Investigation on a Non-cyanide Plating Process of Ni-P Coating on Magnesium Alloy AZ91D

In this research we presented a non-cyanide plating process of Ni-P alloy coating on Mg alloy AZ91D. By applying a new process flow of electroless nickel plating in which zinc coating is used as transition of Ni-P coating on Mg alloy AZ91D, the process of copper transition coating plated in the cyanides bath can be replaced. A new bath composed of NiSO4 was established by orthogonal test. The results show that zinc transition coating can increase the adhesion and pH 4.0 and 95℃, respectively. The present process flow is composed of ultrasonic cleaning→alkaline cleaning→acid pickling→activation→double immersing zinc→electroplating zinc→electroless nickel plating→passivation treatment.The present non-cyanide process of electroless nickel plating is harmless to our surroundings and Ni-P coating on Mg alloy AZ91D produced by present process possesses good adhesion and corrosion resistance.     

A study of corrosion performance of electroless Ni–P and Ni–W–P coatings on AZ91D magnesium alloy

The autocatalytic nature of the electroless nickel‐based alloy coating process will inevitably produce H₂ bubbles which may be left in electroless nickel‐based alloy coating. If the H₂ cannot be removed and left in the coating, it can lead to its poor corrosion resistance due to hydrogen cracks. So, the post treatment is an essential step for electroless deposition process. In this paper, electroless Ni–P and Ni–W–P coatings with chromium‐free pretreatment and dehydrogenation post treatment have been successfully prepared on AZ91D magnesium alloy, and the corrosion behaviors of the two kind coating samples in NaCl solution, HCl solution, and H₂SO₄ solution have been investigated. Both the polarization test and immersion tests show that the electroless Ni–W–P coating has better corrosion resistance than that of electroless Ni–P coating.     

Electrochemical characteristics of calcium-phosphatized AZ31 magnesium alloy in 0.9 % NaCl solution

Magnesium alloys suffer from their high reactivity in common environments. Protective layers are widely created on the surface of magnesium alloys to improve their corrosion resistance. This article evaluates the influence of a calcium-phosphate layer on the electrochemical characteristics of AZ31 magnesium alloy in 0.9 % NaCl solution. The calcium phosphate (CaP) layer was electrochemically deposited in a solution containing 0.1 M Ca(NO₃)₂, 0.06 M NH₄H₂PO₄ and 10 ml l^?1 of H₂O₂. The formed surface layer was composed mainly of brushite [(dicalcium phosphate dihidrate (DCPD)] as proved by energy-dispersive X-ray analysis. The surface morphology was observed by scanning electron microscopy. Immersion test was performed in order to observe degradation of the calcium phosphatized surfaces. The influence of the phosphate layer on the electrochemical characteristics of AZ31, in 0.9 % NaCl solution, was evaluated by potentiodynamic measurements and electrochemical impedance spectroscopy. The obtained results were analysed by the Tafel-extrapolation method and equivalent circuits method. The results showed that the polarization resistance of the DCPD-coated surface is about 25 times higher than that of non-coated surface. The CaP electro-deposition process increased the activation energy of corrosion process.     

Corrosion behaviour of some vapour deposited magnesium alloys

Abstract

Binary magnesium alloys containing chromium, manganese, or titanium were made using a physical vapour deposition technique. The corrosion resistance of the alloys was assessed in aqueous chloride solutions using total immersion tests in quiescent 600 mmol L^?1 NaCI solutions. Alloying with manganese or titanium was found to lower the corrosion rate of magnesium over most of the compositional ranges of interest, whereas addition of chromium had a detrimental effect on the corrosion resistance of magnesium. The lowest corrosion rate was recorded for a Mg–Ti alloy where the value obtained was about 80 times lower than that found for vapour deposited pure magnesium. Open circuit corrosion potential measurements conducted in 600 mmol L^?1 NaCl solution showed that additions of chromium, titanium, and manganese also produced deposits which were significantly more noble than pure magnesium, suggesting that these alloys would be less susceptible to galvanic corrosion.

MST/3064     

The influence of HF treatment on corrosion resistance and in vitro biocompatibility of Mg-Zn-Zr alloy

The samples made of a Mg-2.5wt.%Zn-0.5wt.%Zr alloy were immersed in the 20% hydrofluoric acid (HF) solution at room temperature for different time, with the aim of improving the properties of magnesium (Mg) alloy in applications as biomaterials. The corrosion resistance and in vitro biocompatibility of untreated and fluoride-coated samples were investigated. The results show that the optimum process is to immerse Mg alloys in the 20% HF solution for 6 h. After the immersion, a dense magnesium fluoride (MgF₂) coating of 0.5 μm was synthesized on the surface of Mg-Zn-Zr alloy. Polarization tests recorded a reduction in the corrosion current density from 2.10 to 0.05 μA/cm² due to the MgF₂ protective coating. Immersion tests in the simulated body fluid (SBF) also reveal a much milder corrosion on the fluoride-coated samples, and its corrosion rate was calculated to be 0.05 mm/yr. Hemolysis test suggests that the conversion coated Mg alloy has no obvious hemolysis reaction. The hemolysis ratio (HR) of the samples decreases from 11.34% to 1.86% with the HF treatment, which meets the requirements of biomaterials (HR < 5%). The coculture of 3T3 fibroblasts with Mg alloy results in the adhesion and proliferation of cells on the surface of fluoride-coated samples. All the results show that the MgF₂ conversion coating would markedly improve the corrosion resistance and in vitro biocompatibility of Mg-Zn-Zr alloy.     

Electroless deposition of Ni–P–B4C composite coating on AZ91D magnesium alloy and investigation on its wear and corrosion resistance

In the present work, the effect of applying ternary Ni–P–B₄C composite coating from an electroless plating bath containing sulfate nickel, sodium hypophosphate and suspended B₄C particles, on the corrosion and wear resistance of an AZ91D, high aluminum cast magnesium alloy, was investigated. Regarding low corrosion resistance of magnesium alloys, chromium oxide plus HF (Hydro Fluoric Acid) pretreatment was applied to prepare the substrate for coating treatment in electroless bath. The pH value and temperature of the electroless bath were 9 and 82 °C, respectively. The coating was characterized for its micro structure, morphology, microhardness, wear and corrosion resistance. SEM (Scanning Electron Microscope) observation showed dense and coarse nodules in the ternary composite coating and the cross section of Ni–P–B₄C coating offered presence of well dispersed B₄C particles in the coating. The hardness of the Ni–P–B₄C composite coatings was around 1200 MPa, more than what can be obtained for Ni–P coatings (about 700 MPa). The wear test which was carried out by using pin on disc method, showed that ternary Ni–P–B₄C composite coating had a good wear resistance and more superior than Ni-P coating. The polarization test results for ternary Ni–P–B₄C composite coating exhibited good corrosion resistance properties in protecting the AZ91D magnesium alloy, but not better than Ni–P coating.     

Microstructure and properties of laser clad high-entropy alloy coating on aluminium

An AlCrFeNiCuCo high-entropy alloy (HEA) coating was synthesised on an aluminium substrate by laser cladding. Samples were characterised using an optical microscope, X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, a microhardness tester, and an electrochemical workstation. The results showed that the interface between the cladding layer and matrix was sound, while the HEA coating consisted of BCC and FCC solid solutions and an Al-rich phase resulting from substrate dilution. The microstructure of the clad layer comprised both columnar and equiaxed grains. The average microhardness of the coating was 550 HV_0.2, and it exhibited better corrosion resistance than the aluminium matrix in a 1?mol?L^?1 H₂SO₄ solution. The typical corrosion characteristic of the coating was pitting and localised corrosion.     

Insights into structure and high-temperature oxidation behavior of plasma electrolytic oxidation ceramic coatings formed in NaAlO<Subscript>2</Subscript>–Na<Subscript>2</Subscript>CrO<Subscript>4</Subscript> electrolyte

Plasma electrolytic oxidation (PEO) ceramic coatings were formed on Ti₂AlNb alloy in various NaAlO₂ electrolytes containing 2 g L^?1, 4 g L^?1, and 6 g ^?1 Na₂CrO₄ additive, respectively. The influence of Na₂CrO₄ additive in NaAlO₂ electrolyte on structure and high-temperature oxidation behavior at 800 °C was investigated. The Na₂CrO₄ additive in the NaAlO₂ electrolyte not only promotes the formation of γ-Al₂O₃ phase and densification of ceramic coatings, but also participates directly in the growth of ceramic coating to form new Cr₃O and (Al_0.948Cr_0.052)₂O₃ phases. The PEO ceramic coatings formed in NaAlO₂ electrolytes with 2 g L^?1 and 4 g L^?1 Na₂CrO₄ additive show better oxidation resistance than those PEO coatings formed in a NaAlO₂ basic electrolyte based on isothermal oxidation tests at 800 °C up to 150 h. A thin and uniform isothermally oxidized layer is formed in the interlayer adjacent the substrate, which protects the substrate from the inward diffusion of oxygen and the outward diffusion of metallic elements. The PEO ceramic coatings formed in NaAlO₂ electrolyte with 4 g L^?1 Na₂CrO₄ additive exhibit the least mass gain among all the specimens, which is only a half of the ceramic coating formed in a NaAlO₂ basic electrolyte without any Na₂CrO₄ additive.     

Effect of cobalt content on microstructure and property of electroplated nickel‐cobalt alloy coatings

Nickel‐cobalt alloys were electrodeposited on copper sheets in sulfate bath containing 288.5 g/l NiSO₄·6H₂O, 30 g/l CoSO₄·7H₂O, 40 g/l HBO₃, 15 g/l NaCl and 0.08 g/l lauryl sodium sulfate. The effects of cobalt content on microstructure, microhardness, and wear resistance of electroplating nickel‐cobalt alloys were studied by using SEM and XRD techniques, and microhardness tester and wear tester. The relationship between the microhardness of nickel‐cobalt alloy coatings and heat treatment procedures was also investigated. The experimental results show that cobalt content (W_t) in coating increases with Co²⁺/(Co²⁺ + Ni²⁺)% (X) in plating solution. Fitted regression equation is as following: W_t = –0.7399 + 2.2847X – 0.0133X². The increase of cobalt content leads to that the longitudinal section morphology of coating transforms from the cone into sphericity and at last into the shape of willow leaf, and its structure transforms from face centered cubic (fcc) nickel solid solution into fcc cobalt solid solution and at last into hcp cobalt solid solution. The increase of cobalt content results in the increase of microhardness of nickel‐cobalt alloy coatings, and the hardness reaches a maximum value (363 HV) when cobalt content is 54.9%. After heat treatment at 400°C and 600°C, the microhardness of coatings begins to decrease except the coating containing 79.2% Co. Moreover, the wear resistance of electroplated coatings increases with the increase of cobalt content.     

Ceramic coated Y1 magnesium alloy surfaces by microarc oxidation process for marine applications

The magnesium alloys occupy an important place in marine applications, but their poor corrosion resistance, wear resistance, hardness and so on, have limited their application. To meet these defects, some techniques are developed. Microarc oxidation is a one such recently developed surface treatment technology under anodic oxidation in which ceramic coating is directly formed on the surface of magnesium alloy, by which its surface property is greatly improved. In this paper, a dense ceramic oxide coating, ∼20 μm thick, was prepared on an Y1 magnesium alloy through microarc oxidation in a Na₃SiO₃-Na₂WO₄-KOH-Na₂EDTA electrolytic solution. The property of corrosion resistance of ceramic coating was studied by CS300P electrochemistry-corrosion workstation, and the main impact factor of the corrosion resistance was also analysed. Microstructure and phase composition were analysed by SEM and XRD. The microhardness of the coating was also measured. The basic mechanism of microarc coating formation is explained in brief. The results show that the corrosion resistance property of microarc oxidation coating on the Y1 magnesium surface is superior to the original samples in the 3·5 wt% NaCl solutions. The microarc oxidation coating is relatively dense and uniform, mainly composed of MgO, MgAl₂O₄ and MgSiO₃. The microhardness of the Y1 magnesium alloy surface attained 410 HV, which was much larger than that of the original Y1 magnesium alloy without microarc oxidation.     

镁合金表面电镀前处理新工艺

常用的镁合金电镀前预处理时需预镀锌、预镀铜,还需使用氰化物,不仅污染环境,且预处理工艺复杂.为此,研究了AZ91D镁合金焦磷酸盐体系电镀铜的前处理工艺.分析了酸洗、活化、浸锌液主盐、配位剂、浸锌温度、浸锌时间等工艺参数对浸锌层质量的影响,确定了前处理工艺的优化条件.采用增重法测试了浸锌层的膜重,用电化学方法研究了镁合金的浸锌过程及浸锌层的耐蚀性能,用划痕、热震和锉刀法测试了经该前处理后所得镀铜层与基体的结合力.结果表明:采用该工艺电镀铜所得镀层细致、光亮,镀层与基体结合力良好,对镁合金基体有较好的防护作用.     

全光亮化学镀镍磷合金工艺研究

研究出了一种新型不含铅的全光亮化学镀镍工艺,获得了全光亮的镍磷合金镀层.通过试验分析镀液中添加剂、无机盐、主盐、施镀时间、pH值和施镀温度对化学镀镍磷合金层光亮度的影响;检测了有关性能.结果表明:所得化学镀镍磷合金镀层的光亮度、耐蚀性等性能优于常规化学镀镍磷合金镀层.CuSO4、TaSO4无机盐的添加使溶液稳定性(氯化钯稳定试验)从30 s提高到90 s,同时也提高了化学镀镍磷合金镀层耐蚀性,在5%NaCl溶液中的年腐蚀量从1.1 mg/cm2降为0.     

Passivity,pitting and corrosion of anodically polarized Fe-Ni alloys in 0.5 M H2SO4 containing Cl−

Iron-nickel alloys containing 0, 20, 40, 60, 80 and 100% Ni (wt%) have been anodically polarized in 0.5 M H₂SO₄ containing Cl^?, and the conditions for passivity, pitting and corrosion with respect to alloy composition and Cl^? concentration broadly defined. Breaks occur in the values of the corrosion properties at about 30% and 70% Ni. It is considered that the corrosion properties of alloys containing up to 30% Ni are determined by the ferrite in the alloy and the low corrosion resistance of its surface film, that alloys containing 30 to 70% Ni have a corrosion resistant film probably similar to a nickel ferrite spinel, and that alloys containing over 70% Ni have properties similar to nickel and probably have a surface film based on a solid solution of iron in NiO.     

镁合金硫酸镍体系化学镀镍工艺及镀层性能研究

采用酸性钼酸盐酸洗、碱性钼酸盐活化工艺,研究了AM60镁合金上硫酸镍溶液体系化学镀镍的方法.采用扫描电子显微镜(SEM)观察镀层表面形貌,电子探针(EDX)分析镀层成分,电化学方法研究镀层腐蚀性能,锉刀试验测试镀层与基体结合力.结果表明,所得镀层为Ni-P合金镀层,磷质量分数为10%～14%;镀层均匀致密,无明显缺陷;镀层的自腐蚀电位接近-0.4 V(vs SCE),阳极极化曲线有明显的钝化区;Ni-P镀层耐蚀性好,与基体结合牢固.     

Adsorbents made from textile scraps: preparation,characterization and application for removal of reactive dye

Textile scraps from the clothing industry were used to prepare a low-cost adsorbent to remove anionic dye from textile effluents. Adsorbents were prepared through pyrolysis and chemical activation with K₂CO₃. These samples were characterized through thermogravimetric analysis, scanning electron microscopy, N₂ adsorption/desorption isotherms, Fourier transform infrared spectroscopy, point of zero charge, isoelectric point, elemental composition and proximate analysis. Batch kinetic experiments and adsorption isotherm modeling were conducted in different conditions. The surface properties of the adsorbents were significantly influenced by the activation process. The highest BET surface area (S_BET = 358.55 m² g^?1) was attributed to the sample with chemical treatment. The results indicate that activation process raised 700% the adsorption capacity. The adsorption was strongly dependent on the pH. For the activated adsorbent, 6 g L^?1 was sufficient for the complete removal of 40 mg L^?1 Reactive Black 5 (RB5) solution. The monolayer capacity was up to 10.3 mg g^?1 and was higher than a commercial activated carbon commonly used in textile sector, which was 9.7 mg g^?1.     

Preparation of Dark-Red Membrane by Micro-Arc Oxidation on AM50 Alloys

The dark-red membrane was prepared on AM50 magnesium alloys by using chemical conversion and micro-arc oxidation (MAO). The effects of MAO electrolyte and chemical conversion solution were researched through L₉(3⁴) orthogonal test to obtain formulation for the aforementioned process. Surface morphology, composition, and corrosion resistance were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical test. A dark-red membrane can be obtained by the following steps: first, a chemical conversion was carried out on AM50 alloys by placing them in a solution of 50 g/L KMnO₄ and 100 g/L NH₄H₂PO₄ at 50°C for 11 min; second, the prepared alloys were placed into MAO electrolytes containing 10 g/L Na₂SiO₃ and 5 g/L NaOH in order to obtain the dark-red-colored MAO ceramic membranes. The membranes that are mainly composed of SiO₂, MnO, MnP, MgSiO₃, and Mg₂SiO₄ have porous structures. MnO and MnP were identified to be responsible for giving color to the membrane.     

Effect of chromium carbide coating on thermal properties of short graphite fiber/Al composites

A chromium carbide coating was synthesized onto graphite fibers by molten salts method to improve the interfacial bonding and thermal properties of short graphite fiber/Al composites which were fabricated by vacuum pressure infiltration technique. The graphite fiber/Al composites with different thicknesses of chromium carbide coatings were prepared through varying plating times to investigate the influence of chromium carbide layer on the microstructures and thermal properties of the composites. The combined Maxwell–Garnett effective medium approach and acoustic mismatch model schemes were used to theoretically predict thermal conductivities of the composites. The results indicated that the chromium carbide coating formed on graphite fiber surface in molten salts consists mainly of the Cr₇C₃ phase. The Cr₇C₃-coating layer with plating time of 60 min and thickness of 0.5 μm was found to be most effective in improving the interfacial bonding and decreasing the interfacial thermal resistance between graphite fiber and aluminum matrix. The 40 vol% Cr₇C₃-coated graphite fiber/Al composite with Cr₇C₃ thickness of 0.5 μm exhibited 45.4 % enhancement in in-plane thermal conductivity of 221 W m^?1 K^?1 compared to that of uncoated composite, as well as the coefficient of thermal expansion of 9.4 × 10^?6 K^?1, which made it as very interesting material for thermal management applications.     

The influence of HF treatment on corrosion resistance and <Emphasis Type="Italic">in vitro</Emphasis> biocompatibility of Mg-Zn-Zr alloy

The samples made of a Mg-2.5wt.%Zn-0.5wt.%Zr alloy were immersed in the 20% hydrofluoric acid (HF) solution at room temperature for different time, with the aim of improving the properties of magnesium (Mg) alloy in applications as biomaterials. The corrosion resistance and in vitro biocompatibility of untreated and fluoride-coated samples were investigated. The results show that the optimum process is to immerse Mg alloys in the 20% HF solution for 6 h. After the immersion, a dense magnesium fluoride (MgF₂) coating of 0.5 μm was synthesized on the surface of Mg-Zn-Zr alloy. Polarization tests recorded a reduction in the corrosion current density from 2.10 to 0.05 μA/cm² due to the MgF₂ protective coating. Immersion tests in the simulated body fluid (SBF) also reveal a much milder corrosion on the fluoride-coated samples, and its corrosion rate was calculated to be 0.05 mm/yr. Hemolysis test suggests that the conversion coated Mg alloy has no obvious hemolysis reaction. The hemolysis ratio (HR) of the samples decreases from 11.34% to 1.86% with the HF treatment, which meets the requirements of biomaterials (HR < 5%). The coculture of 3T3 fibroblasts with Mg alloy results in the adhesion and proliferation of cells on the surface of fluoride-coated samples. All the results show that the MgF₂ conversion coating would markedly improve the corrosion resistance and in vitro biocompatibility of Mg-Zn-Zr alloy.     

Corrosion and drug release properties of EN-plating/PLGA composite coating on MAO film

The electroless nickel plating/poly(dl-lactide-co-glycolide) composite coating (EN-plating/PLGA composite coating) was fabricated on the surface of the micro-arc oxidation (MAO) film of the magnesium alloy AZ81 to double control the corrosion and drug release in the hanks' solution. The EN-plating was fabricated on the MAO coating to improve the corrosion resistance by overlaying most pores and micro-cracks on the surface of the MAO film. Meanwhile, a double layered organic poly(dl-lactide-co-glycolide)/paclitaxel (PLGA/PTX) drug releasing coating with a top layered PLGA drug controlled releasing coating on EN plating was prepared to control the drug release rate by adjusting the different lactide: glycolide (LA:GA) ratio of PLGA. Scanning electron microscopy (SEM) and the X-ray powder diffraction (XRD) were used to analyze the morphology and the composition of the EN-plating. The corrosion behavior of the magnesium alloy substrate and the status of the drug in the PLGA matrix were respectively evaluated by Potentiodynamic polarization and Differential scanning calorimetry (DSC). The drug release was determined by ultraviolet–visible (UV–visible) spectrophotometer. EN-plating coating which was composed of compact cauliflower nodules was uniform in size and defect free with no pores or cracks. EN-plating could seal the microcracks and microholes on the outer layer of the MAO coating effectively. The corrosion resistance was improved by preventing the corrosive ions from diffusing to the magnesium alloy substrate. The drug release rate of PTX exhibited a nearly linear sustained-release profile with no significant burst releases.     

Electrochemical deposition and evaluation of electrically conductive polymer coating on biodegradable magnesium implants for neural applications

In an attempt to develop biodegradable, mechanically strong, biocompatible, and conductive nerve guidance conduits, pure magnesium (Mg) was used as the biodegradable substrate material to provide strength while the conductive polymer, poly(3,4-ethylenedioxythiophene) (PEDOT) was used as a conductive coating material to control Mg degradation and improve cytocompatibility of Mg substrates. This study explored a series of electrochemical deposition conditions to produce a uniform, consistent PEDOT coating on large three-dimensional Mg samples. A concentration of 1 M 3,4-ethylenedioxythiophene in ionic liquid was sufficient for coating Mg samples with a size of 5 × 5 × 0.25 mm. Both cyclic voltammetry (CV) and chronoamperometry coating methods produced adequate coverage and uniform PEDOT coating. Low-cost stainless steel and copper electrodes can be used to deposit PEDOT coatings as effectively as platinum and silver/silver chloride electrodes. Five cycles of CV with the potential ranging from ?0.5 to 2.0 V for 200 s per cycle were used to produce consistent coatings for further evaluation. Scanning electron micrographs showed the micro-porous structure of PEDOT coatings. Energy dispersive X-ray spectroscopy showed the peaks of sulfur, carbon, and oxygen, indicating sufficient PEDOT coating. Adhesion strength of the coating was measured using the tape test following the ASTM-D 3359 standard. The adhesion strength of PEDOT coating was within the classifications of 3B to 4B. Tafel tests of the PEDOT coated Mg showed a corrosion current (I_CORR) of 6.14 × 10^?5 A as compared with I_CORR of 9.08 × 10^?4 A for non-coated Mg. The calculated corrosion rate for the PEDOT coated Mg was 2.64 mm/year, much slower than 38.98 mm/year for the non-coated Mg.