| Magnesium alloys are widely employed in automotive, aerospace, electronics and various industries because of its excellent comprehensive performance. It is expected that this trend in its applications will continue to grow in the coming years. However, the poor corrosion resistance is one of main obstacles for the wide application of magnesium alloys. So far, all magnesium alloys must choose appropriate method of surface treatment in order to improve their properties before applied. Electroless plating technology is one of the most appropriate methods to greatly improve the excellent corrosion resistance, wear resistance, conductibility, electromagnetic shielding of the metal coatings. In addition, chemical nickel-phosphorus (Ni-P) coating could offer uniform thickness on the parts with a complex shape. However, there is a larger amount of fluorine in the baths. The existence of fluoride is harmful to process operators, such as the attack of teeth and human bone. In addition, the discharge of large amount of bath will also cause serious environmental pollution due to difficult treatment. Therefore, it is an interesting subject to develop a new environmentally friendly electroless nickel plating bath. In the present work, we still selected cheap nickel salt as the main salt to carry out duplex nickel plating of first alkaline electroless nickel plating and subsequent acidic plating. At the same time the paper researched the growth mechanism of Ni-B coating in alkaline fluoride-free electroless nickel plating bath, which can further directs the industrial production of coating. The experimental results are obtained as follows:1. It clearly demonstrated that thiourea (TU) content and plating temperature had a significant effect on plating rate. In the alkaline fluorine-free electroless Ni-P bath. Plating rate significantly impacts on the coating quality. Too high deposition rate will decrease coating adhesion and increase porosity. Too low deposition rate will lead to an incomplete coverage on the substrate surface. It has been found that the optimal concentration of TU is 0.5 mg-dm-3 and plating temperature is 348.15 K.2. A tight and compact Ni-P double-layer coating was obtained on Mg alloy AZ91D after alkaline fluoride-free electroless Ni-P deposition for 40 min and subsequent acidic fluoride-free electroless Ni-P deposition for 50 min. About 28 ?m thickness of double-layer coating consists of about 15 ?m thick inner alkaline coating and about 13 ?m thick outer acidic coating. It could be found that the coating was compact, uniform and well adhered to the magnesium alloys substrate. The inner alkaline coating not only plays a protective role on Mg alloy substrate in acidic bath, but also provides a good active substrate for the further electroless plating. The outer acidic coating possessed a good corrosion resistance.3. The growth mechanism was investigated in the alkaline fluoride-free electroless Ni-B bath. The result shows different phase with different chemical properties on substrate surface after pretreatment, including porous coarse activation film and bare magnesium alloy. In the initial deposition stage, the main electroless plating process is rapid Ni replacement and nucleation processes. The substrate surface is completely covered by nickel grains in a very short period of time. The reaction is mainly from Ni replacement to nickel growth in the later stage and characterized by the increase of coating thickness. A compact Ni-B/Ni-P coating is produced via alkaline Ni-B plating 60 min and acidic Ni-P plating 60 min. Electrochemical measurements results show that the corrosion resistance of AZ91D magnesium alloy was significantly improved by the composite coating. |
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