Electrochemical aspects of the roles of Na2MoO4 conversion treatment: Galvanic corrosion inhibition of AA5083 coupled to AISI 1045 carbon steel

• The compositional change of surface films on the Al-matrix and Al 6 (Fe, Mn) was observed due to the Na 2 MoO 4 conversion treatment. • Na 2 MoO 4 conversion treatment inhibited the galvanic corrosion damage on AA5083 coupled to AISI 1045 carbon steel in diluted synthetic seawater. • The inhibition was caused by the decrease in the cathodic activity of the Al 6 (Fe, Mn) particles and the increase in the alkaline dissolution resistance of the surrounding Al-matrix. This study demonstrates that a 10-min immersion treatment in Na 2 MoO 4 can effectively decrease the number of pitting corrosion damage on AA5083 aluminum alloy coupled to carbon steel in a 100-times diluted synthetic seawater environment at 298 K (pH 8.2) by modifying the surface films to suppress galvanic corrosion. A 10-min conversion treatment was particularly effective compared with the effect on as-polished and 1-min treated specimens. After the conversion treatment, the surface films on the Al-matrix and Al 6 (Fe, Mn) particles became thicker and Mo-enriched. Additionally, the removal of Fe was observed in the 10-min conversion-treated film on Al 6 (Fe, Mn). These compositional changes caused a decrease in the cathodic activity of the Al 6 (Fe, Mn) particles and an increase in the alkaline dissolution resistance of the surrounding Al-matrix. These electrochemical properties suppressed the galvanic corrosion of AA5083. Additionally, the pitting potential of the 10-min conversion-treated AA5083 was higher than the electrode potential during galvanic coupling. Consequently, the intersection of the anodic polarization curve of 10-min treated AA5083 and the cathodic polarization curve of AISI 1045 carbon steel in diluted synthetic seawater was in the passive region of AA5083.