Effect of coating thickness and grain size on the electrochemical properties of hydrothermally deposited nano-ZrO2coatings on stainless steel surface

Highlights: • Nano-ZrO₂ coatings of various thicknesses are deposited on pre-oxidized steel. • Tetragonal and monoclinic phases are formed at different stages of the growth. • Corrosion current reduces with the increase in coatings thickness. • Corrosion resistance of coating with grain size of ~1 μm reduces. -- Abstract: Nano-crystalline Zirconia (ZrO₂) coatings of different thicknesses were grown on pre-oxidized stainless steel (SS) by hydrothermal method in a static autoclave. Hydrothermal deposition process was repeated multiple times in order to enhance the coating thickness. Four cycles of the deposition process led to the development of ~1 μm thick ZrO₂ coating on the pre-oxidized SS surface. Corrosion resistance properties of the pre-oxidized SS and ZrO₂ coated samples were studied by potentiodynamic polarization (PDP) technique and electrochemical impedance spectroscopy (EIS). First round of ZrO₂ deposition resulted in ~200 nm thick, continuous but porous ZrO₂ coating. Porosity in the coating reduced drastically after the second round of deposition. PDP and EIS analyses showed that the corrosion resistance of SS specimens improved with coating thickness. Growth of both tetragonal and monoclinic phases of ZrO₂ in the coating was observed after second round of deposition process. Intrinsic stresses due to change in the phases were accommodated at the grain boundaries when grain size was small (~ 200 nm), but it deteriorated the coating's corrosion resistance properties when the grain grew to the size of around 500–1000 nm.