Electrochemical Modification of the Aluminum Surface by Cathodic Treatment in Chitosan-Containing Aqueous Phosphate-Molybdate Solutions

The state the morphology of the surface of the aluminum electrode, the elemental composition of the surface layer before and after cathodic treatmentin the galvanostatic mode are investigated. It was found that in the range cathodic current densities less than 0.1 mA/cm2 on the aluminum cathode mainly adsorption of polymolybdate and polyphosphate molybdate ions occurs, accompanied by the formation in the adsorbed layer of polymer chains from molybdenum oxides of intermediate valence Mo (VI) → Mo (IV) → Mo (II) double oxides, aluminates, aluminum polyoxophosphate molybdates, providing free movement of alkali metal and hydrogen cations. According to the data of currentless chronopotentiometry secondaryionic mass-spectrometry and scanning electron microscopy, the formation of a layer of heteropolyoxophosphatemolybdates occurs already in the absence of current. The decisive role in the formation of the morphology modificating layer and its elemental composition is played by the introduction of phosphoric acid sodium molybdate solution. At 0.5 mA/cm2 and higher cathodic current densities, the process of hydrogen evolution predominantly occurs both by the reaction of the discharge of hydrogen ions and water molecules, and due to the chemical interaction of aluminum and the resulting alloy of aluminum with sodium with water molecules and phosphoric acid. This is indicated by alkalinization of the near-electrode layer of the solution. The rate of hydrogen and sodium intercalation ions into the structure of the polyoxophosphatemolybdate layer increases sharply with an increase in the current density to a value of 5... 10 mA/cm2, when a potential of –1.3 ... –3.0 V is established on the electrode. This is facilitated not only by the fibrous structure of the forming layer of aluminum and sodium polyheterooxophosphate-molybdates, but also the formation of protonated cations and anions of phosphoric acid in solution, which facilitate the interaction of chitosan with the surface of the electrode, due to the transition to the phosphate complex, and provide an enhancement of the film-forming effect and ordering of the structure of the modifying layer. This is indicated by a decrease in the angle of contact.