A Study of Crevice Corrosion Susceptibility of Zn-Al Alloys in a High-pH Environment

Abstract

This paper aims to clarify the effect of polymer coating damage of dual polymer–zinc coating used to protect rebar reinforcement from corrosion. The coating damage can result in crevices between the two materials. At these crevices, corrosion–passivation plays an important role in the integrity of the intended coating and the protectiveness of the steel base metal. An experimental design was developed to replicate a crevice of Zn-2%Al alloy. This alloy is commonly used for the dual coating protection of rebars. Experiments in this investigation were performed to test several crevice sizes and conditions to assess the state of crevice corrosion. Prepared electrodes were submerged in a 1 M NaOH solution and connected to a data logger to monitor the potential. A special reference electrode was prepared using activated titanium, against which the potential was measured. Additionally, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization were carried out to assess the corrosion rate of the different specimens. The results suggest that, in the absence of a crevice, corrosion occurred on bare specimens. However, whenever a crevice was present, a shift in potential and corrosion rate values indicated that the specimen shifted from an active corrosion regime to a passive one. This shift (gain) in potential was measured as approximately 0.9 V, resulting in a shift in the electrode potential to −0.6 V (SCE). The analysis showed that the solution inside the crevice shifted toward lower pH values, with pH = 12 suggested as the level that supports more passivity inside the crevice according to the Pourbaix diagram.