Electrodeposition of high corrosion resistance Cu/Ni-P coating on AZ91D magnesium alloy

High corrosion resistance Cu/Ni-P coatings were electrodeposited on AZ91D magnesium alloy via suitable pretreatments, such as one-step acid pickling-activation, once zinc immersion and environment-friendly electroplated copper as the protective under-layer, which made Ni-P deposit on AZ91D Mg alloy in acid plating baths successfully. The pH value and current density for Ni-P electrodeposition were optimized to obtain high corrosion resistance. With increasing the phosphorous content of the Ni-P coatings, the deposits were found to gradually transform to amorphous structure and the corrosion resistance increased synchronously. The anticorrosion ability of AZ91D Mg alloy was greatly improved by the amorphous Ni-P deposits, which was investigated by potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS). The corrosion current density (I_corr) of the coated Mg alloy substrate is about two orders of magnitude less than that of the uncoated.     

Electroless deposition of Ni-W-P coating on AZ91D magnesium alloy

Ternary Ni-W-P alloy coating was deposited directly on AZ91D magnesium alloy by using an alkaline-citrate-based baths. Nickel sulfate and sodium tungstate were used as metal ion sources, respectively, and sodium hypophosphite was used as a reducing agent. The pH value of the electroless bath was tailored for magnesium alloy. The coating was characterized for its structure, morphology, microhardness and the corrosion properties. SEM observation showed the presence of dense and coarse nodules in the ternary coating. EDS analysis showed that the content of tungsten in the Ni-W-P alloy was 4.5 wt.%. Both the electrochemical analysis and the immersion test in 10% HCl solution revealed that the ternary Ni-W-P coating exhibited good corrosion resistance properties in protecting the AZ91D magnesium alloy.     

Studies on influence of zinc immersion and fluoride on nickel electroplating on magnesium alloy AZ91D

The effect of zinc immersion and the role of fluoride in nickel plating bath were mainly investigated in nickel electroplating on magnesium alloy AZ91D. The state of zinc immersion, the composition of zinc film and the role of fluoride in nickel plating bath were explored from the curves of open circuit potential (OCP) and potentiodynamic polarization, the images of scanning electron microscopy (SEM) and the patterns of energy dispersive X-ray (EDX). Results show that the optimum zinc film mixing small amount of Mg(OH)₂ and MgF₂ is obtained by zinc immersion for 30-90 s. The corrosion potential of magnesium alloy substrate attached zinc film will be increased in nickel plating bath and the quantity of MgF₂ sandwiched between magnesium alloy substrate and nickel coating will be reduced, which contributed to produce nickel coating with good performance. Fluoride in nickel plating bath serves as an activator of nickel anodic dissolution and corrosion inhibitor of magnesium alloy substrate. 1.0-1.5 mol dm⁻³ of F⁻ is the optimum concentration range for dissolving nickel anode and protecting magnesium alloy substrate from over-corrosion in nickel plating bath. The nickel coating with good adhesion and high corrosion resistance on magnesium alloy AZ91D is obtained by the developed process of nickel electroplating. This nickel layer can be used as the rendering coating for further plating on magnesium alloys.     

A novel process of electroless Ni-P plating with plasma electrolytic oxidation pretreatment

A novel Ni based coating - plasma electrolytic oxidation (PEO) pre-treatment followed by electroless nickel (EN) plating - has been developed to produce pore free Ni coatings on AZ91 magnesium alloy. The application of the PEO film between the nickel coating and the substrate acts as an effective barrier and catalytic layer for the subsequent nickel plating. The potentiodynamic tests indicated that the corrosion current density of the PEO + EN plating on AZ91 decreased by almost two orders of magnitudes compared to the traditional EN coating. Salt fog spray testing further proved this improvement. More importantly, the new technique does not use Cr⁺⁶ and HF in its pretreatment, therefore is a much environmentally friendlier process.     

A novel process for electroless nickel plating on anodized magnesium alloy

In this paper, a novel palladium-free activation electroless nickel (EN) plating process, by which a TiB₂ powders contained intermediate film was used as catalyst, was introduced for anodized magnesium alloy AZ91D. The corrosion behavior of AZ91D without and with coating was compared and the bonding strength of the EN plating to the substrate was also measured. The results showed that the EN plating could easily take place on the intermediate catalytic layer, directly on which a smooth and compact Ni–P alloy layer without obvious flaws, about 20 μm thickness, was successfully deposited. The catalytic function was principally from TiB₂ powder. The adhesive tensile test indicated a good bonding strength of about 11 MPa between the substrate and the catalytic layer. An obvious passivation range and higher E_corr (−0.323 V) for the EN plating during anodic polarization in 3.5 wt.% NaCl solution, implied a typical character of a compact Ni–P alloy layer, with an effective protection for the substrate.     

Effect of α-Al2O3 on the properties of cold sprayed Al/α-Al2O3 composite coatings on AZ91D magnesium alloy

Composite coatings using pure Al powder blended with α-Al₂O₃ as feedstock were deposited on AZ91D magnesium alloy substrates by cold spray (CS). The content of α-Al₂O₃ in the feedstock was 25 wt.% and 50 wt.%, respectively. The effects of α-Al₂O₃ on the porosity, microhardness, adhesion and tensile strength of the coatings were studied. Electrochemical tests were carried out in neutral 3.5 wt.% NaCl solution to evaluate the effect of α-Al₂O₃ on the corrosion behavior of the coatings. The results showed that the composite coatings possessed lower porosity, higher adhesion strength and tensile strength than cold sprayed pure Al coating. The corrosion current densities of the composite coatings were similar to that of the pure Al coating and much higher than that of bare AZ91D magnesium alloy.     

Correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys

X-ray photoelectron spectroscopy (XPS) was used in order to investigate the correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys exposed to 98% relative humidity at 50 °C. Commercially pure magnesium, used as the reference material, revealed MgO, Mg(OH)₂ and tracers of magnesium carbonate in the air-formed film. For the AZ80 and AZ91D alloys, the amount of magnesium carbonate formed on the surface reached similar values to those of MgO and Mg(OH)₂. A linear relation between the amount of magnesium carbonate formed on the surface and the subsequent corrosion behaviour in the humid environment was found. The AZ80 alloy revealed the highest amount of magnesium carbonate in the air-formed film and the highest atmospheric corrosion resistance, even higher than the AZ91D alloy, indicating that aluminium distribution in the alloy microstructure influenced the amount of magnesium carbonate formed.     

Tribological properties of heat-treated electroless Ni-P coatings on AZ91 alloy

Influence of heat treatment regime on adhesion and wear resistance of Ni-P electroless coating on AZ91 magnesium alloy is investigated in this work. The pretreated substrate was plated using a bath containing nickel sulphate, sodium hypophosphite and sodium acetate as main constituents. The coated samples were heat treated at 400-450 °C for 1-8 h. Adhesion of coating was estimated from the scratch test with an initial load of 8.80 N. Wear resistance was studied using the pin-on-disc method. It was found that there is no significant dependence of the coating wear resistance on heat treatment regime, as the formation of Al-Ni intermetallic sub-layers that reduce coating adhesion is limited to regions where Al₁₇Mg₁₂ phase is present in the substrate. Moreover, the coating shows good sliding properties due to the formation of oxide glazes in the wear track.     

Plasma electrolytic oxidation coating on AZ91 magnesium alloy modified by neodymium and its corrosion resistance

Ceramic coatings on the surfaces of Mg-9Al-1Zn (AZ91) magnesium alloy and Mg-9Al-1Zn-1Nd magnesium alloy (AZ91 magnesium alloy modified by neodymium, named as AZ91Nd in this paper) are synthesized in aluminate electrolyte by plasma electrolytic oxidation (PEO) process, respectively. X-ray diffraction and X-ray photoelectron spectroscopy analyses show the PEO coating on the Mg-9Al-1Zn-1Nd alloy comprises not only MgO and Al₂O₃, which are found in the coating on the AZ91 alloy, but also a trace amount of Nd₂O₃. Microstructure observations indicate the addition of Nd can decrease the sizes of β phases and form Al₂Nd intermetallics in the AZ91 alloy. The fine β phases can effectively restrain the formation of unclosed-holes and greatly decrease the sizes of pores in the coating during the PEO process. In addition, the Al₂Nd intermetallics can be completely covered due to the lateral growth of the PEO coatings formed on the α and β phases. As a result, the coating on the AZ91Nd alloy possesses a dense microstructure compared with that on the AZ91 alloy. The following corrosion tests indicate the corrosion resistance of the PEO coating on the AZ91Nd alloy is evidently higher than that of the PEO coating on the AZ91 alloy.     

The microstructure, mechanical and friction properties of protective diamond like carbon films on magnesium alloy

Protective hard coatings deposited on magnesium alloys are believed to be effective for overcoming their poor wear properties. In this work, diamond-like carbon (DLC) films as hard protective films were deposited on AZ91 magnesium alloy by arc ion plating under negative pulse bias voltages ranging from 0 to −200 V. The microstructure, composition and mechanical properties of the DLC films were analyzed by scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and nanoindentation. The tribological behavior of uncoated and coated AZ91 magnesium alloy was investigated using a ball-on-disk tribotester. The results show that the negative pulse bias voltage used for film deposition has a significant effect on the sp³ carbon content and mechanical properties of the deposited DLC films. A maximum sp³ content of 33.3% was obtained at −100 V, resulting in a high hardness of 28.6 GPa and elastic modulus of 300.0 GPa. The DLC films showed very good adhesion to the AZ91 magnesium alloy with no observable cracks and delamination even during friction testing. Compared with the uncoated AZ91 magnesium alloy, the magnesium alloy coated with DLC films exhibits a low friction coefficient and a narrow, shallow wear track. The wear resistance and surface hardness of AZ91 magnesium alloy can be significantly improved by coating a layer of DLC protective film due to its high hardness and low friction coefficient.     

Preparation of the Ni-P composite coating co-deposited by nano TiC particles and evaluation of it's corrosion property

In current research, low carbon steel plates were coated by Ni-P electroless method. The effect of adding different concentrations (ranging from 0.01 g/l to 0.5 g/l) of TiC nano-sized particles to the plating bath on deposition rate, surface morphology and corrosion behavior of Ni-P-TiC composite coatings were investigated. The surface morphology and the relevant structure were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Corrosion behavior of the coated steel was evaluated by electrochemical impedance spectroscopy (EIS) and polarization techniques. The results showed that addition of TiC nano-particles to Ni-P electroless bath not only changes the surface morphology of Ni-P coating, but also improves corrosion resistance of the steel in comparison with TiC free Ni-P electroless coating. In addition, the deposition rate of coating was also affected by incorporation of TiC particles. It was also found that improvement in corrosion resistance largely depends on the phosphorous and TiC concentrations on the coatings.     

Electrochemical behaviors of the magnesium alloy substrates in various pretreatment solutions

Interface reactions and film features of AZ91D magnesium alloy in pickling, activation and zinc immersion solutions have been investigated. The surface morphologies of the specimens were observed with scanning electron microscope (SEM). Electrochemical behaviors of AZ91D magnesium alloy in the baths of pickling, activation and zinc immersion were analyzed based on the open circuit potential (OCP) - time curves in various solutions. The results show that the corrosive rate in HNO₃ + CrO₃ or HNO₃ + H₃PO₄ pickling solution was more rapid than in KMnO₄ pickling-activation solution. Both α phase and β phase of the substrates were uniformly corroded in HNO₃ + CrO₃ or HNO₃ + H₃PO₄ pickling solution, the coarse surface can augment the mechanical occlusive force between the subsequent coatings and the substrates, so coatings with good adhesion can be obtained. In HF activation solution, the chromic compound formed via HNO₃ + CrO₃ pickling was removed and a compact MgF₂ film was formed on the substrate surface. In K₄P₂O₇ activation solution, the corrosion products formed via HNO₃ + H₃PO₄ pickling were removed, a new thin film of oxides and hydroxides was formed on the substrate surface. In KMnO₄ pickling-activation solution, a film of manganic oxides and phosphates was adhered on the substrate surface. Zinc film was symmetrically produced via K₄P₂O₇ activation or KMnO₄ pickling-activation, so it was good interlayer for Ni or Cu electroplating. Asymmetrical zinc film was produced because the MgF₂ film obtained in the HF activation solution had strong adhesive attraction and it was not suitable for interlayer for electroplating. However, the substrate containing compact MgF₂ film without zinc immersion was fit for direct electroless Ni-P plating.     

Corrosion protection of Mg/Al alloys by thermal sprayed aluminium coatings

The protective features of thermal sprayed Al-coatings applied on AZ31, AZ80 and AZ91D magnesium/aluminium alloys were evaluated in 3.5 wt.% NaCl solution by electrochemical and gravimetric measurements. The changes in the morphology and corrosion behaviour of the Al-coatings induced by a cold-pressing post-treatment were also examined. The specimens were characterized by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. The as-sprayed Al-coatings revealed a high degree of porosity and poor corrosion protection, which resulted in galvanic acceleration of the corrosion of the magnesium substrates. The application of a cold-pressing post-treatment produced more compact Al-coatings with better bonding at the substrate/coating interface and higher corrosion resistance regardless of the nature of the magnesium alloy.     

Electrodeposition of Al-Mn alloy on AZ31B magnesium alloy in molten salts

The Al-Mn alloy coatings were electrodeposited on AZ31B Mg alloy in AlCl₃-NaCl-KCl-MnCl₂ molten salts at 170 °C aiming to improve the corrosion resistance. However, in order to prevent AZ31B Mg alloy from corrosion during electrodeposition in molten salts and to ensure excellent adhesion of coatings to the substrate, AZ31B Mg alloy should be pre-plated with a thin zinc layer as intermediate layer. Then the microstructure, composition and phase constituents of the coatings were investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), and X-ray diffraction (XRD). It was indicated that, by adjusting the MnCl₂ content in the molten salts from 0.5 wt% to 2 wt%, the Mn content in the alloy coating was increased and the phase constituents were changed from f.c.c Al-Mn solid solution to amorphous phase. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization measurements in 3.5% NaCl solution. It was confirmed that the Al-Mn alloy coatings exhibited good corrosion resistance with a chear passive region and significantly reduced corrosion current density at anodic potentiodynamic polarization. The corrosion resistance of the alloy coatings was also related with the microstructure and Mn content of the coatings.     

Composition design and laser cladding of Ni-Zr-Al alloy coating on the magnesium surface

The cluster line criterion was used for optimized design of a Ni-Zr-Al alloy used as coating on the AZ91HP magnesium alloy by laser cladding. Results show that the coating mainly consists of an amorphous, two ternary intermetallic phases with Ni₁₀Zr₇ and Ni₂₁Zr₈ type structures resulting in high hardness, good wear resistance and corrosion resistance. The interface between the clad layer and the substrate has good metallurgical bond.     

Comparison of the coating properties and corrosion rates in electroless Ni-P/PTFE composites prepared by different types of surfactants

The effects of the addition of three types of surfactants (cationic, anionic, non-ionic) at different concentrations in the plating bath on the deposition rate, PTFE content and surface morphology of electroless Ni-P/PTFE composite coatings were investigated. It was demonstrated that the cationic and non-ionic surfactants created a uniform distribution of PTFE particles in the coatings. The effects of the surfactant type and concentration on the corrosion properties of Ni-P/PTFE coatings were also studied. The corrosion resistance was increased by the incorporation of PTFE particles into the Ni-P matrix. The level of improvement depended largely on the type and concentration of the applied surfactants.     

激光熔覆Al+SiC涂层对镁合金表面耐磨性能的改性

通过脉冲激光器(Nd-YAG)在AZ91D镁合金基底上熔覆Al+SiC粉体。采用扫描电子显微镜、能量色散谱(EDS)和X-射线衍射测定分析熔覆层的显微组织、化学成分和物相组成。研究表明:Al+SiC涂层主要由SiC,β-Mg_(17)Al_(12)及Mg和Al相组成,激光熔覆层与镁合金基底表现出良好的冶金结合。所有样品都具有树枝状结构,且随着SiC质量分数的增大,树枝状和胞状结构的间隔变得更大。熔覆涂层的表面硬度高于基底,并且随着熔覆层中的SiC质量分数的增加而增大,SiC质量分数为40%的熔覆层具有最大的显微硬度,达到180 HV,然而质量分数为10%的熔覆层硬度为136 HV。销盘滑动磨损试验表明,复合涂层中的SiC颗粒和原位合成的Mg_(17)Al_(12)相显著提高了AZ91D镁合金的耐磨损性,其中,SiC质量分数从10%增加到30%过程中磨损体积损失逐渐减少,SiC质量分数在20%~30%时熔覆层具有最好的耐磨性。     

Multi-layered electroless Ni–P coatings on powder-sintered Nd–Fe–B permanent magnet

This paper has shown a successful protective coating scheme for powder-sintered Nd–Fe–B permanent magnet using multi-layered electroless nickel (EN) deposition. A low-phosphorus nickel layer is plated with an alkaline EN solution first, followed by a high-phosphorus nickel layer plated with an acidic solution. An additional topcoat by medium-phosphorus nickel on the high-phosphorus coating is also explored. It is shown that the high-phosphorus nickel layer coated in acidic solution provides the best corrosion protection because of its dense amorphous structure. The medium phosphorus topcoat is also dense and is able to provide reasonable corrosion resistance. The low-phosphorus layer itself does not have enough corrosion resistance; its main role is to provide an intermediate coating on the powder-sintered magnet. X-ray diffraction measurement shows that the low-phosphorus coating consists of nano-crystallines, and the high- and the medium-phosphorus coatings are dominated by amorphous structure. Microscopic observation and scratch test on these composite coatings demonstrate good adhesion between the magnet and the coatings. Remanence and coercivity of the plated magnet decrease with the applied coatings, but measured values are still very attractive for practical applications among known hard magnets.     

Investigation of plasma electrolytic oxidation process on AZ91D magnesium alloy

Ceramic coatings oxidized for different time periods were prepared to characterize the plasma electrolytic oxidation (PEO) process of AZ91D magnesium alloy. The coatings were analyzed using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscope and potentiodynamic polarization measurement. The results show that the PEO coatings perform different growth behaviors at different PEO stages, and different morphologies are exhibited on α- and β-phase of Mg substrate. The corrosion resistance measurement predicates that within the first 30 min oxidation, coating oxidized for 20 min is the best corrosion resistant.     

Morphological study of ternary Ni–Cu–P alloys by atomic force microscopy

Ternary electroless Ni–Cu–P alloy films were deposited by using nickel sulphate (B1)- and nickel chloride (B2)-based alkaline baths. Alloy films were characterized for their structure, morphology, chemical composition and microhardness. A single broad peak was obtained in XRD for both B1 and B2 films and the calculated grain sizes are 1.6 and 1.9 nm, respectively. Optical microscopic examination of the deposited coatings revealed a less nodular structure for B2-based coatings. SEM micrographs showed that films were smooth and nodular. Compositional analysis made on these deposits using EDX and the chemical state identification by XPS showed that the coatings are almost identical. AFM studies showed that the deposits from B2 bath are comparatively smoother with less nodular structure. Microhardness measurements and potentiodynamic polarization studies in 3.5% NaCl solution showed that both deposits have similar properties.