New bio-based phosphorylated chitosan/alginate protective coatings on aluminum alloy obtained by the LbL technique

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Bio-based coatings were obtained by the layer-by-layer (LbL) technique from a native chitosan (Mw ≈ 30,000 g mol−1) or from two synthesized phosphorylated chitosan of different molecular weights (Mw ≈ 30,000 g mol−1 and Mw ≈ 250,000 g mol−1) combined with alginate and used for corrosion protection of an aluminum alloy (AA) 3003. First, the LbL growth was monitored by Fourier transform infrared spectroscopy on silicon wafers. Cross-sections of Si wafers were examined by scanning electron microscopy allowing the films thickness to be measured. With the native chitosan and in the presence of 10 bilayers, the LbL thickness was about 200 nm, whereas it was about 300 nm with the phosphorylated chitosan of low molecular weight and about 500 nm with the phosphorylated chitosan of high molecular weight. Then, the bio-based coatings were deposited on the AA3003 by the LbL technique and their protective properties were evaluated by electrochemical impedance spectroscopy (EIS) during immersion in a 0.1 M Na2SO4 solution. It was shown that the three LbL coatings improved the corrosion resistance of the AA3003. The resistance values, extracted at low frequency on the impedance diagrams were attributed to the resistance of the aluminum oxide layer (Rox). The Rox values were at least two orders of magnitude higher for the coated samples (108 Ω cm2) compared to the uncoated one (5 105 Ω cm2). There was no significant difference between the different LbL coatings, which mainly acted by decreasing the active surface area and have a limited barrier effect.