We describe an experiment whose results are interpreted as a direct measurement of the chemical reaction kinetics of atoms being removed from high coordination crystal terrace sites, at least in the particular restricted case of electrochemical dissolution of metal alloys. The central idea is that there are binary metal alloys such as Ag/Au with the property that they can be subjected to electrochemical dissolution (corrosion) conditions where one element, the less noble component, dissolves and the remaining (minor, more noble) element diffuses on the surface, passivating all step edges and other low coordination sites. Further dissolution, particularly under conditions leading to porosity evolution, is rate-limited by dissolution from nine-coordinated terrace sites in face-centered cubic alloys. By dealloying Ag from Ag-Au alloys in ionic liquids, we have extrapolated the activation barrier for Ag dissolution from alloy terrace sites over the composition range 0-20 atom % Au.