The study of corrosion on carbon steel and zinc under thin-film electrolyte

Abstract

Atmospheric corrosion is one of the most widespread types of metal corrosion in the world. Commonly, atmospheric corrosion is affected by the electrochemical properties of the metal and environmental factors, such as humidity, temperature and the properties of the electrolyte on the metal surface. Currently, there still lack of laboratory studies which investigated the effect of changing electrolyte thickness on corrosion behaviours. In this study, a corrosion cell with adjustable electrolyte thickness was used to simulate the atmospheric corrosion process of carbon steel and zinc under thin-film sodium chloride electrolyte conditions. The effects of electrolyte thickness and chloride ion concentration on the atmospheric corrosion of these two metals were investigated by open circuit potential, polarization and galvanic coupling techniques. To perform the experiments under thin-film conditions a novel electrochemical cell was employed. Experiments confirmed the effect of electrolyte thickness on the corrosion rate of the metals. Particularly, it was established the role of oxygen transport in the kinetics of the process. At large electrolyte thickness (several hundred micrometers), oxygen is limited at the proximities of the metal, leading to corrosion rates comparable to the values under bulk electrolyte conditions. With the reduction of the electrolyte thickness, a higher concentration of oxygen is available, due to the faster oxygen equilibrium between air-electrolyte, leading to a substantial increase in both corrosion rate and the diffusion control limiting current. Corrosion current density under approximately 40μm thin-film electrolytes were two orders of magnitude higher than the currents measured for both carbon steel and zinc metals under immersed conditions (bulk electrolyte). The effect of chloride ion concentration on corrosion rate under thin-film electrolytes was also revealed. The higher solubility of oxygen on low chloride-concentration electrolytes showed also an acceleration of the process. Carbon steel and zinc metals presented the highest corrosion rates under 0.01M NaCl electrolyte with 40μm thickness.