Electroless nickel plating on ZM6 (Mg–2.6Nd–0.6Zn–0.8Zr) magnesium alloy substrate

Hou

et al. 2016

Materials & Corrosion

An economical formulation is introduced for electroless Ni-P coating on AZ31B Mg alloy, and the multifunctional additive polyvinylpyrrolidone as the only additive is used for the electroless bath. The effect of PVP concentration on electroless Ni-P coating is researched in detail. The total deposition rate of the Ni-P coating is estimated by the gravimetric method. It is shown that with the increases of PVP concentration in electroless solution, the total deposition rate for Ni-P coatings decreases.The microstructure and surface morphologies of Ni-P coatings are characterized by X-ray diffraction and scanning electron microscopy. Moreover, electrochemical impedance spectroscopy and polarization measurement are used to evaluate the anticorrosion property of the Ni-P coatings in 3.5% NaCl solution. The results show that a mixture of crystalline and amorphous Ni-P coating with medium phosphorus content is successfully deposited on the AZ31B Mg alloy. Furthermore, the Ni-P coating displays preferable properties in both micro-hardness and corrosion resistance when the concentration of PVP is 10 g/L.

Section: Resultsmentioning

confidence: 99%

Characteristics and corrosion behavior of nickel‐phosphorus coatings deposited by a simplified bath

Hou

et al. 2016

Materials & Corrosion

“…The cathodic branch is the main reaction corresponding to the evolution of hydrogen; and the anodic branch is the most important features related to the corrosion resistance. 49 For substrate of the bare Mg alloy (curve a in Fig. 9), the current density for the anodic polarization increases with increasing applied potential; and an activation-controlled anodic process is observed, indicating that no obvious passivation film formed.…”

Section: Pickling In H 3 Po 4 Solution Followed By F − Activation (Mimentioning

confidence: 97%

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

Xie

Chen

Xiang

et al. 2014

J. Electrochem. Soc.

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...

“…Electroless nickel coatings are widely studied and used due to their special advantages, such as uniform deposition, good corrosion and wear resistance, good electrical and thermal conductivity, and good solder ability [5]. According to Ayushi Thakur [6] Ni/Au deposits on magnesium substrates for aerospace applications have also been witnessed recently [7], and Ni electroless deposits have also been serviceable on engine cranksets, fuel pump cover and shell components in aeronautical applications [8]. The most widely used are electroless nickel-phosphorous alloys (Ni-P) that are obtained from hypophosphite-reduced baths [9,10].…”

Section: Introductionmentioning

confidence: 99%

The Influence of Directly Plating Time on the Structure of Electroless Nickel on AZ91

Zhang¹,

Li²

2021

In this paper, the influence of deposition time on the surface morphology, composition, structure, grain size and hardness of electroless nickel plated directly in neutral pH bath on AZ91 magnesium alloy are discussed by using SEM, EDS, XRD and microhardness test machine measurements. The results show that with the increasement of the plating time, the surface tends to be smooth, dense, and complete. The coating is composed of Ni and P and the content of P increases with the increasing the plating time. Furthermore, the XRD peaks for the coatings shift to the right and broaden gradually with prolonged plating time, which indicates that the interplanar spacing of the crystal and the grain size are simultaneously decreased. At the same time, the microhardness of the samples is also increased with increasing plating time.