Passivity and Pitting Susceptibility of type 304 Stainless Steel in Acidic Sulphate Solutions

Passivation and its breakdown reactions of type 304 stainless steel have been studied in acidic sodium sulfate solutions using different electrochemical techniques. Results from potential and impedance measurements under open circuit conditions confirmed the formation of a bilayer as a primary passive film. The results from cyclic polarization curves showed that E_pit−E_prot increases with increasing Cl⁻ concentration indicating failure of the steel sample due to pitting. Impedance spectra recorded at different passivation potentials showed a maximum at 0.4 V due to film thickening. Above 0.4 V film breakdown occurs.     

Inhibition of chloride pitting corrosion of Zircaloy-4 alloy in highly radioactive water by radiolytic nitrate and hydrogen peroxide

We report an investigation of the pitting corrosion susceptibility of Zircaloy-4 alloy in presence of radiolytic chloride, nitrate and hydrogen peroxide. The electrochemical behavior of Zircaloy-4 was essentially studied using cyclic voltammetry and electrochemical impedance spectroscopy to provide an indication of mechanisms and oxide layer modifications. The experiments have shown that the pitting corrosion behavior is dependent on the concentration of these radiolytic species in tritiated water. Nitrate shows pronounced inhibitory action due to adsorption of the ammoniac formed on the passive oxide layer buffering the pH, which stops pit initiation and assists repassivation of the oxide surface. The presence of both hydrogen peroxide and NO_{3 –} produces other effects. The passive oxide layer is thicker and its characteristics change in the bulk. Also, pit initiation is stopped whereas it is more difficult to obtain repassivation of existing pits. This can be explained by the capability of ³H₂O₂ to oxidize the surface and thus enhance passive oxide formation. But, in this case, NO_{3 –}/N³H⁺ ₄ adsorption should be hindered by the ³H₂O₂ present and consequently there is less buffering of the surface pH limiting repassivation of the existing pits. However, with these two combined effects: pH kept constant on the Zircaloy-4 surface and enhancement of the oxide layer intrinsic characteristics, less pitting is observed than in presence of chloride ions alone.     

Long-term assessment of the implant titanium material—artificial saliva interface

This paper studies the long-term (20,000 exposure hours) behavior of titanium and Ti–5Al–4V alloy—Carter–Brugirard saliva interface and the short-term (500 exposure hours) resistance of titanium and Ti–5Al–4V alloy—Tani&Zucchi saliva interface. Potentiodynamic polarization method was applied for the determination of the main electrochemical parameters. Linear polarization measurements for to obtain the corrosion rates were used. Monitoring of the open circuit potentials (E _oc) for long-term have permitted to calculate the potential gradients due to the pH, ΔE _oc(pH) and to the saliva composition ΔE _oc(c) changes which can appear “in vivo” conditions and can generate local corrosion. Atomic force microscopy (AFM) has analyzed the surface roughness. Ion release was studied by atomic absorption spectroscopy (AAS). In Carter–Brugirard saliva both titanium and Ti–5Al–4V alloy present very stable passive films, long-term stability, “very good” resistance, low values of the open circuit potential gradients, which cannot generate local corrosion. In Tani&Zucchi artificial saliva, pitting corrosion and noble pitting protection potentials (which cannot be reached in oral cavity) were registered; titanium ion release is very low; surface roughness increase in time and in the presence of the fluoride ions, denoting some increase in the anodic activity.     

Behaviour of 316 Ti stainless steel in deuterium oxide with chloride

Since tritiated water contains deuterium oxide, we require a better understanding of stainless steel corrosion in tritiated water and thus we have compared the behaviour of 316 Ti stainless steel in ²H₂O and H₂O with and without chloride. This was done by anodic polarization curves, cyclic voltammetry and electrochemical impedance spectroscopy. The corrosion potential of 316 Ti stainless steel in deuterium oxide changes and is related to pH modification due to the dissociation constant of this aqueous medium which shows the importance of pH in passivity. Without chloride, the insulating properties of the passive oxide layer depending on the pH and passive potentials are enhanced with ²H₂O. With deuterium oxide containing chloride at near neutral pH, the repassive potential is lower than that obtained with H₂O, consequently localized corrosion in grain boundaries and pit propagation, which lead to crevice corrosion, are greater. The critical pitting potential is in transpassivity indicating that pitting is less likely to occur. Comparison with and without Cl^- for the passive potentials near the corrosion potential, shows that although chloride reduces the insulation provided by the passive oxide layer it is still greater than that obtained with H₂O. This revised version was published online in November 2006 with corrections to the Cover Date.     

Corrosion behavior of SiC p /6061 Al metal matrix composites in chloride solutions

The corrosion behavior of silicon carbide particulates-aluminum metal matrix composites was studied in chloride solution by means of electrochemical techniques, scanning electron microscope (SEM), transmission electron microscope (TEM) and optical microscope. The materials under investigation were compocasting processed 6061 Al reinforced with increasing amounts of SiC particulates. Electrochemical tests such as potentiostatic polarization were done in 0.1 kmol·m^–3 NaCl solutions that were aerated and deaerated to observe overall corrosion behavior. In addition, pit morphology was observed after immersion tests. It was seen that the pitting potentials did not vary greatly or show definite trends in relation to the amounts of SiC _p reinforcement. However, the degree of corrosion increased with increasing SiC _p content; probably mainly due to galvanic couple. No intermetallics layer was found at the SiC _p /Al interface. Based on pitting potentials of Al-Si alloys, a pitting process around SiC particulate was proposed.Abbreviations SiC _p (silicon carbide particulates) - SiC _f (silicon carbide fibers) - SiC _w (silicon carbide whiskers) - E_pit (pitting potential) - E_prot (protection potential) - E _corr (corrosion potential) - i _galv (galvanic current density) - E _galv (galvanic potential)     

Extremely high pitting resistance of NiTi shape memory alloy thin film in simulated body fluids

In this paper the corrosion behavior of NiTi thin films fabricated by sputtering from Ni and Ti targets has been studied by cyclic potentiodynamic polarization tests in Hank's and Ringer's solution at 310 K. For comparison, bulk NiTi Shape Memory Alloy (SMA) has also been studied to elucidate the different corrosion behavior of bulk and thin film material. The electrochemical experiments reveal that thin film NiTi SMA has comparable corrosion current density (i_corr), much higher pitting corrosion potentials and wider passive range than the bulk NiTi. We show that NiTi SMA vapour deposited thin films are less susceptible to pitting corrosion than the bulk.     

Electroplating of zirconium and aluminum hydroxide thin films following anodic dissolution of corresponding metal anodes in organic medium

Zirconium (IV) hydroxide or hydrate oxide films, which are typically difficult to prepare by electrochemical methods using aqueous solutions, are easily fabricated in an acetone bath using Zr anodes as the metal sources and a metal-free solvent containing halide ions as the supporting electrolyte. This method is also confirmed to be applicable to aluminum anodes. In the early stage of electrolysis, anodic oxidation of the metal anode proceeds in the presence of water as an impurity in the solvent. Subsequently, pitting corrosion of the oxide film on the metal anode occurs as a result of the action of halide ions. The corrosiveness of the halogen additive appears to be an important factor determining the dissolution or deposition of metal species in this stage. That is, Br^– is more active for electrochemical dissolution of a passive oxide film on the anode compared to I^–. Finally, Zr species are deposited on the cathode surface via reactions with cathodically generated hydroxide ions. In these processes, the metal plate acts as a soluble anode and as a metal source for electrodeposition. The coating of Zr (IV) hydroxide film on a stainless steel substrate is shown to act as an effective barrier against electrolytic corrosion.     

Effects of sulfidation of passive film in the presence of SRB on the pitting corrosion behaviors of stainless steels

Pitting corrosion of stainless steels is a very complex process in the media with sulfate-reducing bacteria (SRB). Bacterial activities not only instantaneously affect the pit-initiation and the pit-growth, but also induce the changes of passive film in structure/properties and in turn affect the pitting corrosion behaviors of stainless steels as well. In this work, sulfidation of passive film in the cultures of SRB and its effects on the pitting corrosion behaviors of stainless steels were investigated by electrochemical techniques and X-ray photoelectron spectroscopy (XPS). As results showed, anodic cyclic polarization curves demonstrated that sulfidation caused a loss of passivity of stainless steels, but sulfidation happening in the short term is more detrimental than that in the long term; from standing point of cathodic depolarization of corrosion, sulfidation polarized the cathodic reactions to a greater extent, which means that sulfidation helps impede the pit-growth; XPS revealed that almost only iron oxides in the passive film were converted into sulfides in forms of FeS and FeS₂, while elements of Cr and Ni still remained essentially as oxides/hydroxides. So, in understanding the mechanisms of pitting corrosion of stainless steels in the media with SRB, effects of sulfidation of passive film must be taken into consideration.     

Effect of Cl<Superscript>−</Superscript>on the corrosive wear of AISI 321 stainless steel in H<Subscript>2</Subscript>SO<Subscript>4</Subscript> solution

The effect of Cl⁻ on the corrosive wear behaviour of AISI 321 stainless steel in H₂SO₄ solution was studied via the corrosive wear rate, the load bearing capacity of passive film and the relationship between pitting and corrosive wear. There is a critical load at natural potential, below which the corrosive wear rate is slightly lowered by Cl⁻, while above which is increased. At natural potential there are more pits at low load than that at a higher one in the wear tracks and the pits are also deeper. The load bearing capacity is lowered by Cl⁻ at passive region and then the corrosive wear rate increased.     

Pitting initiation of 316L stainless steel in the media of sulfate-reducing and iron-oxidizing bacteria

Pitting corrosion behavior of stainless steel 316L in the presence of aerobic and anaerobic bacteria isolated from cooling water system in oil refinery was investigated using open circuit potential measurement, electrochemically impedance spectroscopy, scanning electron microscopy examinations, and energy dispersive spectrum analysis. The results show the corrosion potential (E _cor) and polarization resistance (R _p) decrease in the presence of sulfate-reducing bacteria (SRB), iron-oxidizing bacteria (IOB), and a combination of SRB and IOB, in comparison with those observed in the sterile medium for the same exposure time. The presence of SRB demonstrated higher corrosion rates than IOB. The combination of SRB and IOB created the highest corrosion rate. The metabolic activity of bacteria and the integrality and compactness of biofilm influenced the pitting corrosion process, increased the corrosion damage degree of the passive film, and accelerated the pitting corrosion. It is suggested that SRB and IOB in influencing the pitting corrosion of 316L SS is highlighted. The text was submitted by the authors in English.     

交变电场对点蚀破坏不锈钢电极的修复研究

用电化学方法研究了不同参数设置下的交变电场对点蚀破坏18－8SS电极的修复效果，讨论了各参数的敏感取值范围，并通过正交试验寻找了18－8SS在1．0mol/L H2SO4溶液中的最佳修复电场条件，即EH＝1000mV,EL=-200mV,R=1,F=10Hz。在此基础上，提出了该技术的应用条件，即适用于在修复介质中出现明显钝化区的体系。     

Corrosion properties of plasma-sprayed Al2O3-TiO2 coatings on Ti metal

Three different oxides of CrO₂-TiO₂, Al₂O₃ and Al₂O₃-TiO₂ were plasma-sprayed on Ti substrate to evaluate the crystal structure and the corrosion properties of the coatings. No phase change of the coatings after corrosion test in 0.5 M H₂SO₄ solution at 25°C was found regardless of the presence of the NiCoCrAlY bond layer. Electrochemical measurements and SEM results revealed that the single coatings without the bond layer were always effective against corrosion resistance due to lower current density within the passive region. Pitting corrosion of the surface was observed for the Al₂O₃ coating. It can be concluded that the Al₂O₃-TiO₂ coating without the bond layer may be the best oxide among the oxides investigated due to low porosity (5.4%), smooth surface roughness (4.5 μm), low current density (6.3 × 10^‒8 A/cm²) in the passive region, low corrosion potential (E_corr, ‒0.55 V) and no pitting corrosion.     

Chemical synthesis and characterization of hydrous tin oxide (SnO<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>:H<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>O) thin films

In the present investigation, we report chemical synthesis of hydrous tin oxide (SnO ₂ :H ₂ O) thin films by successive ionic layer adsorption and reaction (SILAR) method at room temperature ( \thicksim \thicksim 300 K). The films are characterized for their structural and surface morphological properties. The formation of nanocrystalline SnO ₂ with porous and agglomerated particle morphology is revealed from X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies, respectively. The Fourier transform infrared spectroscopy (FTIR) study confirmed the formation of Sn–O phase and hydrous nature of the deposited film. Static water contact angle studies showed the hydrophilic nature of SnO ₂ :H ₂ O thin film. Electrical resistivity showed the semiconducting behaviour with room temperature electrical resistivity of 10 ⁵  W\boldsymbol\Omega cm. The electrochemical properties studied in 0·5 M Na ₂ SO ₄ electrolyte showed a specific capacitance of 25 F g ^− 1 at 5 mVs ^− 1 scan rate.     

Chloride induced pitting corrosion of nickel in alkaline solutions and its inhibition by organic amines

Cyclic voltammetry, potentiodynamic and potentiostatic anodic polarization techniques complemented by SEM investigation are used to study the initiation and inhibition of pitting corrosion of nickel in NaOH solutions. The presence of Cl⁻ ions locally destroys the passive film formed on the nickel electrode at the pitting potentials, E_p, which are more active the higher the concentration of the Cl⁻ ions. E_p varies with the logarithm of Cl⁻ ions concentration according to sigmoidal S-shaped curves.     

Numerical investigation on stress concentration of corrosion pit

Localized (pitting) corrosion has been observed in steel and high-strength aluminium alloys in aqueous environments and has been identified as a potential origin for fatigue crack nucleation. In the present study, under uniaxial tension loading, stress distribution at the semi-elliptical corrosion pits has been investigated by conducting a series of three-dimensional semi-elliptical pitted models, systematically. Based on the finite element analyses, it is concluded that pit aspect ratio (a/2c) is a main parameter affecting stress concentration factor (SCF). An attempt has been made to construct simple equations to SCF depending on characteristic of pit parameters. At the bottom of hemispherical pit, contribution of secondary (premature) pit formation to SCF is very pronounced.     

Pitting susceptibility of UNS NO. 8904 stainless steel in bromide-sulphide solution

The effect of sulphide ion on the pitting corrosion behaviour of UNS* no. 8904 (904L) stainless steel (SS) in 0.6 M NaBr at 25 °C has been investigated by using potentiodynamic anodic polarization and electrochemical impedance spectroscopy techniques. The pitting potential, E _pit, in 0.6 M NaBr + 10^–2 M Na₂S was more negative than that obtained in 0.6 M NaBr, and decreased with increasing temperature in the range 25–60 °C. Scanning electron microscopy observation showed that elemental sulphur formed on the steel surface before E _pit was reached in 0.6 M NaBr + 10^–2 M Na₂S at 60°C. E _pit of 904L SS pH independent in the pH range from 3–10 of 0.6 M NaBr + 10^–2 M Na₂S at 25 °C, while at high pH values the pitting was suppressed. The impedance measurements showed that the charge transfer resistance, R _t, decreased with time, when the controlled potential became higher than E _pit.     

Corrosion behavior of surface-modified titanium in a simulated body fluid

We investigate the influence of micro-sandblasting and electrochemical passivation on properties such as corrosion rate and surface roughness, which are important to the biocompatibility of titanium (Ti), using surface analysis techniques and electrochemical measurements. Results of microscopy and surface profilometry experiments reveal roughened but uniform surface topography with an average surface roughness in the 0.87–1.06 μm range, depending on the alternating current passivation voltage applied to the micro-sandblasted samples. Open circuit potential versus time measurements in Hank’s Balanced Salt Solution (HBSS, a simulated body fluid) allow determination of the corrosion potential (E _corr) and reveal a shift of E _corr toward higher values upon passivation, thus pointing to increased corrosion stability. Corrosion rates in HBSS range between 0.049 and 0.288 μm year⁻¹ for micro-sandblasted and passivated Ti, as compared to that for the micro-sandblasted and non-passivated surface that is 0.785 μm year⁻¹. Results from this study demonstrate that micro-sandblasting coupled with electrochemical passivation provides a roughened surface with increased corrosion stability and a low corrosion rate in HBSS. Application of this technique to Ti in medical and dental applications may be expected to result in an improvement of biocompatibility.     

Anticorrosion behavior of ultrafine-grained Al-26?wt% Si alloy fabricated by ECAP

Ultrafine-grained (UFG) Al-26 wt% Si alloy was obtained through multipass equal-channel angular pressing (EACP) procedure and subsequently tested in 3.5 wt% NaCl solution for the evaluation of electrochemical corrosion. The results show that the ECAPed alloy with increased number of pressing passes obtain lower mass-loss ratios, nobler E _corr and E _pit, lower I _corr values, and higher anode polarization. The improved corrosion resistance of the ECAPed alloy results from the homogeneous UFG structure with the breakage of brittle large primary silicon crystals, which contributes to a higher pitting resistance. The oxidation product with improved adhesion force and protection efficacy can be formed with greater ease on UFG alloys. It implies that grain refinement through severe-plastic-deformation can enhance anticorrosion behavior of hypereutectic Al–Si alloys, besides the well-known strengthening and toughening effects.     

Corrosion and stress corrosion cracking behavior of 316L austenitic stainless steel in high H2S–CO2–Cl− environment

In oil and gas production environments, H₂S and Cl^? can coordinate to cause pitting or stress corrosion cracking (SCC) of stainless steels. There has been limited work conducted on corrosion and SCC of autenitic stainless steels in high H₂S–CO₂–Cl^? environments. In this paper, by four-point bending test method and scanning electron microscopy analysis, SCC of 316L steel was investigated under high H₂S–CO₂ pressures with 150,000 ppm Cl^? at 60 °C. The effect of high H₂S–CO₂ pressure was discussed. The results indicated that the higher H₂S–CO₂ pressure can accelerate anodic dissolution process, deteriorate passive films, and aggravate SCC sensitivity. Using cyclic potentiodynamic polarization measurements, the corrosion behavior of 316L steel was studied in high H₂S–CO₂–Cl^? environments. The effect of pH on pitting corrosion was discussed. Lower pH can promote both cathodic and anodic actions on 316L steel and facilitate passive film breakdown.     

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.