Magnetic Fields Induced by Electrochemical Reactions:  Aluminum Alloy Corrosion Sensing by SQUID Magnetometry on a Macroscopic Scale

Abstract

Corrosion of aluminum alloy AA 2024 at room temperature was studied in NaCl solutions by a SQUID magnetometer operating in liquid helium. Macroscopic electronic current flow along an asymmetric U-shaped sample was indicated by distinctive magnetic field images with a spatial resolution of the order of 1 mm. It was concluded that the macroscopic current originated from a corrosion potential gradient within the sample because of its asymmetric geometry; this current was negligible on an equipotential (symmetric) sample. Electrochemical quartz crystal microbalance measurement demonstrated that oxygen plays a significant part in this corrosion process, allowing control of corrosion activity of AA 2024 by changing the oxygen content in the solution. The asymmetric sample was magnetically sensitive to corrosion: the higher corrosion rate with increased oxygen concentration resulted in higher magnetic activity. By contrast, the sample that was a system of single sections did not produce a magnetic field that reflected increases in corrosion rate, and magnetic activity of this sample was much less than that of its monolithic counterpart. The obtained data demonstrated that a uniformly corroding surface could be magnetically silent on a macroscopic scale if no special measures are undertaken to distort the equipotentiality of the corroding surface.