Together with my colleague Dr. Ken Vasanth and my Ph.D. Adviser Prof. Jerry Frankel we have recently published a new original reserach paper exploring the use of Scanning Kelvin Probe Force Microscopy (SKPFM) to study the mechanics of localized corrosion of nickel aluminum bronzes (NABs). In this article, we investigated the effect of microstructure on the early stages of localized corrosion of a NAB. SKPFM was conducted in air before exposure and ex situ after immersion to ammoniacal and NaCl solutions. Whereas topography maps clearly revealed different localized corrosion resistance of the various phases present in NABs, Volta potential maps showed an inverse correlation with the expected corrosion resistance.
Why is it important?
Jerry and Dr. Patrik Schmutz introduced SKPFM in corrosion research in the late 1990s for the study of localized corrosion and corrosion inhibition of high-strength aluminum alloys. Since then, SKPFM has been used to examine many alloy families, including carbon and stainless steels, magnesium and nickel alloys, to name a few. In all cases, the materials exhibited a morphology after exposure that could be predicted by the relative practical nobility determined by SKPFM done in air in the as-polished condition. For example, particles that had a high or noble Volta potential difference exhibited higher pitting potentials than the adjacent matrix, acted as local cathodes, or both.
To our initial surprise, SKPFM results contradicted the well-documented correlation between areas of low and high Volta potential difference and the expected anodic or cathodic behavior or practical nobility or reactivity. The localized corrosion morphology was, however, in agreement with the accepted behavior reported for similar NABs. In this paper, we present a plausible explanation and discuss the implications of these findings in detail.
As we conclude, the SKPFM results on NAB strongly suggest that Volta potential difference maps in air should not be universally correlated with practical nobility or reactivity.
Abstract
Scanning Kelvin probe force microscopy was used to investigate the influence of microstructure on the corrosion behavior of nickel aluminum bronzes in ammoniacal and NaCl solutions as well as under potential control in \(Na_{2}SO_{4}\).The results showed an inverse correlation between the measured Volta potential difference and the observed corrosion behavior. In other words, the phase with initially higher Volta potential was preferentially attacked whereas those with the lowest Volta potential difference values remained intact. This behavior suggests that Volta potential difference maps in air cannot be universally correlated with practical nobility or reactivity in solution, as proposed by other investigators. The implications of these findings are discussed.
Citation
M. Iannuzzi, K.L. Vasanth, G.S. Frankel, “Unusual Correlation between SKPFM and Corrosion of Nickel Aluminum Bronzes”, Journal of The Electrochemical Society, 164 (9), (2017): p.C488-C497. doi: 10.1149/2.0391709jes.
M. Iannuzzi, K.L. Vasanth, G.S. Frankel, “Unusual Correlation between SKPFM and Corrosion of Nickel Aluminum Bronzes”, Journal of The Electrochemical Society, 164 (9) (2017) C488-C497. doi: 10.1149/2.0391709jes
The article has ben published as OpenAccess, distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.You can download a copy here or by clicking on the figure above.
