Search

The ratio of the twin decreased after thermomechanical process for the CoCrNi medium-entropy alloy. The grain refinement decreased the passive current and increased the breakdown potential of the CoCrNi medium-entropy alloy in sodium chloride solution. The electrochemical impedance spectroscopy analysis showed that the grain refinement of the CoCrNi medium-entropy alloy facilitated to form thicker passive films and larger passive film resistance, which thus resulted into better corrosion resistance in sodium chloride solution. More nucleation sites after the grain refinement could enhance diffusion rate of alloying element and promote to form thicker and more stable passive films on refined grain CoCrNi medium-entropy alloy. Moreover, sufficient recrystallization level within refined grains for the CoCrNi MEA also improved its corrosion resistance.

The ultrafine grained pure Ni and Ni-W alloy were obtained by electrodeposition technique and subsequent annealing treatment. The effects of annealing on the microstructures, the mechanical properties and the corrosion resistance of pure nickel and Ni-W alloy were investigated. The EBSD analyses showed that annealed pure Ni displayed preferred grain orientation, while annealed Ni-W alloy showed relatively more random grain orientation. Moreover, abnormal grain growth was observed in annealed pure Ni, while annealed Ni-W alloy displayed a more uniform grain size distribution. It could be predicted that solid solution of tungsten significantly suppressed grain growth and induced uniform grain size distribution during low temperature annealing process. Moreover, comparing with annealed pure Ni, the tungsten in annealed Ni-W alloy changed the semiconducting characteristic of surface passive films. Annealed Ni-W alloy showed higher corrosion resistance than annealed pure Ni in 3.5 wt% NaCl solution and in borate buffer solution.

A large number of pitting pits were formed on the surface of Ni foam by immersion corrosion in NaCl solution. The graphene was deposited on Ni foam templates, then 3D graphene foams were obtained by removing Ni foam in the mixed solution of ferric chloride and hydrochloric acid. It was found that pitting pits on Ni foam facilitated to form fewer-layers graphene, however, pitting pits on Ni foam induced more defects on 3D graphene foam. The graphene with fewer-layers exhibited better electrical and mechanical properties for strain sensor devices. Moreover, fewer-layers graphene effectively inhibited degradation of resistance during cycling stability tests.

The effect of strain and low temperature sensitization on corrosion resistance of AISI 321 stainless steel in elevated borate buffer solution was investigated. The study found that the AISI 321 stainless steel with 20% strain showed the lowest degree of sensitization due to the fact that sample with 20% strain had high dislocation density which promoted chromium diffusion and a small amount of α'-martensites. The results from electrochemical impedance spectroscopy indicated that sensitized sample with 20% strain showed the best corrosion resistance after immersion test at 290 °C. A good agreement was found between the electrochemical impedance spectroscopy and the degree of sensitization. The oxide film was composed of three layers according to the results of the XPS and Raman spectra. In addition, the outer part compositions of the oxide film were mainly γ-Fe2O3 and Fe-Cr spinel and iron hydroxide, while the inner and intermediate layers were mainly Fe-Cr spinel and the mixtures of the oxides of Fe, Cr, Ni. The ratio of hydroxide in oxide film increased due to large strain, especially for chromium hydroxide. Mott-Schottky analysis showed that more hydroxides induced more donor and acceptor concentrations in oxide film. More hydroxides reduced corrosion resistance of oxide film.

The microscopic structures of different burnished layers in 2024 aluminum alloy were characterized by Transmission Electron Microscopy. The corrosion resistance of the burnished 2024 aluminum alloy was improved by grain refinement. The decreasing of electron work function improved corrosion resistance of the burnished 2024 aluminum alloy, moreover, less donor and acceptor concentrations in passive film also improved corrosion resistance of the burnished 2024 aluminum alloy. However, the substructures with high density dislocations deteriorated significantly the corrosion resistance of burnished 2024 aluminum alloy. More aluminum oxides improved the corrosion resistance of nanocrystalline 2024 aluminum alloy based on result of XPS.

The corrosion resistance of coarse grained and nanocrystalline Ni Fe alloys in chloride solutions and proton exchange membrane fuel cell environment was investigated. The nanocrystalline Ni Fe alloys were obtained by electrodeposition, while coarse grained Ni Fe alloys were obtained by annealing. The results from electrochemical tests showed that coarse grained Ni Fe alloys had higher corrosion resistance than nanocrystalline samples in NaCl solution due to more compact passive films formed on the surface of coarse grained Ni Fe alloys. In addition, Raman spectra test showed that many nanocrystalline boundaries in electrodeposited Ni Fe alloys promoted the diffusion of nickel, which resulted in the formation of more nickel oxides and nickel hydroxides. However, the electrochemical study showed that coarse grained sample had lower corrosion resistance than nanocrystalline one in a typical proton exchange membrane fuel cell environment. The present study showed that nanocrystalline Ni Fe alloys were more suitable for the bipolar plates in proton exchange membrane fuel cell environment.

The nanocrystalline pure nickels with different grain orientations were fabricated by direct current electrodeposition process. The grain size slightly decreased with the increasing of electrodeposition solution temperature. However, grain orientation was affected significantly. Comparing with samples obtained at 50 °C and 80 °C, sample obtained at 20 °C had the strongest (111) orientation plane which increased electrochemical corrosion resistance of this sample. At the same time, the lowest (111) orientation plane deteriorated electrochemical corrosion resistance of sample obtained at 50 °C.

The corrosion behaviors of NiTi shape memory alloy in NaCl solution, H2SO4 solution and borate buffer solution were investigated. It was found that TiO2 in passive film improved the corrosion resistance of NiTi shape memory. However, low corrosion resistance of passive film was observed in low pH value acidic solution due to TiO2 dissolution. Moreover, the corrosion resistance of NiTi shape memory alloy decreased with the increasing of passivated potential in the three solutions. The donor density in passive film increased with the increasing of passivated potential. Different solutions affect the semiconductor characteristics of the passive film. The reducing in the corrosion resistance was attributed to the more donor concentrations in passive film and thinner thickness of the passive film.

Surface enriched molybdenum 316L stainless steel was obtained by ball milling technique. The electrochemical tests showed surface enriched molybdenum improved the corrosion resistance of the 316L stainless steel. This was attributed to a fact that more molybdenum oxides improved the corrosion resistance of the 316L stainless steel in borate buffer solution. In addition, more chromium oxides in the passive film after ball milling also improved corrosion resistance of the 316L stainless steel.

The effect of grain refinement on the corrosion resistance of the passive film formed on 2099 Al–Li alloy was investigated. The microscopic structures of cold rolled 2099 Al–Li aluminum alloy were characterized by Transmittance Electron Microscopy and surface activity was analysed by electron work function. The corrosion resistance of the passive film was studied by electrochemical techniques and Raman spectra. It was found that nano/ultrafine grained 2099 Al–Li alloy with refined precipitation phases was obtained by several plastic deformation process. The corrosion resistance of the 2099 Al–Li aluminum alloy was improved by grain refinement and refined precipitation phases. The improved corrosion resistance of 2099 Al–Li alloy after grain refinement was attributed to decreased electron work function. The grain refinement promoted the forming of thicker passive film and decreased acceptor concentration in passive film.

The results from double loop electrochemical potentiokinetic reactivation technique, potentiostatic critical pitting temperature technique and electrochemical impedance spectroscopy showed that the resistance of the 2205 duplex stainless steel to pitting corrosion decreased with the increasing of sensitization time due to a decreasing in passivation ability. The 2205 duplex stainless steel showed the increase in pitting corrosion in the ferritic phase and in intergranular corrosion between the austenitic and ferritic phases with the increasing of sensitization time. The precipitated phases changed due to the increasing of the sensitization time, which affected degree of sensitization of the 2205 duplex stainless steel.

The effect of grain refinement on corrosion resistance of alloy 690 was investigated. The electron work function value of coarse grained alloy 690 was higher than that of nano/ultrafine grained one. The grain refinement reduced the electron work function of alloy 690. The passive films formed on coarse grained and nano/ultrafine grained alloy 690 in borate buffer solution were studied by potentiodynamic curves and electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The results showed that the grain refinement improved corrosion resistance of alloy 690. This was attributed to the fact that grain refinement promoted the enrichment of Cr 2 O 3 and inhibited Cr(OH) 3 in the passive film. More Cr 2 O 3 in passive film could significantly improve the corrosion resistance of the nano/ultrafine grained alloy 690.

The passivation properties of deformed 304 stainless steels after immersion in borate buffer solution containing 0.2821 mol/L Cl− at 288 °C were investigated. The spinel and magnetite oxides were formed on all the samples. However, the hematite oxides reduced significantly with the increasing of strain. The sample with maximum strain possessed the poorest corrosion resistance. The hematite oxide could offer high corrosion resistance, while magnetite evidently deteriorated corrosion resistance. Moreover, the influence of the donors in outer layer of oxide film on corrosion resistance was more important than that of the acceptors in inner layer.

The passive film formed on the surface of the alloy 690 in borate buffer solution was studied by potentiodynamic curves and electrochemical impedance spectroscopy. With the increasing of the passivation potential, the corrosion resistance of the alloy 690 reduced. Moreover, the corrosion resistance of the passive film was the lowest in the vicinity of 0.6 V SCE . These results were supported by XPS and Mott–Schottky analyses. The corrosion resistance of the alloy 690 increased with the increasing of passivated potential in borate buffer solution with chloride ion. The chloride ion decreased corrosion resistance of the alloy 690 according to point defect model.

The stain-induced nanocrystalline α'-martensite was obtained by cryogenic cold rolling at liquid-nitrogen temperature for 316L stainless steel. The electrochemical results showed nanocrystalline 316L stainless steel deteriorated its corrosion resistance in a typical proton exchange membrane fuel cell environment compared with coarse grained one. However, comparing with electrochemically nitrided coarse grained stainless steel, electrochemically nitrided nanocrystalline stainless steel improved significantly corrosion resistance in the same environment, which was supported further by Mott–Shottky analysis. X-ray photoelectron spectroscopy analysis revealed that the nanocrystalline promoted the enrichment of nitrogen and chromium and inhibited form of NH 3 on the surface, which could significantly improve the corrosion resistance of the 316L stainless steel. The present study showed that the electrochemically nitrided 316L stainless steel was more suitable for the bipolar plates in proton exchange membrane fuel cell environment than the untreated one, especially for nanocrystalline stainless steel.

The nano/ultrafine grained stainless steel promoted the form of more deformation nanotwins and inhibited dislocation slip and strain induced α′-martensite transformation during deformation compared with coarse grained stainless steel. The diffusivity of the point defects in passive film in coarse grain stainless steel was lower than that in nano/ultrafine one. The corrosion resistance of nano/ultrafine grained stainless steel was improved by reducing content of strain induced α′-martensite. Moreover, the donor and acceptor densities in passive film formed on the stainless steel increased due to more strain induced α′-martensites and more chloride ions. Fewer strain induced α′-martensites and more nanotwins in nano/ultrafine grained stainless steel improved its corrosion resistance.

The present work investigates the corrosion resistance of as-cast FeCoCrNiMn high-entropy alloy in borate buffer solution. Compared with 304 and 316L stainless steels steel, as-cast FeCoCrNiMn high-entropy alloy exhibited excellent passivation ability. This should be attributed to homogenization of alloying elements. Highly corrosion-resistant elements in as-cast FeCoCrNiMn alloys, for example, Cr, Co and Ni, could promote the formation of protective passive film in borate buffer solution. Moreover, combined effect of high mixing entropy and sluggish diffusion also could improve the corrosion resistance. The formation mechanism of the passive films on the surface of as-cast FeCoCrNiMn alloys satisfied the point defect model.

The effects of electrochemical nitridation on the corrosion resistance of the 2205 duplex stainless steel were investigated. It was found that increased nitrogen content in the 2205 duplex stainless steel affected donor and acceptor concentration distribution in the passive films on the surface of the 2205 duplex stainless steel. Moreover, the total donor and acceptor concentration in passive films decreased with the increasing of nitrogen content. The electrochemically nitridation facilitated to induce compact passive film and thus improved significantly the corrosion resistance of the 2205 duplex stainless steel in borate buffer solution.

The effect of the strain of 316L stainless steel on its corrosion resistance in PEMFC environment was investigated. Higher tensile strain facilitated the formation of more dislocations. The 316L stainless steel with 20% strain showed the best corrosion resistance in PEMFC environment containing a few fluoride ions, however, the corrosion resistance of the stainless steel decreased with the increasing of engineering strain in PEMFC environment with a considerable amount of fluoride ions. The morphology and density of the dislocations in strained 316L stainless steel could both affect significantly its corrosion resistance in PEMFC environment. This conclusion was supported by Mott–Schottky measurement and XPS analysis.

The effect of cold rolling temperature on grain boundary characteristic of the 304 austenitic stainless steels in thermomechanical treatment was examined. Comparing to cold rolling at room temperature and annealing, the cryogenic cold deformation and annealing promoted grain refinement and reduced fraction of coincidence site lattice. Although more coincidence site lattice restrained sensitization, the grain refinement accelerated sensitization. The sensitization deteriorated corrosion resistance of 304 austenitic stainless steel obtained by thermomechanical treatment.

The effects of cold rolling temperature on grain size and grain orientation of pure iron were investigated. Comparing with sample rolled at room temperature, the grain refinement was facilitated in sample obtained by cryogenic cold rolling at liquid-nitrogen temperature. However, the grain orientation changed little for two samples. It was shown that cathodic hydrogen evolution reaction could govern the corrosion reaction for pure iron in sulfuric acid solution. The grain refinement obtained by rolling improved the corrosion resistance of iron in sulfuric acid solution, borate buffer solution and borate buffer solution with chloride ion. However, comparing with iron rolled at room temperature, the corrosion resistance of iron obtained by cryogenic temperature rolling was lower. Comparing with iron rolled at room temperature, higher dislocation density in iron rolled at cryogenic temperature reduced its corrosion resistance.

The effects of solution temperature, concentration of chloride ions, and grain refinement on the corrosion resistance of the passive film formed on 304 stainless steels in borate buffer solution were investigated by electrochemical impedance spectroscopy and Mott-Schottky measurements. The results showed that the passive films formed on the surface of 304 stainless steels possessed n- and p-type semiconductor characteristics. With the increase of solution temperature, the corrosion resistance of the passive film of coarse and nano/ultrafine grained stainless steels decreased. However, the corrosion resistance of the passive film in nano/ultrafine grained stainless steel was higher than that in the coarse grained one. Moreover, in cyclic potentiodynamic polarization tests, chloride ions changed semiconductor characteristics of the passive film formed on the surface of stainless steel.

The passive film of iron showed n-type semiconductor characteristic in borate buffer solution, and its donor concentration increased slightly after tensile strain in the present study. However, comparing with solution-annealed sample, the anodic passive film formed on tensile-strained one was highly protective. The more dislocations on tensile-strained sample promoted the diffusion of iron and oxygen vacancy. Moreover, more donor density (mainly oxygen vacancies) promoted the diffusion of oxygen. They all facilitated tensile-strained sample to form Fe2O3 and thicker passive film on the surface. More Fe2O3 and thicker passive film on the surface of tensile-strained iron could improve corrosion resistance. Copyright © 2014 John Wiley & Sons, Ltd.

{011} planes which were medium close-packed planes were the dominating orientation in A sample (without burnishing) and B sample (a burnished sample) surfaces. There was not a common close-packed plane dominating orientation parallel to C sample (the other burnished sample) surface, and texture in this sample was abnormal. The corrosion property by electrochemical impedance spectroscopy was improved by burnishing process. Considering similar dislocation density obtained by XRD and grain boundary characteristic of two burnished samples, although the surface energy quantity for Al was (111)

The nano/ultrafine grain (NUG) with an average grain size of 230 nm was obtained by cold rolling down to 94% reduction in thickness and reversion annealing at 800 °C for 200 s. The NUG sample exhibited a lower corrosion resistance than coarse grain (CG) sample in 0.1 M NaCl solution at room temperature, indicating that the passive film formed on the surface of the NUG austenite did not improve corrosion resistance in the solution. However, the corrosion resistance of the former was higher than that of the latter in 0.5 M H 2 SO 4 solution at room temperature, which was proved by electrochemical impedance spectroscopy and Mott–Schottky plots in conjunction with the point defect model. Comparing slightly difference of acceptor density (i.e. cation vacancies) between CG and NUG samples, higher corrosion resistance of NUG sample was probably attributed to significant decreased donor density (i.e. oxygen vacancies and cation interstitials) in 0.5 M H 2 SO 4 solution. Moreover, the corrosion resistance of the passive films formed on CG and NUG samples in borate buffer solution at room temperature showed little difference.

The effect of burnish process on 2024 aluminum alloy was studied by electron work function and electron backscattered diffraction (EBSD). Moreover, the corrosion resistance of thin passive films formed on 2024 aluminum alloy in borate buffer solutions was studied by the electrochemical impedance spectroscopy (EIS), the Mott–Schottky plots and the galvanostatic techniques. The composition of passive films was analyzed by Raman spectra. The results obtained indicated that the impedance increased due to burnish and this was attributed to decreased electron work function and higher current efficiency in the burnished aluminum alloy which led to thicker passive films. It was further supported by Raman spectra experiment. Moreover, the donor and acceptor concentration of passive films and their the semiconductor type have changed due to burnish.

The passive behaviors of coarse and nano/ultrafine grain austenitic stainless steels processed by cold rolling and reversion annealing were investigated. The average grain about 200 nm was obtained by cold rolling down to 94% reduction in thickness and reversion annealing at 800 °C for 250 s. Passive films formed in borate buffer solution (pH 9.2) have been characterized by electrochemical impedance spectroscopy and Mott–Schottky plots. The impedance results indicated a distinct inflection was observed with the increasing of potential, however, the Mott–Schottky analysis did not show such inflection point in the same potential range. There were not evident differences in passive properties between coarse and nano/ultrafine grain samples. This was attributed to synergistic effect of grain refinement and orientation. However, the corrosion resistance of the passive film in nano/ultrafine grain sample was higher than that in coarse grain sample after immersion test by the three-point-bend loading.

The electrochemical measures in deformed AISI 304 stainless steels ranged from 0% to 24% were investigated. With the increasing of the strain α′-martensite was gradually increasing. Moreover, the morphologies and densities of dislocations changed with increasing strain. The Mott–Schottky plots showed that the maximum total of donor and acceptor densities in passive film occurred in borate buffer solution when ɛ-martensite was most, while in borate buffer solution with 5000 ppm Cl− the maximum total of donor and acceptor densities occurred when the content of α-martensite was most. These results were proven by further impedance spectroscopy experiments.

The ultrafine grained 2205 duplex stainless steel was obtained by cold rolling and annealing. The tensile properties were investigated at room temperature. Comparing with coarse grained stainless steel, ultrafine grained sample showed higher strength and plasticity. In addition, grain size changed deformation orientation. The strain induced α'-martensite was observed in coarse grained 2205 duplex stainless steel with large strain. However, the grain refinement inhibited the transformation of α'-martensite;nevertheless, more deformation twins improved the strength and plasticity of ultrafine grained 2205 duplex stainless steel. In addition, the grain refinement improved corrosion resistance of the 2205 duplex stainless steel in sodium chloride solution.

The effect of the strain of 316L stainless steel on its corrosion resistance in borate buffer solution was investigated by in site tensile test and the electrochemical impedance spectroscopy measurements. It was found that the corrosion resistance of the 316L stainless steel decreased with the increasing of in site strain. The lower corrosion resistance of the stainless steel during in site strain was mainly attributed to the higher doping concentration in passive film. Especially, with the increasing of in site strain, the concentrations of acceptor (i.e., cation vacancies) in the passive films significantly increased. More acceptor concentrations reduced the compactness of the passive film and its corrosion resistance. Moreover, two exponential relationships were found between in site strain and the charge transfer resistance of the passive film and between in site strain and total doping concentrations in passive film, respectively.