Preparation and properties of Ni-NixB and Ni-NixB-Al2O3 coatings on AZ91D magnesium alloy by electrolytic dip method

Magnesium and its alloys are abundant in the Earth's crust, low density, high relative strength and are easy to recycle. Low corrosion, wear resistance and hardness are the most important factors limiting the use of magnesium alloys. Coating of surfaces is a commonly used method for improving the properties of materials. With the electroless coating process developed about 70 years ago, dense and homogeneous coatings are obtained on many different material surfaces. High hardness and wear resistance are the two most important reasons for choosing nickel boride (Ni-B) coatings. In addition, good corrosion resistance, lubrication properties, solderability, magnetic shielding are other remarkable properties of Ni-B coatings. All of these properties make Ni-B coatings a type of coating that has been studied extensively in recent years. In this study, Ni-B and Ni-B-Al2O3 coatings were obtained by electroless coating method on the surface of AZ91D magnesium alloy. Before the production of the coatings, a suitable pretreatment design was made considering the disadvantages of magnesium. After determining the appropriate pretreatments, the effects of ambient temperature and coating time on coating thickness and surface properties were investigated and appropriate parameters were determined. In another part of the studies; NaBH4, which is a boron source for Ni-B coatings, and nano Al2O3, which is a reinforcing element for Ni-B-Al2O3 coatings was added in different proportions and its effect on coating properties was investigated. The microstructures and surface properties of the produced coatings were evaluated by 2 dimensional profilometer, scanning electron microscope (SEM) and optical microscope (OM). Depending on the changing coating composition; the changes in chemical composition were analyzed by energy dispersive X-ray spectroscopy (EDS) analysis and formed phases, compounds, crystallite dimensions and lattice parameters by X-ray diffraction (XRD). Hardness measurements were made by Vickers micro hardness method. The dry friction and wear behavior of the coated materials were examined by ball-on-disc wear tests by applying different loads against alumina balls. Corrosion experiments were carried out by using Tafel extrapolation method and electrochemical impedance spectroscopy by using potentiostat / galvanostat device and 3 electrode horizontal cell in 0.1% M NaCl.