A New Pre-Treatment with Phosphoric Acid and Hydrofluoric Acid of Electroless Ni-P Plating on Mg Alloy

Abstract:

In order to develop a chromium-free pre-treatment method for electroless Ni-P (ENP) plating on magnesium alloy from an acidic plating bath, several various pickling and activation processes were studied in the present work. The surface morphologies of the matrix before and after etching or activation as well as the surface and cross sectional appearance of the Ni-P coatings were observed by scanning electronic microscope (SEM), while the elemental compositions were detected by energy dispersive X-ray spectroscopy (EDS). Those surface characterizations along with the scribe and grid testing results showed that the coatings obtained from etching only in H₃PO₄ solution display poor adhesion due to the existance of an interlayer between the matrix and the coatings. When fluoride was added into the pickling solution, the adhesion of the Ni-P coatings was also unsatisfactory because of the bad mechanical interlocking. By comparison, the Ni-P coatings obtained from pickling in 400 cm³·dm⁻³ H₃PO₄ solution followed by 10 cm³·dm⁻³ HF activation exhibit good characteristics in both adhesion and anti corrosion. The polarization test in 3.5wt.% NaCl aqueous solution showed that the corrosion potential (E_corr) and corrosion current density (I_corr) of the new achieved coatings increases positively from −1.65 V to −0.87 V and decreases remarkably from 2.09 mA⋅cm⁻² to 3.58 μA⋅cm⁻², respectively (both compared with the bare Mg alloy).

• Periodical: Applied Mechanics and Materials (Volume 729)
• Pages: 21-26
• DOI: https://doi.org/10.4028/www.scientific.net/AMM.729.21
• Online since: January 2015
• Authors: Zhi Hui Xie*, Jun Li Zhou, Shu Rong Xiang, Fang Chen
• Keywords: Activation, Corrosion, Electroless Plating, Magnesium Alloy, Pickling

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