Chemical Conversion Treatment of AA5083 Aluminum Alloy and AISI 1045 Carbon Steel under Galvanically Coupled Condition in Na₂MoO₄: Effect of pH on Corrosion Resistance

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To ascertain the effect of solution pH of Na₂MoO₄ chemical conversion treatment for aluminum/steel joints on corrosion resistance, AA5083 aluminum alloy and AISI 1045 carbon steel were immersed in 50 mM Na₂MoO₄ at pH ranges of 8–12 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 Al₆(Fe, Mn) decreased with increasing solution pH of the conversion treatment. The Al₆(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 Al₆(Fe, Mn) particles, resulting in the improved galvanic corrosion resistance of AA5083.

Fig. 8 Effect of solution pH used in the conversion treatment on the number of localized corrosion damages on as-polished and conversion-treated AA5083 after galvanic current and potential measurements as shown in Fig. 6.