Acid- and Alkali-Resistant Film Generation on an Al-Mn-Ce Alloy

Abstract

The growth of anodic films on non-equilibrium Al-16.2 at.% Mn-2.7 at. Ce alloy has been examined in ammonium pentaborate, sodium hydroxide and sulphuric acid electrolytes, providing a pH range from 0.2 to 12. Barrier-type anodic films are developed during anodizing to high voltages, the film growth resulting in formation of a main film material, comprising most of the film thickness, composed of units of Al 2 O 3 , MnO and CeO 2 . The Mn 2+ and Ce 4+ ions migrate outward faster than Al 3+ ions, enabling formation of a thin, manganese-rich and cerium-rich, outermost film layer during anodizing in alkaline conditions. This layer sustains barrier film growth to high pH by allowing ingress of O 2- but preventing ejection of Al 3+ ions to the electrolyte. In sulphuric acid electrolyte, the outer film layer does not form. Furthermore, the outer 50% of the film contains a reduced amount of cerium, possibly due to transient porosity. However, the film remains relatively resistant to the acid electrolyte, thus hindering the transition to growth of porous film material.