Methods—On the Application of Ambient Scanning Kelvin Probe Force Microscopy to Understand Micro-Galvanic Corrosion Phenomena: Interpretation and Challenges

The capabilities of scanning Kelvin probe force microscopy (SKPFM) to directly measure local Volta potential differences with relatively high spatial resolution, has made it a very popular technique to assess the relative nobility of intermetallic phases and therefore to understand corrosion susceptibility and microgalvanic corrosion activity in several alloys. However, the direct correlation between galvanic corrosion activity and Volta potential differences measured in air is not straightforward. Several factors, which can easily go unnoticed, can directly affect the potential values measured by SKPFM, hence compromising their validity and relevance in explaining certain galvanic corrosion activities. Several recent studies presenting conflicting/varied results or failing to properly interpret/discuss the obtained data can be found. This work discusses specific factors that can compromise the validity of SKPFM measurements and/or the understanding acquired from them. It presents a discussion on the SKPFM working principle, the basics of galvanic corrosion, and the relevant terminology around these topics, emphasising the possible correlations between them, with the intention of increasing comprehension of SKPFM-based micro-galvanic corrosion studies. A summary of best recommended practices is presented, aiming at standardizing the use of the SKPFM technique and the representation of the data, and to ensure consistency of the results.