Magnetoelectropolishing for metal surface modification

Abstract

This paper aims to present the preliminary results of magnetoelectropolishing, a process derived and named by application of an external magnetic field coupled with the process being controlled and maintained under oxygen evolution to achieve an electropolished surface of a workpiece. The obtained surfaces exhibit reduced microroughness, better surface wetting resulting from increased surface energy, reduced and more uniform corrosion resistance, minimisation of external surface soiling and improved cleanability. The externally applied magnetic force for use with the enhanced electropolishing process may be selected from a group consisting of either permanent magnetic or electromagnetic devices. It has been shown that the enhanced electropolishing process results in better surface performance. In the present paper, for comparison purposes, results of surface roughness, hydrophilicity and corrosion behaviour are presented. Further studies, concerning the corrosion characteristics, the morphology and surface film chemistry, are in progress.     

Effects of a magnetic field on the anodic dissolution, passivation and transpassivation behaviour of iron in weakly alkaline solutions with or without halides

The effects of a 0.4 T horizontal magnetic field on the anodic dissolution, passivation and transpassivation behaviour of iron in bicarbonate solutions of various concentrations and in dilute bicarbonate solutions with or without halides are investigated by electrochemical polarisation measurements. The applied magnetic field does not affect the activation-controlled anodic current, the steady passive current and the transpassive current, but significantly affects the activation-passivation transition processes for iron in bicarbonate solutions without halides. The effects of magnetic field are strongly dependent on passivation mechanisms that result in different types of surface films and corresponding rate determining steps of film dissolution. There is a synergistic effect between the applied magnetic field and halides, chlorides or bromides, on attacking the passivation of iron in dilute bicarbonate solutions. The effects of the magnetic field are analysed based on the previously proposed electrochemical kinetics equations. The magnetic field affects the anodic polarisation behaviour through its enhancing effects on mass transport processes at the precipitation-dissolution type surface film/solution interfaces. The magnetic field shows little or no effects on continuous and steady passivation films where the oxidation rate is controlled by mass transport processes within surface films. Magnetoelectrochemistry measurements are suggested as a prospective method for researches on corrosion or passivation mechanisms.     

Nitinol surface finishing by magnetoelectropolishing

Abstract

A new advanced technology is proposed to obtain bare metal surfaces for peripheral stents, endodontic files and another medical devices. The effect of an externally applied magnetic field during electropolishing (magnetoelectropolishing) of Nitinol on its surface properties was investigated. The results are compared with standard electropolished Nitinol in the same electrolyte under the same process conditions. To evaluate surface properties, the following techniques were used: SEM and EDX, AFM, contact angle measurement, Auger electron spectroscopy, electrochemical impedance spectroscopy and fatigue resistance testing. All of the above analyses showed significant differences between magnetoelectropolished and conventional electropolished Nitinol surfaces. The two most prominent differences, which are likely to have the most profound implications on increased use of this intermetallic material in medical application are improved wettability and fatigue resistance.     

Influence of long-term ageing in solution containing chloride ions on the passivity and the corrosion resistance of duplex stainless steels

The influence of long-term ageing in NaCl on the passivity and the electrochemical behavior of UNS S32304 is studied. The passive film thickness, the Cr/Fe ratio and the chloride content were significantly increased after ageing. The chloride distribution depends on residual stresses, sample microstructure and surface preparation. Local electrochemical measurements revealed that pitting potentials are between 250–550 mV vs. SCE after electropolishing. The higher the chloride content, the lower the local pitting potential. It was also shown that the presence of chloride was balanced by the enrichment in chromium after ageing. Then no pitting potential could be measured.     

Electropolishing of nickel and cobalt in deep eutectic solvents

Electropolishing is a common method for decreasing surface roughness and removing surface irregularities. In this paper the electropolishing of nickel and cobalt are successfully demonstrated in a deep eutectic solvent, comprising a 2:1 molar mixture of ethylene glycol and choline chloride. Voltammetric and electrochemical impedance studies were used to characterise the polishing mechanism and show that film formation occurs prior to polishing. Scanning electron microscopy and atomic force microscopy were used to characterise the morphology before and after polishing and 3D optical microscopy was used in-situ to observe film formation during polishing. This study shows that the impact of film formation and subsequently mass transport are responsible for electropolishing of both metals in the choline chloride-based ionic liquid.     

Effects of an applied magnetic field on the dissolution and passivation of iron in sulphuric acid

The effects of an applied magnetic field (MF) on the electrochemical state, anodic dissolution and passivation of iron in sulphuric acid solution were studied by potentiodynamic scanning polarisation measurements, potentiostatic polarisation measurements and scanning electron microscopy observation. The magnetic field reduced the fractional surface film coverage on the electrode by enhancing the film dissolution process. This made the electrode prone to active dissolution. With increasing applied potentials the magnetic field accelerated the anodic dissolution at relatively low potentials, changed the oscillation or passivation to permanent active dissolution at intermediate potentials, and maintained the passive state at high potentials. Potentials for the onset of passivation moved in the noble direction when the magnetic field was imposed. An electrode kinetics formulation for the effects of the magnetic field on the dissolution and passivation is proposed. In the presence of a magnetic field and at specific anodic potentials, scalloping occurred due to accelerated localized dissolution. The scalloping areas were on both sides of the electrode and oriented parallel to the direction of the earth’s gravitation field. The ratios of the scalloping area caused by a 0.4 T magnetic field on the whole electrode surface were 0.69 (at 200 mV), 0.66 (at 350 mV) and 0.75 (at 400 mV), respectively. In contrast, uniform electrode surfaces were observed at these anodic potentials in the absence of the magnetic field. Uneven dissolution of iron in the presence of a magnetic field was related to the relative configuration between the magnetic field direction and the electrode surface and also to the special concentration gradient of reactive species at the electrode circumferential area.     

Copper Corrosion Under Non-uniform Magnetic Field in 0.5 M Hydrochloric Acid

The influence of a magnetic field on the electrochemical reactions taking place at the surface of a copper electrode immersed in a 0.5 M HCl solution at room temperature has been studied. The symmetry axis of the magnetic field was lined up in the same direction of the ion flow to minimize the Lorentz forces. Measurements of potentiodynamic polarization curves, electrochemical impedance spectroscopy and electrochemical noise allow concluding that the magnetic field significantly affects the cathodic reactions, with corrosion rates increasing under the presence of oxygen in acid media and decreasing when oxygen is eliminated.     

Magnetic field techniques for the inspection of steel under concrete cover

The results of a study comparing residual magnetic field measurements to magnetic flux leakage (MFL) measurements as methods to detect broken pre-stressing steel are presented. Analysis of two- and three-dimensional magnetic field plots shows that the residual magnetic field technique has strong potential in this application, with detectable signals from a single broken wire on a seven strand cable being found up to 70 mm from the cable surface. MFL measurements with the same yoke produced detectable signals only when the cable was completely severed, indicating that the yoke was not driving the cable into saturation during the magnetisation process. However, the residual field technique was also found to be very sensitive to the technique used to magnetise the object being inspected, indicating that considerable care would be necessary to use the technique successfully in the field.     

钛合金管内表面的电化学磁力研磨复合光整试验

针对热挤压成型对钛合金管的内表面会产生微裂纹、褶皱、毛刺等表面缺陷的问题,提出了一种高效率的电化学磁力研磨复合光整加工方法。设计了电化学磁力研磨复合光整加工的实验装置,分别与纯磁力研磨加工和纯电化学加工进行了光整加工试验对比,检测分析了不同工艺加工前后表面的粗糙度、微观形貌、摩擦磨损行为、表面残余应力和能量谱。结果表明:在相同的加工时间内,与单纯电化学加工和磁力研磨加工相比,电化学磁力研磨复合光整加工的表面粗糙度Ra可达到0.2μm,材料去除量和加工效率显著提高;表面显微形貌要明显优于其他两种加工方式;且加工后表面很好地维持了原有材料的化学成分和表面性质;能够使表面由拉应力转变为约–200 MPa的压应力状态,从而获得更好的表面应力状态。     

Electropolishing parameters of NiTi alloy

Electropolishing has been used in NiTi alloy in several fields for its special characteristics, but its essential details and electropolishing mechanism have not been reported yet as a demand from business competition, which, to a great degree, restricts the application and extension of the electropolishing technology. The effect of processing parameters on nitinol electropolishing was explored. Besides the electrolyte, other factors that influence the electropolishing are temperature, current density, time, spacing between anode and cathode, electrolyte stirring, etc. Studies on the effect of the temperature on the electropolishing process show that the higher the temperature is, the bigger the electropolishing rate is, following the near Gauss law. The relationship between the temperature and the surface roughness follows a near parabolic law, and the relationship between the temperature and the surface reflectivity follows a near sigmoidal law. The relationship between the electropolishing voltage and the current density follows a near cubic law, while that between the electropolishing rate and current density follows a near linear law. The relationship between the electropolishing rate and the time follows a near sigmoidal law. The practical spacing between anode and cathode is confirmed by the Hall bath experiment.     

基于磁场复合和高频、窄脉冲电流电解加工的试验研究

通过磁场复合和高频、窄脉冲电流电解加工工艺,实现了薄壁大面积(10 000mm2)型面铝基材料零件的稳定小间隙加工.试验研究中采用高频、窄脉冲电流,解决了加工间隙流场不均匀的问题;采用磁场复合电解加工工艺方法,解决了试件的表面质量差和杂散腐蚀严重的问题.     

Electropolishing of high-purity aluminium in perchloric acid and ethanol solutions

The electropolishing behaviour of high-purity aluminium with ultrasonic agitation was investigated in ethanol-perchloric acids by using anodic polarization measurement, electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), and atomic force microscope (AFM). The results show that the electropolishing behaviour of the aluminium is under mass-transfer control and Al³⁺ is suggested as the governing species for the salt film mechanism during anodic dissolution in limiting current plateau.     

Large-area self-ordered aluminium sub-micrometre dot arrays prepared by electropolishing on polycrystalline aluminium at constant current

The effects of electropolishing on the sub-micrometre structure pattern on the surface of polycrystalline aluminium were investigated. Under constant current, the electropolishing process results in large-area uniform sub-micrometre dot arrays despite the polycrystalline aluminium surface. Effects of temperature on the pattern of the surface structures were also investigated. Experimental temperature was shown to have a great influence on the structure pattern. When the electropolishing temperature increases, the structure of aluminium surface tends to transform from a dot pattern to stripe one and then to a corrosion structure.     

金属大气腐蚀实验技术进展

综述了金属大气腐蚀的实验方法和分析手段及各种方 法的优缺点，探讨了金属大气腐蚀研究的发展趋势.     

Standards news

Abstract

Electrochemical machining of titanium in conventional electrolytes such as chlorideshas some disadvantages since high voltages are required and surface finish can be poor especially in the stray current regions. This paper compares the electrochemical polarisation behaviour of titanium in chloride, bromide and iodide electrolytes and predicts that much lower voltages would be required for higher dissolution rates when electrochemical machining is carried out in bromide or iodide solutions.

Observations of surface finish show that bromide electrolytes give much better results than chloride or iodide. Although good surface finish was obtained in bromide solutions, the use of mixtures of chloride and bromide could give even better surfaces,equivalent to bright electropolishing. The mechanism of these phenomena was investigated by electrode polarisation in bromide solutions with. varying pH, ranging from acid to alkali conditions. From this work it was possible to recommend improved electrolytes for the electrochemical machining of titanium.     

Investigation of the surface reactivity on a 304L tensile notched specimen using scanning electrochemical microscopy

Local electrochemical reactivity around a notched tensile sample of 304L stainless steel under applied stress was investigated using scanning electrochemical microscopy (SECM). The plastic strain field around the notch was evaluated by finite element model (FEM). Microscopic observations of local plasticity related to grain morphology were correlated with the effective plastic strain field calculated with FEM around the notch root. Numerical results, surface observations and experimental electrochemical investigations showed the significant effect of plastic strain gradient on the surface reactivity. The effect of roughness induced by the triaxial plastic strain field around the notch was investigated with surface topography measurements.     

磁场作用下的微生物腐蚀研究进展

磁体周围存在磁场,磁体间的相互作用就是以磁场作为媒介的,磁场力包括洛伦兹力和安培力.概述了磁场对微生物和金属腐蚀过程的影响,包括对单一细菌和混菌体系、电化学控制和浓差极化控制的电化学过程的影响.归纳了磁场作用于微生物金属腐蚀的防控方法,包括微生物竞争手段、缓蚀杀菌剂的应用以及存在的不足等问题.在此基础上,重点综述了近年来磁场条件下金属微生物腐蚀的研究进展,分析了磁场对于微生物生长特性和生物膜形成的影响,分别以不同微生物存在的体系、电化学过程控制的类型等方面展开详细讨论.针对磁场存在的环境,总结了微生物对金属的腐蚀机理,包括生物膜理论、离子干涉理论、自由基理论等.最后结合磁场对金属腐蚀过程的相关机理,展望了后续磁场对微生物腐蚀防控的研究方向.     

水下钢铁构筑物腐蚀监／检测电化学传感系统研制

设计制作了能对水下大型钢铁构筑物直接测试的小孔限流型腐蚀传感器,讨论了影响其测试面积的结构因素,设计了相应的测试仪器,并成功地在塘沽集装箱码头泊位裸露钢管桩上进行了现场测试。     

Modeling and simulation of cylindrical electro-chemical magnetic abrasive machining of AISI-420 magnetic steel

The objective of the present research is to simulate cylindrical electro-chemical magnetic abrasive machining (C-EMAM) process for magnetic stainless steel (AISI-420). C-EMAM is a new hybrid machining process used for high efficiency finishing of cylindrical jobs made of advanced engineering materials. The material is removed from the workpiece surface due to simultaneous effect of abrasion and electrochemical dissolution. Finite element method is used to calculate the distribution of magnetic field between the magnetic poles in which cylindrical shaped workpiece is placed. The cutting forces responsible for abrasion are calculated from the magnetic forces due to gradient of magnetic field in the working gap. The effect of electrochemical dissolution and abrasion-assisted dissolution are incorporated into the C-EMAM process model using empirical relation for average anodic current. The empirical relation is correlated with the input parameters in the present system based on experimental results. Finally a surface roughness model is developed by considering total volume of material removed with the assumption of triangular surface profile. The simulation results for material removal and surface roughness are validated using experimental results. The simulated results agree with experimental observations.     

Novel strategies for assessing the pitting corrosion resistance of stainless steel surfaces

Pitting corrosion is one of the most common mechanisms of surface damage on stainless steels. Electrochemical methods have been preferentially applied for the evaluation of the pitting corrosion resistance of stainless steels in the laboratory. Nevertheless, some of them are not reliable enough and in general the application of electrochemical methods in the field becomes difficult because of required deep understanding of corrosive phenomena and measurement technology. Therefore, new approaches for the evaluation of the pitting corrosion susceptibility of stainless steel surfaces in the laboratory as well as in the field are necessary. In the present paper two novel strategies including electrochemical noise measurements under anodic polarization for laboratory testing, and an indicator test to assess the susceptibility of stainless steel surfaces to pitting corrosion in the field are introduced. Experimental results concerning the influence of surface treatments on the pitting corrosion resistance on stainless steels have confirmed that final surface condition has a significant effect on their future pitting corrosion susceptibility. In addition, the pitting corrosion resistance of stainless steel surfaces was observed being specifically dependent on the achieved surface topography and in some cases independent on the roughness parameters of the surface.