SYNTHESIS AND CHARACTERIZATION OF ELECTROLESS Ni-P ALLOYS FOR ANTI-WEAR AND ANTI-FOULING COATINGS

Introduction: Fouling is a typical problem in compressors used for processing of hydrocarbon gasses and it is caused by solids adsorption onto the internal surfaces of the machinery. This phenomenon can influence the performance of the facility and, in some cases, it causes the stop or the failure of the system. Surface coating can be a suitable solution to this kind of problem. Anti-fouling and anti-wear coatings can be obtained by electroless-nickel plating technique. It allows for depositing very uniform and dense Ni-P alloy coatings on irregular shaped surface and on different kind of substrate. Materials and methods: Ni-P amorphous alloy was deposited on a steel substrate. The electroless plating bath is an acid solution and consists of a nickel source, a reducing agent (P compounds), stabilizing agents and a buffer. The temperature and the pH were continuously monitored during the deposition and kept constant. Plating parameters were optimized and several coatings were deposited. Nano-particles were added in order to improve the performance of the coating, in particular particles of polytetrafluoroethylene (PTFE) were chosen to improve anti-fouling behavior while nano-zirconia particles were added to improve wear resistance. In order to prevent agglomeration of nano-ZrO2, nano-particles were previously surface modified with methacrylic acid. Ni-P coatings were characterized through SEM imaging and EDS analysis. Coating thickness and roughness were measured. Top view and cross section Vickers hardness was evaluated. The morphology of the samples was compared through a surface profilometry analysis. The wettability was evaluated considering the drop contact angle for different fluids. Results: All deposited coatings (Ni-P, Ni-P with ZrO2, Ni-P with PTFE, Ni-P with ZrO2 and PTFE) are dense layers with a very good adhesion with the substrate. As expected the morphology of the coating follows the morphology of the substrate. SEM micrographies show an optimal dispersion and distribution of nano-particles. Nano-zirconia addiction allows for obtaining small values of roughness and high values of Vickers hardness. Nano-ZrO2 and PTFE particles cause a reduction of drop contact angle for different fluid. In particular PTFE results to be more beneficial for a decrease of the wettability. Discussion: Electroless Nickel Plating technique was well optimized resulting in dense coatings and good rate of deposition. The results show improvements of the desired coating properties in particular when PTFE and ZrO2 are both added.