Initial stages of multi-phased aluminium alloys anodizing by MAO: micro-arc conditions and electrochemical behaviour

Abstract

The electrochemical behaviour of AA1050 and AA2214 alloys was studied in a KOH/silicate in order to investigate the first stages of the coating formation during MAO process (by SEM and electrochemical measurements). Before the sparking initiation, occurring around 300 V for both alloys, a thin inner layer of aluminium oxide grows by a classical anodizing mechanism, and induces a great enhancement of the electrochemical resistance. Then, the micro-arc regime beyond 300 V leads to the formation of a dense, thicker but cracked external aluminium oxide layer. Afterwards, from 400 V, a sharp increase of current density named “current wall” and due to an increase of oxide conductivity, leads to a significant oxygen release and the occurrence of large and energetic sparks. Microscopic analyses reveal that the presence of copper-rich intermetallic phases in the multi-phased alloy (AA2214) induces the incorporation of copper nanoparticles in the γ-alumina layer formed by the sparking phenomenon. These copper particles increase the reaction of water oxidation during the classical anodizing stage, and result in a delay of the sparking initiation during the galvanostatic anodizing and in a lower electrochemical resistance of the anodized layers.