Electrochemical Investigation of Oxygen Reduction Reaction on Type 316L Stainless Steel in NaCl Solutions in a Confined Space

Abstract

Steel plate joints are commonly produced in the manufacturing process of steel structures such as automobiles and bridges. The steel plate joints suffer from aqueous corrosion in atmospheric environments. Especially, severe corrosion is often observed in the confined spaces between the steel plates because aqueous electrolytic solutions can be kept in the spaces. In this case, corrosion reaction rate of the steels in the confined space is basically controlled by the reduction reaction rate of dissolved oxygen. Therefore, it is important to estimate oxygen reduction reaction rate in the confined spaces to predict corrosion deterioration of steel plate joints. In this study, electrochemical investigation of oxygen reduction reaction rate in a confined space was performed. The material used in this study was austenitic stainless steel type 316L bar with various diameters from 5 to 20 mm. The type 316L bar was cut into a small cylindrical piece and the piece was embedded in epoxy resin after being contacted with a lead wire. The surface of the embedded stainless steel sample was ground with SiC papers down to JIS200 grit and then cleaned ultrasonically in EtOH. Test solutions were aqueous NaCl of different concentrations from 0.05wt% to 5wt%. The solutions were prepared with Milli-Q water and reagent grade sodium chloride. For the electrochemical measurements, the sample was set horizontally in a plastic vessel and then an acrylic rod of 20 mm in diameter, which was used as a crevice former, was placed above the sample surface to make a confined space between the sample and the rod. The gap between the sample surface and the rod was changed from 20 um to 200 um by using a laser displacement meter. Electrochemical measurements were conducted by a conventional three electrode method with a platinum wire (Pt) and a Ag/AgCl in a saturated KCl (SSE) as a counter electrode (CE) and a reference electrode (RE), respectively. Cathodic polarization curves for the sample were measured with various gaps at a potential sweep rate of 0.5mV/s. Cathodic polarization behavior for the sample measured in the confined spaces was similar as that measured in a bulk solution; a limiting current region due to oxygen reduction reaction was observed on the samples of various diameters in a confined space. It was confirmed that the limiting current density decreased with decreasing in the gap. This is attributed to be the difference in the oxygen diffusion rate on the sample in the confined space.