Open Circuit Corrosion Potential Research Articles

Cyclic-anodic-polarization studies of a Zr 41.2Ti 13.8Ni 10Cu 12.5Be 22.5 bulk metallic glass

Cyclic-anodic-polarization tests were conducted on a Zr-based bulk metallic glass (BMG) with a chemical composition of Zr 41.2Ti 13.8Ni 10Cu 12.5Be 22.5 [Liquidmetal-001, LM-001 (at.%)]. Samples were compared in two different electrolytes at two different temperatures. Tests were conducted at 22 and 37 °C in both, an aerated 0.6 M NaCl electrolyte and a phosphate-buffered saline (PBS) electrolyte with a physiologically relevant dissolved oxygen content. For all tested conditions, the alloy demonstrated passive behavior at the open-circuit potential with low corrosion rates. However, a susceptibility to localized pitting corrosion was observed in all of the tested conditions. At both temperatures, statistical analyses revealed that the LM-001 BMG exhibited comparable or improved values for every corrosion parameter in the PBS electrolyte, as compared to the NaCl electrolyte. The mean differences between the protection potentials and the open-circuit corrosion potentials were determined to be positive values with greater magnitudes for the PBS electrolyte. Thus, the material may undergo pitting corrosion at surface flaws or after incubation-time periods, depending on the local and bulk environments, with lower probabilities for the PBS electrolyte.

Imaging metastable pits on austenitic stainless steel in situ at the open-circuit corrosion potential

Images describing metastable pits on 304 stainless steel obtained in situ in chloride solution at the open-circuit corrosion potential are presented. These images were obtained by scanning electrochemical microscopy and represent the oxidation of Fe 2+ emanating from the metastable pits as the probe tip passes over them. Each anodic transient on the tip is succeeded by a transient in the cathodic direction. We ascribe this to an indirect observation of the cathodic reaction. Reduction of oxygen as the cathodic reaction causes peroxide to be produced adjacent to the propagating pits. This reacts with Fe 2+ produced by the background passive current locally around the pit site. The microscope tip, set to detect Fe 2+ , detects the reduced background current as a transient negative wing immediately after the anodic wing associated with the pit itself. The signal represents a reduced anodic background current. This negative wing on the probe current is not detected when the metastable pits are generated under potentiostatic polarisation. It is shown that the lifetime of the detected pits is a maximum of a few seconds.

Cyclic-anodic-polarization studies of a Zr41.2Ti13.8Ni10Cu12.5Be22.5 bulk metallic glass

Cyclic-anodic-polarization tests were conducted on a Zr-based bulk metallic glass (BMG) with a chemical composition of Zr 41.2 Ti 13.8 Ni 10 Cu 12.5 Be 22.5 [Liquidmetal-001, LM-001 (at.%)]. Samples were compared in two different electrolytes at two different temperatures. Tests were conducted at 22 and 37 °C in both, an aerated 0.6 M NaCl electrolyte and a phosphate-buffered saline (PBS) electrolyte with a physiologically relevant dissolved oxygen content. For all tested conditions, the alloy demonstrated passive behavior at the open-circuit potential with low corrosion rates. However, a susceptibility to localized pitting corrosion was observed in all of the tested conditions. At both temperatures, statistical analyses revealed that the LM-001 BMG exhibited comparable or improved values for every corrosion parameter in the PBS electrolyte, as compared to the NaCl electrolyte. The mean differences between the protection potentials and the open-circuit corrosion potentials were determined to be positive values with greater magnitudes for the PBS electrolyte. Thus, the material may undergo pitting corrosion at surface flaws or after incubation-time periods, depending on the local and bulk environments, with lower probabilities for the PBS electrolyte.

Surface characteristics and corrosion resistance properties of TiNi shape memory alloy coated with Ta

Ti-50.6 at.% Ni alloy samples were coated with tantalum by the arc ion plating method with the aim to improve their radiopacity. Surface characteristics and corrosion resistance properties were investigated in the present study. The results of XRD show that the film as-deposited was composed of a metastable tetragonal β-phase tantalum which can transform into α-phase tantalum after annealing at 700 °C, 800 °C, 900 °C for 1 h, respectively. TEM observations show that thin nano-structured Ta layers were obtained in the as-deposited samples. Mixture of microcrystallite (approx. 40 nm) and large-size crystal (approx. 150 nm) α-phase tantalum was formed in the film after annealing at 700 °C. When annealed at 900 °C, the film is composed of dominantly uniform large-size α-phase tantalum. The results of XPS survey and high resolution spectra show that a thin oxide film with Ta 2O 5 in the outmost layer and tantalum suboxides in the inner layer, are formed inside tantalum coating as a result of natural passivation of Ta in the atmosphere. The corrosion resistance was determined by electrochemical methods in 0.9% NaCl solution at 37 °C. Both the as-deposited and annealed samples exhibit excellent corrosion resistance property. Compared to the untreated coating samples, the annealed Ta coating samples exhibit higher open-circuit corrosion potential ( E corr), breakdown potential ( E br), and lower passive current density ( I p). The results indicate that the annealing treatment can improve the corrosion resistance property.

Interface capacitance at mercury and iron electrodes in the presence of organic compound

A new experimental setup that determines simultaneously the electrochemical impedance spectrum and the modulation of interface capacitance induced by the impedance measurement itself is described. Then, the signal processing used to determine the traditional impedance spectrum and the modulation of interface capacitance at the same frequency as for the impedance measurement is discussed. Two model systems have been used to illustrate the implications of faradaic processes for the interface capacitance. First, an ideally polarisable interface, an Hg electrode in KCl solution, was examined. It was found that the interfacial capacitance changed instantaneously, and the sign of the capacitance modulation changed when the electrode potential crossed the potential of zero charge (PZC). In the presence of n-butyl alcohol in the same system, however, the capacitance modulation response was quite different. A negative slope of the capacitance - potential curve was verified only at extremely low frequencies, although the system still remained an ideally polarisable one. The second system model examined was Fe in H2SO4 solution at the open circuit corrosion potential. In the absence of inhibitor, only one time constant was observed, and this phenomenon was attributed to the relaxation of surface coverage by Hads. In the presence of propargylic alcohol, two relaxation phenomena were observed, both in the impedance spectrum and in the capacitance modulation. The faster relaxation process was attributed to that of the reaction intermediate of Fe dissolution, and the slower phenomenon to the adsorption - desorption of the inhibitor.

Surface modification of TiNi alloy through tantalum immersion ion implantation

TiNi alloy samples were implanted with tantalum at a dose of 3.1×10 17 N/cm 2 by means of Plasma immersion ion implantation (PIII) method. The surface morphology, chemical composition was characterized by Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The corrosion resistance, were determined by electrochemical methods in 0.9% NaCl solution at 37 °C. The results show that PIII treatment can increase the open-circuit corrosion potential ( E corr) and decrease the corrosion current density ( I corr). The pitting potential ( E br) for the tantalum implanted TiNi alloy is approximately 810 mV, more than 200 mV higher than that of the unimplanted one. The corrosion hystersis loop of the unimplanted TiNi alloy is larger than that of the implanted material, indicating improvement of the repassivating ability and pitting resistance of the TiNi alloys.

Biocompatibility and charge injection property of iridium film formed by ion beam assisted deposition

Iridium thin film formed by electron-beam evaporation with simultaneous bombardment of Ar ion beam was evaluated for a stimulating neural electrode. The electrochemical behavior of as-deposited Ir film on Ni–Ti sample was almost identical to bulk Ir by producing much higher open-circuit corrosion potential and much lower anodic current density than the uncoated Ni–Ti in both 1 n sulfuric acid and saline solution. The charge injection capability of Ir film was compared with that of Pt electrode currently used mostly as a stimulating neural electrode. The charge density of Pt was small and unchanged with increasing number of activating cycles in 0.1 m H 2SO 4, whilst the Ir film continuously produced increases in charge density. The charge injection density of Ir film in physiological solution was higher for the more activated sample under the identical stimulating condition. Attachment and proliferation with PC12 cells on Ir-coated CP Ti without applying electrical stimulation was similar to the polished CP Ti. A network of neurons and extending axons were formed on Ir film.

Corrosion behavior of phosphorus ion-implanted Ni 50.6Ti 49.4 shape memory alloy

Pretreatment of NiTi alloy by plasma-based phosphorus implantation (PBII) is of interest in order to make beta-particle-emitting radioactive stents. Corrosion properties of PBII-treated and un-treated NiTi alloys have been tested with anodic polarization curves in 1% NaCl solution at 37 °C. PBII treatment can increase the open-circuit corrosion potential (Ecorr) and decrease the corrosion current density (Icorr). The anodic polarization curves of PBII-treated NiTi alloy show a ‘secondary passivation’ phenomenon in the potential range of 0.72 to 1.28 V. The existence of P 2O 5 can probably be connected with the so-called secondary passivation phenomenon.

Comparison of the influence of nitrate ions on the electrochemical behaviour of iron and carbon steels in sulphate solutions

The effect of nitrate ions on the electrochemical behaviour of iron (ferrite) and two carbon steels (martensite and pearlite) in sulphate solutions of different pH values was investigated by cyclic voltammetry polarization and electrochemical impedance spectroscopy. The pitting inhibiting effect of nitrate ions on ferrite in sulphate media is pH dependent. Nitrate ions were unable to inhibit the pitting on ferrite in neutral sulphate solutions, but did effectively protect passivated ferrite from pitting in acidic sulphate solutions. No pitting occurred on the surface of the martensite and pearlite specimens in sulphate solutions regardless of the pH of the solutions. At the open-circuit corrosion potentials, the three materials underwent general corrosion. The impedance spectra for the three materials measured in neutral sulphate solutions containing nitrates and chlorides at the corrosion potentials all showed a capacitive loop, while in acidic sulphate solutions their impedance spectra were greatly reduced in size and displayed at least a low frequency impedance loop (inductive or capacitive loop) besides the well-known high frequency capacitive loop. The variation of the impedance behaviour with pH is explained.

Electrochemical dissolution of nichrome in sulphuric acid

The paper reports polarisation studies on the dissolution of nichrome in different concentrations of H 2SO 4 at room temperature. The cathodic current density ( i c) and anodic current density ( i a) at fixed over-potential, together with the corrosion current density ( i corr), are found to be proportional to H 2SO 4 concentration, whereas they decrease progressively at higher Na 2SO 4 concentration, the H 2SO 4 concentration being maintained at 1.0 M. Anodic polarisation studies indicate that passivity becomes more difficult at increased acid concentrations, whereas an increase in the sodium sulphate concentration decreases not only the open-circuit corrosion potential, but also the passive current ( i p). The Langmuir adsorption isotherm was used to establish the mechanism of the process.

Pin-on-Disk Corrosion-Wear Test

Abstract An electrochemical pin-on-disk corrosion-wear apparatus was developed at the Albany Research Center of the U.S. Department of Energy. The instrument was qualified on a low-alloy T1 tool steel [ASTM A 514] and a 304 stainless steel (Type 304). The apparatus incorporates simple specimen and counterface geometry and is instrumented for simultaneous corrosion and wear testing. The electrochemical and wear parameters of potential, current, charge, sliding speed, frictional force, and normal acceleration can be continuously displayed and recorded. After a break-in period, the electrochemical pin-on-disk produced constant wear rates independent of path length for both ASTM A 514 steel and 304 stainless steel. Results for 304 stainless steel in sulfate solutions show that abrasive wear causes the corrosion potential to shift by 0.4 V in the active direction and the passive current density to increase by three orders of magnitude, compound with the condition of no wear. Current density was a linear function of the sliding speed at a constant applied anodic potential. The open circuit corrosion potential exhibits a decay function behavior with respect to the sliding speed. Volume loss and corrosion measurements showed that mechanical removal of material was responsible for 95% of the corrosion-wear losses for 304 stainless steel. Continual corrosion exposure, however, increased the mechanical removal of material by 35 to 48%.

Noble metal cathodic arc implantation for corrosion control of Ti-6A1-4V

Ti-6A1-4V samples have been implanted with iridium utilizing a cathodic arc metal ion implantation system. The bias voltage of the substrates was held at −40 kV, resulting in implantation energies of 40 keV for singly charged ions, 80, 120 and 160 keV for ions that are ionized two, three or four times, respectively. Samples were prepared with ion doses of 0.6 × 10 16 and 1.2 × 10 16 ions cm −2. The samples were analyzed through anodic polarization tests and open circuit corrosion potential measurements in N 2 deaerated 1 N H 2SO 4 solution. The results of these tests indicate that the potential of the sample surface increases in potential toward that of pure iridium, indicating continuous surface enrichment until a passivated state is reached. SIMS analysis, before and after corrosion tests confirm that the surface concentration of iridium increases from 2 and 4% to ca 65 and 90% (for doses of 0.6 × 10 16 and 1.2 × 10 16 ions cm −2, respectively).

Corrosion behavior of high energy-high rate consolidated graphite/copper metal matrix composites in chloride media

The corrosion behavior of particulate reinforced graphite/copper (Grp/Cu) metal matrix composites (MMCs) was studied in 3.5 wt.% sodium chloride solution using electrochemical techniques, ionic solution analysis, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) techniques. The materials under investigation were high energy-high rate (HEHR) consolidated Grp/Cu metal matrix composites. HEHR processing employs a 10 MJ homopolar generator that supplies a 100 kA pulse to rapidly heat and solidify the composite powder compact. This short time at high temperature and the preferential heating and melting at the graphite-copper interface serve to encapsulate the graphite reinforcement, thus providing a highly densified composite product. Initially the open circuit potential corrosion behavior of 1.2, 5, 15, 25, and 40 volume percent GrpCu composites was studied in aerated and deaerated 3.5 wt.% NaCl solution using SEM and EDX. Subsequently, the environmental stability of these composites was studied using electrochemical techniques such as polarization resistance and potentiodynamic polarization. The severity of corrosive attack increased with increasing graphite content and in aerated solutions. In addition, solutions from these tests were analyzed to determine the relative amounts of copper and carbon present in the electrolyte after polarization tests. Microscopic analysis techniques were used to characterize the corrosion morphologies and the extensive localized corrosion occurring at the graphite-copper interface. The effectiveness of benzotriazole as a corrosion inhibitor for the copper MMCs was also studied.

Laser surface alloying of ferritic Fe-40Cr alloy with ruthenium

An investigation is made of the laser surface alloying of an experimental ferritic Fe-40Cr alloy with a rutherium powder coating using a continuous wave CO 2 laser. Themicrostructure and corrosion behaviour of the laser surface alloying layer and the Fe-40Cr bulk allyo were examined by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, microhardness, potentiodynamic and corrosion potential measurements. The results of the microstructural examination showed that fine cellular dendrites with a ruthenium content as high as 51.84wt.% were produced by laser surface alloying. The extended solubility of ruthenium in the cellular dendrites resulted in a dramatic increase of the hardness in the laser alloying layer. Potentiodynamic and open circuit corrosion potential measurements indicated that the ruthenium-containing surface layer spontaneously passivated in hydrochloric and sulphuric acid solutions, whereas the bulk Fe—40Cr alloy remained in the active state when exposed in these reducing acid solutions.

Corrosion behaviour of some vapour deposited magnesium alloys

Binary magnesium alloys containing chromium, manganese, or titanium were made using a physical vapour deposition technique. The corrosion resistance of the alloys was assessed in aqueous chloride solutions using total immersion tests in quiescent 600 mmol L−1 NaCI solutions. Alloying with manganese or titanium was found to lower the corrosion rate of magnesium over most of the compositional ranges of interest, whereas addition of chromium had a detrimental effect on the corrosion resistance of magnesium. The lowest corrosion rate was recorded for a Mg–Ti alloy where the value obtained was about 80 times lower than that found for vapour deposited pure magnesium. Open circuit corrosion potential measurements conducted in 600 mmol L−1 NaCl solution showed that additions of chromium, titanium, and manganese also produced deposits which were significantly more noble than pure magnesium, suggesting that these alloys would be less susceptible to galvanic corrosion.MST/3064

Corrosion behaviour of some vapour deposited magnesium alloys

Abstract Binary magnesium alloys containing chromium, manganese, or titanium were made using a physical vapour deposition technique. The corrosion resistance of the alloys was assessed in aqueous chloride solutions using total immersion tests in quiescent 600 mmol L−1 NaCI solutions. Alloying with manganese or titanium was found to lower the corrosion rate of magnesium over most of the compositional ranges of interest, whereas addition of chromium had a detrimental effect on the corrosion resistance of magnesium. The lowest corrosion rate was recorded for a Mg–Ti alloy where the value obtained was about 80 times lower than that found for vapour deposited pure magnesium. Open circuit corrosion potential measurements conducted in 600 mmol L−1 NaCl solution showed that additions of chromium, titanium, and manganese also produced deposits which were significantly more noble than pure magnesium, suggesting that these alloys would be less susceptible to galvanic corrosion.MST/3064

Fe–Cr–Ni–Mo–Ru ALLOYS PRODUCED BY LASER SURFACE ALLOYING

Laser surface alloying of duplex 2205 stainless steel with prep laced ruthenium powder has been investigated. The microstructure and corrosion behaviour of the laser surface alloyed layers were examined by scanning electron microscopy, energy dispersive X-ray spectrometry, microhardness, and open circuit corrosion potential measurements. The microstructure of the surface alloyed layers consisted mainly of fine cellular dendrites; spectrometric analyses indicated that ruthenium contents as high as 23·30 wt-% could be produced. These features are presumed to be responsible for the higher hardness values observed in the alloyed layer. The open circuit corrosion potentials of the layers containing ruthenium were displaced towards more noble values in reducing acid solutions, resulting in spontaneous passivation of the alloy layer surface.

Electrochemical behaviour of zinc-rich epoxy paints in 3% NaCl solution

Electrochemical impedance spectroscopy (EIS) in the 100kHz–1mHz frequency range was employed as the main electrochemical technique to study the corrosion protection behaviour of a zinc-rich epoxy paint in 3% NaCl solution. The EIS results obtained at the open-circuit corrosion potential have been interpreted using a model involving the impedance of particle to particle contact to account for the increasing resistance between zinc particles with immersion period, in addition to the impedance due to the zinc surface oxide layer and the electrical resistivity of the binder. Galvanic current and dc potential measurements allowed us to conclude that the cathodic protection effect of the paint takes some time to be achieved. The loss of cathodic protection is due to a double effect: the decrease of the Zn Fe area ratio due to Zn corrosion and the loss of electric contact between Zn to Zn particles. Even when the cathodic protection effect by Zn dust became weak, the substrate steel is still protected against corrosion due to the barrier nature of the ZRP film reinforced by Zn corrosion products.

Studies on the influence of chloride ion and pH on the electrochemical behaviour of aluminium alloys 8090 and 2014

The influence of chloride ion concentration and pH (2.0, 6.0 and 11.0) on the corrosion behaviour of 8090 (A1-Li-Cu-Mg-Zr) and 2014 (A1-4.4%Cu) alloys has been studied in NaC1 solution using a potentiodynamic polarization technique. The corrosion rate for both the alloys was high at pH values of 2.0 and 11.0 as compared to that at pH 6.0, and the rate increased in chloride ion concentration at all pH levels. A similar result was found for the passive current density. Increase in pH changed the slope of the cathodic polarization curve by changing the cathodic reaction. Increasing the chloride ion concentration decreased the cathodic reaction rate. On the other hand, the anodic reaction rate increased with increase in chloride ion concentration. The open circuit corrosion potential and the pitting potential shifted in the active (negative) direction with increasing pH and chloride ion concen- tration. The $i_p$ values for 8090-T851 were slightly lower than those for 2014-T6.

Passivation and repassivation kinetics of iron-aluminum alloys in 1 N H 2SO 4 using potential step and scratch tests

Experiments were conducted by US Bureau of Mines researchers with iron-aluminum alloys (5–25wt.%Al) in 1 N H 2SO 4 to determine the amount of charge required to repassivate their surfaces following scratching with a diamond indenter, to gain a better understanding of how wear and corrosion interact. This understanding is aimed at reducing the corrosion and wear of materials. Solutions saturated with air were used at ambient temperature. Formation of the passive films was studied by measuring the current transient resulting from stepping from the open circuit corrosion potential to the passive region of +1.175 V(NHE). The repassivation kinetics were studied by measuring the current transient caused by scratching a rotating disc electrode with a diamond indenter while holding at the passivation potential. Comparison of the charge density required to form or repair the passive film was made for the potential step and scratching experiments. The amount of charge required to repassivate the film was determined as a function of indenter load, shape, velocity and aluminum content.