The replacement of chromate by molybdate in phosphoric acid-based etch solutions for aluminium alloys

Abstract

Corrosion inhibition of aluminium alloys in the acidic solutions is important in the aluminium finishing industry (stripping solutions for anodic films). Sodium molybdate is a potential replacement for chromate-containing species used in the stripping solutions. It was tested as the corrosion inhibitor of 2024 aluminium alloy in orthophosphoric acid solutions. A corrosion rate of the alloy as a function of an initial concentration of sodium molybdate and temperature was determined by using gravimetric and gasometric methods. The corrosion rate was significantly lower in the solutions containing over 5 mM of sodium molybdate when compared to the uninhibited solution. The inhibition efficiency decreased with the increase of the temperature for the constant concentration of sodium molybdate. Sodium molybdate reduced the corrosion of the alloy by adsorption as well as formation of insoluble corrosion products on the alloy.

Corrosion of structural constituents of 2017 aluminium alloy in acidic solutions containing inhibitors

“…It can be concluded that Al 2 Cu, significantly enhancing dissolution of the matrix and adjacent intermetallics, determines the corrosion behaviour of the studied alloy. This behaviour can be compared to Al-Cu cast alloys, 2024 [5] 7075 [7] the dendrites increases outward from their centre. [24] Unfortunately, the corrosion mechanism of these alloys was rarely studied in acidic solutions.…”

“…The first one is insufficient corrosion protection of aluminium alloys, especially with a high content of copper. [ 5 ] This requires further studies of corrosion behaviour of intermetallics, existing in aluminium alloys, in H 3 PO 4 –Na 2 MoO 4 system. The second one is the precipitation of an insoluble Mo‐containing salt on the metallic surface.…”

“…[10] Corrosion inhibition in the H 3 PO 4 -Na 2 MoO 4 system has already been reported for 1050, 2024 and 7075 aluminium alloys, but the roles of the heteropolyoxomolybdate and heteropoly blue species remained unclear. [4][5][6][7]11] Recently, it was shown that heteropolyoxomolybdate species, when adsorbed onto a metallic surface, inhibit anodic reaction in the corrosion cell, whereas heteropoly blue formation is detrimental. [7] The most important heteropolyoxotungstate is [PW 12 O 40 ] 3 13 O 41 ] 7− .…”

“…[12,13] So far, heteropolyoxomolybdates have been used as the corrosion inhibitors of various aluminium alloys, but these studies have been mostly focused on the influence of the chemical composition of the environment on the corrosion kinetics and inhibition efficiency. [4][5][6][7]11] This study is focused on the corrosion behaviour of structural components of 2017 aluminium alloy in H 3 PO 4 aqueous solutions containing heteropolyoxoanions. The alloy, formerly called duralumin, was used for the construction of the first all-metal transport aircraft, Junkers F 13.…”

Orthophosphoric acid solutions of sodium orthovanadate, sodium tungstate, and sodium molybdate as potential corrosion inhibitors of the Al2Cu intermetallic phase

“…Heteropolyoxoanions can be formed by about 60 elements on the periodic [24,[27][28][29]. Interestingly, iso-and heteropolyoxomolybdates also decrease the dissolution rate of anodic coatings in solutions of orthophosphoric acid [30].…”