Improving wear resistance and corrosion resistance of AZ31 magnesium alloy by DLC/AlN/Al coating

A novel protective coating, consisting of three layers (top: diamond-like carbon, middle: aluminum nitride, bottom: aluminum), was deposited on the surface of AZ31 magnesium alloy layer by layer. Nano-indenter, electrochemical system and tribological tester were performed to investigate the hardness, wear resistance and corrosion resistance of the coated AZ31 magnesium alloy, respectively. The DLC/AlN/Al coating improved the magnesium alloy's surface hardness and reduced its friction coefficient, which consequently induced a great improvement of the magnesium alloy's wear resistance. Furthermore, the corrosion resistance of the AZ31 magnesium alloy with the DLC/AlN/Al coating was also enhanced with the corrosion current density decreasing from 2.25 × 10⁻⁵ A/cm² to 1.28 × 10⁻⁶ A/cm² in a 3.5 wt.% NaCl solution.     

Electroless Ni-Sn-P coating on AZ91D magnesium alloy and its corrosion resistance

An electroless Ni-Sn-P coating was deposited on AZ91D magnesium alloy in an alkaline-citrate-based bath where nickel sulphate and sodium stannate were used as metal ion sources and sodium hypophosphite was used as a reducing agent. The phase structure of the coating was amorphous. SEM and attached EDS observation revealed the presence of dense and uniform nodules in the ternary coating and the content of tin was 2.48wt.%. Both the electrochemical analysis and the immersion test in 10% HCl solution proved that the ternary Ni-Sn-P coating exhibited better corrosion resistance than the Ni-P coating in protecting the magnesium alloy substrate.     

Polyoxadiazole-based coating for corrosion protection of magnesium alloy

In this study, polyoxadiazole-based coatings were molecularly designed by attaching two different functional groups, i.e., diphenyl-ether and diphenyl-hexafluoropropane, in the main polymer chain for the purpose of low water permeability and eventually for high corrosion protection of AM50 magnesium alloy. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) were used to evaluate the coating performance of the two polymers. Electrochemical experiments showed that POD-6FP (poly(4,4′-diphenyl-hexafluoropropane-1,3,4-oxadiazole)) coated alloy exhibited 3-4 orders of magnitude higher corrosion resistance as compared to the POD-DPE (poly (4,4′-diphenyl-ether-1,3,4-oxadiazole)) coated alloy. The high coating performance of the POD-6FP polymer can be attributed to the hydrophobic group attached to the polyoxadiazole chain.     

Chemical conversion coating on AZ31B magnesium alloy and its corrosion tendency

The morphology change of the magnesium matrix after pre-treatment and the mor-phology as well as the phase composition of chemical conversion coating formed by phosphate were studied using scanning electron microscope and X-ray diffraction. The corrosion resistance of the coating was studied by salt spray and damp test, and the corrosion tendency during salt immersion test was analyzed. The results show that the phase composition before and after pre-treatment is almost change- less, and the deep microflaw appears between α and β phases during acidic pickling. The phosphate conversion coating is mainly composed of Mg, MgO, and some amor-phous phase, and it can provide a good protection for the AZ31B alloy. Results from corrosive morphology indicate that the growth and the corrosion resistance of the phosphate conversion coating are related to the forming process of the AZ31B matrix.     

The effect of interlayer on corrosion resistance of ceramic coating/Mg alloy substrate in simulated physiological environment

Ceramic coatings are deposited on biodegradable magnesium alloys by physical vapor deposition to reduce the electrochemical activity in the simulated physiological environment. Although an interlayer is generally used to reduce the mismatch between the hard coating and soft substrate, the effects of the interlayer on the electrochemical corrosion behavior have seldom been explored. In this work, AlO_xN_y ceramic coatings were deposited on AZ31 magnesium alloys with Al or Ti interlayers. Polarization tests and electrochemical impedance spectroscopy (EIS) were conducted to evaluate the corrosion resistance in the cell culture medium. The AlO_xN_y ceramic coating significantly improved the bio-corrosion resistance of the magnesium alloy, but the Ti interlayer accelerated the corrosion rate. In comparison, although the addition of an Al interlayer led to smaller enhancement in the surface mechanical properties of the AlO_xN_y coating, corrosion could be impeded effectively. Our results indicate that an Al interlayer is preferred over Ti and the corrosion failure mechanism is discussed from the perspective of defects.     

Influence of magnesium nitrate on the corrosion performance of sol-gel coated AA2024-T3 aluminium alloy

Traditional anticorrosion technology has relied heavily on using reducible metal species, predominantly hexavalent chromium (Cr(VI)), for protecting reactive metal alloys such as aluminium which is extensively used in the aerospace sector. However, the impending changes in the use of Cr(VI) in Europe and the United States have forced aerospace manufacturers to examine alternative materials for protecting aluminium. One of the most promising alternatives being investigated are organosilane based sol-gels containing anticorrosion additives. In this work the anticorrosion properties of magnesium (II) nitrate (Mg(NO₃)₂) as an inhibitor were investigated at different concentrations (0.1%-1.0 wt.%) in a methyltrimethoxysilane (MTEOS) sol-gel on the aluminium alloy AA 2024-T3 and compared to Alodine^TM 1200 (the established Cr(VI) pre-treatment). Electrochemical evaluation of the coating system by electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) measurements correlated strongly with results obtained from Neutral Salt Spray (NSS) exposure data. The surface morphology of the coating was studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS). The results indicated that the optimum performance was achieved with a Mg (NO₃)₂ level of 0.7% w/w. It is proposed that the superior anticorrosion properties of the Mg²⁺ rich sol-gel are due to the pore blocking mechanism of insoluble magnesium precipitates formed during the hydrolysis process.     

Microstructure and corrosion resistance of magnesium alloy ZE41 with laser surface cladding by Al-Si powder

The microstructure and corrosion behavior of commercial alloy ZE41 modified by surface laser cladding with Al-Si powder mixture was studied by SEM, TEM, X-ray diffraction and electrochemical methods. The coating is composed of an Al-Mg matrix and dendrite precipitates of Mg₂Si. In function of the laser speed, the matrix is formed by a Mg solid solution in Al or by the intermetallic phase Mg₁₇Al₁₂. The presence of different matrixes is responsible for galvanic corrosion and decrease of corrosion resistance in interfacial area between coats. Isolated samples of the bulk coatings material showed similar corrosion potentials inspite of different matrix composition. This interpreted in terms of a mechanism involving two steps: (1) an initial dissolution of anodic Mg₂Si particles followed by (2) pitting in the formed crevices. The proposed mechanism corresponds well with the experimental observations and the mechanisms of localized corrosion observed for aluminium alloys in the chloride media described in the literature. Improved corrosion resistance can be achieved by the microstructure homogenization through the optimization of laser parameters and/or following heat treatment.     

Sol-gel silica coatings on ZE41 magnesium alloy for corrosion protection

Silica coatings have been applied on the surface of ZE41 magnesium alloy following the organic sol-gel route and the dip-coating technique. Three different concentrations of sol solution and two densification temperatures of the coating (400 °C and 500 °C) were used to optimize the compaction of the coatings and as a result reach the corrosion protection of the metallic substrate tests in 3.5 wt.% NaCl aqueous solution. Crack-free coatings with thickness in the 2-3 μm were obtained on the ZE41 magnesium alloy. The combination of high alkoxide concentration in the sol-gel formulation, and the high sintering temperature (500 °C) leads to coating (D500) with the optimal physical barrier against the corrosion process. This coating was capable of resisting more than 7 days in contact with the aggressive electrolyte suffering minor corrosion degradation. A corrosion mechanism for each of the tested specimens has been proposed.     

Anticorrosive magnesium phosphate coating on AZ31 magnesium alloy

A novel anticorrosive film with a thickness of approximately 50 μm was successfully coated on an AZ31 magnesium alloy by chemical and low-heat treatments (50 °C). The film was a single-phase system of newberyite (MgHPO₄•3H₂O) having an orthorhombic crystal structure. The corrosion current density of the newberyite film coated on the AZ31 magnesium alloy decreased by more than two orders of magnitude as compared to that of the AZ31 magnesium alloy. The static water contact angle of the newberyite film was less than 10°. The average value of the scratch critical load for the newberyite coating was estimated to be approximately 15 mN.     

Effect of electroless-Ni-plating on corrosion fatigue behavior of magnesium alloy

Fatigue tests were performed on electroless nickel-plated magnesium alloy specimens in laboratory air and 3% sodium chloride solution. In laboratory air, the effect of surface treatments (plating, blasting and polishing) on the fatigue lives of specimens was found to be minimal. However, in 3% sodium chloride solution, the electroless Ni-plated specimens were found to have shorter fatigue lives than those of the polished and blasted specimens. In order to study the fatigue mechanisms, successive observations of the specimen surfaces were conducted during the fatigue process in both laboratory air and sodium chloride solution. Observations of the fracture surfaces were also conducted to clarify the fatigue mechanism.     

Stearic acid coating on magnesium for enhancing corrosion resistance in Hanks' solution

Magnesium is potentially a desirable metallic material for degradable orthopedic implants if its corrosion rate can be reduced so that the implants can maintain sufficient mechanical integrity in the initial period of bone fracture healing. To achieve this aim, an organic coating was prepared on magnesium samples by a two-step process: (i) hydrothermal treatment to form a thick hydroxide layer, followed by (ii) immersion coating using stearic acid (SA, CH₃(CH₂)₁₆COOH, an endogenous long-chain saturated fatty acid which is the main component of fat). The hydroxide layer formed in the first step provided sites to anchor the SA coating in the subsequent step, with the formation of magnesium stearate, which would enhance adhesion of the coating to the substrate. The corrosion behavior of the coated samples in Hanks' solution (a simulated body fluid) was studied by electrochemical methods. The corrosion resistance of the coated samples was much higher than that of bare magnesium, especially in the initial period, due to the barrier effect provided by the coating. Degradation of the coating occurred due to cracking in long-term immersion, with the formation of magnesium hydroxide and apatite.     

Influence of carbon sputtering power on structure, corrosion resistance, adhesion strength and wear resistance of platinum/ruthenium/nitrogen doped diamond-like carbon thin films

Platinum/ruthenium/nitrogen doped diamond-like carbon (PtRuN-DLC) thin films were deposited on conductive p-Si substrates using a DC magnetron sputtering deposition system by varying carbon sputtering power. The chemical composition, bonding structure, surface morphology and adhesion strength of the PtRuN-DLC films were investigated using X-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy, atomic force microscopy (AFM), and micro-scratch test, respectively. The corrosion behavior of the PtRuN-DLC films in a 0.1 M NaCl solution was investigated using potentiodynamic polarization test. The corrosion results indicated that the corrosion resistance of the PtRuN-DLC films increased with increased carbon sputtering power probably due to decreased porosity level in the films with increased growth rate and film thickness. The wear performance of the PtRuN-DLC films was investigated with a ball-on-disc micro-tribometer. It was found that the increased carbon sputtering power significantly improved the wear performance of the films by enhancing the adhesion strength of the films.     

2-Hydroxy-4-methoxy-acetophenone as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy

The inhibition behavior of 2-hydroxy-4-methoxy-acetophenone (paeonol) as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy was investigated in 0.05 wt.% NaCl solution by means of polarization curve, AC impedance, weight loss measurement, scanning electron microscopy, Fourier transformation infrared spectroscopy, ultraviolet analysis, and computer molecular simulation. The results show that paeonol can inhibit the corrosion of AZ91D. The maximum inhibition efficiency is achieved when paeonol concentration is 50 ppm by weight in this study. It is proposed that paeonol chelates with Mg to form a paeonol-Mg complex mixing with the original Mg(OH)₂ film on the surface to inhibit the anodic dissolution of AZ91D.     

Structure, scratch resistance and corrosion performance of nickel doped diamond-like carbon thin films

Nickel doped diamond-like carbon (DLC:Ni) thin films were fabricated on conductive p-Si (100) substrates with DC magnetron sputtering deposition by varying DC power applied to a Ni target. The bonding structure, surface morphology, scratch resistance and corrosion resistance of the DLC:Ni films were studied using X-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy, atomic force microscopy (AFM), micro-scratch test, potentiodynamic polarization test and open circuit potential measurement. The results indicated that the corrosion resistance of the DLC:Ni films in a 0.6 M NaCl solution decreased with increased Ni content in the films though the films showed good passivation behavior in the NaCl solution.     

Study on corrosion resistance of pure magnesium with CaSiO3 contained coating in NaC1 solution

A composite coating was fabricated on pure magnesium by hydrothermal treatment in order to reduce its degradation in body environment.The coating was characterized by scanning electron microscopy(SEM) and X-ray diffraction(XRD).The XRD pattern showed that the main composition of the coating was a mixture of CaSiO3,MgSiO3 and Mg(OH)2.Electrochemical test showed that the corrosion current density(icorr) of the coated magnesium was decreased by about two orders of magnitude compared with that of the bare magnesium,and the EIS measurement also showed that the corrosion resistant performance of the coated magnesium was significantly enhanced.Meanwhile,weight loss test showed that the weight loss of the coated magnesium was lower than that of the bare magnesium.Hence,the present study indicated that the composite coating could greatly slow down the degradation of pure magnesium.     

羟基磷灰石/氧化石墨烯/氢氧化镁复合涂层镁-钙合金的耐蚀性及骨折修复效果研究

镁金属在生理环境中的降解速率和生物活性改善有助于其在生物医用领域的应用推广,合金化处理和表面改性是两种有效的途径。本文针对镁-钙合金(ZQ)提出一种新的改性方法,利用水热处理、电泳沉积和电化学沉积的结合,在其表面上构建羟基磷灰石/氧化石墨烯/氢氧化镁(HA/GO/ Mg(OH)2)复合涂层。扫描电镜(SEM)和X-射线衍射(XRD)分析证实,该复合涂层样品(ZQ-HEP)表面逐层形成了外层纳米薄片HA、中层絮片状GO、及内层Mg(OH)2纳米片的复合涂层。体外电化学耐腐蚀性测试分析表明,涂层化的ZQ-HEP比未处理ZQ样品在磷酸缓冲盐溶液(PBS)中有更优的耐蚀性。通过构建的体内兔股骨髁骨折模型,评价表面改性前后ZQ样品的骨折修复效果。术后大体观察、影像学和组织学分析证实,体内生理环境下ZQ-HEP螺钉比ZQ螺钉能降低析氢速率,减少皮下气肿现象。同时,ZQ-HEP螺钉因其更优的耐蚀性,在术后4周内可较好地保持其材料的完整性,同时介导更多的新骨长入,进而实现骨折的快速愈合。本研究提出的GO/HA/ Mg(OH)2复合涂层构建策略,可在调控镁金属降解速率的同时,显著提升其成骨修复效果,具有良好的临床应用前景。     

Improvement of thermal shock resistance for thermal barrier coating on aluminum alloy with various electroless interlayers

NiCoCrAlY/8YSZ coating was firstly directly deposited on aluminum alloy 5A06 by atmospheric plasma spray to make it applicable to short-time high temperature condition. The failure after thermal shock test was mainly due to the stress caused by thermal expansion mismatch between the bond coat and the substrate as well as the galvanic corrosion of the aluminum alloy. Ni-P, Ni-W-P and Ni-Cu-P as interlayers were electrolessly deposited on the substrate in order to mitigate the thermal stress. The composition and thermal transformation of the interlayers were investigated. Thermal shock resistance and bonding strength of multilayer coatings (interlayer/NiCoCrAlY/8YSZ) were tested. Diffusion layers mainly composed of AlNi, Al₃Ni₂ and Al₃Ni were observed between the interlayers and the substrate after thermal shock test. The oxidation of the substrate was effectively inhibited. Ni-P interlayer obtained at lower pH value was superior to the other two interlayers and enhanced the thermal shock life from 38 to more than 200 cycles. With the application of the Ni-P and Ni-Cu-P interlayers, the bonding strength examined by pull-off test was also largely improved from 11.7 MPa to 18.8 and 19.0 MPa, respectively.     

A composite coating for corrosion protection of AM60B magnesium alloy

Oxide films have been produced on AM60B magnesium alloy by micro-arc anodic oxidation in an environmentally friendly alkaline solution, with and without addition of nanoparticles (TiO₂, ZrO₂ and Al₂O₃). Because of the anodic oxide porosity, inherent in the sparking process, organo-functional silanes top coat has been applied to seal pores and cracks, and achieve an efficient protective coating system. The surface and cross-section morphology of samples were analyzed by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS). Scratch tests were performed for evaluating the adhesion strength and scratch hardness of the anodic oxides to the AM60B substrate. The corrosion resistance of both anodic oxides and oxide/silane composite coatings was evaluated in 0.6 M NaCl solution using potentiodynamic polarization tests. The addition of nanoparticles to the anodizing solution doesn't affect significantly the corrosion resistance in comparison with anodic oxides produced in nanoparticles free solutions. Conversely, the adhesion strength and scratch hardness of the anodic oxides to the substrate is quite scattered, and it is higher for the samples produced in ZrO₂ and in Al₂O₃ rich solutions. For this reason specimens anodized in ZrO₂ and Al₂O₃ containing solutions were chosen for silane deposition. Two silanes were used, namely octyltrimethoxysilane (OSi) and 1, 2-bis [triethoxysilyl] ethane (BTSE). The anodizing treatment carried out in oxides nanoparticles containing solutions (ZrO₂ or Al₂O₃), followed by a silane top coat treatment performed using OSi precursor, is an interesting way, suitable for industrial applications, to synthesize adherent corrosion resistant coatings on magnesium alloy AM60B in a short process time.     

镁合金微弧氧化陶瓷膜的制备及耐蚀性能

利用微弧氧化技术在AZ91D镁合金表面原位生成含有钙、磷元素的陶瓷膜层.用SEM、XRD、EDS等研究陶瓷膜微观形貌、相组成及元素含量,利用Tafel和EIS技术来评价陶瓷膜的腐蚀性能.结果表明,所制备的陶瓷膜层成功地引入了钙和磷元素,陶瓷膜层主要由Mg2SiO4和MgO相组成.增加钙盐浓度,可以使膜层内的钙元素含量增多,微孔增加并且出现了微裂纹.电化学测试表明陶瓷膜使得镁合金在0.9%NaCl生理盐水中的耐蚀性提高了1～2个数量级,当钙盐浓度为0.3 g/L时,陶瓷膜层的耐蚀性最好.     

Effect of laser surface treatment on corrosion and wear resistance of ACM720 Mg alloy

A creep resistant Mg alloy ACM720 was subjected to laser surface treatment using Nd:YAG laser equipped with a fiber optics beam delivery system in argon atmosphere. This treatment was found to be beneficial for the corrosion and wear resistance of the alloy. Long-term linear polarization resistance and Electrochemical Impedance Spectroscopy measurements confirmed that the polarization resistance values of laser surface treated alloy were twice as high as that for the untreated alloy. The improved corrosion resistance was attributed to the absence of the second phase Al₂Ca at the grain boundary, microstructural refinement and extended solid solubility, particularly of Al, in α-Mg matrix owing to rapid solidification. The laser treatment also increased surface hardness two times and reduced the wear rate considerably due to grain refinement and solid solution strengthening.