Application of Electroless Ni-P and Ni-P-Zn Coating on Magnesium Alloy Via Maleic Acid Pre-Treatment and Investigation of Their Properties

Heshmati, Mehrdad and Seifzadeh, Davod (2017) Application of Electroless Ni-P and Ni-P-Zn Coating on Magnesium Alloy Via Maleic Acid Pre-Treatment and Investigation of Their Properties. Masters thesis, University of Mohaghegh Ardabili.

Text (اعمال پوشش های بدون برق نیکل- فسفر و نیکل – فسفر- روی بر روی منیزیم آلیاژی با استفاده از پیش تیمار مالئیک اسید و بررسی خواص آن ها) Mehrdad Heshmati.pdf Download (1MB)

Abstract

Due to the high strength to weight ratio, magnesium alloys have potential applications in the industrial field. Unfortunately, low corrosion resistance and hardness of the magnesium alloys are restricted the widespread applications of them. Electroless plating is a suitable way to overcome these problems and improve the corrosion resistance and hardness of the alloys. Surface pretreatment layer is including the chromate conversion coating and surface activation by hydroflouric acid. Due to high toxity of the hexavalent chromium compound and severe corresivity of the HF, finding of an environmentally- friendly processes is necessary. In this work, the electroless Ni-P plating of the AM60B magnesium alloy by maleic acid as an environmentally-friendly pretreatment layer is investigated. After applying the maleic acid pretreatment layer, the sample was immersed in sulphate containing electroless Ni-P plating bath. Scanning Electron Microscopy (SEM) showed that the surface of the sample was uniformlly covered by a cauliflower- like electroless coating after 3h plating. Chemical composition of the Ni-P coating which was studied by Energy Dispersive X-ray Spectroscopy (EDX) demonstrated that the surface has about 6.64 wt.% phosphorus. Moreover the microstructure of the coating was studied by X-ray Diffraction(XRD). The corrosion resistance of the Ni-P coating was studied by potentiodynamic polarization and electrochemical impedance spectroscopy in 3.5 wt.% NaCl solution. The result showed that the Ni-P coating was improved the corrosion resistance of the magnesium alloys. Porosity and thermal shock tests revealed that the coating was not porous and has a good adhesion to the substrate. Furtheremore, in this study the electroless Ni-P-Zn coating, with a widespared potential applications in the automotive industry was applied. Similar to the Ni-P coating the electroless Ni-P-Zn coating, was applied on the AM60B alloy by the maleic acid pretreatment layer. The results showed that the coating was amorphous to some extent and the average amount of the P and Zn were about 10.25 and 7.55 wt% respectively. Moreover, the thickness of the coating was about 22 µm. The corrosion results showd that the Ni-P-Zn in comparsion with the applied Ni-P coating has a better hardness and good corrosion resistance which is due to the more compact and amorphous structure of the coating.

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