Porosity tailoring of electrophoretically derived zirconia coatings using acidic and alkaline-based sol-gel post-treatment to enhance anti-corrosion performance

Abstract

In this work, the post-treatment sol-gel methods based on acidic and alkaline catalyzed tetraethylorthosilicate (TEOS) solutions were applied to refine the porosity network of electrophoretically deposited ZrO2 coating on anodically oxidized 316 L substrate to improve the corrosion resistance. The ZrO2 coating dipped in alkaline TEOS solution manifested the best barrier performance, arising from the formation of a dense continuous SiO2 interlayer at the coating/substrate interface as well as narrowing the interconnected pores in the coating's body. This was related to the easily inward diffusion of the alkaline hydrolyzed TEOS molecules with an inert surface charge into the negatively charged ZrO2 coating through the open pores toward the interface. However, the acidic-catalyzed TEOS molecules mostly preferred to form a relatively thin SiO2 layer on the surface of ZrO2 coating, which provided a poor barrier performance. This corresponded to the established electrostatic repulsion forces between the positively charged ZrO2 particles and acidic-hydrolyzed TEOS molecules with positive surface charge, which restricted the unlimited diffusion of TEOS molecules. Based on the microscopic observations and electrochemical impedance results, a possible mechanism for refinement of the porous network of ZrO2 coating and formation of SiO2 layers in the acidic and alkaline TEOS solutions was suggested.