Vanadate - based conversion coatings for the aluminium alloy 2014A - T6
The aluminium alloy 2014A-T6 is used as an aerospace alloy due to its strength and rigidity. The disadvantage of this alloy is that it is susceptible to galvanic corrosion, this occurs due to the fact that the main alloying element, copper, forms discreet precipitates principally of CuAl2 which are noble to the main aluminium matrix. One of the main ways in protecting the alloy from corrosion is by the application of a passive chromate conversion coating. Due to legislation, chromates are being outlawed, so replacements are being sought Investigations into sodium orthovanadate as an alternative conversion coa ing, showed that it provided a degree of protection against chloride ion attack from both salt spray testing and immersion in 3.5% sodium chloride. Tests were carried out to see if coating of the copper intermetallic could reduce the amount of galvanic attack the 2014A-T6 Al alloy was subjected to. Investigations were carried out in to the inhibition the copper surface by the use of sulphur-based compounds. The results showed that inhibition did not occur. However, further investigations into organic acids such as sebacic acid showed that when added to the sodium orthovanadate solution they promoted greater corrosion protection by reacting with the alkaline sodium orthovanadate solution to form sodium sebacate. The sodium sebacate was able to block cracks on the coating surface thereby inhibiting ingress of chloride ions to the surface of the 2014A-T6 Al alloy. This was proved by the use of scanning electron microscopy, which showed the presence of fine needle like features of precipitated sodium sebacate. X-ray photoelectron spectroscopy results showed that there was a high possibility that sodium sebacate was present on the surface due to the presence of both carbon and sodium which were not present when the alloy was treated from a solution containing just sodium orthovanadate. A variety of coatings were analysed using D. C. electrochemical polarisation, it was found that the sodium orthovanadate treatment, containing the sebacic acid, gave a considerable increase in the corrosion resistance in 3.5% NaCl compared to all other coatings except chromate. However, the sodium orthovanadate coating containing sebacic acid showed a reasonably close corrosion. resistance to the Alochrom 1200 chromate coating when subjected to salt spray corrosion tests. Simulated samples of 2014A-T6 Al alloy were produced by coupling laboratory produced CuAl2 and commercially produced aluminium. The results obtained using D. C. electrochemical polarisation gave similar trends to that obtained by a commercial sample of 2014A-T6 Al alloy showing that the sodium orthovanadate/sebacic acid coating solution was highly effective. This was backed up by the use of zero resistance ammetery which showed that the sodium orthovanadate/sebacic acid at 60 seconds at 60°C showed broadly comparable results with chromates up until 190 hours, when the coated couples were immersed in 3.5% NaCl at 25°C.