Application of induced current potential drop technique for measurements of cracks on internal wall of tube-shaped specimens

This paper presents the applicability of induced current potential drop (ICPD) technique to the detection and sizing of defects in objects with a shape of real components. The shape of tube was selected as one of the shape of real components and an ICPD sensor for the inspection of tube-shaped specimens was developed. The usefulness of the developed sensor was verified by experiments. Finite element analysis (FEA) was conducted to obtain calibration data to convert potential drop (PD) data measured using the sensor into the depth of defects. It was shown that the crack depths estimated based on measured PDs and FEA showed good agreement with the crack depths estimated by ultrasonic testing.     

Profile reconstruction of simulated natural cracks from eddy current signals

Based upon the fact that internal conductivity of the crack has a serious effect on the eddy current signals, two kinds of inversion schemes that can reconstruct cracks with non-vanishing conductivity from eddy current signals are proposed in this paper. One is a physics-based approach that modifies crack parameters iteratively and the other is a neuronet-based approach. Two models that are supposed to be appropriate models of a natural crack are also proposed. After detailed explanation of the schemes, several reconstruction results using simulated data are presented. The results show these schemes can reconstruct crack profile with high accuracy where not only shape of the crack but also internal conductivity is unknown. Advantages and disadvantages of both schemes are discussed at the end of the paper.     

Computer simulation of pitting potential measurements

A deterministic model for the growth of single pits in stainless steel has been combined with a purely stochastic model of pit nucleation. Monte-Carlo simulations have been used to compare the predictions of this model with potentiodynamic experimental measurements of the pitting potential. The quantitative agreement between model and experiment is reasonable for both 304 and 316 stainless steel, and the effects of varying surface roughness, solution chloride concentration and potential sweep rate have been considered.     

Magnesium: Current and potential automotive applications

In this paper, the material properties, structural performance, mass saving potential, design and manufacturing characteristics of magnesium are compared with various competing materials such as cast iron, steel sheet, aluminum alloys, and polymers. The current and potential automotive applications of magnesium are reviewed, and the technical challenges for these applications are also discussed.     

Modelling and experimental measurements of idealised and light-moderate RCF cracks in rails using an ACFM sensor

Alternating current field measurement (ACFM) sensors can be used to detect surface breaking defects in metal components. In rails rolling contact fatigue (RCF) cracks form due to the wheel–rail contact stresses. These cracks are surface breaking and can have complex shapes. A COMSOL model has been developed for a commercial ACFM system and RCF cracks in rails. In this paper model results are compared to experimental measurements using an ACFM pencil probe for calibration defects machined into a rail and real RCF defects (light and moderate categories; <20 mm surface length) in a rail removed from service. X-ray tomography has been used to determine the size and morphology of the real RCF cracks for input into the model. It has been shown that the model can be used to determine the change in normalised Bx signal due to the presence of calibration defects machined into a new rail. The model has also been used to compare the experimental data for the real RCF cracks to the reconstructed model cracks and a semi-elliptical approximation to their shape.     

Simplified NDE of multiple cracks by means of the potential drop technique

A method is developed for the simplified evaluation of multiple cracks, based on the d.c. potential drop technique. A subregion containing cracks is considered. The distribution of the potential drop for the cracks except for end cracks in the subregion is measured and calculated numerically by assuming a value for the depths of the end cracks and by assuming any value for the depths of other cracks. Modifying the depths of the other cracks is repeated until the difference between measured and calculated potential drops is minimized, where the depths of the end cracks are fixed. Evaluation of the centre crack in the subregion is made for different assumed depths of the end cracks. To make the evaluation independent of the depths of the end cracks, a subregion is extended to increase successively the number of cracks considered. Multiple cracks are sized well enough by this method.     

Experimental and simulated transmittance versus potential curves for poly(o-methoxyaniline) films

Experimental transmittance versus potential (T-E) curves were obtained for poly(o-methoxyaniline) electrochromic films in aqueous acidic medium. T-E curves assumed different forms such as oval and figure-eight shapes, depending on the scan rate and the wavelength used during spectroelectrochemical experiments. A theoretical model was proposed to explain experimental results and to construct T-E curves by computer simulation. The model assumes current transients to be an exponential process with a single relaxation time that explains the curve shapes. An alternative model using more than one relaxation time is commented on to describe more than one species of migrating ions and to produce a closer fit.     

Electrochemical noise measurements of AISI 316L during wear in simulated physiological media

Electrochemical noise has been used to study the tribocorrosion behaviour of austenitic stainless steel AISI 316 in two different simulated body fluid solutions [phosphate buffer solution (PBS) and PBS plus bovine serum albumin). The analysis of potential and current transients permits not only the determination of the contribution of the aggressive medium to the total volume loss due to synergism between wear and corrosion but also the detection of changes in the mechanism produced by the mechanical/electrochemical attack.     

Crack interaction in NDE of multiple cracks by means of potential drop technique

A method is developed to evaluate crack interaction in NDE of multiple cracks by d.c. potential drop technique. A subregion containing m(=1,3,5,…) cracks are considered. The potential drop of the middle crack for each subregion are considered for two successive cases (m and m+2) and compared. The ratio of these two potential drops shows the effect of the first and last cracks of the subregion on the potential drop of the middle crack. Comparison of the potential drop of the middle crack with the potential drop of a single crack with the same depth and the same positions of potential probes shows the crack interaction. The method is based on the FEM. Experimental data verify the numerical results.     

Considerations on reproducibility of potential and corrosion rate measurements in reinforced concrete

This communication analyses information supplied by the electrochemical parameters related to rebar corrosion in reinforced concrete structures (RCS). Corrosion potential and corrosion current density are determined for different sets of conditions. Tests are performed to gather data on conflictive aspects of the two electrochemical parameters, with regard to evaluating the results of RCS inspections. Consideration is made of the limitations of potential measurements if not accompanied by complementary indications such as concrete resistivity, rebar surface area involved in the measurements, or the instrumentally determined corrosion current. The capacity of galvanostatic pulses applied directly on large RCS to offer a reliable guide to corrosion rate of steel reinforcements is discussed.     

The electrical potential difference across cracks in PZT measured by Kelvin Probe Microscopy and the implications for fracture

An indentation crack in a poled PZT ceramic subjected to an electric field is investigated using AFM and KFM to determine the crack opening displacement and the electrical potential difference across the crack. The experimental results are used to calculate the crack tip stress and dielectric displacement intensity factors and the crack tip energy release rate. From the applied electric field and the measured field interior to the crack, the dielectric constant of the crack interior is determined to be 40. The consequences of this permittivity on the crack tip energy release rate are illustrated for a Griffith crack. The theoretically predicted effect of an applied electric field in retarding crack growth decreases significantly with increasing permittivity. In practical situations in terms of crack length, applied load and electric field level, the retardation of crack growth is negligible when the dielectric constant of the crack interior is higher than 20.     

NDE of multiple cracks on the surface of materials by means of the potential drop technique

A method is developed to evaluate nondestructively multiple cracks on the surface of materials. The method is based on the d.c. potential drop technique. Multiple two-dimensional cracks of unknown depth are inspected, where the distance between the cracks is known. First the distribution of the potential drop between both sides of each crack is measured on the cracked surface. Next the distribution of the potential drop is calculated numerically by assuming the depth of every crack. Then by comparing the measured and calculated potential drop, a correction factor is obtained for the assumed depth of the respective cracks. Modifying the crack depth by the correction factor is repeated until the difference between the measured and calculated distributions of the potential drop is minimized. It is shown that multiple cracks are sized accurately by the present method.     

An alternative method for crack interaction in NDE of multiple cracks by means of potential drop technique

A procedure of calculating the crack interaction in NDE of multiple cracks by d.c. potential drop technique is newly developed. This method can be used when the distances between cracks are small or large. The equations obtained evaluate crack interaction for any case regardless of the distances between the cracks. A subregion containing m (=1,3,5,…,) cracks are considered. The potential drop of the middle crack for each subregion are considered for two successive cases (m and m+2). The ratio of these two potential drops shows the effect of the first and last cracks of the subregion on the potential drop of the middle crack. The crack interaction is shown by the ratio of the potential drop of the middle crack to the potential drop of a single crack with the same depth and the same positions of potential probes. The equations showing the crack interaction are obtained based on the numerical analysis followed by an experimental validation.     

温度和外加电位对X70管线钢在土壤模拟溶液中应力腐蚀行为的影响

采用动电位扫描、慢应变拉伸（SSRT）和扫描电镜观察研究了温度和外加电位对X70管线钢在成都土壤模拟溶液中的应力腐蚀行为的影响。结果表明,在不同温度和不同电位下,X70管线钢在土壤模拟溶液中表现出不同的应力腐蚀敏感性。在温度和外加电位的交互试验中,电位的变化对X70管线钢在成都土壤模拟溶液中的应力腐蚀敏感性的影响占主导地位,应力腐蚀敏感性在不同温度下的变化趋势保持一致。在－450 mV(vs SCE,下同)的阳极电位下,SCC的机理为阳极溶解;在－850 mV电位下,阴极保护作用抑制了阳极溶解;当电位负移至－1200 mV时,表现出较强的应力腐蚀敏感性,SCC机理以氢脆为主。温度对应力腐蚀敏感性的影响主要体现在对阴极极化电位的影响,但是各种因素综合在一起导致应力腐蚀敏感性随温度变化的复杂性。     

Kinetics of the passivation of molybdenum in salt solutions as inferred from impedance and potential measurements

The oxide film formed on the molybdenum electrode in sodium salt solutions of different anions was found to thicken according to a logarithmic growth law and the electrode potential varied with time according to the equation E_h = α + β log t. The rate of oxide film thickening in the respective solutions was in the order Na₃PO₄ > Na₂SO₄ > NaCl > NaNO₃. The equilibrium potential was found to be independent of anion concentration in the case of Cl^- and NO^-₃ and was slightly dependent on the PO^3-₄ and SO^2-₄ concentrations. Complex plane analysis showed that the double-layer capacitance affects the measured capacitance and an electrical model was proposed to explain the alternating current behaviour of the electrode-electrolyte interface.     

锻造裂纹的分析与防治

介绍了锻造裂纹的主要试验研究方法:高温拉伸、热镦粗、热扭转、热弯曲和应变诱导裂纹,并结合裂纹附近微观组织和热力学理论探讨了多种材料锻件裂纹形成的原因。得出锻件裂纹不仅取决于材料本身的内因,而且与力学性能、组织结构、制件尺寸等因素有关。阐述了目前锻造裂纹的预防原则、内裂修复原理以及应用情况。     

外加电位下X80双相管线钢在模拟沿海土壤环境中的应力腐蚀行为

采用交流阻抗谱、极化试验、慢应变拉伸试验研究了不同外加电位下在模拟沿海土壤环境中X80双相管线钢的应力腐蚀行为,对拉伸断口和极化后试样进行SEM表面形貌及能谱分析。结果表明,与慢扫极化(模拟的非裂尖区域)相比,X80双相管线钢快扫极化模拟的裂尖腐蚀电位较负且腐蚀电流较大。-750 mV外加阴极电位处于裂尖自腐蚀电位范围,不足以起到阴极保护的作用,对应力腐蚀仍十分敏感。外加电位为-1050 mV时,阴极反应速率显著大于阳极反应,阴极反应产生的氢被金属吸收且扩散,慢应变拉伸未经颈缩即发生断裂,为准解理断裂。外加阴极电位为-900 mV,阴极电流有效抑制了阳极溶解反应,因此管线钢在模拟沿海土壤溶液中慢应变拉伸抗拉强度和断面收缩率都最高,断口表现为韧性断裂,侧面裂纹细小,阻抗模值最大,应力腐蚀敏感性最小。     

Kinetics of the passivation of molybdenum in acids and alkali solutions as inferred from impedance and potential measurements

The electrochemical behaviour of mechanically polished molybdenum electrodes in different acid and alkali solutions was investigated using open-circuit potential and impedance measurements. The effect of the concentration of acid (HCl) and alkali (NaOH) on the electrochemical behaviour of the molybdenum electrodes was also studied. The various kinetic parameters, i.e. capacitance, resistance and potential, controlling the mechanism of oxide film growth were monitored as functions of time.Complex plane analysis reflects the high passivation properties of the naturally formed oxide film on molybdenum irrespective of the dissolution medium. The results showed also that the passive films undergo structural changes during the course of measurements which may be attributed to a further oxidation of the passive MoO₂ film to MoO₃.The electrode potential was found to be sensitive to variations in pH. In highly concentrated NaOH solution, the passive film is subject to continuous dissolution.     

The fundamentals of nano- and submicro-scaled ceramic particles incorporation into electrodeposited nickel layers: Zeta potential measurements

Nano- and submicro-scaled ZrO₂ particles were codeposited into Ni matrix by an electrochemical plating process using a Watts bath. The incorporation has been studied in relation to dispersion stability. The ZrO₂ particles were characterized through zeta potential electro-acoustic measurements and particle size distribution. SEM and EDX analysis of the codeposited layers were performed to characterise the coating morphology and to determine the particle concentration in the layer. A content up to 10 vol.% of ZrO₂ was found in Ni matrix. As electrostatic repulsion within the dispersion increases, particle agglomeration slows down and the concentration of non-agglomerated particles rises resulting in an enhanced amount of particles incorporated in the electroplated Ni layers. The effect of electrophoresis and diffusion on particle transport to the cathode is discussed.