Effects of alloying elements on the electrochemical characteristics of iron aluminides

Effects of alloying elements on the electrochemical characteristics of iron aluminides in the H2SO4, H2SO4+KSCN and HCl solutions were investigated using electrochemical tests. The corrosion morphologies in iron aluminides were analysed by utilising optical microscopy. It was found that the addition of Cr and Mo to iron aluminides increased the corrosion potential, pitting potential and repassivation potential. The active current density, passive current density and reactivation current density decreased as Cr and Mo were added. In the case of Mo addition, the passive current density was slightly higher in the H2SO4 solution than in solutions containing SCN- and Cl-. When B was added to samples, the corrosion potential and repassivation potential decreased, whereas the active current density, passive current density, reactivation current density and pitting potential increased. Iron aluminides containing Mo and Cr showed remarkably improved intergranular and pitting corrosion resistance to SCN- and Cl- solution. On the other hand, B addition accelerated granular and intergranular corrosion by precipitation of borides. This revised version was published online in November 2006 with corrections to the Cover Date.     

块体纳米晶工业纯铁在盐酸溶液中的电化学腐蚀行为

通过静态失重试验,动电位极化曲线,电化学阻抗谱(EIS)实验,研究了块体纳米工业纯铁(BNIPI)和粗晶工业纯铁棒(CGPIR)在室温1mol/l盐酸溶液中的腐蚀行为.结果表明,BNIPI与CGPIR相比,开路腐蚀电位Ecorr正向移动114mV,平均腐蚀速度和腐蚀电流Icorr变小,极化电阻Rp增大为1.58倍.BNIPI抗盐酸的腐蚀能力与CGPIR相比,不但没有下降,相反有所增强.使用扫描电子显微镜(SEM)对静态腐蚀失重试样的形貌进行了观察,显示BNIPI上几乎没有点蚀坑出现.     

Effects of 5-(3-aminophenyl)-tetrazole on the inhibition of unalloyed iron corrosion in aerated 3.5% sodium chloride solutions as a corrosion inhibitor

The effects of 5-(3-aminophenyl)-tetrazole (APT) on the inhibition of unalloyed iron corrosion in aerated 3.5% NaCl solutions as a corrosion inhibitor have been studied using open circuit potential (OCP), cyclic potentiodynamic polarization (CPP), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) measurements. The inhibited iron surface was characterized by scanning electron spectroscopy (SEM) and energy dispersive X-ray (EDS) investigations. The OCP showed positive shifts of potential in the presence of APT and the increase of its concentration. CPP and CA measurements indicated that APT molecules decrease the pitting and uniform corrosions through decreasing the pitting and absolute currents, and corrosion rate as well as shifting the corrosion and pitting potentials of iron towards the noble values. EIS plots revealed that APT increases the surface and polarization resistances of iron. SEM/EDS investigations proved that the inhibition of iron corrosion in NaCl containing APT solutions is achieved by the adsorption of APT molecules onto iron to preclude the dissolution process by blocking the active sites on its surface.     

Hydrogen transport and environmental embrittlement effects in iron aluminides

Environmental embrittlement types of six iron aluminides have been systematically evaluated using electrochemical hydrogen permeation measurement, hydrogen microprint technique and tensile test in this study. The results of hydrogen permeation and microprint technique show that three - disordered solid solution structure of binary and ternary iron aluminides (Fe-10Al, Fe-18Al, and Fe-18Al-5Cr) have higher effective diffusivity and permeation rate than B2 ordered structure of binary and ternary iron aluminides (Fe-28Al, Fe-28Al-5Cr, and Fe-40Al). The tensile test of six iron aluminides in air, vacuum treatment and hydrogen precharged were analyzed and concluded. Three - disordered solid solution structure of iron aluminides suffer in hydrogen environment which are quite different embrittlement mechanisms from those B2 ordered structure of iron aluminides with serious tensile loss in air by moisture induced embrittlement.     

The preparation and properties of iron base self-fluxing alloy spray-welding coatings with different rare earths and chromium content

This investigation studied the effects of rare earths (RE) and chromium on the high temperature oxidation resistance and aqueous corrosion resistance of iron base self-fluxing alloy coatings. Four coatings were prepared through smelting-atomizing and oxide-acetylene flame spraying. The properties of the coatings were evaluated by cyclic oxidation tests, weight loss experiments, electrochemical impedance spectroscopy (EIS), potentiodynamic scanning technique, scanning electron microscopy (SEM) and electron dispersive X-ray analysis (EDAX). The addition of RE greatly enhanced the oxidation resistance of the coatings. This effect was attributed to the fact that RE changed the ion diffusion patterns of the coatings in the process of scale forming, resulting in more protective scales with high adhesion. The increase of chromium content in the coatings enhanced the corrosion resistance of the coatings in nitric acid solution, but in hydrochloric acid and sulfuric acid solutions the result was reversed. Rare earth addition had a beneficial effect in sulfuric acid and nitric solutions, but in hydrochloric acid, the samples with RE had a corrosion rate slightly higher than that of the samples without RE addition. These results are explained by the effect that the addition of RE minimized the cathodic area of the coatings.     

Influence of laser surface alloying with chromium and nickel on corrosion resistance of type 304L stainless steel

Abstract

The influence of laser surface alloying (LSA) with Cr and Cr + Ni on the corrosion behaviour of type 304L stainless steel (SS) was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in chloride (0·5M NaCl) and acidic (1 N H₂SO₄) media. Surface alloying was carried out by laser cladding type 304L SS substrate with premixed powders of AISI type 316L SS and the desired alloying elements. The results indicated that Cr surface alloyed specimen exhibited a duplex (γ + α) microstructure with Cr content of ～24 wt-%, whereas Cr + Ni surface alloyed specimen was associated with austenitic microstructure with Cr and Ni contents of ～22 wt-% each. The potentiodynamic polarisation results in chloride solution indicated that LSA with Cr + Ni considerably enhanced the pitting corrosion resistance compared with LSA with Cr alone. In acidic media, such beneficial effects were not observed. Electrochemical impedance spectroscopy results showed an increase in semicircle arc for both chloride and acidic media for both Cr and Cr + Ni clad samples indicating improvement in the oxide film stability compared with untreated specimen. The polarisation resistance was higher and capacitance values of the laser clad specimen were lower than those in the untreated specimen. The microstructural changes and compositional variations produced by LSA are correlated to the corrosion behaviour.     

Deciphering the potentiodynamic polarization curves of iron aluminides Fe3Al and Fe3Al + Cr

The potentiodynamic polarization curves of Fe₃Al and Fe₃Al + Cr intermetallics obtained in aerated pH 4 H₂SO₄ acidic solution have been theoretically analyzed. The role of chromium in minimizing the hydrogen embrittlement (HE) of the intermetallic Fe₃Al (resulting in its poor ductility) has been addressed based on the analysis. In the case of the chromium-alloyed iron aluminide, calculations indicate that hydrogen liberation does not occur on the surface due to the shift of the corrosion mixed potential to a value nobler than the electrode potential for the hydrogen evolution reaction. This shift occurs due to the induction of passivity on alloying with Cr resulting in the formation of a passive film. The minimization of HE of iron aluminides on alloying with Cr can thus be understood.     

The abrasive corrosion behavior of plasma-nitrided alloy steels in chloride environments

Both corrosion and abrasive corrosion behavior of plama-nitrided type 304 and 410 stainless steels and 4140 low alloy steel were investigated in 3% NaCl solution (pH = 6.8) by electrochemical corrosion measurements. Surface morphology and alloying elements after corrosion and abrasion corrosion tests were examined by scanning electron microscopy and energy dispersive analysis of X-rays. The results indicated that the plasma-nitrided SAE 4140 steel containing -(Fe,Cr)_{2 – 3}N and -(Fe,Cr)₄N surface nitrides which produce a thick and dense protective layer exhibited a significant decrease of corrosion currents by inhibition of the anodic dissolution of iron, whereas the plasma-nitrided type 304 and 410 stainless steels containing the segregation of chromium nitride CrN exhibited a extensive pitting corrosion by acceleration of the anodic dissolution of iron. It is concluded that the susceptibility to pitting is consistent with the degree of chromium segregation, and decreases as follows: 304 stainless steel > 410 stainless steel > 4140 steel. Also, the results of abrasive corrosion testing for the plasma-nitrided alloys are strongly related to the subtleties of the nitrided microstructures resulting in a pitting and spalling type of abrasive corrosion of type 304 and 410 stainless steels, and excellent abrasive corrosion resistance for SAE 4140 steel.     

Compatibility of ferritic and duplex stainless steels as implant materials: in vitro corrosion performance

The pitting corrosion, crevice corrosion and accelerated leaching of iron, chromium and nickel of super-ferritic and duplex stainless steels, and for effective comparison the presently used 316L stainless steel, have been studied in an artificial physiological solution (Hank's solution) by the potentiodynamic anodic polarization method. The results of the above studies have shown the new super-ferritic stainless steel to be immune to pitting and crevice corrosion attack. The pitting and crevice corrosion resistances of duplex stainless steel were found to be superior to those of the commonly used type 316L stainless steel implant materials. The accelerated leaching study conducted for the above alloys showed very little tendency for the leaching of metal ions when compared with 316L stainless steel. Thus the present study indicated that super-ferritic and duplex stainless steels can be adopted as implant materials due to their higher pitting and crevice corrosion resistance.     

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.     

Austenitizing treatment influence on the electrochemical corrosion behavior of 0.3C-14Cr-3Mo martensitic stainless steel

Effects of austenitizing treatment temperatures on aqueous corrosion properties of martensitic stainless steels were investigated by electrochemical tests (potentiodynamic test, potentiostatic test and electrochemical impedance spectroscopy), and surface analyses (optical microscopy and XRD). The results of potentiodynamic test revealed that the breakdown potential increased with the increased austenitizing temperature, indicating increased relative resistance to initiation of localized corrosion. EIS measurements showed that MSS3 (1030 °C) exhibits larger polarization resistance value than MSS1 (970 °C) and MSS2 (1000 °C) at passive and breakdown states. This was caused by decreasing the amount of Cr-rich M₂₃C₆ carbide which acts as preferential sites for pitting corrosion.     

Effects of Cl- and SO2-4 Ions on Corrosion Behavior of X70 Steel

Corrosion behaviors of X70 steel were studied by means of electrochemical experiments and morphology observation.First, through potentiodynamic polarization in solution of various Cl- ions concentration, it was found that Epit began to appear in solution of Cl- concentration above 0.1 mol/L, and there was a critical point of Cl- concentration between 0.05 mol/L and 0.1 mol/L, below which the extent of pitting and general corrosion were trivial, while in solution of Cl- concentration above 0.1 mol/L, general and pitting corrosion became greater as the increasing of Cl- concentration. All of them were confirmed by the SEM observations after anodic polarization. Second, via the potentiodynamic polarization curves of X70 steel in 0.5 mol/L Cl- solution with 0, 0.05, 0.5 and 1 mol/L SO42-ions, it was found SO42- ions were able to inhibit corrosion aroused from Cl- ions, accordingly a model was set up to describe the process. In addition, to further explore the inhibited effect of SO42- ions, EIS was used in solutions of different Cl- and SO42- concentrations, the results revealed that the e(ectrochemical resistance has a relation with the [SO42-]/[Cl-], that was, the bigger the value of [SO42-]/[Cl-], the greater the electrochemical resistance.     

Influence of aging temperature on corrosion behavior of Al–Zn–Mg–Sc–Zr alloy

Al–Zn–Mg–Sc–Zr alloy sheets were prepared using water chilling copper mould ingot metallurgy processing which was protected by active flux. The effect of aging temperature on the corrosion characteristics of Al–Zn–Mg–Sc–Zr alloy was investigated by means of exfoliation corrosion testing, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) combined with transmission electron microscopy, scanning electron microscopy and optical microscopy observation. It is found that with increasing the aging temperature the susceptibility to exfoliation corrosion decreases. Electrochemical measurements reveal that at early stage of immersion in testing solution, EIS plots of the samples are composed of a capacitive arc and an inductive loop. Inductive loop disappears with the increasing of immersion time and two time constants in impedance diagrams appear. Moreover, the trends of corrosion resistance are further confirmed by polarization curve and EIS test. In addition, transmission electron microscopy observations show that the improved corrosion resistance from increasing aging temperature is duo to the coarsening of matrix and separated precipitates at the grain boundary, and the increased spacing of grain boundary precipitates.     

氯离子对球墨铸铁管土壤腐蚀影响机理研究

基于埋片加速腐蚀试验,采用电化学阻抗谱测量和微观分析技术,研究了氯离子(Cl~-)对球墨铸铁腐蚀规律、腐蚀层结构以及腐蚀产物的影响。主要结论如下:(1)球墨铸铁主要发生局部腐蚀,且随Cl~-含量的增加,腐蚀速率增大,出现严重腐蚀坑;其腐蚀坑深与腐蚀时间成幂函数关系,当Cl~-含量高于0.515%时,局部腐蚀情况尤为严重。(2)球墨铸铁腐蚀经历点蚀诱导期、点蚀发展期和稳定腐蚀期3个阶段;在前两个阶段电荷传递为腐蚀速率控制步骤,第三个阶段转变为氧的扩散控制。(3)腐蚀产物主要为铁的氧化物和羟基氧化物;高Cl~-环境下产生的β-Fe OOH和铁的羟基氯化物均能加速球墨铸铁的电化学腐蚀。     

Effect of microstructure on corrosion behavior of TiN hard coatings produced by a modified two-grid attachment magnetron sputtering process

The introduction of an electrically biased two-grid attachment inside a conventional physical vapor deposition process system produces a reactive coating deposition and increases the metal ion-to-neutral ratio in the plasma, resulting in denser and smoother films.The corrosion behavior of titanium nitride (TiN) coatings was investigated by electrochemical methods, such as potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) in deaerated 3.5% NaCl solution. Electrochemical tests were used to evaluate the effect of microstructure on the corrosion behavior of TiN coatings exposed to a corrosive environment. The crystal structure of the coatings was examined by X-ray diffractometry and the microstructure of the coatings was investigated by scanning electron microscopy.In the potentiodynamic polarization test and EIS measurement, the corrosion current density of TiN deposited by the modified two-grid attachment magnetron sputtering process was lower than for TiN deposited by conventional magnetron type and also presented higher charge-transfer resistance values during 240 h total immersion time. The modified process promotes the grain refinement, which yields lower porosity.     

Corrosion behaviour of CrN/Cr multilayers on stainless steel deposited by unbalanced magnetron sputtering

This paper reports the results of the influence of bilayer period (Λ) and total thickness (f) on the corrosion resistance of magnetron-sputtered CrN/Cr multilayers. Corrosion tests were carried out by potentiodynamic polarization with 0.5 M H₂SO₄ + 0.05 M KSCN solution and electrochemical impedance spectroscopy (EIS) with 3% NaCl solution. Measurements were also taken on the uncoated substrate and hard chromium coatings for comparison. Multilayer microstructure and morphology were studied by X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) and chemical composition was studied by energy dispersive X-ray analysis (EDX).The experiments showed that CrN/Cr coatings having lower bilayer period and lower thickness increased their efficiency as a barrier and improved the corrosion resistance of all coatings evaluated.     

Lochkorrosion an passiven Legierungssystemen der Elemente Eisen,Chrom und Nickel. Teil 2: Prüfverfahren zur Ermittlung der Lochkorrosionsbeständigkeit - Chemische Prüfungen - elektrochemische Untersuchungen

Pitting corrosion on alloy systems of the elements iron, chromium and nickel. Part 2: Test methods to determine resistance to pitting corrosion - chemical tests - electrochemical investigations The paper offers a survey on those publications concerning the examination of pitting corrosion on passive alloying systems. The different kinds of test methods - chemical, electrochemical or special procedures - are clearly described. A. detailed discussion about the limits of the diverse methods - especially their practical interpretation - finishes the second part.     

Laser surface hardening of AISI 420 stainless steel treated by pulsed Nd:YAG laser

The AISI 420 martensitic stainless steel was surface-hardened by a pulsed Nd:YAG laser. The influences of process parameters (laser pulse energy, duration time and travel speed) on the depth and hardness of laser treated area and its corrosion behavior were Investigated. In the optimum process parameters, maximum hardness (490 VHN) in the laser surface treated area was achieved. The pitting corrosion behavior was studied by potentiodynamic polarization technique in 3.5% NaCl solution at 25 °C. Metallographical and electrochemical corrosion studies illustrated beneficial effects of laser surface hardening by refining the microstructure and enhancing the pitting corrosion resistance of the martensitic stainless steel. The pitting corrosion resistance of laser surface treated samples in 3.5% NaCl solution depends on the overlap ratio clearly. The pitting potential (E_pp) decreased significantly by increasing the ratio of pulse overlapping.     

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.