Sacrificial Zn-Ni coatings by electroplating and hydrogen embrittlement of high-strength steels

Abstract

A review of sacrificial Zn-Ni electroplating coatings on high-strength steels is presented. These steels are used for heavy structural applications such as landing gears, etc., that are subjected to high stresses and corrosive environments in service. The electroplating process involving aqueous electrolytes invariably produces hydrogen. The emitted hydrogen can diffuse into substrate steel, contributing to the delayed failures by hydrogen embrittlement. Microstructural inhomogeneities arising from the heat treatments and defects produced during coatings and those inherently present in the steels can trap hydrogen emitted during plating. Dissolved and trapped hydrogen can slowly diffuse to the stress concentrations or crack tips, contributing to the delayed structural failures. Baking after plating helps to eliminate hydrogen to some extent, though it may introduce some thermomechanical stresses at the bimaterial interfaces. This review discusses a) the current state of sacrificial Zn-Ni coatings, b) their protection against corrosion of the substrate, c) the associated hydrogen embrittlement predominately under cyclic loads, and d) recent advances in terms of the compositionally modulated coatings for enhanced protection.