Studies of Several Pickling and Activation Processes for Electroless Ni-P Plating on AZ31 Magnesium Alloy

Abstract

Several chromium-free pickling and activation processes were investigated in order to develop an eco-friendly pretreatment for electroless Ni-P plating on magnesium alloy. The coatings obtained from etching only in H 3 PO 4 solution exhibit poor adhesion due to the existance of an interlayer. Meanwhile, a corrosion model of Mg alloy in H 3 PO 4 solution was proposed. When fluoride was added into the pickling solution, the adhesion of the coatings was also unsatisfied because of the bad mechanical interlocking action. By comparison, the coatings obtained from pickling in H 3 PO 4 solution followed by NH 4 Magnesium (Mg) alloy is one of the lightest structural metals and a potential candidate to replace the heavier aluminum or steel in manufacturing of mechanic components due to its low density, excellent physical and mechanical properties. [1][2][3][4][5][6] However, the use of Mg alloy for structural applications has been limited owing to their poor corrosion resistance and high chemical reactivity. 7-12Electroless Ni-P plating (ENP) is one of the most effective surface treatment techniques to overcome the defect of Mg alloy, because ENP can provide excellent properties of hardness, good wear resistance and great corrosion resistance by forming a uniform deposit on the Mg alloy substrate surface. [13][14][15][16] However, ENP on Mg alloy presents many problems still because of the rapid formation of oxide film upon Mg alloy exposed to air/water, and forming loose immersion layers on the substrate surface by replacement reactions at the initial deposition stage. The loose oxide film and immersion layers hinder successful ENP and are detrimental to the adhesion and uniformity of subsequent coatings.17-20 Therefore, appropriate pretreatment steps must be taken before ENP to remove the natural oxide tarnish, embedded sand, heavy metal impurities and burned-in lubricants on the surface of the Mg alloy, and to activate the deposition of nickel on the matrix. [21][22][23] There are mainly two types of preferred pretreatments for Mg alloy, including zinc immersion and direct ENP.18,24 The direct ENP is simpler and more convenient.11 However, acid pickling in a chromiumcontaining (VI) solution, followed by activation in a hydrofluoric acid solution is usually employed in the conventional pretreatment processes of direct ENP. [25][26][27][28][29][30] Several modified pretreatments have been devised to avoid the use of such process, which contains some species detrimental to the environment and harmful to human health. 11,[31][32][33] A method of avoiding the use of toxicant was carried out by employing a pre-plating step with an alkaline ENP bath prior to subsequent common ENP in an acidic bath.31 Molybdate (MoO 4 2− ) chemical conversion coating was also attempted for replacing the chromium acid pickling. [32][33] In the work of Ref. 32, strong volatile hydrogen fluoride acid was applied in the activation process and the molybdate conversion bath contained carcinogenic NaNO 2 . 32 The adhesion, which is a very important...

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“…Hydrofluoric acid (HF), a kind of weak acid that can lead MgO to dissolve into water or ethanol, i.e. being able to remove the oxides on Mg alloy powders, so it is widely used to pre-treat Mg alloy workpiece surface prior to electroplating [27,28]. The concentration of HF in solution and corresponding treating time not only have large effect on oxide removal, but also on alloy loss resulted from corrosion [29,30].…”