Effect of organic additives on the performance of titanium-based conversion coatings

Abstract

The performance on aluminium–manganese alloy 3003 of hexafluorotitanic acid (H 2TiF 6) conversion treatments incorporating either poly(acrylic acid) (PAA) or tannic acid (TA) has been studied and compared with coatings without organic additions and coatings containing only the two organic compounds. Polarisation curves were first used to assess the performance, and elucidate protective mechanisms in NaCl solutions. It is shown that for short immersion times, the addition of the organic components gives pitting potentials (defined here as the potential at which the pitting current passes 0.01 mA cm −2) higher than for H 2TiF 6 alone, with pitting currents increasing more slowly as the scan continued. However, after longer immersion, whereas the H 2TiF 6 based coatings without inhibitor shows steadily increasing pitting potentials, improvement with inhibitors was less marked, so that after three days the uninhibited coating showed the highest pitting potential. Pitting inhibition for the combined systems (defined here as the degree of separation between corrosion potential and pitting potential) is limited, and after longer immersion times is usually worse than H 2TiF 6 alone, and sometimes worse than bare alloy. Corrosion currents fall markedly with time for most samples, but the TA- and PAA-containing coatings also show strong inhibition when first exposed. To investigate the separate protective effects of the inorganic Ti-containing film and the organic additives, AC impedance spectra were measured for coatings of PAA or TA without the inorganic component and compared with spectra for coatings of H 2TiF 6 alone [Surf. Eng. 15 (1999) 407]. All three coatings show polarisation resistances increasing significantly over time, indicating falling corrosion rates.