| Magnesium alloy has many excellent physical and mechanical properties, such as small density, high specific strength, small elastic medullar ratio, rigid, and easy machining, easy-casting, and some other advantages. Therefore magnesium alloys are being widely used in automobiles, machinery manufacturing, aerospace, electronics, etc. areas. However the application of magnesium alloy is restricted due to the electrochemical activity of magnesium which leads to the corrosion resistance of magnesium alloy is very poor.Although there are lots of method to reduce the corrosion tendency of magnesium alloy including controlling impurities content, or alloying, etc. Corrosion resistance of magnesium alloy is often enhanced by applications of surface coatings or treatments due to the virtue of simple, low cost, access to functional coatings. And compared to other surface treatments, electroless nickel plating can not only access to high hardness, high wear and corrosion resistant, but also achieves uniform coatings on the shape of complex castings, which has been widely applied to various fields. In recent years, the direct electroless nickel plating that adding fluoride to the bath and controlling the pH value has been drawn attention. The main steps of direct electroless nickel plating are mechanical pre-treatment→caustic washing→pickling→activation→electroless nickel plating. In this process the various procedures of pre-treatment have decisive impacts on the success of nickel plating and ultimately on the quality of nickel coating.It is found that the surface defects like black stripes appeared in certain surface regions of AM60B by studying the performance of different matrix of magnesium alloy in the pickling process. These black stripes will be difficult to eliminate even by extend degrease time before pickling or enhance pickling process, and the stripes are detrimental to proceed the follow-up electoless nickel plating. However AZ91D magnesium alloy will not appear such surface defects in the process of pickling. The microstructure and composition of the defects has been observed by means of stereomicroscopy, metallomicroscopy and scanning electron microscopy combined with energy spectrum analysis. The formation mechanism of the defect has also been discussed.Chromic acid pickling solution is commonly used in the process of electroless nickel plating on magnesium alloy. The impact of a variety chrome acid washing process on the electroless nickel plating has been studied in this paper through testing the coating interface morphology, deposit adhesion and corrosion resistance. The surface topography and elemental composition after different pickling process has also analyzed. The conclusion are :different pickling process obtain different state of corrosion, surface roughness is increasing in the process of pickling, thus increasing the mechanical match effect between the follow-up electroless nickel coating and the substrate, therefore enhancing the coating's adhesion. The sample's morphology changes mainly in chrome acid washing step, and HF activation step has little impact. After chromic acid - nitrate pickling the Mg matrix is optional corroded, appearing the Cr2O3, Al2O3, etc. oxides. If we do not erase this oxide layer, it would has a negative impact on the following nickel plating. Based on the analysis of mechanism of the chromium acid pickling, a chromium-free pre-treatment process which introduces by corrosion inhibit treatment + diluted acid pickling has been proposed. In order to obtain the optimally corrosion inhibiting process and diluted acid pickling process, the various steps of the chromium-free pre-treatment have been discussed.Hydrofluoric acid activation is an essential procedure in the process of electroless nickel plating on magnesium alloys. By testing the surface topography and elemental composition after activated by different concentrations of HF, combining XPS analysis of the surface chemical and its chemistry state, trying to explain the impact of composition and structure of fluoride film which formed in the procedure of HF activation on the process of electroless nickel plating. The results show that: the structure of fluoride film is greatly affected by HF concentration, high concentration of HF activation obtains dense and thin fluoride film, and difficult to proceed nickel plating on fluoride film; low concentration of HF activation obtains osteoporosis and thicker fluoride film, and the matrix Mg is readily to corrosion in the bath. The fluoride film is formed by MgF2 and Mg(OH)2 that composing hydroxyl MgF2: Mg(OH)2-xFx by XPS analyzing. The initial deposition mechanism is proposed: fluoride film is dissolved in electroless nickel plating bath, exposing the magnesium matrix and exchanging with Ni ion in the bath, precipitating autocatalytic Ni. Then the process of nickel deposition is beginning.
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