Unexpected higher corrosion in the gas phase region of metals caused by calcium and magnesium ions compared to sodium ions

Abstract

In the marine environment, Na⁺ ions have been the focus of attention owing to their high content, which is one of the important factors causing marine corrosion. With reference to the content of macro ions in seawater, circular iron samples were semi-immersed in 0.04 M MgCl₂ and 0.6 M NaCl solutions containing different proportions of ethanol. Unexpectedly, we observed more severe corrosion effects in the gas phase region and at the gas–liquid interface of metal samples semi-immersed in the MgCl₂ solution. Although the concentration of the MgCl₂ solution was only 1/15 of that of the NaCl solution, the iron corrosion induced by MgCl₂ was significantly more severe than that caused by NaCl when the ethanol content was increased. Mg²⁺ ions outperform Na⁺ ions in metal gas phase corrosion. Especially in the oxygen content of the gas phase corrosion product, MgCl₂ caused an increase by up to 52.7%, while NaCl only resulted in a 10.3% increase. Ethanol is normally regarded as a corrosion inhibitor and exists in the liquid phase. Interestingly, in the gas phase and at the gas–liquid interface, ethanol aggravated rather than reducing iron corrosion, particularly in the presence of Mg²⁺ ions. In addition, we observed that Ca²⁺ ions produced more severe corrosion effects.