Galvanic Corrosion Behavior between Aluminum and Iron in Chloride Solutions at Near-Neutral pH and Corrosion Prevention By Anodizing
Authors: Kosaba Takumi , Muto Izumi , Sugawara Yu 
Published: December 2020

The joints between steels and aluminum alloys are expected to be used as multi-material structures due to their low cost, high stiffness, and high corrosion resistance. When aluminum alloys are connected to steels, galvanic corrosion is expected to occur because the corrosion potentials of aluminum alloys are lower than those of iron and steels. In the case of the galvanic corrosion in chloride solutions at near-neutral pH, the anodic reaction on aluminum alloys are oxide film formation and/or pitting. For pure Al and aluminum alloys, the generation of localized corrosion is evaluated by the critical potential for localized corrosion, such as pitting potential. In this study, the electrode potential under galvanic coupling and pitting potential were compared. Surface modification is effective methods for improvement on corrosion resistance of aluminum alloys. However, there is little research related to the galvanic corrosion between steels and aluminum alloys. In this research, the galvanic corrosion behavior of pure aluminum coupled to pure iron in chloride containing near-neutral pH solutions and the effect of anodizing on galvanic corrosion were investigated.

Pure iron and pure aluminum were prepared as specimens in this research. The specimen surface of aluminum was polished down to 0.25 µm with a diamond paste. The iron surface was also polished down to 1 µm with a diamond paste. Diluted synthetic seawater (200 mg/L Cl-, pH 8.2) was used as the electrolyte for electrochemical measurements. The electrode area was ca. 10 mm2. Ag/AgCl (3.33 M KCl) was used as the reference electrode. Galvanic currents and electrode potentials were measured. The distance between iron and aluminum electrodes was kept at 10 mm. A zero resistance ammeter was used to measure the galvanic currents. The electrode potential of pure aluminum was also measured. Additionally, anodizing in sulfuric acid was conducted to pure aluminum in various voltages and times. Galvanic current measurements were carried out between these anodized specimens and pure iron to evaluate the effect of anodizing on galvanic corrosion prevention.

The galvanic currents and electrode potentials of pure aluminum coupled to pure iron were measured. The galvanic currents were anodic currents. The oscillations of galvanic currents and electrode potentials were generated during the measurements. Filiform corrosion was observed on pure aluminum after the measurements. The generation of filiform corrosion was caused by higher electrode potentials of pure aluminum coupled to pure iron compared with the initiate potential for filiform corrosion.

The time variation of galvanic currents and electrode potentials of anodized aluminum coupled to pure iron were also measured in the diluted synthetic seawater (pH 8.2). Pure aluminum specimens were anodized at 4, 8 and 16 V at 18 s. The oscillations were observed in the galvanic currents of anodized aluminum (4 and 8V); however, no oscillation was measured on the specimen anodized at 16 V. After the measurements, pitting was observed on anodized aluminum (4 and 8V). However, no pitting was observed on the specimen anodized at 16 V. The thickness of anodic oxide film was measured. It was confirmed that the thickness of the oxide film increased with anodizing voltage. Consequently, it was indicated that the galvanic corrosion resistance of pure aluminum for galvanic corrosion was enhanced by the increase of the thickness of the oxide film.