Corrosion Inhibition Study of Aqueous Vanadate on Mg Alloy AZ31

Abstract

Corrosion inhibition of AZ31 Mg alloy with aqueous vanadate was studied and has been attributed to the pH dependence of vanadate speciation. Immersion in tetrahedral coordinated vanadate species, present in neutral and alkaline solution, was shown to decrease corrosion current density and increase the breakdown potential, both of which were enhanced with longer immersion times. Exposure to octahedral coordinated vanadate, predominant in acidic solution, only slightly decreased corrosion current density. An acidic solution was adjusted to alkaline conditions and samples were immersed in the adjusted alkaline solution. Inhibition of these samples was weaker than that of samples immersed in initially alkaline solutions. Anodic inhibition was observed on samples treated in solutions containing tetrahedral species. SEM images showed that vanadate formed a film across secondary particles and the Mg matrix, and provided qualitative evidence that inhibition efficiency increased as the pH increased. XPS results indicated that film formation was associated with the reductive adsorption of vanadium oxoions. Exposure at pH 5.0 produced a film predominated by V 4+ . Exposure at pH values of 7.7 and higher, however, produced a film containing predominantly V 3+ . Mg alloys have attracted great attention due to their high strength/weight ratio, good thermal conductivity and other physical properties.1-3 They have been widely used in the automotive industry, as aerospace components, and in the field of electronics.4-9 However, Mg alloys have high chemical activities, and are susceptible to corrosion, especially galvanic corrosion arising from interactions with more noble materials such as steel.10 Their active corrosion behavior is a barrier to wider applications. Therefore, protection under various application conditions is a critical issue strongly worthy of investigation. 11,12Extensive research has been conducted with the goal of inhibiting corrosion and improving the corrosion resistance of Mg alloys. Many corrosion mitigation methods have been reported, including alloying, surface modification, coatings, and inhibitors.13-15 Birbilis et al. studied the effect of numerous alloying elements on the corrosion kinetics of Mg, graphically presenting the effect of anodic and cathodic kinetics on over 30 alloying elements. 16,17 These detailed studies of alloying elements provided beneficial information for the fabrication, modification and development of corrosion-resistant Mg alloys. Surface modification of Mg alloys includes anodizing, 18 hot diffusion, 19 laser treatment 20 and other treatments that affect the surface, but do not change the original mechanical properties of the alloy.15 Surface modification can increase polarization resistance and reduce the localized galvanic interactions that arise from the heterogeneous structure found in these alloys. 15,[18][19][20] Conversion coatings are another effective method to improve the corrosion resistance of Mg alloys.21 These coatings can provide excellent protection of the s...

The Effect of Vanadate, Phosphate, Fluoride Compounds on the Aqueous Corrosion of Magnesium Alloy AZ31 in Dilute Chloride Solutions

“…In moderately alkaline solutions, vanadate alone has the effect of increasing the corrosion potential, increasing the breakdown potential slightly, and decreasing the corrosion rate. [21] When mixed together under the conditions of these experiments, hydrophsophate and vanadate combine to strongly decrease the corrosion potential, increase the breakdown potential and decrease slightly the corrosion rate of AZ31 compared to chloride-only exposures.…”

“…Because the surface conversion of magnesium alloys is an essential part of coating systems for magnesium alloys, and due to restrictions on the use of chromate-based conversion processes, protective non-chromate conversion coating materials and processes are needed. For example, surface conversion chemistries based on phosphate, cerium, fluoride, vanadate, selenite and molybdate have demonstrated a protective effect on magnesium [15][16][17][18][19][20][21][22][23].…”

“…Vanadate has been shown to be a good inhibitor of the 2024-T3 aluminum alloy [24][25][26], and a vanadate conversion coating process that demonstrates a self-healing response has been reported for that alloy [19]. Prior studies of vanadate inhibition on Mg alloys indicate that vanadate alone does not provide a sufficiently potent inhibiting effect on Mg by itself, and one or more companion inhibitors may be needed for technological applications [20,21,27]. Vanadium is not a cancer-causing chemical in mammals, but may do damage to the respiratory system under high concentration exposure [22].…”

The Effect of Vanadate, Phosphate, Fluoride Compounds and Their Effect on the Aqueous Corrosion of Magnesium Alloy AZ31 in Dilute Chloride Solutions

“…Because surface conversion of magnesium alloys is an essential part of coating systems for magnesium alloys, and due to restrictions on the use of chromate-based conversion processes, protective non-chromate conversion coating materials and processes are needed. For example, surface conversion chemistries based on phosphate, cerium, fluoride, vanadate, selenite and molybdate have demonstrated a protective effect on magnesium [12][13][14][15][16][17][18][19][20].…”

“…Vanadate has been shown to be a good inhibitor on 2024-T3 aluminum alloy [21][22][23], and a vanadate conversion coating process that demonstrates a self-healing response has been reported for that alloy [16]. Prior studies of vanadate inhibition on Mg alloys indicate that vanadate alone does not provide a sufficiently potent inhibiting effect on Mg by itself, and one or more companion inhibitors may be needed for technological applications [17,18,24]. One candidate companion is phosphate or a hydrophosphate species.…”