Quantitative analysis of indium and iron at the surface of a commercial Al-Zn-In sacrificial anode

X-ray photoelectron spectroscopy (XPS) was employed to analyse the surface of commercial Al-Zn-In sacrificial anodes in order to quantify the concentrations of indium and iron. Analysis was tarried out on the as-cast and cut surfaces prior to corrosion testing, followed by short-term 5 h corrosion tests in a 3.5 wt,% NaCl solution in order to determine the difference, if any, in the activity of the as-cast and cut surfaces, One year corrosion tests were also carried out to monitor the changes at the surface in [In] and [Fe], and how these changes were related to anode performance, Indium was found at a concentration of similar to 1 wt,% at the as-cast surface compared to a concentration of 0.02 wt,% in the bulk, defined by XPS analysis and optical emission spectroscopy, This surface enrichment was ascribed to inverse segregation, The as-cast surface was activated spontaneously compared to the cut surface. It is proposed that this behaviour was due to higher [In] and [Fe] at the surface, it is believed that the enrichment of indium and iron is situated at the metal/oxide interface, which leads to disruption of the oxide film and results in a more active surface, Indium enriches to a maximum concentration of 4 wt,% at the surface when the anodes are corroded over 1 year, The enrichment supports the indium dissolution-redeposition mechanism for activating passive aluminium, proposed by Reboul et al,(1) Corrosion tests and XPS analysis suggest that mechanical loss becomes the more dominant mechanism in the loss of anode efficiency beyond 1 month of operation, Copyright (C) 2000 John Wiley & Sons, Ltd.