Corrosion and Adhesion Performance of Ferric Ion-Based Hybrid Nano-Conversion Coatings on Aluminum Alloy

Efforts at partial or total replacement of environmentally unfriendly and carcinogenic chromate in conversion coatings on Al and its alloys are yielding some positive outcomes. This study investigated the corrosion and adhesion performance of novel conversion coatings on Al6061 alloy using an electrochemical test and microstructural examination of the alloy’s surface. The results of the electrochemical assessments of ferric ions and hybrid coatings based on ferric ion/chromate as well as ferric ion/molybdate in 3.5% NaCl revealed the superiority of ferric coatings over hybrid coatings. Corrosion rate for ferric coatings was 0.037 mm/yr, while 0.246 mm/yr was recorded for ferric ions/chromate, and the rate for ferric ions/molybdate coatings was 0.304 mm/yr. From scanning electron microscopy, the coatings consisted of irregularly shaped materials that highlighted and decorated the grains and sub-grain boundaries of their substrates. However, cracked morphologies, which were thought to be weaknesses in the coatings, prevailed in hybrid coatings. Energy Dispersive X-ray analyzer, within the limits of detection, revealed that the elemental components of the coatings were Al and O for ferric coatings; Al, O, and Cr for Ferric/chromate; and ferric/molybdate had Al, O, and Mo, respectively. There were no obvious disparities in the adhesion performance of the coatings as revealed by pull-off adhesion tests after exposure to an atmospheric environment for 168 h. These pretreatments are suitable replacements for highly concentrated generic chromate conversion coatings due to lower costs for process effluent treatments compared to effluents from the latter.