Degradation of anti-rust oil film in a simulated coastal atmosphere: Inhibition mechanism and in-situ monitoring

Anti-rust oils (ARO) are widely used to inhibit atmospheric corrosion of metal structures, but their degradation mechanisms are unclear, particularly in coastal environments. In this work, we first prepared a kind of ARO made of sodium petroleum sulfonate (SPS) and white oil, then coated the ARO on a mild steel plate to investigate its deterioration process beneath saline water droplets by Quartz crystal microbalance (QCM) and electrochemical mapping techniques. It shows that the SPS concentration is crucial to the ARO inhibition property. When the SPS content is lower than its critical micelle concentration (CMC), saline droplets may intrude the barrier formed by the nonpolar paraffin groups of SPS and then adsorb on the steel surface, resulting in localised corrosion . However, once the SPS content exceeds its CMC, the saline droplets will be emulsified and captured in the oil phase by the reverse micelle effect, thus inhibiting the corrosion of steel substrates. Moreover, the atmospheric corrosion rate of mild steel obeys a power decaying under ARO film. Finally, a capacitive sensor was invented to evaluate the degree of degradation of ARO in situ by dielectric constant measurement.