Galvanic Corrosion Processes of Aluminum Coupled to Iron in Chloride Solutions at Near-Neutral pH

Abstract

Multimaterial structures are effective for weight reduction of automobiles, which contributes to the prevention of global warming and the decrease in CO2 emission. Steel-aluminum joints have many advantages such as low cost, high strength, and high corrosion resistance. However, galvanic corrosion was expected to occur because the corrosion potential of aluminum alloys is usually different from those of iron and steels. Considering the galvanic corrosion processes in chloride-containing near-neutral pH solutions, the existence of surface oxides is an important factor affecting the initiation of corrosion. In this research, the galvanic corrosion behavior of aluminum alloy coupled to iron and the steel in near-neutral pH solutions was investigated. Pure iron, AISI 1045 carbon steel, pure aluminum, and A5083-O(Al-Mg alloy) were used as the specimens in this research. The surfaces of pure iron and AISI 1045 were polished down to 1 µm. Pure aluminum and A5083-O were polished down to 0.25 µm with a diamond paste and cleaned ultrasonically in ethanol before electrochemical measurements. The electrode area was ca. 1 cm2. Ag/AgCl (3.33 M KCl) was used as the reference electrode. In diluted synthetic seawater (200mg/L [Cl-], pH 8.2, naturally aerated, galvanic current was measured for 20 h using zero resistance ammeter technique. The distance between two electrodes was 1 cm. Fig. 1(a) shows the time variation of the galvanic current and electrode potential of pure aluminum coupled to pure iron. From the beginning of the immersion to 30 ks, many current transients were generated. However, after 30 ks, the galvanic current became a constant value of approximately 0.15 µA cm-2. The potential gradually decreased with time. The pH of the electrolyte decreased from 8.2 to 6.7 by galvanic corrosion. Fig. 1(b) and 1(c) shows the electrode surfaces of pure iron and pure aluminum after galvanic current measurements. Rust was observed on pure iron, and filiform corrosion was generated on pure aluminum. Anodic polarization curves were measured in diluted synthetic seawater (200mg/L [Cl-], deaerated), and the pH was adjusted to 8.2, 6.7, 5.2, 3.7, and 2.2 with NaOH or H2SO4. Potentiodynamic polarization was started from corrosion potential, and the scanning rate was 23 mV min-1. The electrode potential of pure aluminum under galvanic coupling conditions was found to be lower than the pitting potentials of pure aluminum. But, corrosion was initiated on the aluminum surface (see Fig. 1(b, c)). The galvanic corrosion behavior of pure aluminum and aluminum alloy AA5083 were discussed in terms of the surface properties of the specimens and solution chemistry. Figure 1