In 100-times diluted synthetic seawater at 298 K (pH 8.2), the effect of anodizing on the galvanic corrosion resistance of AA5083 coupled to pure Fe, Type 430, or 304 stainless steel was investigated by measuring the galvanic current densities and electrode potentials. Anodizing in H2SO4 effectively suppressed the galvanic corrosion of AA5083. It was shown that an increase in pitting potential by anodizing alone could not determine whether galvanic corrosion would occur or not. The cathodic activity on Al6(Fe, Mn), which causes alkalization on and around Al6(Fe, Mn) particles, decreased as the anodizing time and voltage increased and the anodic oxide film on the Al-matrix in alkaline environments became stable as the thickness of the oxide film increased. A comparison of these two factors revealed that the dissolution resistance of surface oxide film on Al-matrix contributed the galvanic corrosion prevention of anodized AA5083 coupled to pure Fe. In the case of AA5083 anodized at 16 V for 180 s, no galvanic corrosion damage was observed on the AA5083 coupled to Type 430 or 304.
The role of KMnO 4 -NaF conversion treatment in the galvanic corrosion resistance of AA5083 aluminum alloy coupled to AISI 1045 carbon steel in synthetic seawater (diluted 100 times) was investigated. The 10 min-treated AA5083 was observed to reduce the period of high current density during the early stage of coupling and decrease the number of localized corrosion damages on the AA5083. The 10 min conversion treatment significantly reduced the electrode potential of bulk Al 6 (Fe, Mn), and it was concluded that the Al 6 (Fe, Mn) particles on the conversion-treated AA5083 no longer acted as a local cathode at the electrode potential when the AA5083 was in contact with AISI 1045 carbon steel. The decrease in cathodic activity of Al 6 (Fe, Mn) was attributed to the removal of Fe from the surface film of Al 6 (Fe, Mn), addition of Mn by the conversion treatment, and thickening of the film.
To ascertain the effect of solution pH of Na 2 MoO 4 chemical conversion treatment for aluminum/steel joints on corrosion resistance, AA5083 aluminum alloy and AISI 1045 carbon steel were immersed in 50 mM Na 2 MoO 4 at pH ranges of 812 under galvanically coupled condition. Subsequently, in diluted synthetic seawater, the galvanic corrosion resistance of the AA5083 alloy connected to the AISI 1045 carbon steel was assessed. The number of localized corrosion damages was counted, and AA5083 treated at pH 11 was found to be the better corrosion resistance. The oxygen reduction current on bulk Al 6 (Fe, Mn) decreased with increasing solution pH of the conversion treatment. The Al 6 (Fe, Mn) particles on AA5083 were not preferential cathodes, and alkalization through oxygen reduction would not occur when the treatment was performed above pH 9. Auger electron spectroscopy analysis showed that Mo-accumulation, Fe-removal, and film thickening occurred on the particles of AA5083 treated at pH 11. These factors contributed to the suppression of the cathodic activity of the Al 6 (Fe, Mn) particles, resulting in the improved galvanic corrosion resistance of AA5083.
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.
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