Unexpected iron corrosion by excess sodium in two-dimensional Na–Cl crystals of abnormal stoichiometries at ambient conditions

Abstract

At ambient conditions, we found salt crystals formed from unsaturated solutions on an iron surface; these salt crystals had abnormal stoichiometries (i.e. Na2Cl and Na3Cl), and these abnormal crystals with Cl:Na of 1/2–1/3 could enhance iron corrosion. Interestingly, we found that the ratio of abnormal crystals, Na2Cl or Na3Cl, with ordinary NaCl was relative to the initial NaCl concentration of the solution. Theoretical calculations suggest that this abnormal crystallisation behaviour is attributed to the different adsorption energy curves between Cl−–iron and Na+–iron, which not only promotes Na+ and Cl− adsorbing on the metallic surface to crystallise at unsaturated concentration but also induces the formation of abnormal stoichiometries of Na–Cl crystals for different kinetic adsorptionprocess. These abnormal crystals could also be observed on other metallic surfaces, such as copper. Our findings will help elucidate some fundamental physical and chemical views, including metal corrosion, crystallisation and electrochemical reactions.