Aluminum alloys (AAs) such as 5000 and 6000 series have relatively good corrosion resistance and plastic workability. The use of these alloys for automobile body parts have been increased significantly because of reducing the weight of the body and improvement of fuel consumption. Although aluminum alloys themselves have relatively good corrosion resistance in atmospheric environments, galvanic corrosion between aluminum alloys and other materials should be taken into consideration in automotive application. This is because the body parts made of Al alloys should be jointed with other materials such as steel parts by welding or other methods and because the jointed parts can be exposed to atmospheric corrosion environments. In this study, galvanic corrosion between AA5052 aluminum-magnesium alloy and SS400 carbon steel is investigated in aqueous NaCl solutions.

A galvanic couple used in this study 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 3-mm 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 up to JIS 2000 grit and rinsed in EtOH. Galvanic corrosion experiments were performed for 2 hours in aqueous NaCl solutions at 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.

Corrosion potentials measured during the immersion in various concentrations of NaCl were almost constant for 2 h. The corrosion potentials changed in the less noble direction with increasing NaCl concentrations. Galvanic currents between AA5052 and SS400 showed positive values during the immersion in the solutions, indicating that aluminum-alloy side was basically an anode. However, the galvanic currents were almost the same independent of NaCl concentrations. From the results of polarization behaviors for AA5052 and SS400, a significant anodic current increase due to the propagation of localized corrosion on aluminum alloy and a diffusion-limiting current for the reduction of dissolved oxygen on steel were observed at the corrosion potentials of the galvanic corrosion tests. These results indicated that galvanic corrosion between AA5052 and SS400 in the early stage of immersion progressed under a diffusion-limiting condition for the reduction of dissolved oxygen on steel.