Investigations of conductive polymer coatings for corrosion protection of aluminum alloys

The anti-corrosion properties of a novel double strand conductive polymer coating on aluminum alloys in salt environments as a possible replacement for chromium conversion coatings was investigated. The conductive coating consisted of a double helix backbone structure of polyaniline wrapped with an esterified polyacrylic acid strand. This polymer complex offers advantages over other conductive polymer systems in material stability and processability. The polymer is soluble in single organic solvents and was sprayed onto square AA7075-T6 and AA2024-T3 aluminum alloy samples.; The coatings were investigated for corrosion protection properties using cyclic polarization, electrochemical impedance spectroscopy (EIS) and long term exposure in aggressive salt environments. The polymer coated samples were tested against two commercial chromate conversion coatings and uncoated alloy samples. The cyclic polarization tests show a two order of magnitude lowering of the corrosion current over the uncoated samples, a one order of magnitude lowering of I{dollar}sb{lcub}rm corr{rcub}{dollar} over the second type of chromate coating and a one order of magnitude higher I{dollar}sb{lcub}rm corr{rcub}{dollar} over the first type of chromate coating. Impedance results show a two order of magnitude higher impedance at low frequencies over the uncoated and equivalent performance for both types of chromate coatings.; EIS and cyclic polarization tests of the conductive state (green colored) versus a non-conductive state (blue colored) of the polymer coating shows that the electrically conductive state is more effective than the insulating state for corrosion protection. This data indicates that the electro-active nature of the polymer coating plays a role in the protection of the alloys. After two months exposure in a.5N NaCl solution, there is no evidence of pitting at the alloy surface for the polymer coated samples under scanning electron microscopy (SEM) images, indicating a change in the typical corrosion mechanism for aluminum alloys.; The molecular structure of the polymer coating may offer the ability to chelate with the aluminum oxide layer, the metal layer or reaction products. Other researchers studying inhibitor effects have shown that this chelation is one possible way to prevent or slow the pitting process on aluminum alloy surfaces.