Investigation of Mg/Al alloy sacrificial anode corrosion with Scanning Electrochemical Microscopy

Corrosion of Mg/Al alloy was investigated with the scanning electrochemical microscope (SECM) using a Mg2+ ion selective microelectrode (ISME) as measuring tip. Mg/Al alloys have practical application as sacrificial anodes to prevent the corrosion of various metals susceptible to corrosion. A typical application of such sacrificial anodes is in water boilers. A model (Mg/Al)/Fe target was fabricated using boiler sacrificial anode and high purity iron wires. Corrosion current was measured between the electrically connected galvanic pair, and Mg2+ ion flow rate was calculated with Faraday's law of electrolysis. However, Mg2+ flow rate could not be calculated with this method while the two metals were left electrically-disconnected. For this reason, fixed height lateral scans and retreating scans were performed above the Mg2+ sample with Mg2+ ion-selective electrodes employed as SECM tips. Mg2+ flow rate was estimated using the concentration profiles obtained with SECM. Relatively high-speed scans were possible with robust, solid contact Mg2+ micropipette electrodes, owing to their low resistance compared to conventional micropipettes of the same size.