A study on the electroless Ni–P deposition on WE43 magnesium alloy

• Published in: Surface & coatings technology Vol. 205; no. 7; pp. 2281 - 2286
• Main Authors: Iranipour, N., Azari Khosroshahi, R., Parvini Ahmadi, N.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 25.12.2010; Elsevier
• Subjects: Annealing; Applied sciences; Coatings; Cross-disciplinary physics: materials science; rheology; Deposition; Deposition mechanism; Electroless plating; Eutectics; Exact sciences and technology; Magnesium alloy; Magnesium base alloys; Materials science; Metallic coatings; Metals. Metallurgy; Microhardness; Microstructure; Nickel; Physics; Production techniques; Surface treatment; Surface treatments; Electroless plating; Deposition mechanism; Magnesium alloy; Nickel base alloys; Metal coating; Chemical deposition; Surface treatments; Mechanism
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2010.09.006

In this paper an attempt has been made to understand the mechanism of the deposition process of an electroless Ni–P (EN) coating on WE43 magnesium alloy. Also a number of properties concerning the deposited coatings have been reviewed. The results show that the starting microstructure of the alloy consists of a primary α phase together with some eutectic β phase at triple points and along with some grain boundaries. Microstructural studies reveal an uneven distribution of alloying elements in the phases and they are predominantly segregated to the eutectic β phase. This phenomenon can result in galvanic coupling between eutectic β and primary α phases. Detailed studies prove that the replacement reaction takes place at the early stages of coating, and is followed by the autocatalytic reaction at the next stages of deposition. The X-ray diffraction patterns of primitive coatings show a broad peak around 2θ of 45°, which is an indication of an amorphous or an extremely fine crystalline structure. Annealing at 400°C for an hour led the nature of deposits to be changed to crystalline phases of Ni, Ni3P, and NiP3. Microhardness values of coatings are considerably higher than those of the bare substrate. These further increase when they are annealed at 400°C for 1h. Electrochemical polarization curves and calculated corrosion values reveal higher corrosion potential (Ecorr) for the coating than for the bare substrate. This decreases again when the coating is annealed at 400°C for 1h. ► Deposition process is divided into steps of the thin film formation and the bulk coating. ►Replacement reaction between Ni and Mg at the early stages of the coating promotes Ni nucleation and growth. ► Autocatalytic reaction plays the major role after nuclei formation and is the main mechanism of the bulk coating. ► Although microstructural changes during annealing treatment improves microhardness of the coating but deteriorates its corrosion resistance.