Modeling of Corrosion and Hydrogen Embrittlement of ZnNi Coated High Strength Steel 4340 Under Atmospheric and Immersion Conditions

High Strength steels are extensively used for aerospace structural forgings such as landing gear, fasteners and airframe fittings. ZnNi coatings can be used to protect the steel. This coating can provide equal to or greater corrosion protection than cadmium coatings, while at the same time eliminating the risk of exposure to a carcinogenic material. Previous research has shown that the ZnNi plating process does not cause Hydrogen Embrittlement (HE) when performed properly, but it can lead to problems if damaged. No systematic modeling has been performed to understand the effect of defect size on HE under atmospheric (ATM, 84% RH and 49.2 g/m2 NaCl deposition) and immersion (IMM, 0.6 M NaCl) conditions. The main objective of this work is to provide insightful information through modeling to understand the effect of coating defect size and exposure environment on the corrosion and HE of ZnNi plated high strength steel 4340. To accomplish this objective, Elsyca Corrosion Master software was used to model the electrochemical kinetics of a ZnNi plated tensile bar with and without coating defects. Solidworks was used to draw and modify the computer-aided design (CAD) model indicating the base and coating materials. Cad2Mesh processes the model to generate a surface mesh with automatic refinement near edges and dissimilar material boundaries for Elsyca Corrosion Master. Experimental Polarization data under IMM and ATM conditions, found in the literature, was utilized for this model. This data was deconvoluted using Curve Analyzer software to use it as an input data for the Corrosion Master software. It has been observed that the defect size and exposure environment (electrolyte film thickness, conductivity, diffusion layer thickness and solubility of oxygen) have a noticeable effect in the corrosion and HE of ZnNi coated steel. The corrosion rate, corrosion potential, and hydrogen current density were obtained utilizing the Xplorer software. The results indicated that as the defect size decreases the corrosion rate of the ZnNi coating decreases. In addition, the steel potential decreases, resulting in higher hydrogen current density on 4340 steel, which translates to higher potentiality for HE. In terms of exposure environment, corrosion rate and hydrogen current density were higher for the ATM condition than for the IMM condition, leading to a higher susceptibility for HE under ATM than under IMM condition.