Formation Mechanism and Properties of Titanate Conversion Coating on AZ31 Magnesium Alloy

Abstract

A titanate conversion coating was made on AZ31 magnesium alloys in a titanium chloride and hexafluorosilicic acid solution. The titanate conversion coating formation proceeded via the formation of a porous magnesium hydroxide and fluoride layer. Compact silicon hydroxide and titanium hydroxide were then formed locally on top of the porous layer. The fluorine ion dissociated from hexafluorosilicic acid contributed to the incorporation of magnesium fluoride in the porous layer. Moreover, a pH rise due to hydrogen discharge caused the precipitation of silicon hydroxide and titanium hydroxide. With continued immersion, the porous layer thickened and the coverage of the compact silicon hydroxide and titanium hydroxide proceeded to a larger extent, which, in turn, reduced the corrosion area fraction and enhanced the polarization resistance of the AZ31. However, prolonged immersion resulted in more cracks with larger openings, which reduced the corrosion resistance of the titanate conversion coating. As a result, the titanate conversion coating formed at immediate immersion times provided the best corrosion protection on the AZ31.Eco-awareness has been gradually raised in recent years, such as the need for energy saving and carbon reduction in transportation systems. Magnesium alloys are known as one of the green materials in the 21th century. They have excellent physical and mechanical properties, including low density, high specific strength and stiffness, excellent damping effects, and good electromagnetic shielding characteristics. 1-3 Because magnesium resource is abundant on earth, magnesium alloys have the potential to be used widely in the industry. However, magnesium has a low reduction potential (-2.37 V) and is readily oxidized in nature. According to the Pourbaix diagram of magnesium, 4 magnesium ion stably exists in the aqueous solution with pH value less than 8.5, signifying its poor corrosion resistance in the atmosphere. To enhance the corrosion resistance of magnesium alloys is thus crucial before their extensive applications. 2,3,5,6 Several surface modification treatments on magnesium alloys have been developed, including chemical conversion coating, anodization, electroplating, electroless plating, and nitrogen ion implantation. 7-12 Among them, chemical conversion coating treatment is known as an efficient and low-cost process. Moreover, the conversion coating improves the corrosion resistance of magnesium alloys and the adhesion of the organic paint. Chromate conversion coating is known for its excellent corrosion resistance and unique self-healing properties. 13,14 However, its usage has been banned by several environmental directives due to the toxicity to human health and ecological environment. Several non-chromate conversion treatments have thus been studied, including phosphate, phosphate/permanganate, rare-earth metal salt, stannate, and the solution containing transition metal compounds such as vannadate, molybdate, zirconate, and titanate. 7, 8, 15-36 Among the various conversion coating t...

Advanced characterization study of commercial conversion and electrocoating structures on magnesium alloys AZ31B and ZE10A

“…However, relatively few studies have pursued advanced microstructural characterization of the local chemistry, morphology, and structure of the coating and coating/substrate interface, e.g. for Mg substrates [42,43] and for Al substrates [44][45][46]. Such understanding is of particular importance for conversion coatings, as the resultant surface and its corrosion resistance are influenced not only by the coating solution chemistry and process conditions, but also by the substrate Mg alloy composition and microstructure [17,19,40].…”

Electrochemical behaviors of magnesium alloy with phosphate conversion coating in NaCl solutions

“…0, the equivalent circuit will be simplified into a representative impedance model for the coated systems (Fig. 4e) [44,45]. In Fig.…”

Corrosion protection of magnesium and its alloys by metal phosphate conversion coatings

“…The minimum corrosion area fraction (0% and 0?2 mg cm 22 day 21 ) was seen for Sr-P coating, which in fact offers corrosion protection better than that of the DOW chromate based conversion coating and other conversion coatings. 6,34 It has been depicted that after 72 h salt spray exposure, the Sr-P coating demonstrated the best anticorrosion performance.…”