J. Galland

Preparation and investigation of nanostructured SiC-nickel layers by electrodeposition

Nanocomposite layers have been obtained by electrochemical deposition of silicon carbide nanoparticles (mean diameter 20 nm) in a nickel plating bath. The influence of nanoparticles on nickel electroplating is studied by electrochemical impedance spectroscopy (EIS) at cathodic potential. Cathodic polarization diagrams have been determined in the electrolyte with and without nanoparticles. We show that incorporation of 20 nm SiC particles into nickel deposition modifies the impedance and cathodic polarization diagrams in correlation with the modification of the surface morphology and crystallinity of nickel matrix.

Composite Electrodeposition to Obtain Nanostructured Coatings

Nanocomposite coatings are obtained by electrochemical deposition of silicon carbide nanoparticles (mean diameter 20 nm) in a nickel plating bath. The influence of nanoparticles on nickel electroplating is studied by electrochemical impedance spectroscopy at cathodic potential, and cathodic polarization diagrams are performed in the electrolyte with and without nanoparticles. We show that incorporation of 20 nm SIC particles into nickel deposits modifies the impedance and cathodic polarization diagrams in correlation with the modification of the surface morphology and crystallinity of the nickel matrix

Intergranular corrosion of AISI 316L steel

Abstract The degree of sensitization (DOS) of austenitic stainless steel AISI 316L to intergranular corrosion (IGC) was determined by means of electrolytic etching in oxalic acid and electrochemical potentiokinetic reactivation (EPR) tests completed by metallographic inspection. An analytical electron microscope, equipped with an energy-dispersive X-ray spectrometer, was used to examine the microstructure of the steel investigated, which had been annealed for 3.60×10 4 to 1.91×10 7 s at a temperature of 650°C. The kinetics of precipitation of secondary phases (M 23 C 6 carbides, Laves phase Fe 2 Mo) were studied by means of quantitative metallography. The volume fractions of chromium-rich M 23 C 6 carbides along grain boundaries and Laves phase Fe 2 Mo in the matrix were determined. Chromium concentration profile measurements across the grain boundary, in the vicinity of M 23 C 6 carbides, have been performed using the analytical electron microscope. All results concerning microstructural changes were quantitatively related to the IGC characteristics by statistical normalization.

Electrochemical impedance of pits. Influence of the pit morphology

The evolution of the impedance of a AISI 430 steel electrode affected by pitting corrosion was studied. Open pits were obtained in sulfuric acid solution containing sodium chloride and closed pits were formed in pure sodium chloride solution. Some changes affecting the shape of the impedance diagrams during the development of pits could be related to geometrical effects due to the morphologies of the pits. The results obtained with open pits are explained by the increase in the roughness induced by their development. With closed pits, the electrode impedance behaves like the impedance of a porous electrode. A simple electrical circuit model is proposed to account for the evolution of the impedance diagrams.

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. I. Characterisation of hydrogen evolution on vertical unstressed electrodes

For investigating stressed carbon-steel electrode (AISI 4140) cracking induced by hydrogen embrittlement with the electrochemical noise (EN) technique, the contribution of the evolving hydrogen bubbles to the EN has to be determined. Under cathodic current control, various reasons may explain the fluctuations of the electrode potential. In the first paper of this series the electrolyte resistance (ER) fluctuations, which yield ohmic-drop fluctuations, were analysed for unstressed vertical tensile specimens. A simplified model was proposed to tentatively derive the characteristic parameters of the gas evolution, such as the bubble mean size and evolution mean rate, from the power spectral density (PSD) of the ER fluctuations. However, results in only qualitative agreement with optical observations were obtained, indicating that the complicated screening and dragging effects of rising bubbles have to be taken into account in the modelling for vertical electrodes. The electrode potential fluctuations of unstressed tensile specimens were investigated in the second paper of this series and the influence of a stress was examined in the third paper.

A thermal outgassing method (T.O.M.) to measure the hydrogen diffusion coefficients in austenitic, austeno-ferritic and ferritic-perlitic steels

Abstract A thermal outgassing method (T.O.M.) and a baking method have been applied to measure the hydrogen diffusion coefficient at high temperature (150–600°C) in an austenitic stainless steel (AISI A286), an austeno-ferritic stainless steel (Z2CND2205) and a ferritic–perlitic steel (FM35). The results obtained by the above mentioned methods are in good agreement with those measured by the permeation technique at high temperature. Furthermore, in the austeno-ferritic steel Z2CND2205 the inverse of the total hydrogen diffusion coefficient 1\ D H( α+γ) is equal to f α \ D H α + f γ \ D H γ , where f α and f γ are, respectively, the volume fraction of α and γ phases and D H α and D H γ are the hydrogen diffusion coefficients for each phase.

Examination of hydrogen interaction in carbon steel by means of quantitative microstructural and fracture descriptions

The relations between the quantitative microstructural characteristics and the resistance of carbon steels to hydrogen-induced cracking (HIC) were studied for plates used in the oil and refinery industry. The width of the pearlitic bands and the degree of banding were considered if the testing of the resistance to HIC was performed in accordance with the NACE TM 0284 standard. The role of the degree of banding was important while that of the width of pearlitic bands was negligible. Additional hydrogen embrittlement testing of tensile specimens oriented in longitudinal and through-thickness directions revealed that hydrogen strongly increased the anisotropy of mechanical properties. These changes could be correlated with the geometric characteristics of nonmetallic inclusions (MnS) and the pearlitic bands in different metallographic sections. A quantitative description of fracture surfaces has been made by means of a profilometric method.

Quantitative measurements of the degree of exfoliation on aluminium alloys

Abstract The paper deals with quantitative measurements of the degree of exfoliation on three aluminium-lithium alloys (Al-Li-Cu, 8090 and 2091) in under-aged and over-aged conditions and on 7075-T7351, 7175-T6511 and 2024-T351. A special device was used in this work to measure the forces developed by the corrosion products during exfoliation in Exco-solution. Potentiokinetic curves were obtained on these alloys in Exco-solution and sodium chloride solutions (0.6, 1, 2 and 4 M Cl−) at a sweep rate of 1000 mV min−1, to determine the pitting potential,Ep, of each alloy. The curve obtained by plotting the forces developed by the exfoliation process as a function of time could be divided into two stages; an incubation period,t1, was followed by an increase in the forces to a level corresponding to a time of exfoliation equal to 240 h (F240h). The results obtained were compared to those observed in the conventional Exco-test (ASTM G34). Finally, stress-corrosion cracking tests using the slow strain-rate tensile technique were carried out on the aluminium-lithium alloys, either in Exco-solution or in the sodium chloride solutions containing various chloride concentrations. The results obtained show that, when the time of incubation,t1, increases, the force of exfoliation,F240h, decreases and the resistance to stress-corrosion cracking,RSCC, increases. This resistance is sensitive to the copper content, i.e.RSCC decreases also when the copper content increases. It is concluded thatt1 may be taken as an exfoliation criterion.

Evaluation of the long-term corrosion behavior of dental amalgams: influence of palladium addition and particle morphology.

OBJECTIVES The purpose of this investigation was to evaluate the long-term corrosion behavior of experimental amalgams as a function of particle morphology and palladium content. METHODS Samples of four experimental high copper amalgams were prepared according to ADA specifications. Two of them had the same chemical composition but one had lathe cut particles (LCP) and the other had spherical particles (SP). The two others had spherical powders with an addition of 0.5 wt% of palladium (SP 0.5) and 1 wt% of palladium(SP 1) for the other. Corrosion resistance was evaluated by electrochemical techniques in Ringers solution in a thermostated cell at 37 degrees C for samples aged 5, 8, 12, 16 months and 10 years. Potentiokinetic curves were drawn and the potential and the current density corresponding to the first anodic peak were registered. RESULTS For all the amalgam samples the corrosion behavior improves over the 10-year period. SP samples exhibit a better behavior than LCP. Palladium addition improves corrosion behavior as compared to samples without palladium. No real difference is found regarding the amount of palladium between 0.5 and 1%. The potentials progress from a range between 0 and 20 mV/SCE to a range of 60-80 after 10 years. The stabilization of the potential begins after only 16 months. Except for the LCP, all the values converge to the same level of 80 mV/SCE. SIGNIFICANCE The addition of no more than 0.5 wt% Palladium in a high copper amalgam powder improves the corrosion behavior of the amalgam up to a period of 10 years. The potential of the first anodic peak increases for each amalgam, probably in relation to the evolution of the structure of the material. Clinically, it is of interest to consider the good electrochemical behavior of older restorations when contemplating the repair or replacement of such fillings. At the same time, galvanic current can occur when a new amalgam restoration is placed in contact with an old one even if the same amalgam is used. In this situation, the new filling will be anodic and its degradation will be accelerated. The evaluation of the corrosion behavior of dental amalgams has to take into account the age of the samples.

Use of quantitative metallography in the evaluation of hydrogen action during martensitic transformations

Experiments based on image analysis method and electrochemical potentiostatic tests were performed with particular attention to the influence of hydrogen on the stabilisation of austenite of Fe–32 wt.%Ni alloy. This alloy exhibits great hydrogen absorption capacity (25 wt. ppm) and this presence modifies the martensitic transformation essentially at temperatures near MS (−40°C). In the whole range of the quenching temperatures, the difference between the volume fraction of martensite, measured on specimens with hydrogen or without hydrogen, is constant. It was found that the value of the interfacial area of the martensite plate groups SV is the best parameter describing the interfacial area between martensite and austenite. It was also found that there is a general relationship, expressed by a linear function, between this morphological parameter and an anodic current I.