To answer this question, we have applied the atomic emission spectroelectrochemistry (AESEC) technique. This allows us to measure directly the dissolution rate of each element during either a spontaneous, open circuit corrosion experiment (dissolution-corrosion profile), or to decompose the polarization curve into elemental currents (elemental polarization curve). Examples of the later are shown in the Figure below. We also use AC polarization so as to measure charge transfer resistance for each element around the open circuit potential.
The approach is to compare the dissolution-corrosion profile with the elemental polarization curve and the AC elemental polarization to see to what extent the corrosion rate of each element may be predicted from the polarization techniques. The question of microstructure is addressed directly by analyzing the role of phase structure on the dissolution of the coating. This is done by performing experiments on pure phases, intermetallic compounds, and a multiphase Zn-Al-Mg alloy galvanized steel coating. Nominally pure materials include: Zn, Al, and Mg; synthetic η-phase (Zn 0.7-Al), β-phase Al (Zn-22Al), and α-Al phase (Zn-68Al); and synthetic Zn2Mg intermetallic compound. The results are interpreted in terms of the mixed potential theory on an element by element basis, which allows prediction of the corrosion behavior of a system over a wide range of experimental conditions.
Fig.1. Elemental AESEC polarization curves of pure metals, Zn2Mg, and alloy coating in deaerated 10 mM NaCl solution at pH=10.1 at 25 °C.
1. T. Prosek, J. Hagstrom, D Persson, N. Fuertes, F. Lindberg, Ondrej Chocholaty, C. Taxén, J. Serák, D Thierry, Effect of microstructure of Zn-Al and Zn-Al-Mg model alloys on corrosion stability, Corrosion Science 110 (2016) 71-81.