Investigation of the corrosion and mechanical properties of electroless nickel-based composite coatings using the response surface methodology

Although many metals are produced, not all of these metals have properties suitable for direct use or processing. For example, the surfaces of metals such as steel, aluminum and copper are susceptible to corrosion as their manufactured forms. In order for these metals to be used for their intended purposes, their surfaces need to be properly prepared and subjected to various modification processes. In this context, the metal coating industry plays an important role in these preparation and development processes. The electroless plating method stands out among the plating methods due to its advantages such as its low cost, its ability to coat even materials with complex geometries with uniform thickness and being a simpler process compared to other methods. Aluminum alloys have a wide range of applications in modern industry and are among the critical materials that meet the needs of different sectors. Properties such as light weight, high strength, superior corrosion resistance and easy machinability make these materials offer significant advantages in terms of cost and performance. However, aluminum materials have disadvantages such as low hardness and low wear resistance. The choice of aluminum and its alloys as an alternative to heavier materials such as steel and copper offers significant weight savings, especially in the machinery manufacturing and construction sectors. However, although the natural aluminum oxide layers formed on the aluminum surface act as a protective barrier, in some cases this layer may be insufficient. Deterioration and corrosion of aluminum surfaces are usually caused by chemical contamination, physical factors or electrochemical reactions. These factors cause the aluminum oxide layer to be damaged and deformations to start on the surface. In this thesis, it is aimed to improve these disadvantageous properties of aluminum materials by applying electroless coating method. Today, with technological advances, the performance criteria expected from materials have also diversified. Some of these requirements can only be met by coating methods or these methods offer more economically efficient solutions. Within the scope of the studies, it is aimed to develop Ni-B-P-TiO₂ coatings on 6XXX series aluminum alloys using electroless coating method and to investigate the properties of these coatings in detail. 6XXX series aluminum alloys are widely preferred in engineering applications due to their superior properties such as high strength, easy formability and good castability. These properties make the use of these alloys attractive, especially in parts where durability and performance requirements are at the forefront. Electroless plating is a process that does not require electric current and offers significant advantages such as uniform coating thickness, the ability to be applied to complex geometries and high corrosion resistance. This method, which is widely used in industrial applications, is preferred to protect the surfaces of parts from corrosion, abrasion and other chemical effects. Electroless coating has a wide range of applications thanks to its applicability to all kinds of material surfaces. Especially coatings that offer superior wear and corrosion resistance have an important place in the industry. While the coating thickness varies depending on the immersion time, the part leaves the solution when the reaction is complete. The plating bath can be reused; the process can be continued by adding as the components of the solution decrease. Nickel alloys are of great importance in scientific and industrial fields thanks to their advantages such as high hardness and wear resistance. The effect of the reductant used in electroless nickel coatings on the coating properties is quite significant. In this context, electroless nickel coatings are generally classified according to the type of reductant used. In coatings where hypophosphite is used as a reducing agent, phosphorus accumulation occurs in the nickel matrix and these coatings are called Ni-P coatings. When boron-containing reductants (for example, boron hydrides and amino borons) are used, boron is also reduced in the nickel matrix and Ni-B alloys are formed. Although there are many studies on multi-alloy nickel platings in the literature, studies on electroless platings where boron and phosphorus coexist are limited. In addition, within the scope of the studies, it was aimed to improve the hardness, wear resistance and corrosion performance of the coatings by adding TiO₂ particles to Ni-B-P coatings. Ni-B-P electroless coatings have a wide range of applications in various industries such as electronics, automotive, aerospace and manufacturing. These coatings act as protective layers and increase the durability of critical components thanks to their superior wear-resistant properties. In addition, by simultaneously depositing boron and phosphorus in the nickel matrix, a ternary alloy system is formed to simultaneously improve corrosion and wear resistance. In this context, the Ni-B-P-TiO₂ coatings developed in this study are expected to contribute to engineering applications by exhibiting superior mechanical and corrosion properties. Response Surface Methodology (RSM) was used to understand and optimize the effects of coating process parameters. RSM involves the design and execution of a series of experiments by varying the process parameters within certain ranges. The data obtained from these experiments are used to construct mathematical models representing response surfaces. These models provide a visual representation of the interactions of process factors and the effects of these interactions on the results. Furthermore, the analysis of the model coefficients provides important information to determine the level of importance of each factor and interactions between factors on the results. Response Surface Methodology is an approach consisting of statistical and mathematical techniques used to reveal functional relationships between one or more response variables and multiple independent variables. This methodology provides a systematic evaluation process, facilitating both process optimization and the identification of effective parameters. In this study, TiO₂ concentration, stirring speed and surfactant (SDS) concentration were selected as bath parameters for the development of electroless Ni-B-P-TiO₂ composite coatings. The fixed parameters included nickel sulfate, sodium hypophosphite, sodium acetate, lactic acid, DMAB, thiourea, coating time and temperature. Based on these parameters, 20 different experimental designs were performed using Response Surface Methodology (RSM). Within the scope of characterization studies, the best performing samples were selected and their surface morphology (SEM), crystal structures (XRD), cross-sectional images (SEM), hardness values (HV) and corrosion resistance (Tafel analysis) were examined. NiBPT3 was selected for the best corrosion resistance, NiBPT5 for the best hardness and NiBPT12 for the best thickness. For obtain cross-sectional SEM figures, the samples were bakalited and then sanding and polishing processes were applied. The hardness of each of these samples was measured at 5 different points of the coating and the final hardness was determined by averaging these 5 values. The hardness values of the selected NiBPT3- NiBPT5- NiBPT12 coatings were 752 HV, 768 HV and 748 HV, respectively. In NiBPT5 coating, 20 g/L TiO2, 400 rpm mixing speed and 0.01 g/L surfactant concentration were used and a hardness value of 768 HV was obtained. It was observed that the hardness value increased as the amount of surfactant concentration, which is one of the variable parameters, decreased, while the hardness value of the coating increased as the stirring speed increased. In order to determine the crystallographic structure of the coatings obtained within the scope of the studies, X-ray diffraction analysis was performed using CuKα (λ = 1.54050 Å) radiation in the range of 10-90°. According to the results of XRD analysis, with the increase in the amount of TiO2, the ratio of TiO2 in the coating increased and the intensity of the diffraction peaks of TiO2 increased accordingly. The thickness values of NiBPT3- NiBPT5- NiBPT12 were measured as 24.8 μm, 23.7 μm and 26.5 μm, respectively. While the TiO2 powder concentration in the samples was kept constant, the stirring speed was applied at two different values, namely 200 and 400 rpm. The investigations revealed that the change in stirring speed affected the coating thickness. When SEM images are examined, it is seen that the NiBPT5 coating is more homogeneous and uniform, which means that SDS provides more homogeneous and uniform coatings by balancing surface roughness and agglomeration in composite coatings. In addition, platinum counter electrode and saturated calomel electrode were used as reference electrodes and Ni-B-P-TiO2 based coating samples were used as working electrodes in potentiodynamic polarization analysis. Before the Tafel extrapolation tests, all samples were immersed in 3.5 wt% NaCl solution for 30 minutes to stabilize the open circuit potential (OCP). Tafel extrapolation tests were performed at a potential range of 100 mV to 100 mV and a scan rate of 1 mV/s. The Tafel curves and SEM images showed that TiO2 particles with surface movement made a significant contribution to the improvement in corrosion protection. The TiO2 concentration ratio of experiments NiBPT12- NiBPT14- NiBPT17 are different and are 20, 5, 10 g/L respectively. Mixing speeds and SDS powder concentrations are constant. The hardness values of NiBPT12- NiBPT14- NiBPT17 were measured as 748 HV, 619 HV, 639 HV respectively and the coating thicknesses of NiBPT12- NiBPT14- NiBPT17 were 26.5mm, 14.4mm and 16mm respectively. Based on these data, it is observed that the hardness values and coating thicknesses of the materials increase as the TiO2 powder concentration increases within the scope of this thesis. As a result of the experiments, it was determined that Ni-B-P-TiO₂ coatings exhibited superior performance in terms of surface hardness, coating thickness and corrosion resistance. In addition, mathematical models generated by Response Surface Methodology (RSM) were evaluated as an effective method to understand the effects of process parameters on coating properties and to determine the ideal process conditions. This study highlights the potential of electroless coating technologies in advanced material design and offers important contributions towards the use of Ni-B-P-TiO₂ coatings in industrial applications. The electroless coating method provides a homogeneous coating of materials and offers an economical solution as the coating baths can be used more than once. Aluminum, which has limitations such as low hardness and low melting temperature, is frequently preferred in sectors such as automotive, aerospace and defense industries due to its light weight. In order to improve the properties of such materials, high hardness and high melting temperature Coating processes using TiO₂, a ceramic material with superior qualities, are considered as an effective method in terms of performance improvement.