Challenges to local potential measurement for chemical mapping with scanning microelectrochemical techniques due to the occurrence of galvanic coupling processes on magnesium alloys

• Limitations in the measurement of local potential in the presence of corrosion by galvanic coupling. • The uncertainty arises from the assumption of constant conductivity in the electrolyte. • The interaction between concentration and potential gradients varies in intensity with distance from the corrosion site. • It is possible to model the local potential by taking into account the effect of local concentration gradients. • Validation cells for scanning microelectrochemical techniques cannot be based on a noble metal current source. The interaction between concentrations and potentials was modeled and quantified in relation to local potential measurements during corrosion processes. This modeling aimed to study the uncertainty generated by the description of potential distributions in the vicinity of active corrosion sites, assuming hemispherical symmetry of the currents and the transport of metal ions radiating from a point electrode. This work demonstrates the coupling of electrical and concentration gradients around anodic and cathodic sites in galvanic corrosion processes, a coupling that, in practice, will limit the interpretation of local electrochemical data and the quantification of corrosion mechanisms if it is not taken into account. A critical distance is defined beyond which the signal measured by a probe deviates from the linear dependence with a slope of −1, generally asccepted to describe the variation of the probe signal with the tip-sample distance during scanning microelectrochemical measurements using passive microelectrodes as probes. Furthermore, the value of this critical distance depends strongly on the conductivity of the electrolytic test solution, being larger for more dilute solutions. In addition, the implications for estimating local corrosion currents using a noble metal, such as a platinum wire as a model current source for calibrating the potential signal in a validation cell, are also presented.