Study of Microstructure and Corrosion Behavior of Multilayered Ni Coatings by Ultrasound-assisted Electrodeposition

Abstract

Multilayered Ni coatings were successfully deposited on martensitic stainless-steel substrates by electrodeposition method with intermittent ultrasound. It is a novel kind of multilayered microstructure of stacked-up sandwiches, whose compositions of each monolayer are the same to those of its adjacent layers. The grains of ordinary Ni layers are columnar, vertically to the substrate surface, but the grains of ultrasonic Ni layers grow parallel along the substrate surface. By multilayered structure, the pinholes running through the coating were obviously inhibited, because the multilayered Ni coating can cover the whole surface of the substrate without the direct pinhole passages down to the substrate. Therefore, its corrosion mechanism is the uniform corrosion. For the ordinary Ni coating, the direct corrosion on the substrate can occur through the pinholes, which are the convenient passages for corrosive media to reach the substrate easily. This behavior can lead to the Fe/Ni galvanic corrosion, coating adhesion reduction, and quick formation of coating cracks in the ordinary Ni coating.

Keywords:
multilayered coatings; ultrasound; corrosion; pinhole