Evaluation of Initial Corrosion Phenomenon of Magnesium Alloys by SKPFM

Abstract

Magnesium (Mg) has the lowest standard electrode potential in structural materials, and then it easily corroded for that property contacting with other metals. The corrosion resistance of Mg alloys was generally investigated by conducting saltwater immersion test, salt spray test, and electrochemical corrosion test. However, these tests give only macroscopic information but don’t give microscopic (i.e. galvanic corrosion at the interface). So, the origin of galvanic corrosion at the interface between dispersoids and a-Mg in Mg alloys was investigated for fundamental clarification. In this paper, the surface potential difference (VSPD) at the interface between dispersoids and a-Mg using AZ91D Mg alloy is measured by using Scanning Kelvin Probe Force Microscope (SKPFM). Surface potential of pure metals measured by SKPFM had good correlation with standard electrode potential (SEP), and then VSPD also positively correlated with difference of SEP. Salt water immersion test was conducted using AZ91D in order to compare VSPD with corrosion phenomenon. Topographic map around the interface between dispersoids and a-Mg was measured by using AFM after corrosion test. Corroded phase corresponded to the anodic phase indicated by surface potential.

Unusual Correlation between SKPFM and Corrosion of Nickel Aluminum Bronzes

“…For example, particles that had a high or noble Volta potential difference exhibited higher pitting potentials (E Pit ) than the matrix, acted as local cathodes, or both. 23,24,45,47,48 In all cases, noble IMCs or phases were enriched in elements that have a relatively high work function such as Cu, Cr, Mo, Ni, and W. 59 The implications of the partitioning of noble metal elements to the various constituents of a heterogeneous alloy will be discussed in more detail below. The objective of this work is to explore the use of SKPFM as a tool to investigate the early stages of NAB corrosion in simulated marine environments.…”

“…Others have used SKPFM to study corrosion and corrosion inhibition mechanism of aluminum alloys, 28,42,43 magnesium alloys, [44][45][46] stainless steels, [47][48][49][50][51] AlSiMg alloys produced by additive manufacturing, 52 nickel-based alloys, 53 low alloy steels, 54,55 and filiform corrosion mechanisms, [56][57][58] to name a few examples. It has now become a common practice to refer to low ψ values (i.e., dark areas in Volta potential difference maps) as anodic regions and high ψ (i.e., bright areas) as cathodic.…”