Corrosion Resistance and Formation Analysis of a Molybdate Conversion Coating Prepared by Alkaline Treatment on Aluminum Alloy 6063

Abstract

A molybdate-based conversion coating (MBCC) was prepared on aluminum alloy 6063 (AA6063) by treatment in an alkaline conversion bath. The obtained MBCC is a gray and uniform conversion coating. Scanning electron microscope and electrochemical measurements were used to analyze the basic properties of the coating. X-ray photoelectron spectroscopy (XPS) was utilized to investigate the composition and formation process of the coating. The surface morphology and cross-sectional observation show that the coating grows rapidly under given conditions, its thickness can reach more than 30 μm in 10 min, and the MBCC prepared with the alkaline method is a non-cracked coating. The potentiodynamic polarization test indicates that the corrosion current of MBCC is more than one order of magnitude lower than the corrosion current of an untreated surface. The XPS measurement illustrates that the coating is made up of Al, Mo, O and F. According to the high-resolution XPS spectral analysis, the elements detected in the coating are in the form of Al 2 O 3 , AlF 3 , MoO 3 and Al 2 (MoO 4 ) 3 . Based on the XPS results, the formation process of the coating under alkaline conditions was discussed.

Optimizing corrosion resistance of Fe35Ni20Cr12Mn28Al5 high-entropy alloy: synergistic effect of Mo inhibitor, Al content and cold rolling

“…Additionally, Mo can be oxidized to form molybdate ion (MoO 4 2− ) with Al, and hydrogen ions may be deposited in an insoluble compound, Al 2 (MoO 4 ) 3 , in the conversion coating as the following reaction [equation (6)] (Huang et al , 2019): …”

Growth process of molybdate conversion coating on the surface of aluminum foil and its adhesive mechanism

“…The peak at 73.5 eV is attributed to Al, whereas the other peak is the combination of two peaks at 75.6 and 76.1 eV, respectively, 5…”

“…However, the use of hexavalent chromium is restricted by its high toxicity to humans and environmental contamination. 5 In order to overcome these problems, a number of alternative treatments are proposed using chrome-free solutions such as phosphate (P), 6,7 rare earth salt, 8 permanganate, 9 titanate/zirconate (Ti/Zr), 10,11 and molybdate (Mo). 12,13 However, some problems remain to be solved for these methods.…”

Fabrication and corrosion resistance research of stearic acid-doped silane composite passivation film on electrolytic manganese

“…However, the wastewater in this corresponding process containing hexavalent chromium and phosphate enrichment, results in serious water pollution [12], hazarding human health, and severely going against the requirements of environmental protection. Therefore, researchers have been exploring the various of environmentally friendly passivation films with excellent anti-corrosion effects, such as molybdate [13,14], silicate [15,16], titanate [17,18], vanadate [12,19,20], rare earth salt [21][22][23] and so on. For instance, Qiu et al [18] successfully prepared a titanium passivation film on the surface of electrolytic manganese by adjusting the conditions of the titanium solution, and its corrosion resistance is better than that of the Cr coating.…”