Constant magnetic field refinement of chromium-free conversion coatings on AZ31 magnesium alloy

By the electrochemical protective efficiency of conversion coatings on an AZ31 alloy, it is shown that the corrosion stability of AZ31 alloy depends on the activating solution treated with magnetic field. Scanning electron microscopic studies demonstrate that the surface morphology of conversion coatings is identical irrespective of the magnetic field treatment. The cross-section composition of conversion coatings is determined by x-ray photoelectron spectroscopy (XPS). XPS data demonstrate that a conversion coating treated with magnetic field is thicker than coatings nonsubjected to this treatment. It is also shown that a conversion coating of AZ31 alloy mainly consists of oxides and hydroxides, namely, MgO, Mg(OH)₂, MgF₂ and γAl₂O₃, MgAl₂O₄. However, when magnetic field is applied, the aluminum concentration increases from 0.73 to 6.65 at. %, and the fluorine concentration decreases from 12.06 to 8.11 at. %.     

Characterization of calcium-modified zinc phosphate conversion coatings and their influences on corrosion resistance of AZ31 alloy

Two kinds of phosphate conversion coatings, including zinc phosphate coating and zinc-calcium phosphate coating, were prepared on the surface of AZ31 alloy in phosphate baths. The morphologies of these coatings were observed using scanning electron microscopy. Their chemical compositions and structures were characterized using energy-dispersive X-ray spectrum, X-ray photoelectron spectroscopy and X-ray diffraction. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization technique. The results show that the flowerlike Zn-Ca phosphate conversion coatings are mainly composed of hopeite (Zn₃(PO₄)₂·4H₂O). They have a quite different morphology from the dry-riverbed-like Zn phosphate coatings that consist of MgO, MgF₂, Zn or ZnO and hopeite. Both of the zinc and zinc-calcium phosphate coatings can remarkably reduce the corrosion current density of the substrates. The Zn-Ca coating exhibits better corrosion resistance than the Zn coating. Introduction of calcium into the phosphate baths leads to the full crystallinity of the Zn-Ca coating.     

Looking for a surface treatment to deposit a reactive element with potential industrial use in high temperature oxidation field

In order to introduce a so‐called reactive element simultaneously at all the surfaces of a NiCr (Ni‐20wt.%Cr‐1.5wt.%Si) metallic substrate alloy, an easy to apply surface treatment was used: an yttrium containing thin film (constituted of a hydrated yttrium hydroxide nitrate) was electrodeposited from a mixed water‐ethanol solution. After a thermal conversion treatment at 600 °C under argon, leading to an Y₂O₃ coating, specimens were submitted to high‐temperature oxidation testing, under artificial air at atmospheric pressure, in order to evaluate in these conditions the effect of the coatings on the behaviour of the studied alloy. Expected effects of the introduced reactive element (REE) were effectively shown: decrease of the coated sample weight gain, suppression of the formation of the base metal oxides and change of the oxide grains morphology.     

Preparation and characterization of duplex PEO/MoC coatings on Mg-Li alloy

A combined PEO and chemical conversion process was employed to fabricate duplex plasma electrolytic oxidation/molybdate conversion (PEO/MoC) coating on the surface of Mg-Li alloy. The microstructure and composition of the composite coatings were investigated by SEM, EDX, XRD and XPS. The anti-corrosion properties of duplex PEO/MoC coatings were evaluated by potentiodynamic polarization and EIS. The duplex PEO/MoC coating was composed of crystalline NaMgF₃, Mg₂SiO₄, MoO₃ and MgO. Spherical-like microparticles accumulated and dispersed uniformly on the surface of the PEO coating. The corrosion resistance for Mg-Li alloy was improved by using a combination of plasma electrolytic oxidation and chemical conversion.     

外加磁场下AZ31镁合金磷化膜结构及耐蚀性研究

为了获得优良的AZ31镁合金磷化膜,采用外加磁场作用于镁合金的磷化过程.利用SEM,AFM,XRD和电化学工作站等仪器,研究转化膜的表面形貌、结构及耐蚀性.研究结果表明:磁场方向垂直于镁合金样品情况下显著促进AZ31镁合金转化膜的形成,转化膜主要由晶态Zn3(PO4)2·4H2O与A1PO4和非晶态Mn化合物组成,外加...     

Synthesis and protection of AM50 magnesium alloy and its composites using environmentally pretreatment electrolyte

This article details with the synthesis and surface modification of AM50 magnesium alloy and its composite (AM50-5%ZrO₂ and AM50-10% ZrO₂) substrate to improve the corrosion resistance. Environmentally friendly chemical conversion coating based on permanganate-phosphate solution was used in this study. Deposition of electroless Ni-P alloy and electrodeposition Ni onto AM50 magnesium alloys have been carried out. The influences of pretreatment steps on the surface morphology and composition analysis have been studied using scanning electron microscope/energy dispersed X-ray system (SEM/EDS). Additionally, the surface morphology and phase composition of the coatings were determined with (SEM/EDS) and X-ray diffraction (XRD), respectively. Moreover, potentiodynamic polarization tests have been used to estimate the corrosion behavior of unprotected and protected AM50 Mg and its composite alloys in 5% NaCl solutions.     

Preparation and corrosion resistance of titanium–zirconium–cerium based conversion coating on 6061 aluminum alloy

This paper aims to develop a chromium-free chemical conversion coating with good corrosion resistance. A novel chemical conversion coating was prepared on 6061 aluminum alloy by dipping in the treatment solution containing titanium/zirconium based-ions and sodium metaphosphate and cerium nitrate hexahydrate as additives. The morphology and composition of the conversion coatings were observed by scanning electron microscopy and energy dispersive X-ray spectroscopy. The microarea structure of conversion coatings at different formation stages was analyzed by electron probe microanalyzer. The electrochemical polarization curve revealed that the corrosion potential of the conversion coating was −0.577 V and the corrosion current density was 0.1148 μA/cm². The equivalent circuit fitted by AC impedance showed that the film resistance reaches 68,140 Ω. The formation of coating preferentially grows on the Al (Fe) Si intermetallic to form oxides of Ti and Zr; then TiO₂ formed by a higher concentration of Ti⁴⁺ gradually covered ZrO₂. Ce³⁺ could adsorb on the intermetallic compound, the hydrolysis of which causes the local pH of the solution to decrease and promotes the aluminum alloy dissolved.     

Effects of pre-treatments on the corrosion properties of electroless Ni-P layers deposited on AlMg2 alloy

Amorphous Ni-P layers with 8-10 wt.% phosphorus were deposited by sodium hypophosphite onto AlMg2 type aluminium alloy substrates after different pre-treatments. Prior to the electroless nickel-phosphorus (ENP) deposition in an acetate and lactic acid based nickel bath, the widely applied Zn (zincate) or Ni displacement (Ni strike) pre-treatments for aluminium substrates as well as a non-conventional surface conditioning one (soaking in a warm solution containing only hypophosphite and lactic acid) were all tested and their effects evaluated on the corrosion and other properties of the Ni-P layers developed right afterwards. The surface morphology and structure of the ENP layers were characterized by scanning electron microscopy and X-ray diffraction analysis. Polarization resistances were measured in 0.5 mol dm^− 3 Na₂SO₄ solution at pH 3. Compared to the direct electroless plating on the bare aluminium alloy AlMg2, it was found, that the hypophosphite adlayer (hypophosphite immersion pre-treatment) have also increased the corrosion resistance as the displacement pre-coatings, but without decreasing the deposition rate unlike conventional displacement methods. In the studied ENP deposition systems the decrease of corrosion rate could mainly be attributed to the lower microporosity and smoother morphology of the nickel-phosphorus coatings.     

Microstructure and Corrosion Resistance of Electrodeposited Ni-Cu-Mo Alloy Coatings

This paper deals with the electrodeposition of Ni-Cu-Mo ternary alloy coatings on low-carbon steel substrate from an aqueous citrate sulfate bath. The structures and microstructure of coatings were characterized by scanning electron microscopy and x-ray diffractometry. The corrosion resistance of coatings was investigated by potentiodynamic polarization (Tafel) and electrochemical impedance spectroscopy techniques. The results show that the Ni-Cu-Mo coatings are mainly composed of fcc-Ni phase and a small amount of NiCu phase. Ni-Cu-Mo coatings exhibit a nodular surface morphology, and the roughness of electroplated coating increases with the increasing of Na₂MoO₄·2H₂O in the bath. The corrosion performance of the coatings is significantly affected by the Mo content of the alloy coating and their surface morphology. The coating prepared in bath containing 40 g/L Na₂MoO₄·2H₂O has the highest corrosion resistance in 3.5 wt.% NaCl solution, while that prepared in bath containing 60 g/L (or more) Na₂MoO₄·2H₂O shows a lower corrosion resistance due to the presence of microcracks on the coating surface.     

Corrosion resistance of Mg−Al LDH/Mg(OH)2/silane−Ce hybrid coating on magnesium alloy AZ31

An environmentally-friendly hybrid coating on AZ31 magnesium alloy substrates was reported. The synergic effect was studied on Mg−Al-layered double hydroxide Mg−Al LDH/Mg(OH)₂-coated AZ31 magnesium alloy via an in-situ steam coating process and a subsequent combined surface modification of bis-[triethoxysilylpropyl]tetrasulfide (BTESPT) silane and Ce(NO₃)₃. The microstructure and composition characteristics of the hybrid coatings were investigated by means of X-ray diffraction (XRD), scanning electronic microscopy (SEM), Fourier transform infrared spectrophotometry (FT-IR) and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the coated samples was evaluated by potentiodynamic polarization (PDP), electrochemical impedance spectrum (EIS) and hydrogen evolution rate during immersion in 3.5 wt.% NaCl solution. The results show an improved corrosion resistance of the alloy in the presence of BTESPT silane and Ce(NO₃)₃. This is most likely due to the synergistic effect of steam coating and silane coating to enhance the barrier properties of hybrid coating. In addition, the formation mechanism and anti-corrosion mechanism of coatings were discussed.     

Oxydation De Revetements De Siliciures De Niobium

Isothermal oxdiation in air of silicide coatings on a niobium alloy has been studied between 1100°C and 1400°C. Coating formation was performed by the “slurry” process using two coating compositions, 60%Si, 20%Fe, 20%Cr and 60%Si, 20%Co, 20%Cr. The behaviour under oxidation for these two coatings has been compared.During oxidation for both compositions a surface layer of SiO₂ forms which contains particles of Nb₂O₅, but modifications of the biphased layer morphology causes variations in the oxidation rate. Moreover, additions of boron to the coating composition causes a reduction in the oxidation rate of the samples with cobalt by modifying the oxidation mechanism     

Conversion coating treatment for AZ91 magnesium alloys by a permanganate‐REMS bath

A permanganate‐rare earth metal salt (REMS) chemical conversion bath was applied to a sample of AZ91 magnesium alloy in this study, a red‐brown conversion coating formed subsequently on the sample surface. The test results of this coating with a scanning electron microscope (SEM) showed that there existed net‐like cracks on the surface of the treated magnesium alloy. With the analyses of X‐ray Diffraction (XRD) and X‐ray Photoelectron Spectroscopy (XPS), a further study of this coating indicated that the coating was structurally amorphous and mainly composed of CeO₂, MnO, MnO₂, MgO, Mg(OH)₂ and MgAl₂O₄. Furthermore, the electrochemical polarization tests showed that compared with the samples treated by the chrome‐based method, the open‐circuit potential of the magnesium alloy coated in permanganate‐REMS bath moved from ? 1.34 V_SCE to ? 1.28 V_SCE and the anodic current density of the alloy, at the same potential, decreased evidently in simulated sweat fluid. The cracks in the chemical conversion coating should be caused by the phase structure of the magnesium alloy. During the chemical conversion process, the localized corrosion micro‐cell led to the formation of the net‐like cracks on the surface. Simultaneously, the dehydration of the surface coating after treatment also accelerated the formation of the cracks at the coating surface.     

Synthesis and evaluation of corrosion resistance of molybdate-based conversion coatings on electroplated zinc

Three molybdate-based conversion coatings on electroplated zinc have been prepared and the composition, morphology, and structure of these coatings are measured by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and X-ray diffraction (XRD), respectively. It was found that these coatings with ‘meshwork’ surface were complex coatings composed of multiple compounds. Molybdenum species were present in the conversion coating as Mo (VI) and Mo (IV) compounds. The results of neutral salt spray test showed that molybdate-based conversion coatings with the addition of H₃PO₄, SiO₂ and TiOSO₄ in the passivation baths possess higher corrosion resistance compared with chromate conversion coatings, which was due to the compactness and anti-corrosion essence of the conversion coating.     

Abrasive wear resistance of electroless Ni-P coated aluminium after post treatment

Electroless nickel (EN) coatings are recognised for their hardness and wear resistance in automotive and aerospace industries. In this work, electroless Ni-P coatings were deposited on aluminium alloy substrate LM24 (Al-9 wt.% Si alloy) and the effect of post treatment on the wear resistance was studied. The post treatments included heat treatment and lapping with two different surface textures. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and micro-abrasion tester were used to analyse morphology, structure and abrasive wear resistance of the coatings. Post heat treatment significantly improved the coating density and structure, giving rise to enhanced hardness and wear resistance. Microhardness of electroless Ni-P coatings with thickness of about 15 μm increased due to the formation of Ni₃P after heat treatment.     

Molten salt synthesis of perovskite conversion coatings: A novel approach for corrosion protection of stainless steels in molten carbonate fuel cells

A novel conversion coating process has been developed to meet the stability requirements of stainless steel hardware in the demanding MCFC fuel cell environments. The process applies a perovskite-based coating by exploiting spontaneous oxidizing reactions of the metallic surface with La₂O₃ in eutectic alkali carbonate mixtures. By using well controlled synthesis procedures, conversion coating layers covering the entire metallic surface with a uniform and compact structure could be obtained. The as-formed coatings with a surface morphology of agglomerated crystallite particles consisted of a thin (<5 μm) LaFeO₃ perovskite layer grown over a thicker (>5 μm) LiFeO₂-rich layer. Test coupons of 316L stainless steel with the perovskite conversion coating were analyzed for corrosion protection and interfacial resistivity properties. It was found that the conversion coating is highly conductive while showing excellent long-term corrosion stability in simulated MCFC environments. These results suggested that perovskite coatings formed by molten salt conversion reactions could be particularly attractive to confer optimal protection and electrical continuity to MCFC current collectors.     

电火花沉积非晶涂层的组织结构与摩擦磨损性能

以Zr₅₅Al₁₀Ni₅Cu₃₀非晶态合金棒为电极,利用电火花沉积技术在ZL101铝合金表面制备了锆基合金涂层.利用X射线衍射仪（XRD）、扫描电镜（SEM）、显微硬度计和摩擦磨损实验机等对涂层的微观结构、表面形貌、显微硬度和摩擦磨损性能进行了分析测试.结果表明,沉积层表面较致密、均匀,为典型的“溅射状”花样形貌;沉积层主要由非晶、ZrO₂和Cu₈Zr₃等相组成;沉积层的平均显微硬度为1 555 HV 0.01,约为基材的15倍,摩擦系数仅为0.096,呈现出良好的减摩耐磨特性,沉积层的磨损机制主要为疲劳磨损和磨粒磨损.     

Electrospark alloying for deposition on aluminum surface of Al-Sn coatings and their wear resistance under dry friction

Some aspects of coating deposition on aluminum substrate by electrospark alloying with toolelectrode from Al-Sn alloy stimulating the SnO₂ nanofibers formation in coatings have been studied. Wear resistance of such coatings, under dry friction conditions, in conjunction with a counterbody from hardened steel has been investigated. The conditions under which the coatings thus obtained manifest the of effect of the maximal wear excess of the counterbody compared to the wear of the coatings containing SnO₂ nanofibers have been specified. The effect reaches its maximum value under the dry friction after the treatment of the surface in the mode of “sparking” at a constant energy supply in the spark gap: at high rates of the tool electrode movement with respect to the specimen and at relatively large times of the electrospark effect on the treated surface.     

Influence of pH value on chromate-free conversion coating for magnesium alloys

Many factors were found to have effects on the conversion coatings for AZ31 alloy, alloy the most important one in producing high quality conversion coatings is found to be the control of the pH value. The influence of pH value on the conversion coating including color, thickness, adhesion and surface morphology was studied. The performance of conversion coating was examined by cross cut test, SEM method and salt immersion. The results show that the variation ofpH value causes surface treatment process unstably. The conversion coating can obtain as pH value ranging from 3.0 to 5.0, while it presents dark, thick and bad adhesion under lower pH value. The conversion coatings have good combination of thickness and adhesion when pH value ranging from 4.0 to 4.5, and it exhibits a good corrosion resistance.     

Study of the structure and corrosion behavior of PEO coatings on AM50 magnesium alloy by electrochemical impedance spectroscopy

In this work coatings were developed on the surface of AM50 magnesium alloy using four different electrolytes containing 10 wt.% each of K₃PO₄ and Na₃PO₄ in combination with either potassium or sodium hydroxides. Electrolyte conductivity and breakdown voltage were measured in order to correlate the property of the coating to the nature of electrolyte. Further, the coatings were examined using scanning electron microscopy for surface morphology and cross sectional investigation, X-ray diffraction for phase determination, and electrochemical impedance spectroscopy for corrosion resistance evaluation. The effect of employing different ions in the electrolytes results in different surface morphologies, chemical phases and, consequently, the corrosion resistance of the coatings. The EIS results indicate the presence of porous and compact layers in the structure of the PEO coatings, whilst the overall coating resistance mainly results from the compact layer, the role of the porous layer as a barrier against corrosion is negligible. Finally, a correlation between the passive current density of the bare alloy and the corrosion resistance of the PEO coating is proposed.     

Effects of plasma treatment on bioactivity of TiO2 coatings

In this work, nano-TiO₂ powders were deposited on titanium alloy substrates by atmospheric plasma spraying, followed by plasma immersion ion implantation (PIII) using hydrogen, oxygen and ammonia gases. The bioactivities of PIII-treated TiO₂ coatings were evaluated by the formation of apatite on their surface after soaked in simulated body fluids (SBF) for a period of time. As-sprayed TiO₂ coating is composed of rutile, anatase and TiO_2−x (most of them is Ti₃O₅). After immersion in SBF for two weeks, the hydrogen PIII-treated TiO₂ coating can induce bone-like apatite formation on its surface but apatite cannot be formed on the surface of as-sprayed and oxygen, ammonia PIII-treated TiO₂ coatings. The results obtained indicated that a hydrogenated surface plays a very important role to induce bioactivity of TiO₂ coatings.