ZnNi coating on UHSS for aeronautical applications: effect of base material and coating’s microstructure on the degassing process for preventing hydrogen embrittlement

Ultra-high strength steels (UHSS) are currently used in applications where high mechanical loads are required as it is the case of landing gears in aeronautical applications. Among the wide range of UHSS available in the market, 300M is one of the preferred choices for aeronautical components manufacturing. However, despite the excellent mechanical properties exhibited by this UHSS components, their corrosion resistance is far from fulfilling the requirements of the aeronautic sector. Historically, UHSS have been plated with sacrificial coatings for enhancing the corrosion performance of the components, where cadmium coatings were applied on UHSS for providing corrosion protection. However, due to the increasingly stringent environmental regulations, cadmium coatings are being replaced by the more benign ZnNi alloy coatings. As ZnNi plating presents a lower efficiency with respect to Cd plating, a higher amount H is incorporated into the coating, and it is susceptible to migrate into the base material where it can contribute to hydrogen embrittlement. To prevent the risk of H embrittlement the plated components are subjected to H degassing which is affected both from the initial H content and the coating microstructure. Additionally, H diffusion into the base material and its effusion behaviour is also affected by the nature of the UHSS as well as the pre-treatment and plating process sequence, which might be different depending on the material nature of the substrate.
The aim of this work is to investigate the influence of the base material and the ZnNi coating´s microstructure on H content and degassing efficiency of electroplated ZnNi coatings on different UHSS. Thus, EZ2NKD18 and CUSTOM465 UHSS have been plated with different ZnNi coatings exhibiting different microstructures as those shown in figure 1, namely closed, semi-open and open microstructure, correlating substrate´s nature, chemical composition, thickness, mechanical properties and microstructure with H content and degassing efficiency under different conditions. The coatings features and degassing performance have been compared to those obtained on 300M base materials. Microstructural characterization has been carried out by FESEM. GDOES has been used for determination of H content and its distribution on both the ZnNi and the base material. The effect of hydrogen embrittlement in the mechanical properties of the coated components has been evaluated according to the ASTM F519. These results will boost the understanding of the impact of the ZnNi coatings morphology on the H embrittlement susceptibility of plated components on different UHSS.

Figure 1. ZnNi microstructure as a result of plating conditions where a) closed, b) semi-open and c) open structure.