Influence of molybdophosphoric acid on the kinetics of the anodic coating dissolution

• Published in: Surface & coatings technology Vol. 357; pp. 535 - 542
• Main Authors: Kwolek, Przemysław, Pustuła, Aneta, Nowak, Wojciech J.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15-01-2019
• Subjects: Aluminium alloy; Anodic coating; Corrosion inhibitor; Orthophosphoric acid; Sodium molybdate; Sodium molybdate; Orthophosphoric acid; Anodic coating; Aluminium alloy; Corrosion inhibitor
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2018.10.055

Aluminium finishing industry requires a stripping solution for anodic coatings for determination of their weight. The commonly applied one contains orthophosphoric acid as etchant and chromium trioxide as the corrosion inhibitor of metallic substrate. The latter, due to its toxicity, should be replaced, potentially with sodium molybdate. When sodium molybdate is dissolved in orthophosphoric acid, molybdophosphoric acid is formed. In this work the kinetics of dissolution of anodic coating was studied in 0.5 M H3PO4. Moreover, the influence of the initial concentration of Na2MoO4 in this solution was established. It was shown that molybdophosphoric acid species adsorb onto the surface and within the pores of anodic coating and decrease its dissolution rate in acidic solution. The mechanism of this process was evaluated by using Electrochemical Impedance Spectroscopy. Analysis of the impedance spectra revealed that adsorbed molybdophosphoric acid species increase the capacitance of the porous part of the coating from 0.7 to 1.1 μF and the resistance of its barrier layer from 0.3 MΩ to 2 MΩ in the solutions containing 0 and 25 mM of Na2MoO4 respectively. The corrosion potential of the specimens also increased from −400 mV vs. Standard Hydrogen Electrode to −200 mV in the solutions containing >25 mM of Na2MoO4.
•Molybdophosphoric acid adsorbs onto the surface of the coating.•Molybdophosphoric acid penetrates within the pores of the coating.•Molybdophosphoric acid increases the corrosion potential of the anodised specimens.•The capacitance of the coating and the resistance of the barrier layer increase.•Molybdophosphoric acid decreases the rate of dissolution of the anodic coatings.