Corrosion behaviour and biocompatibility of a novel Ni-free intermetallic coating growth on austenitic steel by hot dipping in an Al-12.6%Si alloy

Commercial 316 LVM austenitic stainless steel samples have been coated by immersion in a bath of molten Al–12.6%Si alloy for 120 s. The coating consists of the Al₁₂(Fe,Cr)₃Si₂ intermetallic. In vitro corrosion behaviour has been evaluated in the Ringer’s solution by means of potentiodynamic curves and electrochemical impedance spectroscopy. The results reveal that the coated specimens exhibit lower susceptibility to localised corrosion with respect to the substrate. XPS analysis suggests that the ennoblement of the pitting potential is due to the formation of a chromium oxyhydroxide containing passive layer. The intermetallic coating shows a good biocompatibility, as demonstrated by culturing human mesenchymal stem cells isolated from bone marrow which attached, grew and differentiated to the osteoblastic lineage to a similar extent on coated and bare steels. In summary, this study proposes a method that generates Ni-free coatings of the stainless steel with useful properties for biomedical applications.     

Microstructure and electrochemical behavior of laser treated Fe---Cr and Fe---Cr---Si---N surface alloyed layers on carbon steel

Laser surface alloyed Fe---Cr and Fe---Cr---Si---N layers on carbon steel were produced by laser irradiation involving preplaced Fe/Cr and Fe/Cr/Si₃N₄ powders, respectively. The effects of Cr content (13–25 wt.%) and microstructure on the corrosion behavior of the laser treated Fe---Cr and Fe---Cr---Si---N alloys in deaerated neutral 3.5 wt.% NaCl solution were studied. Potentiodynamic polarization tests and electrochemical impedance spectroscopy measurements were used to evaluate the corrosion resistance of the surface layers. The results showed that the Fe---Cr---Si---N treated layer with a fine duplex phase microstructure had a higher pitting potential than the Fe---Cr treated layers. The passive film resistances increased and the passive current density decreased with increasing Cr content in the Fe---Cr layer.     

Effect of carburization on electrochemical corrosion behaviours of TiAl alloy

Electrochemical corrosion behaviours of the untreated and the carburized of Ti-46.5Al (mol %) alloy were investigated. X-ray diffractometry (XRD) and scanning electron microscopy (SEM) were applied to characterize the carburized layer. Potentiodynamic polarization curve, electrochemical impedance spectroscopy (EIS) and SEM morphology of the corroded surface were used to evaluate corrosion resistance of both carburized and untreated TiAl alloy in 1 mol/L HCl. The outer layer of the carburized TiAl alloy is a continuous Ti₂AlC scale. Polarization curve and electrochemical impedance spectroscopy (EIS) of the carburized TiAl alloy present a nobler corrosion potential, a more positive pitting potential and a higher polarization resistance, respectively, compared with the untreated sample. After anodic corrosion or immersion corrosion, a deposited layer can be observed on the surface of the carburized titanium aluminide alloy. By contrast, pitting and crevasse corrosion occur on the surface of the untreated TiAl alloy after anodic corrosion and some corrosion products and slight corrosion appear on the surface of the untreated TiAl alloy after immersion corrosion.     

Effect of chromium on the corrosion behavior of iron aluminides in thiosulfate-chloride solution

Aqueous corrosion characteristics of iron aluminides in thiosulfate-chloride solution were studied as a function of chromium addition. Four kinds of iron aluminides, namely, FA-61, FA-77, FA-72 and FA-78, were prepared by arc melting followed by thermomechanical treatment. The corrosion behavior in thiosulfate-chloride solution for the prepared alloys were investigated by electrochemical tests (potentiodynamic test, potentiostatic test and electrochemical impedance spectroscopy (EIS) measurement) and surface analyses. The results of the potentiodynamic test indicated that the breakdown potential increased with increasing Cr content. Cr additions were found to prevent passive film from undergoing pitting corrosion. In EIS measurement, the depression angle was inversely related to pitting resistance, and decreased with increasing chromium content. The SEM observations of the sample surfaces reveal the different forms of pit as a function of chromium content. The AES results give evidence that the thiosulfate ions are reduced on the metallic surface, which inhibits the repassivation process.     

Influence of surface roughness on the corrosion behaviour of magnesium alloy

In this study, the influence of surface roughness on the passivation and pitting corrosion behaviour of AZ91 magnesium alloy in chloride-containing environment was examined using electrochemical techniques. Potentiodynamic polarisation and electrochemical impedance spectroscopy tests suggested that the passivation behaviour of the alloy was affected by increasing the surface roughness. Consequently, the corrosion current and the pitting tendency of the alloy also increased with increase in the surface roughness. Scanning electron micrographs of 24 h immersion test samples clearly revealed pitting corrosion in the highest surface roughness (Sa 430) alloy, whereas in the lowest surface roughness (Sa 80) alloy no evidence of pitting corrosion was observed. Interestingly, when the passivity of the alloy was disturbed by galvanostatically holding the sample at anodic current for 1 h, the alloy underwent high pitting corrosion irrespective of their surface roughness. Thus the study suggests that the surface roughness plays a critical role in the passivation behaviour of the alloy and hence the pitting tendency.     

Effect of micro-alloying chromium on the corrosion resistance of nanocrystalline nickel aluminide intermetallic produced by mechanical alloying process

In this study, Ni₅₀Al_50 − xCr_x nanocrystalline intermetallic compound was synthesized by using the high energy mechanical milling of pure Ni, Al and Cr elemental powders for 16 h. The morphological investigation was done by using the optical and scanning electron microscope. The corrosion behavior of the samples was studied by using the electrochemical impedance spectroscopy in 3.5% NaCl solution. The results showed that when the micro-alloying Cr content is increased, the particles distribution is modified and the size of particles is decreased. Therefore the passive film which is formed on the surface of samples is less porous, so the corrosion resistance is increased.     

铁氧化菌作用下316L不锈钢点蚀电化学特性的研究

采用电化学测量、交流阻抗技术、扫描电镜观察和能谱分析等实验方法,研究了316L不锈钢在铁氧化菌(IOB)溶液中的腐蚀电化学行为,分析了炼油厂冷却水系统微生物腐蚀的特征及机制,结果表明,在含有IOB溶液中的自腐蚀电位(Ecorr)、点蚀电位(Epit)和极化电阻(Rp)均随浸泡时间的增加呈现出降-升-降的变化趋势;在含有IOB溶液中的腐蚀速率均大于在无菌溶液中;IOB的生长代谢活动及其生物膜的完整性和致密性影响了316L不锈钢表面的腐蚀过程,使不锈钢表面的钝化膜层腐蚀破坏程度增加,加速了316L不锈钢的点蚀.     

Exfoliation corrosion of extruded Mg-Li-Ca alloy

Exfoliation on as-extruded Mg-1 Li-1 Ca magnesium alloy was investigated after an immersion in 3.5 wt%NaCl aqueous solution for 90, 120 and 150 days through optical microscope, digital camera, scanning electron microscope, electrochemical workstation, scanning Kalvin probe, X-ray diffraction and Fourier transform infrared spectroscope. The results demonstrated that exfoliation corrosion occurred on extruded Mg-1 Li-1 Ca alloy due to elongated microstructure parallel to surface, and delamination of lamellar structure resulted from galvanic effect and wedge effect. Skin layer with fine grains exhibited better corrosion resistance, whereas the interior with coarse grains and the intermetallic compound,Mg_2 Ca particles existing in a fibrous structure, dispersed along grain boundaries and extrusion direction in a line. Furthermore, galvanic effect between Mg_2 Ca particles and their neighboring a-Mg matrix facilitated dissolution of Mg_2 Ca particles and a-Mg matrix; wedge effect was caused by formation of corrosion products. Exfoliation corrosion of extruded Mg-Li-Ca alloys might be a synergic effect of pitting corrosion,filiform corrosion, intergranular corrosion and stress corrosion. Finally, exfoliation corrosion mechanism was proposed.     

Corrosion resistance of model ultrafine-grained Al–Li alloys produced by severe plastic deformation

The influence of grain boundaries and fine precipitation on the corrosion behavior was investigated in two model aluminum–lithium alloys, namely (in wt%) Al–1.6Li (lithium in a solid solution) and Al–2.3Li (lithium in the form of Al₃Li precipitation), subjected to three different severe plastic deformation (SPD) treatments which refined the microstructure of the alloys to the ultrafine grain size. The SPD techniques used in the experiments were equal channel angular pressing (ECAP), hydrostatic extrusion (HE), and extrusion-torsion (ET). The corrosion behavior was examined using a potentiodynamic polarization test, electrochemical impedance spectroscopy, and an immersion test followed by a SEM surface analysis. The electrochemical tests were conducted in a 0.1 M Na₂SO₄ solution added with 100 ppm of Cl⁻. The immersion tests (48 h) were performed in a 3.5% NaCl solution at room temperature. The results indicate that the pitting potential, pit number, and stability of the passive layer formed on the surface of the substrates undergo changes depending on the average grain size and the presence of precipitation or its lack. The corrosion resistance, examined in the solution mentioned above, appears to increase with decreasing average grain size. The ET method gave the microstructure with the lowest corrosion resistance.     

Enhancement of corrosion resistance of a biomedical grade 316LVM stainless steel by potentiodynamic cyclic polarization

The paper discusses the results on the use of a simple cyclic linear potentiodynamic polarization technique as a method of improving corrosion properties of passive oxide films formed on a biomedical-grade 316LVM stainless steel surface in phosphate buffer. The results demonstrate that the modification of 316LVM surface by cyclic potentiodynamic polarization between the potential of hydrogen and oxygen evolution results in the formation of a passive film that offers significantly increased corrosion resistance (both pitting and general) when compared to the naturally grown passive film. The effect of number of cycles and anodic potential limit on the resulting corrosion properties is discussed. The capacitance analysis demonstrates that the major difference between the electrochemically formed and naturally grown passive film is in the type of semiconductivity in the potential region where pitting on the unmodified surface occurs. The XPS analysis shows that this is due to the presence of Cr(VI)-species in the electrochemically formed passive film, which contribute to the increased density of metal vacancies, and thus to the increased pitting corrosion resistance of the passive film.     

电偶对中6A01铝合金的电化学腐蚀行为研究

为了解决高速列车环境下6A01铝合金的电偶腐蚀问题,利用电化学阻抗谱和极化曲线等电化学方法研究了6A01铝合金在不同溶液中的电偶腐蚀行为.结果表明:6A01铝合金在NaCl溶液和模拟污水溶液中以点蚀为主,模拟污水溶液中铝合金的腐蚀速率较小,腐蚀速率随时间先增大后减小,再增大再减小,最后趋于稳定,铝合金的耐蚀性能减弱.铝...     

Short-term electrochemical corrosion behavior of pipeline steel in saline sandy environments

In this study, we investigate the short-term electrochemical corrosion behavior of X70 pipeline steel in salinized sandy soil using the electrochemical impedance spectroscopy (EIS). The results show that the corrosion behavior of X70 steel in sandy soils with a relatively high water content (~ 18%) is mainly dominated by pitting or localized corrosion, which is controlled by charge-transfer processes. The reason can be attributed to the loose corrosion products (unable to produce an effective protective layer) and discontinuous gradation in sand particles. EIS plots of X70 steel consist of two capacitive loops at the initial stage of corrosion, while a capacitive arc at high-, medium-, and low-frequency is developed after 12 h of corrosion. The capacitive loop formed at the high frequency indicates the resistance and capacitance generated by the penetration of ions into the metal surface through the sand pores and the defects of the corrosion product film.     

Surface modification of aluminum alloys in molten salts containing CeCl3

A process for surface modification of A16061-T6 has been carried out in molten salts containing Ce such as a NaCl---SnCl₂---CeCl₃ melt at 200 °C. Samples treated in molten salts for 2 h were subsequently exposed to a NaCl solution for 30 days to evaluate resistance against pitting corrosion. The coating mechanism in molten salts and the pitting corrosion behavior of the coated material have been studied by electrochemical impedance spectroscopy. The results show that this coating technique enhances the pitting corrosion resistance of aluminum alloys in contact with sea water.     

AZ91D镁合金表面熔盐置换扩散涂层组织及耐腐蚀性能研究

采用AlCl3-NaCl熔盐自发置换扩散工艺在AZ91D镁合金表面制备镁铝金属间化合物涂层,并利用开路电位、电化学阻抗方法对镁合金及熔盐置换扩散改性处理试样在3.5%(质量分数)NaCl溶液中的腐蚀行为进行比较研究。结果表明:熔盐自发置换扩散铝涂层在温度为400℃,保温8h的工艺条件下呈现出了明显的分层结构特征。根据组织结构不同,可将其分为:颗粒状形貌的外层和具有网状结构的内层;经表面熔盐自发置换扩散处理后的镁合金试样,腐蚀电位比未改性的AZ91D镁合金大幅提高,膜层阻抗约为未改性的AZ91D镁合金的10倍。     

Novel quantitative method for evaluation pitting corrosion and pitting corrosion inhibition of carbon steel using electrochemical noise analysis

The electrochemical noise of N80 steel in sodium chloride solution containing 0.5 M bicarbonate with and without inhibitor was recorded. The current signal which corresponded to pitting corrosion was extracted from the records based on wavelet transform, then the intergraded current was calculated. Comparing with the intergraded current with and without inhibitor, the inhibition efficiency on pitting corrosion was calculated. Electrochemical impedance spectroscopy (EIS) measurements were also done to validate the electrochemical noise analysis results. From this paper, it was shown that pitting corrosion and inhibition efficiency on pitting corrosion could be evaluated accurately and quantitatively through intergrading pitting corrosion current which extracted based on wavelet transform.     

pH值对X90管线钢在模拟酸性土壤溶液中腐蚀行为的影响

目前,对X90等第三代管线钢在土壤中的腐蚀研究较少。采用动电位极化曲线、交流阻抗谱和腐蚀失重等研究了不同pH值下,X90管线钢在模拟酸性土壤溶液中的腐蚀行为。结果表明:pH值的变化不影响X90钢的阴阳极反应机理;当pH≤4时,交流阻抗谱低频区出现感抗弧,此过程存在吸附或点蚀形核,当pH≥4时,交流阻抗谱出现Warburg扩散阻抗,传质扩散为反应主要控制步骤;随着溶液pH值降低,腐蚀电流密度增大,溶液中总铁含量增加,试样表面腐蚀程度加剧,均表明X90管线钢的腐蚀速率增大。     

Effect of Cl- on the Properties of the Passive Films Formed on 316L Stainless Steel in Acidic Solution

The corrosion behavior of 316L stainless steel （31 6L SS） has been investigated in solutions containing various concentrations of chloride ions by using potentiodynamic polarization, capacitance measurement and Mott- Schottky relationship analysis （M-S）. The result indicates that passive currents change slightly with the addition of chloride ions. The pitting potential （Epit） decreases linearly with Iog[CI-]. Correspondingly, the point defect diffusion coefficient （Do） of the passive film increases linearly with increasing Iog[CI-]. The results also indicate that the pitting corrosion of 316L SS follows the adsorption mechanism in NaCI solution.     

The corrosion behavior of niobium bearing cold deformed austenitic stainless steels in 3.5% NaCl solution

The corrosion behavior of some austenitic stainless steels was studied in 3.5% NaCl solution. The effects of Nb content and cold deformation on the corrosion resistance were investigated.Electrochemical impedance spectroscopy (EIS) technique was used to investigate the corrosion behavior after two weeks of immersion in NaCl solution. The occurrence of pitting corrosion due to chloride ion attack was also examined by visual inspection, cyclic potentiodynamic, Scanning Electron Microscope (SEM) and microprobe analysis using energy dispersive X-ray (EDS).According to EIS measurements, the corrosion resistance increased by increasing the Nb content. Cold deformation (CD) has a critical effect on the corrosion resistance. The specimens worked at 23% CD showed the best corrosion resistance. The surface corrosion resistance increased double compared with non-deformed specimens. Increasing the deformation to 40% and 50% affects negatively the corrosion resistance. It is argued that on cold working, strain-induced martensite and residual stresses are significantly introduced on the surface, which affects the localized corrosion resistance by increasing the number of active anodic sites on the surface.     

Electrochemical formation of highly pitting resistant passive films on a biomedical grade 316LVM stainless steel surface

The results discussed in the paper demonstrate that a significant improvement in pitting corrosion resistance of a biomedical grade 316LVM stainless steel can be achieved by electrochemically forming highly-protective passive oxide films on the material's surface, under cyclic potentiodynamic polarization conditions. The film formed in a sodium nitrate electrolyte is completely resistant to pitting corrosion in simulating physiological solutions even at high temperatures (60 °C), and after sterilization. The high pitting resistance of the electrochemically-formed films was explained on the basis of their semiconducting properties. Namely, the enrichment of the outer part of the electrochemically formed passive film with Cr(VI)-species results in a decrease in the density of oxygen vacancies, which act as pitting initiation sites, and their ‘replacement’ by metal vacancies formed by the electrochemical oxidation of Cr(III) to Cr(VI). In this configuration, the outer Cr(VI)-rich oxide layer behaves as cation selective, which results in the increased pitting corrosion resistance of the film. The simple electrochemical passivation technique discussed in the paper can be efficiently used to form highly pitting resistant passive films on 316LVM-built medical implant devices of any geometry.