Electrodeposition of Ni-W Alloy from Citric Acid Free Aqueous Electrolyte As a Substitute for Hard Chrome Coating and the Effect of Tungsten Content on Coating Hardness
Authors: Omar Nurul Amanina Binti , Dombrowe Scott , Koester Frank , Lampke Thomas 
Published: November 2022

In the industry, chrome coating is an important player in the functional layer application. Due to its high corrosion and wear resistance as well as high hardness [1], hard chrome plating is a popular choice in the automotive and aerospace industry. However, since September 2017, the application of chrome (VI) in surface treatment is banned by the European Chemical Agency (ECHA) due to chrome (VI) being toxic and environmentally hazardous [2]. Chrome (III) compound is a popular substitute for chrome (VI) due to it being less toxic compared to chrome (VI). However, chrome (III) coating has a lower corrosion resistance, and the electroplating process is more sensitive to metallic impurities.

Many new innovations have since been developed such as the plasma electrolytic oxidation (PEO), extreme highspeed laser deposition (EHLA). Even established technologies such as physical vapour deposition (PVD), thermal spraying and case hardening process have tried to replace hard chrome coating with a degree of success. Admittedly, the above-mentioned technologies have their fair share of disadvantages. PEO, a green procedure [3], causes a metal substrate to develop a highly resistant oxide which is extremely strong and corrosion as well as wear resistant. The process however needs a high temperature and pressure, continuous replacement of electrolytes and consumes a great amount of power which made the process not feasible for a wide industrial application [4-6]. EHLA and thermal spraying on the other hand are restricted to rotational symmetric components. PVD is restricted to substrates that can handle high processing temperature like nitrable ferrous base materials. Lastly, case hardening processes requires long heat treatments which contribute to a higher process cost. [7]

In our presentation, another alternative for chrome coating based on electrodeposition will be introduced. The element tungsten is considered in our experiments because tungsten not only has an excellent corrosion properties and high tensile strength at high temperature, but it also has a microhardness of 650 Vickers. However, it is not possible to deposit tungsten from an aqueous solution because tungstate cannot be directly reduced to metallic tungsten due to a very low overvoltage for hydrogen evolution on tungsten. Thus, an induced co-deposition mechanism is needed, and it is known that tungsten can be easily codeposited with iron group metal. [8-9]

In this study a modified Watt’s bath is used as electrolyte to deposit NiW alloy. Our preliminary tests have shown that tungsten can also be deposited with nickel without the help of citric acid as complexing agent. The usage of citric acid does indeed inhibit the deposition of nickel but show no influence in the deposition of tungsten. Hence, in our study, experiments with the rotating disc electrode (RDE) will be done to see if tungsten content in the coating could be increased when the hydrodynamic and coating parameters are manipulated. The chemical composition of the layer is measured using SEM/EDX and the surface morphology is pictured using SEM. Metallographic cross-sections are made to see if there are any fissures in the layer and the hardness of the layer is measured using the microhardness test. The dependency of hardness on the tungsten content in the layer and the annealing parameter will also be shown in the presentation. XRD measurement is conducted for phase analysis and grain size calculation.

Figure 1