Galvanic Corrosion of AA5052 Aluminum-Magnesium Alloy and Carbon Steel in NaCl Solutions

Abstract

Aluminum alloys (AAs) such as 5000 and 6000 series, which show relatively good corrosion resistance and plastic workability, have been utilized to automobile body parts for reducing the weight of the body and for the improvement of fuel consumption. However, since the body parts made of Al alloys should be joined with steel parts by welding or other methods and the joined parts can be exposed to atmospheric corrosion environments, there has been a concern about degradation of the parts due to galvanic corrosion between Al alloy and steel. In this study, galvanic corrosion behavior of AA5052 aluminum-magnesium alloy and SS400 carbon steel is investigated in NaCl solutions. A galvanic couple used in this study consisted 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). Both materials were 15-mm-square plates of 3 mm in thickness. After lead wire was connected on the surface of the plates, they were embedded in an epoxy resin. At this time, both materials were separated with a 500-um gap. The surface of the galvanic couple was polished with SiC papers down to JIS 2000 grit and rinsed in EtOH. Galvanic corrosion experiments were done in aqueous NaCl solutions of various concentrations ranging from 5 mM to 500 mM. Galvanic current and corrosion potential of the couple were measured during the immersion in the solutions. When a galvanic couple was immersed in a NaCl solution, galvanic current decreased gradually with immersion time and reached almost a constant value. On the other hand, corrosion potential for the galvanic couple showed complex behavior with immersion time. Just after immersion, the corrosion potential decreased in the less noble direction, which may be due to the initiation of corrosion of AA5052, and then after reaching the minimum value, the corrosion potential started to increase to a noble potential. Although the corrosion potential and the magnitude of the galvanic current were different depending on the concentration of the NaCl solutions, they did not change in a regular manner as a function of the NaCl concentrations. The changes in the corrosion potential and the galvanic current may be related to the transition of anodic dissolution behavior of AA5052 in the NaCl solutions of different concentrations.