Spontaneous Passivation Research Articles

Electrochemical micromachining of passive electrodes – Application to bulk metallic glasses

Electrochemical micromachining (ECMM) is a method for micro-forming of metallic materials independent of their hardness and without inducing significant heat into the work piece. Thus, it appears to be well suited for micro-shaping of bulk metallic glasses (BMGs). On the example of a Fe-based BMG, Fe65.5Cr4Mo4Ga4P12C5B5.5, the capability of ECMM exploiting the transpassive dissolution regime is discussed. The glassy alloy exhibits spontaneous passivity in aqueous acidic electrolytes. Due to its low Cr content the electrochemical behaviour during the ECMM is not comparable to that of stainless steel and therefore, established experimental conditions are not applicable. The electronic behaviour of the ECMM setup is discussed in relation with the electrochemical reactions during the machining. Resultant, different ways of machining the glassy Fe65.5Cr4Mo4Ga4P12C5B5.5 alloy in an aqueous acidic electrolyte are derived and ways to enhance the electric efficiency are discussed: the addition of an oxidant to the machining electrolyte and the application of a Schottky diode in the electronic setup.

Corrosion behavior of new ternary zirconium alloys as alternative materials for biomedical applications

The in vitro corrosion behavior of four Zr‐based alloys (Zr97.5Nb1.5VM1.0; VM = valve metal: Ti, Mo, W, Ta; at%) to be used as implant materials has been assessed and compared with that of Grade 2 Ti, which is known to be a highly compatible metal standard. Corrosion resistance was investigated by open circuit potential and electrochemical impedance spectroscopy (EIS) measurements, as a function of exposure time to an artificial physiological environment (Ringer's solution). Open circuit potential values indicated that Zr‐based alloys and Grade 2 Ti undergo spontaneous passivation due to spontaneously formed oxide film passivating the metallic surface, in the simulated aggressive environment. It also indicated that the tendency for the formation of a spontaneous oxide is greater for the Zr97.5Nb1.5Ta1.0 alloy and that this oxide has better corrosion protection characteristics than the ones formed on Grade 2 Ti or on Zr97.5Nb1.5Ti1.0, Zr97.5Nb1.5Mo1.0, and Zr97.5Nb1.5W1.0 alloys. EIS study showed high impedance values for all samples, increasing with exposure time, indicating an improvement in corrosion resistance of the spontaneous oxide film. The fit obtained suggests a single passive film present on the metals surface, improving their resistance with exposure time, presenting the highest values to the Zr97.5Nb1.5Ta1.0 alloy. All these electrochemical results suggest that Zr97.5Nb1.5VM1.0 alloys are promising materials for osteo‐synthesis prosthetic devices, since corrosion stability is directly associated with biocompatibility and is a necessary condition for applying a material as biomaterial.

Corrosion Resistance of Ti3Al/BN Abradable Seal Coating

Ti–Al mixed powder (Ti:Al = 3:1 in atomic ratio) and Ti3Al intermetallic alloy powder mechanically clad hexagonal BN to fabricate TiAl/BN and Ti3Al/BN composite powders. The corresponding porous abradable seal coatings (named as TAC-1 and TAC-2, respectively) were deposited using vacuum plasma spray (VPS) technology, and their corrosion behavior was studied via salt spray corrosion and electrochemical tests. Phase compositions and microstructures of these coatings before and after corrosion were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) facilitated with energy dispersive X-ray spectrometer (EDS). The results showed that spontaneous passivation of TAC-1 and TAC-2 granted the coatings excellent corrosion resistance than that of commercial Al/BN coating. Additionally, TAC-2 exhibited higher corrosion potential (E corr) and breakdown potential (E bp) but a lower corrosion current density (i corr) than TAC-1. A small quantity of the corrosion product (Al(OH)3 and AlO) could be detected on the surface of TAC-1, while no corrosion product appeared in TAC-2. The non-uniform elements distribution in the metal matrix of TAC-1 resulted in localized corrosion and relatively poor corrosion resistance compared to TAC-2.

Microstructure, mechanical and anti-corrosion property evaluation of iron-based thin film metallic glasses

Thin film metallic glasses (TFMGs) represent a class of promising engineering materials for structural applications. Despite the effort that has been made in the development of TFMG materials, the iron-based thin film metallic glasses fabricated by sputtering have gained limited attention. In this work, five iron-based Fe–Zr–Ti thin film metallic glasses with different Fe contents ranging from 37.6 to 49.8at.% were prepared by magnetron co-sputtering system using pure Fe, Zr and Ti targets. Through XRD and TEM analyses, the amorphous phase was confirmed for each coating. The glass transition temperature (Tg) and crystallization temperature (Tx) of TFMG increased with increasing Fe content and reached 963K and 989K, respectively, when Fe content was 49.8at.%. The supercooled liquid region was around 26.3 to 51.6°C, which was shown to be unrelated to Fe concentration. The hardness, elastic modulus, and H/E ratio of TFMGs increased with increasing Fe concentration. Based on the HRC-DB test, adequate adhesion quality was obtained for all TFMGs. The corrosion resistance of TFMGs also increased with increasing Fe content and spontaneous passivation behavior was discovered due to the large content of Zr and Ti valve metals. Nevertheless, the corrosion resistance of Fe–Zr–Ti TFMGs was strongly influenced by surface defects. A series of high hardness Fe–Zr–Ti thin film metallic glasses with good adhesion property and adequate corrosion resistance was reported in this study.

The effect of fluoride ions on the corrosion behavior of pure titanium in 0.05 M sulfuric acid

The corrosion behavior of pure titanium was investigated in 0.05M H2SO4 solution with fluoride ions using various electrochemical techniques, such as open circuit potential (OCP), potentiodynamic polarization, electrochemical impedance measurements (EIS) and surface analyses via X-ray photoelectron spectroscopy (XPS). The results showed that titanium exhibited spontaneous passivation behavior in solutions with 0M and 0.0005M fluoride ions, compared with the active-passive behavior with 0.002M∼0.005M. A critical value of the fluoride concentration was defined between 0.0005M and 0.002M. Multiple corrosion potentials were found under the conditions with fluoride ions higher than 0.002M. The fluoride ion accelerated the corrosion of titanium via changing the structure of the film formed at OCP and destroying the protectiveness of the film, while it had few effects on cathodic hydrogen evolution reaction and passivation process at OCP. A corrosion mechanism associated with the fluoride ion was proposed. The corrosion behavior was well interpreted using the proposed mechanism and a criteria for the critical value of the fluoride concentration was also proposed.

Passive behavior of a bulk nanostructured 316L austenitic stainless steel consisting of nanometer-sized grains with embedded nano-twin bundles

The passive behavior of a bulk nanostructured 316L austenitic stainless steel consisting of nanometer-sized grains (NG) and nano-twin bundles (NT) are investigated. The electrochemical results indicate that the spontaneous passivation ability and growth rate of passive film are improved. The X-ray photoelectron spectroscopy (XPS) shows that a Cr0-enriched layer forms at the passive film/metal interface. More nucleation sites afforded by the nanostructures and the enhanced diffusion rate of charged species across the passive film are believed to be responsible for the improved passive ability. The PDM model is introduced to elaborate the microscopic process of passivation.

Electrochemical study of the corrosion of different alloys exposed to deaerated 80 °C geothermal brines containing CO2

Corrosion of metallic engineering materials accounts for problems during geothermal operation in the Upper Rhine Graben (URG). Herein, we study the electrochemical behaviour of various metal alloys in an 80°C simulated geothermal environment by using potentiodynamic polarisation and open-circuit potential measurements. Two different natural geothermal waters from URG geothermal sites were used for the experiments. The measurements reveal spontaneous passivation to be a key process for all alloys. This ennoblement protects more noble alloys from significant corrosion (e.g. titanium gr. 2, alloy 625) and brings less noble alloys to failure, mostly due to pitting corrosion (e.g. 316L).

A Comparative Evaluation between New Ternary Zirconium Alloys as Alternative Metals for Orthopedic and Dental Prosthetic Devices

We assessed in vitro the corrosion behavior and biocompatibility of four Zr-based alloys (Zr97.5 Nb1.5VM1.0 ; VM, valve metal: Ti, Mo, W, Ta; at%) to be used as implant materials, comparing the results with grade-2 titanium, a biocompatible metal standard. Corrosion resistance was investigated by open circuit potential and electrochemical impedance spectroscopy measurements as a function of exposure time to an artificial physiological environment (Ringer's solution). Human bone marrow stromal cells were used to evaluate biocompatibility of the alloys and their influence on growth kinetics and cell osteogenic differentiation through histochemical and gene expression analyses. Open circuit potential values indicated that Zr-based alloys and grade-2 Ti undergo spontaneous passivation in the simulated aggressive environment. High impedance values for all samples demonstrated improved corrosion resistance of the oxide film, with the best protection characteristics displayed by Zr97.5 Nb1.5Ta1.0. Cells seeded on all surfaces showed the same growth kinetics, although matrix mineralization and alkaline phosphatase activity were maximal on Zr97.5 Nb1.5Mo1.0 and Zr97.5 Nb1.5Ta1.0. Markers of ongoing proliferation, however, such as podocalyxin and CD49f, were still overexpressed on Zr97.5 Nb1.5 Mo1.0 even upon osteoinduction. No relevant effects were noted for the CD146-expressing population of bone progenitors. Nonetheless, the presence of a more differentiated cell population on Zr97.5Nb1.5Ta1.0 samples was inferable by comparing mineralization data and transcript levels of osteogenic markers (osteocalcin, osteopontin, bone sialoprotein, and RUNX2). The combination of passivation, corrosion resistance and satisfactory biotolerance to bone progenitors make the Zr-based alloys promising implant materials. Among those we tested, Zr97.5Nb1.5Ta1.0 seems to be the most appealing.

Activation of Titanium for Synthesis of Supported and Unsupported Metallic Nanoparticles

Titanium (Ti) resists corrosion due to spontaneous passivation and subsequent formation of a stable, substantially inert oxide film. This passive film limits the reducing ability of Ti in wet chemical synthesis. Aggressive anions are applied here to destabilize Ti passivity, with the objective of activating Ti as a reducing agent for wet-chemical synthesis, through the formation of unsupported and supported metallic nanoparticles (MNPs). For instance, within the first minute of Ti immersion in a standard gold solution (SGS), 1000 ± 3 μg Au/mL in 2% HCl, gold nanoparticles (AuNPs) in solution (unsupported) and a considerable population of highly dispersed AuNPs on Ti (Ti-supported AuNPs) are obtained. This occurs at room temperature, without using reducing agents, stabilizers, or any chemical pre-treatments. Electrochemical measurements revealed that the passivity of Ti was destabilized (oxide thinning/dissolution) in the SGS. The addition of F− to the SGS promoted destabilization of the passive film, and hence activated the reducing ability of Ti. This in turn resulted in significant increase in the population of AuNPs on Ti. Reduction of cations in solution by the active nascent (atomic) hydrogen (H) generated by substrate (Ti) dissolution, promoted by the addition of F−, in acidic medium is confirmed here and discussed vs. conductivity of the oxide (passive) film and solution pH. Experimental findings reveal that the relevance of H as the actual reducing agent is limited to instantaneous and non-conductive passive films. Solution pH studies on such passive films show that the Nernst equation (dEH/H+/d(pH) = −0.059 V) controls the wet chemical synthesis of MNPs rather than Eoreduction of the base metal (the substrate). Based on polarization measurements, the prepared Ti-supported AuNPs are demonstrated here as highly active electrocatalysts (better than Pt) for the hydrogen evolution reaction (HER) in 0.1 M HCl solution.

Corrosion resistance of Fe-based amorphous metallic matrix coating fabricated by HVOF thermal spraying

The Fe-based amorphous metallic matrix coating (Fe-AMMC) was fabricated with the powder mixtures of Fe-based metallic glass synthesized with industrial raw materials, NiCr alloy and WC particle by high velocity oxy-fuel (HVOF) spraying. The corrosion resistance of Fe-AMMC was investigated by potentiodynamic polarization tests in 1 mol/L HCl, NaCl, H2SO4 and NaOH solutions, respectively. The surface morphologies corroded were observed by SEM. The results indicate that Fe-AMMC exhibits excellent corrosion resistance, higher corrosion resistance than 304L stainless steel in the chloride solutions. The low corrosion current density and passive current density of Fe-AMMC with a wide spontaneous passivation region are about 132.0 μA/cm2 and 9.0 mA/cm2 in HCl solution, and about 2.5 μA/cm2 and 2.3 mA/cm2 in NaCl solution. The excellent corrosion resistance demonstrates that Fe-based amorphous metallic matrix powder is a viable engineering material in practical anti-corrosion and anti-wear coating applications.

Electrochemical corrosion characteristics of type 316L stainless steel in hot concentrated seawater

The corrosion behaviour of 316L stainless steel was studied in the concentrated artificial seawater at 72°C, i.e., the simulated low temperature-multi effect distillation environments, by using electrochemical measurement techniques. The corrosion state changes from spontaneous passivation to pitting after about 1150h of immersion. Pitting corrosion is under mixed control of charge transfer and diffusion processes in the long-term immersion. The salt deposits retard the diffusion of oxygen to the metal surface. The pit depth only reaches about 38μm after one year of immersion due to the alloying effect of Mo, low dissolved oxygen levels and weak occlusion states.

Effect of platinum group metal addition on microstructure and corrosion behaviour of Ti–47·5 at-%Al

Plain and alloyed titanium aluminides of composition Ti–47·5 at-%Al were prepared with the addition of 1·0 at-% platinum group metals (PGMs). The as cast alloys were subjected to potentiodynamic scans in 5, 15 and 25 wt-%HCl solutions at room temperature, and the PGM containing alloys were assessed for their abilities to spontaneously passivate by cathodic modification. Plain titanium aluminide had a duplex microstructure consisting of lamellar (α2 and γ alternating lamellae) and γ-TiAl phase grains. The introduction of 1·0 at-%PGMs (platinum, palladium and iridium) led to the formation of a new phase, developing more in the γ-TiAl phase grains and a general improvement of corrosion resistance by increasing the corrosion potential to nobler values. Platinum group metal additions to plain TiAl resulted in the corrosion potentials falling in the passive region of plain TiAl, indicating spontaneous passivation of PGM alloyed TiAl in 5 and 15 wt-%HCl solutions. In 25 wt-%HCl solution, the addition of PGMs shifted the cathodic process in the transpassive or active region of plain TiAl, resulting in either case in the dissolution of the alloy due to the absence of an extended passivation region. The cathodic modification of PGM alloyed TiAl occurred as a result of PGM accumulation on the surface of the TiAl alloys, which simultaneously improved the hydrogen evolution efficiency and inhibited anodic dissolution.

In vitro corrosion behaviour of Ti–Nb–Sn shape memory alloys in Ringer’s physiological solution

The nearly equiatomic Ni-Ti alloy (Nitinol) has been widely employed in the medical and dental fields owing to its shape memory or superelastic properties. The main concern about the use of this alloy derives form the fact that it contains a large amount of nickel (55% by mass), which is suspected responsible for allergic, toxic and carcinogenic reactions. In this work, the in vitro corrosion behavior of two Ti-Nb-Sn shape memory alloys, Ti-16Nb-5Sn and Ti-18Nb-4Sn (mass%) has been investigated and compared with that of Nitinol. The in vitro corrosion resistance was assessed in naturally aerated Ringer's physiological solution at 37°C by corrosion potential and electrochemical impedance spectroscopy (EIS) measurements as a function of exposure time, and potentiodynamic polarization curves. Corrosion potential values indicated that both Ni-Ti and Ti-Nb-Sn alloys undergo spontaneous passivation due to spontaneously formed oxide film passivating the metallic surface, in the aggressive environment. It also indicated that the tendency for the formation of a spontaneous oxide is greater for the Ti-18Nb-5Sn alloy. Significantly low anodic current density values were obtained from the polarization curves, indicating a typical passive behaviour for all investigated alloys, but Nitinol exhibited breakdown of passivity at potentials above approximately 450 mV(SCE), suggesting lower corrosion protection characteristics of its oxide film compared to the Ti-Nb-Sn alloys. EIS studies showed high impedance values for all samples, increasing with exposure time, indicating an improvement in corrosion resistance of the spontaneous oxide film. The obtained EIS spectra were analyzed using an equivalent electrical circuit representing a duplex structure oxide film, composed by an outer and porous layer (low resistance), and an inner barrier layer (high resistance) mainly responsible for the alloys corrosion resistance. The resistance of passive film present on the metals' surface increases with exposure time displaying the highest values to Ti-18Nb-4Sn alloy. All these electrochemical results suggest that Ti-Nb-Sn alloys are promising materials for biomedical applications.

Improved surface morphology and corrosion resistance for galvannealed coatings by pre-electroplating iron

Dual-phase steel was electroplated with Fe prior to reduction annealing to improve surface quality and corrosion characteristics after galvannealing. This pre-electroplating process caused remarkable reduction in surface cracks and induced δ-phase formation over the surface of the galvannealed coating. Moreover, the degree of Fe–Zn alloying significantly increased, resulting in a thicker coating. Consequently, the corrosion potential increased and corrosion current decreased in the highly polarized anodic reaction. Further, spontaneous passivation occurred, resulting in a very low corrosion rate.

Corrosion Behavior of (Zr<sub>51.6</sub>Cu<sub>48.4</sub>)<sub>93</sub>Al<sub>7</sub> Bulk Metallic Glass

Corrosion behavior of (Zr51.6Cu48.4)93Al7bulk metallic glass in 3 M HCl, 3M H2SO4, 3M HNO3, 3M NaOH and 3.5 mass%(0.6M)NaCl solutions was studied by immersion test and electrochemical measurements. The bulk metallic glass was spontaneously passivated in the acids，alkali and brine, although it exhibits distinct spontaneous passivation with high passive current density in the Cl-containing solutions. The immersion test indicated, except for 3 M HCl solution, low corrosion rates of the order of 10-2 mm/year for (Zr51.6Cu48.4)93Al7bulk metallic glass in the other solutions, The quantitative X-ray energy disperse spectroscopy(EDS) analysis revealed that the high corrosion resistance of the bulk metallic glass was due to the formation of aluminium and zirconium-enriched passive film. Corrosion products were mainly copper chloride and oxides in the Cl-containing solutions . The growth mechanism of the passive film is also discussed.

A non–destructive compositional analysis of thin surface films formed on W–xTa alloys by angle resolved X–ray photoelectron spectroscopy

Synergistic effect of the simultaneous additions of tungsten and tantalum in the extremely high corrosion resistance properties of the spontaneously passivated sputter–deposited W–xTa alloys was investigated using a non-destructive angle resolved X-ray photoelectron spectroscopy (angle resolved XPS) techniques in this study. In-depth surface analyses of the thin passive films formed on the spontaneously passivated amorphous/nanocrystalline W–xTA alloys using angle resolved XPS analyses revealed that the high corrosion resistance of the alloys is mostly due to the formation of homogeneous passive double oxyhydroxide films consisting of Wox and Ta4+ cations with a small concentration gradients in–depth particularly after immersion between 20–168 h in 12 M HCl solution open to air at 30°C. Consequently, tantalum metal acts synergistically with tungsten in enhancing the spontaneous passivity as well as the high corrosion resistance of the sputter–deposited binary W–xTa alloys in 12 M HCl solution.Keywords: Sputter deposition; W–xTa alloys; 12 M HCl; Take-off angleDOI: http://dx.doi.org/10.3126/bibechana.v8i0.4784BIBECHANA 8 (2012) 8-16

Characterization of a new beta titanium alloy, Ti–12Mo–3Nb, for biomedical applications

In recent years, different beta titanium alloys have been developed for biomedical applications with a combination of mechanical properties including a low Young's modulus, high strength, fatigue resistance and good ductility with excellent corrosion resistance. From this perspective, a new metastable beta titanium Ti–12Mo–3Nb alloy was developed with the replacement of both vanadium and aluminum from the traditional Ti–6Al–4V alloy. This paper presents the microstructure, mechanical properties and corrosion resistance of the Ti–12Mo–3Nb alloy heat-treated at 950°C for 1h. The material was characterized by X-ray diffraction and by scanning electron microscopy. Tensile tests were carried out at room temperature. Corrosion tests were performed using Ringer's solution at 25°C. The results showed that this alloy could potentially be used for biomedical purposes due to its good mechanical properties and spontaneous passivation.

Encapsulation of triethanolamine as organic corrosion inhibitor into nanoparticles and its active corrosion protection for steel sheets

Triethanolamine (TEA), a corrosion inhibitor for zinc and steel, was introduced into nano-sized particles as nanoreservoirs to increase longevity of inhibitive property and prevent degradation caused by direct addition of corrosion inhibitor into coating layer. TEA-incorporated nanoparticles with average particle size around 400–450 nm were successfully synthesized by sequential emulsion polymerization, occupying around 5% of total solid weight of particles during a neutralization process. Encapsulated TEA was released from the capsule inside when the pH level of environment became acidic or alkaline due to an acid–base interaction or ionization of seed material in specific conditions. In the corrosion tests, the encapsulated TEA decreased the corrosion rate of steel substrate owing to its adsorption on steel surface and the resistance of coating layer against corrosive environment was much higher and remained its resistance as immersion time increased when TEA was incorporated in coating layer in the encapsulated form. Based on the scanning vibrating electrode technique (SVET) result, anticorrosive ability of the encapsulated TEA seemed to improve due to the spontaneous passivation of exposed metal on the defected region of coated steel.

The passivation of iron in ammoniacal solutions containing copper (II) ions

In the present study it was found that the presence of millimolar amounts of copper (II) ions in ammoniacal solutions leads to the spontaneous formation of a stable passive layer on metallic iron and iron alloys with nickel and cobalt, under conditions in which they would otherwise remain in active dissolution. This finding may be of significance to industrial processes which employ ammoniacal solutions to leach metal values from materials rich in metallic iron, such as, for example, laterite ores which have undergone a reductive pre-treatment. The spontaneous passivation is found to take place regardless of the presence or practical absence of dissolved oxygen, and occurs more readily the higher the copper concentration and the lower the ammonia–ammonium bicarbonate concentration, though it does not take place in ammonia-free aqueous solutions of similar copper (II) ion concentrations. Electrochemical and scanning electron microscopy / energy dispersive X-ray spectroscopy (SEM/EDX) investigations conducted as part of this study showed that the observed passivation is promoted by the cementation of copper onto the actively dissolving iron surface, and subsequently by its re-dissolution. Such process is thought to create favourable conditions which promote the formation of a stable passive layer on the iron surface.

Corrosion monitoring of type 304L stainless steel in nuclear near-high level waste by electrochemical noise

Electrochemical noise (EN) monitoring of 304L stainless steel (SS) and sensitized 304 SS in 3 N nitric acid and nuclear near-high level waste (HLW) solution was carried out using a three nominally identical electrode configuration under open circuit conditions. EN signals characteristics of passivation process was obtained for 304L SS in 3 N nitric acid throughout the measurement period, while in near-HLW solution, features of passivation and depassivation were observed. Potentiodynamic polarization of 304L SS in 3 N nitric acid and near-HLW showed spontaneous passivation. Noise resistance was evaluated from EN time record and this parameter was used in the present investigation to qualitatively assess the corrosion behavior during the immersion period. The average noise resistance was found to be lower in near-HLW solution when compared to 3 N nitric acid. The results of the investigation are presented in this article.