Toward Improving Ambient Volta Potential Measurements with SKPFM for Corrosion Studies

Abstract

Scanning Kelvin probe force microscopy (SKPFM) is used in corrosion studies to quantify the relative nobility of different mi crostructural features present within complex metallic systems and thereby elucidate possible corrosion initiation sites. However, Volta potential differences (VPDs) measured via SKPFM in the literature for metal alloys exhibit large variability, making inter pretation and application for corrosion studies difficult. We have developed an improved method for referencing SKPFM VPDs by quantifying the closely related work function of the probe relative to an inert gold standard whose modified work function is calculated via density functional theory (DFT). By measuring and tracking changes in the probe vs. gold VPD, this method compensates for some of the complex effects that cause changes in an individual probe's work function. Furthermore, it provides a path toward direct, quantitative comparison of SKPFM results obtained by different researchers. Application of this method to a Cu-Ag-Ti eutectic braze of a steel sample imaged with multiple SKPFM probes of differing compositions led to enhanced repeatability both within and among probe types, as well as enabled the calculation of modified work function values for each of the microstructural constituents present.

On the Volta potential measured by SKPFM – fundamental and practical aspects with relevance to corrosion science

“…Sample preparation can impose large artefacts in the order of 10's to 100's of mV, such as due to tiny polishing marks, which may only be visible with ultra-high resolution microscopic observations. Furthermore, the pyrolysis of carbon on the surface ('burnt carbon'), due to the ubiquitous presence of hydrocarbon in ambient air, which gets deposited onto the surface during sample preparation, have been reported to impose large Volta potential variations across unaffected and carbon-burnt regions [57][58][59]. Hence, the surface condition has utmost importance to be 'controlled' when Volta potential measurements are conducted.…”

Corrosion Initiation and Propagation on Carburized Martensitic Stainless Steel Surfaces Studied via Advanced Scanning Probe Microscopy

“…To quantify the findings from SKPFM mapping, a threshold technique was implemented (see example image in Figure 1 below) that utilized a user-determined cut-off potential based on the distribution of Volta potentials observed in the corresponding data histogram (512 bins). From the resulting thresholded data, the average Volta potential (with corresponding standard deviation) was calculated for each of the two phases present on the surface (i.e., matrix and carbides, identities confirmed through SEM/EDS characterization) [52]. Figure 1a shows a representative SKPFM Volta potential map for HTT P675.…”

A closed-host bi-layer dense/porous solid electrolyte interphase for enhanced lithium-metal anode stability

“…This is employed in a dual pass, frequency modulated (FM), PeakForce (PF, Bruker Corporation) method to acquire spatially resolved Volta potentials, operating in a pseudo capacitor-like method at a constant lift height of 100 nm above the surface. A nominally 5-nm, Si-based probe with an Alcoated cantilever (Bruker Corporation) was used for SKPFM acquisition, calibrated with a relatively inert gold standard for repeatable results [34]. Both techniques were done on a Bruker Dimension Icon AFM equipped with a 64-bit NanoScope V controller, inside an inert glovebox (MBraun, <0.1 ppm O 2 and H 2 O).…”