Effects of inorganic additions on the performance of manganese-based conversion treatments

The electrochemical behaviour of permanganate-based no-rinse conversion coatings on AA3003-H14 aluminium alloy shows that they have promise as an alternative to toxic chromate-based coatings. A selection of inorganic additions has been made here to a simple 0.1 M KMnO₄ formulation investigated previously. The performance of the different coatings has been evaluated from potential-time and polarisation measurements in 0.5 wt % (0.085 M) NaCl solution for periods up to 3 days. The addition of fluoride ions produced no benefit, whereas Al(NO₃)₃ shows an improvement when added in small amounts. Tests with the individual ions reveal that it is Al³⁺ cations, rather than anions, which produce the greatest benefit.