Localised galvanic corrosion processes in thermal spray coated/cast aluminium alloy systems

Cast Aluminium Alloys, because of the propensity of aluminium to react with impurities and alloying elements, are prone to developing IM impurities during the solidification process. These IM phases can act, in some fluids, as initiation sites for localised corrosion processes, resulting in degradation phenomena, such as pitting. Whilst Thermal Spray coatings can improve the wear resistance of Cast Aluminium Alloys, their corrosion performance may be hampered by the presence of through porosity within the coating. The present work details some preliminary studies of the localised corrosion processes occurring at the interface area between a Thermal Spray coating and a cast aluminium alloy. Using the SVP100 SVET system, cross sections of the coated samples, immersed in a NaCl or NaCl/HCl solution, were scanned, over extended periods, in order to map the progressive development of cathodic and anodic areas. EIS sampling, over periods of up to 72 hours, and the results of the Acidified Salt Spray cabinet testing are also presented. Although only preliminary work has been performed thus far, the premise that the presence of more noble metals along the path of open pores in Thermal Spray coatings on cast Aluminium Alloy, LM25, in the presence of an aggressive aqueous solution, results in the expedited corrosion of the substrate is demonstrated. This occurs, preferentially, around the intermetallic phases, predominantly Fe-based, and results in further destruction of the coating, through spalling, exposing additional substrate to corrosive attack.