Novel chromium-free conversion coatings based on Zr/Ti/Mn/Mo compounds were developed at a pilot scale to improve the corrosion resistance of the AA2024-T3 and AA7075-T6 aluminum alloys for aircraft applications. The influence of the presence of Zr and Ti in the Zr/Ti/Mn/Mo conversion bath's formulation on the corrosion resistance of the coated alloys was investigated. The corrosion resistance provided by the conversion coatings was evaluated by salt spray exposure and potentiodynamic sweeps. Optical and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS) and atomic force microscopy (AFM) operating in the Kelvin Probe mode (SKPFM) were used to provide microstructural information of the coated samples that achieved the best results in the corrosion tests. The salt spray test evidenced the higher corrosion resistance of the coated samples compared to the bare surfaces for both alloys. The potentiodynamic tests showed that the corrosion current density decreased for coated AA7075-T6 and AA2024-T3 alloys, which indicated an obvious improvement of the corrosion resistance with all the processes for both alloys. Although the corrosion resistance of the coated samples appeared to be higher for the alloy AA7075-T6 than for the alloy AA2024-T3, both alloys achieved the best corrosion protection with the coatings deposited from conversion bath formulations containing no titanium salts. The microscopy analysis on the coated AA7075-T6 samples revealed that a local deposition of Zr compounds and, possibly, an oxidation process occurred in the vicinity of the alloy's intermetallic particles. The amount of the Zr deposits at these locations increased with coating's formulations without Ti, which provided the best corrosion resistance.The Cr-free conversion coatings developed in this study for the AA7075-T6 and AA2024-T3 alloys do not meet yet the strict requirements of the aircraft industry. However, they significantly improved the corrosion performance with respect to the bare alloys and could be a good starting point for further studies and optimization. (C) 2015 Elsevier B.V. All rights reserved.

Coatings, comprising an inner porous anodic alumina film and an outer polyaniline/TiO2 nanoparticle layer, were electrochemically synthesised on an AA2024-T3 aluminium alloy by single-step, anodic polarisation in an oxalic acid-based electrolyte. The morphology and composition of the coatings were examined by TEM, SEM and XPS, with the size and zeta-potential of the TiO2 nanoparticles in the oxalic acid solution also measured. Observation of the growth of the coating during of anodic polarisation revealed that a distinct, two-layered coating is formed from the early stages of polarisation, with the anodic film forming at a constant rate and the outer layer developing at a rate that decreases markedly with times beyond about 30 min. Nanoparticles, agglomerated in the electrolyte, migrate to the anode due to the negative zeta-potential and form the nanoparticle-containing layer at the surface of anodic film. Such particles are not incorporated in the pores due to agglomeration and disordered film porosity at the outer layer of the anodic film. (C) 2009 Elsevier B.V. All rights reserved.

Anodic alumina films containing polyaniline and TiO2 or ZrO2 nanoparticles were electrochemically synthesised on an AA2024T3 aluminium alloy in a single step anodising process carried out in oxalic acid electrolyte containing aniline and nanoparticles. The morphology and composition of the films were examined by scanning electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The resultant coatings, with a thickness of similar to 2.5 mu m, presented a polyaniline and nanoparticle rich layer in the near surface region, with a thickness in the range of 150-250 nm. The electrochemical polarisation curves revealed that the TiO2 containing films provide improved corrosion protection to the AA2024T3 aluminium alloy, showing a passive current density two orders of magnitude lower than the films with ZrO2 nanoparticles and the films without nanoparticles. The improved corrosion protection offered by the TiO2 nanoparticle containing coatings is attributed to the presence of the nanoparticle rich layer formed on the outer part of the coating, which acts as a barrier layer that blocks the pores of the anodic alumina film.

The corrosion protection afforded to an AA3105 aluminium alloy supporting an anodic film with incorporated polyaniline and TiO2 nanoparticles has been examined. The films were synthesised by simultaneous anodizing and electropolymerisation of aniline in the presence of nanoparticles. The morphology and composition of the films were probed by TEM, SEM, rf-GDOES and XPS. The resultant coatings comprised a thin porous anodic film of 2-3 mu m thickness, with an outer hybrid polyaniline/TiO2 layer of several tens nanometres thickness, with the dimensions of TiO2 nanoparticles being below 10 nm. Electrochemical impedance spectroscopy analysis and salt spray testing revealed that TiO2 containing films provide improved corrosion protection to the AA3105 aluminium alloy compared with the film without nanoparticles. The improved protection provided by the coatings containing TiO2 nanoparticles is attributed to the TiO2 particle-rich thin film layer formed on the outer part of the coating that acts as a blocking barrier layer for the anodic porous aluminium oxide film. (C) 2008 Elsevier B.V. All Fights reserved.

Anodic alumina films containing polyaniline and either TiO2 or ZrO2 nanoparticles were electrochemically synthesised on an AA2024T3 aluminium alloy by a single step anodising procedure in an oxalic acid electrolyte. The morphology and composition of the films were examined by SEM, TEM, GDOES and XPS. The resultant coatings, of thickness about 2.2 mu m, displayed a nanoparticle-rich layer in the near surface-regions, of thickness in the range 100-250 nm. Potentiodynamic polarisation behaviour revealed that the polyaniline and TiO2-containing films on the AA2024T3 aluminium alloy show a passive current density two orders of magnitude lower than for films with ZrO2 nanoparticles; and films without nanoparticles. The coatings with TiO2 nanoparticles, but without polyamiline, showed intermediate behaviour, with a passive current density one order of magnitude lower than the coatings with polyaniline and TiO2 nanoparticles. The improved barrier protection offered by the TiO2 nanoparticle containing coatings is attributed to the presence of the nanoparticle-rich layer formed on the outer part of the coating that blocks access to the pores of the anodic alumina film. (C) 2008 Elsevier B.V. All rights reserved.