REPASSIVATION OF 316L STAINLESS STEEL IN INHIBITED SODIUM CHLORIDE SOLUTION

Abstract

This study investigates the stability of the passive oxide film formed on the surface of 316L stainless steel in the presence of green inhibitors. The potentiostatic and potentiodynamic polarization tests employed were done in 3.5 wt% sodium chloride in the presence of leaves extracts of Musa spp. (MS), Jatropha curcas (JC), and their hybrid at ambient temperature and 70 oC, respectively. For the potentiostatic tests, the coupons were cathodically stripped at -850 mV for 600 s to remove the native oxide on their surfaces and a potential of 50 mV was imposed to observe the repassivation for 200 s. While for the potentiodynamic tests, the pitting potential measured at 100 μA/cm2, the corrosion potential and cathodic current density were used for analysis. The current transients perfectly fitted into the exponential decay curve which has the equation i = is + ipeak exp (-t/τ), where the decay constant, τ relates to the repassivating speed and is measures the extent to which the newly formed film heals and stabilizes. The current transients showed that MS helps in the repassivating process, especially at 300 parts per million (ppm) at the ambient temperature and JC especially at 200 ppm, also at the lower temperature. The potentiodynamic curves mostly showed correlations with the current transients except for the hybrids inhibitor. The green inhibitors increased the pitting potentials anodically at some inhibitor concentrations. The selected SEM images revealed that the pit sizes and densities were reduced in some inhibited environments relative to the uninhibited one