Ultrasonic-Assisted Electroless Ni-P Plating on Dual Phase Mg-Li Alloy

Abstract

An ultrasonic-assisted method for electroless Ni-P plating on dual-phase Mg-Li alloy is proposed. Environmentally-friendly Ce(NO 3 ) 3 -KMnO 4 solution was successfully applied for the pre-treatment. Uniform and compact Ni-P coatings with fine structures have been successfully obtained. The Ni-P coatings are mixed with amorphous-crystalline features and well adhered to the substrates. The corrosion resistances were investigated by electrochemical impedance spectroscopy, open circuit potential, potentiodynamic polarization and hydrogen evolution tests. The results indicate that ultrasonic processing significantly improves the corrosion resistance of Ni-P coatings due to the formation of uniform, dense and fine structures, which strengthen the passive film. Magnesium-lithium (Mg-Li) alloys are the lowest in density of metallic structural materials.1 Mg-Li alloys have excellent properties, such as high specific strength, good formability, damping capacity, impact resistance and energetic particle penetration tolerances.1,2 These properties provide Mg-Li alloys with a great potential for various applications, especially for transportation, in the aerospace industry and for electronic components.3-5 However, the application of Mg-Li alloys is restricted in practice due to undesirable properties, mainly poor corrosion and wear resistance.6-9 Surface treatment, specifically coating, is an effective way to improve corrosion resistance and wear resistance of Mg alloys. Among them, nickel-phosphorous (Ni-P) plating remains of high interest because of its unique advantages, including good corrosion resistance, wear resistance, lubricity, acceptable ductility, electric conduction and high hardness. 26 However, electroless Ni-P plating on Mg-Li base alloys, specifically on dual phase Mg-8 wt%Li base alloys, is much more difficult than on other Mg base alloys because the standard electrode potential of Li (−3.045 V vs SHE) is lower than that of Mg (−2.37 V vs SHE), making the alloy extremely susceptible to galvanic corrosion. This results in an unattractive appearance of plating and in the degradation of mechanical properties.Electroless nickel plating is an autocatalytic reaction for deposition of nickel on a substrate. 27 There are two main methods for electroless nickel plating on magnesium alloys, i.e. direct electroless nickel plating 9,28,29 and zinc immersion. 8,30-32 Direct electroless nickel plating has the advantages of simplicity and suitability for magnesium alloys with high aluminum content. Nevertheless, the main disadvantage of this method is the use of hexavalent chromium compounds [Cr(VI)] and hydrofluoric acid [HF] during the etching and conversion coating treatments. These materials are not friendly to the environment and increase the risk to people during operation. 8,9 The zinc immersion procedure is complicated and the step of plating copper with cyanide is poisonous to humans and the environment. In addition, common electroless nickel plating exhibits other drawbacks, such as energy consumption, fast ev...

Electrochemical impedance spectroscopy (EIS): An efficiency method to monitor resin curing processes

“…3. Constant phase elements (CPE) are used to replace all capacitance elements to correct non-pure capacitance due to surface inhomogeneity and roughness [28][29][30]. R L , R c , CPE c , R ct and CPE d refer to wire resistance, coating resistance, coating capacitance, charge transfer resistance and capacitance at the interface of resin/electrode, respectively.…”

Composite coatings prepared by combined plasma electrolytic oxidation and chemical conversion routes on magnesium-lithium alloy

“…Moreover, the anodic curve of composite coating B shows a relatively longer passivation plateau with a breakdown potential of 1.22 V/SCE, which is 0.05 V/SCE positive than that of the composite coating A (1.27 V/SCE), suggesting a minimized tendency for corrosion attack. Based on the SEM, EDX, XPS, TF-XRD, and potentiodynamic polarization results, the stannate conversion film can effectively seal the porous PEO coating and block the infiltration of the corrosive solution to the alloy substrate, affording the excellent anti-corrosion properties.Notably, the anti-corrosion performance of the composite coating B outperforms other coatings on Mg-Li alloys[3][4][5][6][7]17,18,27,[49][50][51][52]. Comparing to other works[26,[53][54][55], superior corrosion rates for coatings prepared by Shi et al[54] and Song et al[55] are noticed.…”

“…The formation of OHresults in an increase in the pH at the interface between the PEO coating and solution. The OHpreferentially combines with Ce 3+ and La 3+ to form Ce(OH) 3…”

“…Magnesium-lithium (Mg-Li) alloys have become one of the most desirable light-weight structural materials due to their fascinating features such as high strength-to-weight ratio, high specific stiffness, excellent magnetic screen and shock resistance [1][2][3]. However, the poor corrosion resistance of the Mg-Li alloys that stems from intrinsic high chemical reactivity of magnesium and lithium has greatly impeded their potential applications and remains a major limitation [1,2].…”

“…However, the poor corrosion resistance of the Mg-Li alloys that stems from intrinsic high chemical reactivity of magnesium and lithium has greatly impeded their potential applications and remains a major limitation [1,2]. Recently, considerable efforts of experimental studies have been sought to address this difficult challenge, such as ultrasonic-assisted electroless plating coatings [3], phosphate conversion coatings [4], molybdate/permanganate conversion coatings [5], stannate conversion coatings [6], lanthanum conversion coatings [7], epoxy coatings [8], atomic layer deposition films [9], and superhydrophobic coatings [10]. These coatings, however, either suffer from limited corrosion resistance or durability, thus affecting the overall performance of Mg-Li alloys.…”