Effect of Gap Size on Galvanic Corrosion between AA5052 Aluminum-Magnesium Alloy and SS400 Carbon Steel in Aqueous NaCl Solutions

Abstract

Recently, aluminum alloys (AAs) have been utilized to automobile body parts in order to reduce the weight of the body and improve fuel consumption efficiency. Although some aluminum alloys such as 5000 and 6000 series have relatively good corrosion resistance in atmospheric environments, galvanic corrosion between aluminum alloys and other electroconductive materials is one of the critical issues on automotive application of the aluminum alloys. So far, we have investigated galvanic corrosion between AA5052 aluminum-magnesium alloy and SS400 carbon steel in aqueous NaCl solutions and reported that the galvanic corrosion behavior changes depending on the concentration of NaCl solutions [1]. In this study, effect of gap size between aluminum alloys and steel on galvanic corrosion behavior was investigated in aqueous NaCl solutions. A galvanic couple used in this study was almost the same as that reported before, which was made of AA5052 Al-Mg alloy (Mg: 2.65, Si: 0. 08, Fe: 0.27, Cu: 0.01, Cr: 0.17, Zn: 0.01, Al: bal. (mass%)) and carbon steel (JIS SS400). The plate of both materials was 10-mm width and 15-mm length and 5-mm thickness. After lead wires were connected to each plate, both plates were embedded together in an epoxy resin. In this study, both plates were placed separately along the surface with gap sizes from 500 um to 10 mm so that a coplaner galvanic couple was prepared. The surface of the galvanic couple was ground with SiC papers up to JIS 2000 grit and rinsed in EtOH. Galvanic corrosion experiments were done for 72 h in aqueous NaCl solutions of various concentrations ranging from 5 mM to 2 M. Galvanic current and corrosion potential of the couple were measured during the immersion in the solutions. It was found from the results of corrosion potential, galvanic current, and surface observation that galvanic corrosion behavior changes depending on the gap size between AA5052 and SS400 in the NaCl solutions. When the gap size was 10 mm, no corrosion occurred on the SS400 side in all of the test solutions, suggesting that anodic dissolution reaction was dominant on the AA5052 side. However, when the gap size was decreased, the corrosion of the steel occurred in the solutions at lower concentrations, although the aluminum alloy also showed localized corrosion. These results indicated that anodic dissolution reaction was generated on both materials in the smaller gap-size condition with decreasing the NaCl concentrations. In this study, the change in the galvanic corrosion behavior with the gap size was also discussed from the results of change in the solution chemistry and polarization behavior of the materials. 1. K. Ota, E. Tada, A. Ooi, A. Nishikata, ECS meeting abstracts, MA2018-02, 574, 2018.