Microstructure characterization, corrosion and wear resistance of duplex electrodeposited Ni-P/Ni-B-TiO₂ sol-enhanced nano-composite coating

Abstract

Corrosion and wear are responsible for more than 90% of metals failure in industries. Therefore, it is imperative to overcome the detriment caused by these phenomena to save costs, materials resources, manpower and energy efficiency. Applying coatings is one of the most efficient methods to combat wear and corrosion. Among different coating methods, electrochemical deposition is the most common and appreciated method due to its simple application technique and low cost. For the coating material selection, there are many available options that are compatible with electrodeposition methods. Nickel is one of the major preferences, since it has both good corrosion resistance and mechanical properties compared to other materials. To improve the coating characteristics, several developments have been introduced and investigated. This research considers doping enforcing particles into the coating to create a composite. Compared with conventional coatings, composite coatings have exquisite properties. If the size of doping particles decreases and their dispersion through the coating improves, then the overall properties of the coating are enhanced. Thus, instead of simple composite coatings, micro and nano-composite coatings have been developed. Moreover, the development of the coating itself is not the only relevant issue. In fact, each type of coating is mostly effective against either corrosion attacks or wear. In the electrodeposited nickel coatings category, high concentration phosphorous Ni-P coating is believed to have the ultimate corrosion resistance. Ni-B was believed to be the best coating in terms of tribological behavior. This research group has determined that sol-enhanced nano-composite Ni-B-TiO2 coating is the best coating to protect the surface against wear. However, none of the mentioned coatings can effectively withstand both corrosion and wear at the same time. Facing the problem mentioned above, an idea to create a special coating that can be effective both ways was presented and considered as the provision of the current study. The idea was to simply apply a multilayer coating instead of a single-layer one. The multilayer coating was set to consist of two main layers: The first layer, or the inner layer, is to be deposited on the surface of the substrate to be the corrosion-resistance layer. The second layer, or the outer layer, is to be applied on the top of the first layer to act as a wear resistance shield. For the first layer, high-content phosphorous Ni-P coating and for the second layer, sol-enhanced nano-composite Ni-B-TiO2 coating were selected, respectively. The deposition method for the Ni-P coating was electroless plating and for the sol-enhanced nano-composite Ni-B-TiO2 coating, electroplating was conducted. Optimum sol-enhancement parameters were extracted and used from the published results of a previous study from our research group. To make comparison and investigate the enhancement in properties, along with the duplex Ni-P/NiB-TiO2 coating sample, mono-layer Ni-P and Ni-B-TiO2 coating samples were also prepared for further analysis. It was observed that the microstructure of each layer within the duplex coating is not affected by the duplex coating process and remains identical to the mono-layer coating with the same chemical composition. The XRD-EDS revealed that the chemical composition of the inner and outer layers did not change and remained as single-layer Ni-P and Ni-B-TiO2.The outer layer in the duplex coating was completely the same as the mono-layer Ni-B-TiO2 nano-composite coating with almost the same hardness and grain size. In addition, wear resistance for the duplex coating was reported to be approximately the same as the single layer Ni-B-TiO2 coating and much better than the Ni-P coating. There was also an astonishing result from comparing corrosion behavior between Ni-P and sol-enhanced Ni-B-TiO2 nano-composite coatings. No previous study compared the properties of high-concentration Ni-P coatings and sol-enhanced Ni-B-TiO2 nano-composite coatings. This study has revealed that although Ni-P coatings are generally better than Ni-B coatings in terms of corrosion resistance, the corrosion resistance of this Ni-B-TiO2 coating is slightly better than the high content Ni-P coating, given the current study specifications. However, the ultimate result of this study was to identify the duplex Ni-P/Ni-B-TiO2 coating as having the best corrosion resistance among itself, Ni-P and Ni-B-TiO2 coating samples. It was concluded that while having the same corrosion potential as single layer Ni-B-TiO2 coating, the duplex Ni-P/Ni-B-TiO2 coating has almost 10 times lower corrosion current density and thus, its corrosion kinetic rate is much slower in comparison to other coating samples. Hence, we believe the duplex coating with both good wear resistance and corrosion protection will have great application prospect in a wide range of industries.