Correlation between morphology and electrochemical behavior of chromium-free conversion coatings for aluminum alloys corrosion protection

Chromium(III)-based, tungstate-based and cerium(III)-based conversion coatings on AA2024-T3 are compared in terms of morphology and electrochemical behavior. The durability of coatings is studied under neutral salt spray conditions and in a weakly aggressive electrolyte composed of 0.1 mol/L Na2SO4 and 10−3 mol/L NaCl. The use of both microscopic and electrochemical analyses such as fitting of electrochemical impedance spectroscopy diagrams allows one to have a better insight into the influence of the interfacial and conversion layers on the degradation phenomena. The role of the remaining native interfacial oxide layer is particularly highlighted, its resistance being the highest at the beginning of the exposure. The degradation of the barrier effect of this interfacial layer is demonstrated to be closely related to the performances of the conversion layer both in terms of passive protection and a reservoir of corrosion inhibitor. Finally, coupled analyses from EIS fitting and microscopic observations allow one to reach a precise interpretation of the strengths and weak points of such system. This diagnostic is an important step towards the optimization of the chromium-free systems.