphuric acid electrolyte due to enhanced loss of manganese species to the solution. At high pH, the manganese-rich surface layer and, at low pH, the underlying manganese-containing film material, are of significantly reduced reactivity compared with anodic alumina; consequently, barrier-type anodic films can be grown in sodium hydroxide and sulphuric acid electrolytes under conditions that lead to porous films on aluminium. XPS reveals mainly Mn 3+ species in the near surface regions of all films, although there is evidence of Mn 2+ species in the underlying film material.