Polypyrrole Films Doped with Phosphomolybdate Anions on Al Surfaces - Formation and Corrosion Protection Characterisation

The present paper deals with some preliminary experimental results regarding electrodeposition of polypyrrole films doped with phosphomolybdate anions onto Al substrates involving an initial anodizing in nitric acid electrolyte; early studies proved that this stage offers a very good adherence of the further layer on Al surfaces. The phosphomolybdate doped polypyrrole films have been formed both galvanostatically and involving cyclic voltammetry, in aqueous solutions containing phosphomolybdic acid and pyrrole. In several experiments Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt) has been added, acting as mediator and improving the layer morphology and aspect. The films exhibited a very good adherence on the metallic substrate, with a uniform black color and showed classical cauliflower morphology, more dense and ordered in the presence of Tiron, according to SEM investigations. Raman spectroscopy and EDX analysis revealed the presence of doping anion within polypyrrole layer. Also EDX measurements suggested that the film incorporates a 1 : 9 ratio of phosphomolybdate anion to PPy units. When both Tiron and phosphomolybdate anions are present in the solution, phosphomolybdate and other related anions are preferentially incorporated within the layer. Based on the continuous immersion test in NaCl 0.5 M solution for 240 h, the phosphomolybdate entrapping into the polymer matrix was shown to provide a better corrosion protection in chloride medium.