Critical Passivation Current Density Research Articles

銅電解精製におけるアノードの不動態化に及ぼす液組成の影響

To elucidate the effect of solution composition on the passivation of anode in Cu electrorefining, the potentiodynamic polarization curves were measured by polarizing from -0.2 V versus NHE to 1.0 V using the potential sweep method at 1.0 mV·s−1. The diffusion coefficient was measured by rotating disc electrode method under the conditions of 50, 100, 150, 200, 250 and 300 revolutions per minute of disc electrode and polarizing from 0.34 V to -0.40 V at 10.0 mV·s−1. The critical current density for passivation and the passive current density decreased linearly with total concentration of SO4 in solution regardless of solution composition and the presence of impurities such as As3+，Sb3+ and Bi3+, showing the good correlation between the current density for passivation and the total concentration of SO4. The diffusion coefficient of Cu2+ ions decreased with total concentration of SO4 in solution. The increase in passivation with the total concentration of SO4 is attributed to both decreases in solubility of CuSO4 in electrolyte and the diffusion coefficient of Cu2+ ions. On the other hand, focusing on the effect of kinds of cation on the passivation, the harmful effect on the passivation was largest with Cu2+ ions, and was larger in the order with Li+，Ni2+ and Na+ ions, and was smallest with H+ ions. The diffusion coefficient of Cu2+ ions decreased most with Ni2+ ions, and decreased secondly with Li+ and Na+, and decreased least with H+ ions. The diffusion coefficient of Cu2+ ions decreased with increasing the viscosity of solution.

Effect of Residual Stress on the Corrosion Behavior of Austenitic Stainless Steel

In this paper we demonstrate that the residual stress introduced by several different surface finishes affects the critical current density for passivation and the passive current density in the anodic polarization curve of austenitic stainless steel and that those critical current densities can be reduced by controlling the residual stress by applying a cavitating jet to the backs of specimens. The results show that the current density either increased or decreased depending on the surface finish, and that was decreased by introducing compressive residual stress for all surface finishes.

Influence of heat treatment on microstructure and passivity of Cu–30Zn–1Sn alloy in buffer solution containing chloride ions

Tin as an alloying element is of great interest in brasses for dezincification impediment. In this paper, Cu–30Zn–1Sn alloy was submitted to three different heat treatments, viz. A (heating up to 800 °C for 20 h, held at 200 °C for 20 h in salt bath and air cooled), B (heating up to 800 °C for 20 h and water quenched) and C (heating up to 600 °C for 20 h and water quenched). The influence of heat treatment on microstructure was evaluated by OM and SEM–EDS analysis. The corrosion resistance in buffer solution (pH 9), H3BO3/Na2B4O7 ·10H2O, with various concentrations of chloride ions was evaluated by potentiodynamic polarization curves and compared with multi-component Pourbaix diagrams. A correlation between the heat treatment, microstructure and passivity of the heat treated samples was observed. The results indicated that all heat treatment procedures led to formation of α, and γ-Sn-rich phases as microstructure constituents with a small fraction of β′ phase in A. Sn-rich phase appears in grain boundaries and its morphology was slightly changed due to heat treatment. Beneficial influence of low concentration chloride ions on passivity was associated with the formation of copper oxides/hydroxide and chloride complexes. Deterioration was observed at concentrations higher than 0·05 M NaCl due to accelerated dissolution of copper by formation of CuCl\(_{2}^{-}\). As a result of dezincification process, preferential corrosion attack and copper redeposition on α phase (matrix) were observed. However, Sn-rich (γ1) phase in grain boundaries was not attacked due to SnO2 formation. In buffer solution, the higher passivity current density in A was related to the presence of small amount of β′ phase. On the other hand, in 1 M NaCl, lower critical current density for passivation in B and A (about two times lower than C) was attributed to the grain size effect.

Critical Factors for Controlling the Stability of the Passive Film on Stainless Steels

In order to understand the stable passivity of stainless steels, the critical factors including the mechanism of reductive dissolution relating to breakdown and repair of passive film have been reviewed based on recent evidence. The Cr enrichment in the passive film by two stage process and alloying effect are discussed in detail. The importance of the relationship between critical passivation current density, icrit, in acidic solution and pitting potential, Epit, in chloride solution is emphasized for understanding the stability of passive stainless steel.

Pinhole Defect Density of CrNx Thin Films Formed by Ion-Beam-Enhanced Deposition on Stainless Steel Substrates

To improve the corrosion protection performance of films, the relationship between the pinhole defect density of the films and the surface condition of stainless steel substrates was studied. A commercial, heated type 304 stainless steel (ss) and a high purity heated type 316L ss substrate were used. The surface of substrates was finished with wet SiC paper polishing, diamond paste polishing, or electropolishing. films thick were formed by ion-beam-enhanced deposition. The pinhole defect density was evaluated by the critical passivation current density method. The surface roughness was examined by atomic force microscopy. The pinhole defect density of films on a commercial 304 ss substrate decreased with decreasing surface roughness of the substrate in cases of diamond paste polishing and wet SiC paper polishing. Pits were caused by electropolishing on the surface of 304 ss substrate owing to the dissolution of manganese oxysulfide inclusions, which resulted in an increase in the pinhole defect density of film. However, no pits were caused by electropolishing on a high purity 316L ss substrate and an extremely low pinhole defect density of area % was obtained for film on this substrate.

Reduction of Pinhole Defects in DLC Film Prepared with Plasma-based Ion Implantation and Deposition

Diamond-like carbon (DLC) films were prepared on the SUS304 substrate with a hybrid process of plasma-based ion implantation and deposition using toluene plasma. The pinhole defects in DLC films reached to the substrate deteriorate corrosion resistance. The area ratio of pinhole defects to the bare substrate was evaluated by the critical passivation current density (CPCD) method. The CPCD measurement indicated that the area ratio of pinhole defects in the DLC film was decreased with increasing film thickness and reached to about 1 x 10^-3 % at more than 6 micro m in thickness. The apparent area ratio of pinhole defects were also reduced by the interfacial treatment before DLC deposition such as the production of an interfacial mixing layer by carbon ion implantation to the substrate or the formation of SiCx nanolayer in front of substrate in the thinner film thickness. The ultrasonic cleaning in the acetone during the deposition reduced the area ratio of pinhole defects to 6 x 10^-4 % at the DLC film thickness of 1 micro m.

Evaluation of pinhole defect in DLC film prepared by hybrid process of plasma-based ion implantation and deposition

Pinhole defect in diamond-like carbon (DLC) film prepared by a hybrid process of plasma-based ion implantation and deposition using toluene plasma was evaluated by the critical passivation current density in the anodic polarization method. The area ratio of pinhole defects to the SUS304 bare substrate was decreased exponentially with increasing DLC film thickness and reached about 3 × 10 - 6 % at 11 μ m film thickness. As a result, it is found that the corrosion resistance of DLC-coated specimens was improved with increasing film thickness. The production of an interfacial mixing layer by ion implantation from methane and acetylene plasmas between the DLC film and the substrate material reduced pinhole defects in the film.

Evolution of current transients and morphology of metastable and stable pitting on stainless steel near the critical pitting temperature

Type 904L austenitic stainless steel with a critical pitting temperature (CPT) of 56 °C was polarized at 750 mV (Ag/AgCl) in 1 M NaCl at 45, 49 and 54 °C. Current transients due to metastable pitting were recorded at these three temperatures, and a smaller number of stable pit current transients were recorded at 62 °C. The metastable pit current, I, followed a power law relationship I ∼ t n during initial growth prior to repassivation. The exponent n increased with temperature, from 0.5 to 1.5. To grow at the lower temperatures, the pits need to be more occluded (lower value of n; lower pit current density). The repassivated metastable pits showed two morphologies: small occluded cavities associated with the small transients, and open ones with polished surfaces for large transients recorded close to the CPT. A new type of deterministic repassivation was identified for the latter type of pit. The stable pit currents rose in proportion to t 3/2, just like the faster-growing metastable pits, but only for a few seconds; then there was a drop in current and a further noisy increase occurred with a lower exponent. These pits undercut the metal surface, producing a lacy metal cover. The results are broadly consistent with a model that attributes the CPT to the onset of passivation in the saturated pit solution. A new feature requiring explanation is that, very close to the CPT, pits can precipitate a salt film but fail to undercut the surface. It is hypothesized that since a higher anodic current density is required at the undercutting site than at the pit base, there will be a range of temperatures where the former but not the latter would exceed the critical current density for passivation and is thus disallowed.

Effects of Compressive Stress on Corrosion-Protective Quality and Its Maintenance under a Corrosive Environment for TiN Films Deposited by Reactive HCD Ion Plating

Titanium nitride films were deposited onto SUS304 substrates under various film thickness, deposition temperatures, and substrate bias voltages. Their protective quality was evaluated by electrochemical testing in accordance with the critical passivation current density (CPCD) method. Two types of tests were employed to evaluate corrosive behavior of coated substrates: a high-temperature and high-pressure corrosion test; and a measurement of the change in anodic current density with immersion time. A scanning electron microscope was used to examine surface morphology and fractured cross-sections of the films. Residual stress in the films was determined by the sin2 ψ method. An increase in film thickness engendered high protective quality. That protective quality was improved with increasing deposition temperature; micrometer-order pores were observed on all parts of films deposited at lower deposition temperatures, whereas few pores existed on films deposited at higher temperatures. This result indicates that these pores are one factor influencing overall protective quality. A film deposited with no substrate bias voltage displayed morphology with a typical columnar-structure; it also demonstrated complete protective quality. As the bias voltage increased, protective quality deteriorated, whereas an excess increase in the bias voltage gave rise to a slightly higher protective quality. Films with lower compressive stress had only a few pores and possessed higher protective quality, suggesting that pore formation originates in compressive stress. Corrosion tests indicated that the coated substrates corroded more rapidly as compressive stress in the film increased. The effect of compressive stress on maintenance of corrosion-protective quality was treated quantitatively. The rate of increase in the exposed area of the substrate was estimated from variation of an anodic current density with immersion time. This evaluation indicates that a decrease in compressive stress contributes greatly to maintenance of protective quality.

Electrochemical behaviour of sintered, yttria reinforced ferritic stainless steels

The electrochemical behaviour of 434L ferritic stainless steels produced by the powder metallurgy (PM) route has been studied. The effects of sintering temperature (1250 and 1400°C) and dispersoid addition (10%Y2O3) have been investigated by potent iodynamic polarisation and electrochemical impedance studies in 0·05 M H2SO4 solution. All the alloys exhibited active-passive behaviour in the electrolyte. The critical current density for passivation was lower in the case of the PM samples when compared with 430, which has been attributed to the presence of Mo in the samples. The passive current density of the PM samples correlated with the sintered porosities. All the other passivation parameters were similar for the materials. The nature of passive film was investigated using electrochemical impedance spectroscopy. A lower capacitance (i.e. more corrosion resistant) surface film was obtained after sintering at the higher temperature. Yttria dispersoids in ferritic stainless steel decreased the corrosion resistance of the surface film after sintering at the higher temperature, whereas they did not significantly affect the surface film behaviour after lower sintering temperature. The results have been correlated to microstructural parameters of the PM materials.

Electrochemical behaviour of sintered yttria reinforced ferritic stainless steels

The electrochemical behaviour of 434L ferritic stainless steels produced by the powder metallurgy (PM) route has been studied. The effects of sintering temperature (1250°C and 1400°C) and dispersoid addition (10%Y2O3) have been investigated by potentiodynamic polarisation and electrochemical impedance studies in 0·05M H2SO4 solution. All the alloys exhibited active-passive behaviour in the electrolyte. The critical current density for passivation was lower in the case of the PM samples when compared with 430, which has been attributed to the presence of Mo in the samples. The passive current density of the PM samples correlated with the sintered porosities. All the other passivation parameters were similar for all the materials. The nature of passive film was investigated using electrochemical impedance spectroscopy. A lower capacitance (i.e. more corrosion resistant) surface film was obtained after sintering at the higher temperature. Yttria dispersoids in ferritic stainless steel decreased the corrosion resistance of the surface film after sintering at the higher temperature, whereas they did not significantly affect the surface film behaviour after lower sintering temperature. The results have been correlated to microstructural parameters of the PM materials.

Deposition Condition and Pinhole Defect Density of CrNx ( 1 ≧ x ≧ 0 ) Thin Films Formed by Ion-Beam-Enhanced Deposition

In order to produce a thin film with a low pinhole defect density using an ion-beam-enhanced deposition method, the effect of deposition conditions, such as the gas flow rate, the beam current, the accelerator voltage, and the beam voltage of enhancement source, on the pinhole defect density was examined. thin films were deposited on type 304 stainless steel substrates, and then anodic polarization curves of -coated steels were measured in a deaerated M KSCN solution. Pinhole defect density was evaluated by the ratio of the critical passivation current density of -coated steel to that of noncoated steel. It was found that the pinhole defect density was decreased with increasing the beam voltage of the enhancement source. The lowest pinhole defect density, 0.005 area %, was obtained by the following conditions: flow rate 5 sccm, Ar flow rate 3 sccm, beam current 20 mA, beam voltage 200 V, accelerator voltage 600 V, and film thickness 65 nm. © 2003 The Electrochemical Society. All rights reserved.

Effect of preparation conditions on pinhole defect of TiN films by ion mixing and vapor deposition

Pinhole defects of titanium nitride (TiN) films prepared by nitrogen ion irradiation during titanium vapor deposition, ion mixing and vapor deposition (IVD) technique, were evaluated potentiodynamically in a deaerated 0.5 kmol/m 3 H 2SO 4+0.05 kmol/m 3 KSCN solution at 298 K. The nearly stoichiometric TiN films exhibited the columnar structure with the preferred crystal growth of (1 1 1) or (2 0 0) plane, and they contained more or less pinhole defects. The critical passivation current density (CPCD) i crit of TiN films decreased successfully with increasing film thickness and substrate temperature. Here, the area ratio of pinhole defects was evaluated by the ratio of i crit of coated and non-coated specimens, i.e., by CPCD method. Based on such electrochemical evaluation, the optimum preparation conditions for the corrosion-resistant TiN films were discussed.

Pitting Characteristics of Alloy 600 and Alloy 601 in a Chloride-Containing Acidic Environment

Abstract To study the pitting characteristics of two nickel-based alloys in aggressive acidic environments, potentiodynamic anodic polarization of Alloy 600 (UNS N06600) and Alloy 601 (UNS N06601) was carried out in the presence of various concentrations of sodium chloride (NaCl) in 0.1 M sulfuric acid (H2SO4) at 30°C, 60°C, and 90°C. It was found that the addition of NaCl increased the corrosion rate and critical current density for passivation of both alloys significantly. Potentialrelated parameters like open-circuit potential (OCP) and pitting potential (Epit) were also shifted in the active direction with the increase of NaCl concentration. The pitting potential for Alloy 601 was almost 200 mV more positive than that for Alloy 600 in the presence of a particular concentration of NaCl. With the increase of temperature from 30°C to 90°C, the pitting attack on both alloys became more pronounced.

Effects of Al, Si and Mo on Passivation Characteristics of Fe-10Cr Alloys

A study has been made on the effects of Al, Si and Mo on anodic polarization characteristics of Fe-10Cr alloys in 0.05-1.0 kmol.m -3 H 2 SO 4 and NaCl solutions. Potential decay curves have also been measured in order to evaluate the stability of the passive films formed on Fe-10Cr alloys containing Al, Si and Mo. The analysis of the chemical composition of the passive film has been carried out by AES and XPS. The addition of Mo was very useful to decrease the critical passivation current density of Fe-10Cr alloys which contained Al and Si. The passive current density decreased with addition of 2 mass%Mo to Fe-10Cr-3Si alloy, while it increased in Fe-10Cr-3Al alloy. Pitting potential was moved toward the noble direction by the addition of Mo, except for the case of Fe-10Cr-3Al-3Si-2Mo alloy. It seems that Laves phase (Fe 2 Mo) was precipitated in this alloy, and Mo-depleted zone was formed locally. The potential decay time of the passive film was increased by the addition of Al, Si and Mo to Fe-10Cr alloys. Two step potential decay curves were obtained in Si-containing alloys, and this behavior of the decay curve was shown more clearly in Fe-10Cr-3Si-2Mo alloy. The results of AES and XPS analysis indicated that Cr and Al concentrated in the passive film of Fe-10Cr-Al alloy. In the case of Fe-10Cr-Si alloy, Si concentrated in the surface region of the film and Cr in the internal region of the film. The confirmation was not possible for the enrichment of Mo in the passive film.

Pinhole defect evaluation of TiN films prepared by dry coating process

Pinhole defects of TiN films deposited onto stainless steel by a few dry coating processes were evaluated potentiodynamically in a deaerated 0.5 kmol/m 3 H 2SO 4+0.05 kmol/m 3 KSCN solution at 298 K. The critical passivation current density i crit in the TiN films prepared by activated reactive evaporation method decreased considerably with increasing film thickness. For the ⩾1.5 μm thick films prepared by r.f. reactive sputtering, and ion mixing and vapor deposition methods; however, there was an increasing tendency in i crit with cracking and/or peeling. Here, the area ratio of pinhole defects was evaluated by the ratio of i crit of a coated and a non-coated specimen. Such electrochemical evaluation was concluded to be a reliable technique for the pinhole defects of dry coating films.

Characterisation of anodic oxide films formed on titanium and two ternary titanium alloys in hydrochloric acid solutions

The purpose of this paper is to characterise the anodic oxide films formed on titanium and two ternary titanium alloys, Ti-15Mo-5Al and Ti-10Mo-10Al in 1%, 10% and 20% hydrochloric acid solutions at 25, 50, and 75°C. The anodic film on titanium in hydrochloric acid is stable between + 0.6 V (SCE) and + 2.0 V (SCE). For our new ternary titanium alloys, the passive film is formed at about + 0.6 V (SCE) and is stable to + 2.0 V (SCE). The anodic polarization curves for alloys differ from the base metal curve, presenting two peaks for the critical passivation current density in the active-passive potential range. At the first current peak (the first critical passivation potential E cr1 ) a porous titanium pentaoxide (Ti 3 O 5 ) is formed. When the potential reaches the second current peak (the second critical passivation potential E cr2 ) the compact and protective titanium dioxide (TiO 2 ) is formed. The impedance spectra exhibit the typical behaviour for a passive film i.e. a near capacitive response illustrated by a phase angle close to - 90 over a wide frequency range. The oxide film on titanium and its alloys in hydrochloric acid solutions exhibits a high resistance and a low capacitance (with the increase of the potential) attributed to the surface roughness decrease as the oxide layer thickens.

Corrosion properties of amorphous Mo-Si-N and nanolayered Mo-Si-Nn/SiC coatings

Corrosion properties of sputter deposited MoSi{sub 2}, SiC, Mo-Si-N (MoSi{sub 2.2}N{sub 2.5}) and nanolayered Mo-Si-N/SiC coatings on Fe37 low carbon steel have been studied using electrochemical polarization measurements in 1 N H{sub 2}SO{sub 4} solution. A decrease in both critical current density for passivation and minimum current in passive state was observed in annealed nanolayered Mo-Si-N/SiC coating compared to each of its constituents alone as single layer coating. On contrary to MoSi{sub 2} coating, only slight increase in critical current density was observed in Mo-Si-N coated sample after annealing. Molybdenum disilicide source material has good thermal and electrical conductivity, which allows effective dc-magnetron sputter deposition. Therefore this is a relatively simple method to produce amorphous coatings which have a high crystallization temperature and promising properties for corrosion applications.

プラズマＣＶＤ法により作製されたＴｉＮ薄膜の欠陥面積率評価に対するＣＰＣＤ法の適用の検討

Corrosion resistance of coated TiN film is governed by pinhole defect. Therefore, it is necessary to evaluate the existing state of defects. The Critical Passivation Current Density (CPCD) method had some problems though this method could quantitatively estimate defects as a pinhole defect ratio (%). The most important problem is the overestimation in defect ratio evaluation in the cases when large corrosion pits were generated and some parts of TiN films were exfoliated due to its low adhesion strength and high residual stress. Therefore in this study, the adhesion characteristics of TiN films which was coated by plasma CVD was investigated by the scratch test. The applicable range of the CPCD test was made clear for the defect ratio evaluation of TiN films with various film thickness and adhesion strength. And then, some improvements of CPCD test for avoiding the overestimation of defect in film are discussed.

ホローカソード方式イオンプレーティング法で作製したＴｉ／ＴｉＮ多層皮膜の耐食・耐摩耗性

Attempts have been made to reveal a desirable design for a Ti/TiN multilayer as a protector for corrosion and wear at-tacks. Multilayers of Ti/TiN were deposited on two types of chromium molybdenum steels and on a high speed tool steel by switching on and off a reactive gas feed valve. Layer thicknesses were determined by scanning electron microscopy and also estimated from Ti depth profiles obtained by grow discharge atomic emission spectrometry (GD-AES). Corrosion resisitance was evaluated by two types of tests : one is the method where the coated steels were exposed to the gas generated through hydrolyzing a resin mixture of ABS and PVC at 443 K under a pressure of 2.87 MPa, and the other is the critical passivation current density method where a deaerated solution of 0.5 kmol/m3H2SO4-0.05 kmol/m3KSCN was used as an electrolyte. Ttribology tests were carried out under unlubricated conditions with a reciprocating type testing apparatus. Substrate current is a fairly useful indicator for precise control of layer thickness. GD-AES offers an exact estmation of layer thickness in the matter of minutes, for example, only within 0.4 ks for a multilayer of 56 μm in total thickness. Corrosion tests show that TiN coatings with a thicker Ti underlayer are more protective and that the multilayer coatings possess a better protective function when the ratio of cumulative thickness of Ti layers to that of TiN layers, ∑XTi to ∑XTiN, is within 1 to 2. It is possible that an increase in thickness of the outer TiN layer brings about reduction of protection performance. With an increase in intrinsic compressive stress in the TiN layer, abrasive damage progresses more rapidly toward a substrate. The Ti layer contacting to the outer TiN layer suppressed the progression; for multilayers, therefore decreasing the thickness of the outer TiN layer resulted in a decrease in wear loss.