Corrosion Inhibition of Aluminum Alloy 2024-T3 by Aqueous Vanadium Species

Abstract

Nuclear magnetic resonance ͑NMR͒ measurements were made on aqueous vanadate solutions to characterize speciation as a function of pH and vanadate concentration. Additionally, potentiodynamic polarization measurements were carried out on Al alloy 2024-T3 in 50 mM NaCl solutions in which pH and vanadate concentration were systematically varied. Results showed that inhibition by vanadates occurred mainly in alkaline solutions where tetrahedrally coordinated vanadates, metavanadate and pyrovanadate, were abundant. Inhibition was not observed in solutions where octahedrally coordinated decavanadates predominated. Anodic inhibition, in the form of increased pitting potential, was observed in both aerated and deaerated solutions. In contrast, cathodic inhibition was observed only in aerated solutions acting primarily through the suppression of oxygen reduction. Energydispersive spectroscopy, used to collect chemical maps from aluminum coupons exposed to vanadate solutions, showed the suppression of Al 2 CuMg particle dissolution compared to vanadate-free solutions. NMR measurements were also used to track changes in vanadate speciation with time, pH adjustment, and with exposure to metallic aluminum surfaces. NMR showed noninhibiting octahedrally coordinated decavanadates rapidly decompose into inhibiting tetrahedrally coordinated metavanadates and pyrovanadates after alkaline pH adjustment. While decomposition begins immediately upon pH adjustment, equilibrium may not be reached even after significant time periods.

Chromate replacement: what does the future hold?

“…It was observed that firstly, decavanadate ions Several studies demonstrated the presence of Cr(VI) within the conversion coating 45,46 Suspected to be carcinogenic 50 RE-based (Ce or La) AA2024-T3 57,58,63 Mainly works as cathodic inhibitors, precipitation of Ce-oxide on cathodic sites, induced by the local pH increase Vulnerability of RE market as mainly dominated by Chinese production 66,67 Galvanized steel 62 Only minor improvement in pitting potential and corrosion current was noted 56,65 Vanadate-based AA2024-T3 zinc carbon steel 69 Adsorption of inhibitor on cathodic sites (intermetallic particles in the case of AA2024-T3) and stabilization of passive film. Inhibition of oxygen reduction reaction [20][21][22]70 Do not meet environmental and health restrictions as Vanadium and its compounds were proven to be carcinogenic [72][73][74] Li-containing Only tested on AA2024-T3 Good barrier properties provided by both systems Organic: effective inhibitor is still needed to achieve good corrosion resistance Nanocomposites: higher density of inhibitor can be loaded into the system Nanocomposites: cost towards implementation could be too important 99,100 Phosphate-based Steel carbon steel Al alloys zinc were controllably released from the LDH containers and also that the aggressive chloride ions were also entrapped within such containers. Organic inhibitors which have shown potential in substituting chromate-based inhibitors, especially in hybrid sol-gel coatings upon Al alloys include benzotriazole, tolyltriazole, 2-mercaptobenzothiazole, mercaptobenzimidazole, 8-hydroxyquinoline, salicylaldoxime, quinaldic acid.…”

“…17 The remarkable corrosion resistance of chromates is persistent over a very wide range of pH and electrolyte concentrations; 18,19 a feature unable to be matched by alternative inhibiting compounds to date, most having finite operating pH and minimum concentration requirements. [20][21][22] The mechanism of corrosion inhibition from chromates, irrespective of chromates being known utilized for many decades prior, is something that has really only been understood in more contemporary times. 19,[23][24][25] Unlike most inhibiting compounds, chromates are both anodic and cathodic inhibitors, meaning that they can restrict the rate of metal dissolution, 19,[26][27][28] and simultaneously reduce the rate of reduction reactions (both oxygen reduction and water reduction) 19,26,27,29 in many environments (Fig.…”

“…69 A few decades later, promising vanadate-based conversion coatings on Al alloys 2024 were successfully developed, as they were shown to adsorb on the cathodic sites, hence significantly inhibiting the oxygen reduction rate (ORR). [20][21][22]70 However, concerns have been raised towards the carcinogenicity and mutagenicity of vanadium and its compounds, 71 in the context of human administration for diabetes treatments, 72 but also with clear evidence of lung tumor in rat and mice after V 2 O 5 inhalation. 73 Although vanadium compounds display interesting properties, and not only in the materials engineering field, the carcinogenicity of these compounds is now proven by numerous studies 74,75 making this alternative no longer viable.…”

Vanadate post-treatments of anodised aluminium and AA 2024 T3 alloy for corrosion protection

“…It is noteworthy that the levels of corrosion protection provided by the double dip processes are achieved without the requirement for sealing of the films, and also that the corrosion tests were carried out for much longer than the times often employed for neutral salt spray testing of sealed panels treated by CAA and SAA, usually about 300-336 h. 19,20 The protection provided by the double-dip processes is considered to be due to corrosion inhibition provided by vanadate ions that suppress the cathodic reaction [12][13][14] and increase the pitting potential. 15 The low solubility of heavy metal vanadates assists the retention of vanadates in the unsealed pores. Hence, vanadates remain available to provide protection throughout the period of the salt spray tests.…”

“…14 Inhibition was observed mainly in alkaline conditions when tetrahedrally coordinated vanadate, metavanadate and pyrovanadate were present and the pitting potential was increased. 15 An increased pitting potential and suppressed dissolution of S phase was reported for a range of copper-containing intermetallic particles with vanadate addition to alkaline sodium chloride solutions. 16 Monovanadates have been found on the matrix of AA 2024-T3 alloy in an acidic solution and a polyvanadate film on intermetallic particles in a basic solution.…”