Primary Passivation Potential Research Articles

Temperature-Dependence Corrosion Behavior of Ti6Al4V in the Presence of HCl

Ti alloys have been widely used in biomedical field due to good compatibility and corrosion resistance. However, corrosion-related failures of implanted Ti devices and prostheses have been regularly reported within the medical literature. The corrosion of Ti alloys has attracted much attention in vivo and in vitro. In the current study, the corrosion behavior of Ti6Al4V alloy was investigated using surface analysis and electrochemical tests. Corrosion of Ti6Al4V in 2 M hydrochloric acid is temperature dependent within the temperature range studied. It has found that the steady state current density at −510 mV vs. SCE (the primary passivation potential at the physiological temperature of 37°C) becomes higher with increasing temperature. The α phase of Ti6Al4V is preferentially dissolved relative to the β phase after potentiostatic measurement at primary passivation potential in 2 M HCl at 37°C. This investigation provides novel and useful information for Ti corrosion-related failures of biomedical implants and prostheses.

Investigation of Local Hydrogen Uptake in Rescaled Model Occluded Sites Using Crevice Scaling Laws

A major challenge in the quantification of hydrogen (H) assisted cracking is the determination of local H contents in pits and cracks and subsequently the environment assisted cracking behavior as a function of H concentration for the given metallurgical condition. Moreover, the lack of insight on these issues makes it difficult to computational design improved alloys for resistance to H assisted cracking. One difficulty in understanding H uptake at cracks and pits stems from the occluded chemistry and the local current and potential distribution due to ohmic potential drop. The effects of occluded site geometry and applied potential leading to depassivation/activation, and potential drop on H production and uptake in a martensitic precipitation/age hardened stainless steel PH 13-8Mo (Fe-13Cr-8Ni-2Mo-1Al) were explored. A potential (E)-pH diagram for PH 13-8Mo was calculated using the CALPHAD approach which illustrates the H/H+ equilibrium line and the oxide stability regions. On planar electrode surfaces, the total H concentration was found to increase exponentially with H overpotential in simulated acidified pit solutions. Comparison of CALPHAD and experimental findings indicated that the potential and pH range expected in acidified pit bottoms that H production would be thermodynamically possible on metal surfaces without a stable oxide film. This data could later be used to quantify H uptake in pits as a function of potential and pH. The x2/gap scaling law, where x is the pit/crevice depth and “gap” is the pit/crevice width, was computationally determined and used to rescale model pits from micrometer to millimeter dimensions using E-log current density (i) data as a boundary condition for finite element analysis. Such rescaling enabled local H measurements by thermal desorption as a function of pit depth in rescaled pits. Two values of critical depth (xcrit) were identified. Significant local H uptake was observed at x > xcrit under conditions where external surfaces were in a passive state and above the H electrode potential (EH/H+). The local potential drops below EH/H+ at a depth, xcrit1= xHER and reaches the primary passivation potential (EPass) of the stainless steel at a depth, xcrit2 = xPass. Thus, at x>xcrit1, the stainless steel experiences H production and uptake at/near the pit bottom but remains passivated. At x>xcrit2, H absorbs on actively dissolving pit surfaces since the local potential drops below EPass. Concurrent metal dissolution and H uptake lead to significant amounts of local H absorption at simulated pit surfaces. The ability of this rescaled approach to provide insight on the development of crack tip conditions that negatively impact environment assisted fracture are further discussed. Acknowledgments: This work was supported in part as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584. Finite element analysis, CALPHAD calculation and assessments of environment assisted cracking were conducted under the auspices of this grant while corrosion electrochemistry and hydrogen analysis was conducted under funding from NSF.

Corrosion Behavior of Mild Steel in Acetic Acid Solutions

Abstract The corrosion behavior of mild steel in acetic acid (CH3COOH) solutions was studied by weight loss and potentiostatic polarization techniques. The variation in corrosion rate of mild steel with concentrations of CH3COOH, evaluated by weight loss and electrochemical techniques, showed marked resemblance. From both techniques, the maximum corrosion rate was observed for 20% CH3COOH solution at all three experimental temperatures (25, 35, and 45°C). Anodic polarization curves showed active-passive behavior at each concentration, except at 80% CH3COOH. Critical current density (ic), passive current density (in), primary passivation potential (Epp), and potential for passivity (Ep) had their highest values in 20% CH3COOH solution. With an increase in temperature, while the anodic polarization curves shifted toward higher current density region at each concentration, the passive region became progressively less distinguishable. With the addition of sodium acetate (NaCOOCH3) as a supporting electrolyte,...

Quantitative comparison of the influences of tungsten and molybdenum on the passivity of Fe-29Cr ferritic stainless steels

The influence of tungsten on the passivity of Fe-29Cr stainless steels was compared quantitatively with that of molybdenum by examining the effects of these alloying elements on passivation parameters such as the pitting potential ( E pit ), the primary passivation potential ( E pp ) and the critical anodic current density ( I c ), which were determined from anodic polarization responses of the alloys in a chloride or an acid solution. As the content of molybdenum and/or tungsten increased, the passive region of the alloys was expanded by increasing E pit , and simultaneously by decreasing both E pp and I c for the alloys. Quantitative relationships were drawn between the passivation parameters and the content of tungsten or molybdenum in the alloys. It was demonstrated that the effects of tungsten on the passivation parameters of the alloys are almost equivalent to those of molybdenum when compared on the basis of atomic per cent, a result of the similarities in chemical and electrical properties between the two elements.

Applicability of bond percolation theory to intergranular stress-corrosion cracking of sensitized AlSl 304 stainless steel

A model system (isothermally sensitized 304 stainless steel in the SL-EPR test environment) was examined in order to assess the significance of the percentage of active grain boundaries on intergranular stress-corrosion cracking (IGSCC) susceptibility. The relationship between the primary passivation potential and Cr concentration was exploited in order (1) to determine the population distributions of grain boundary minimum Cr contents and (2) to subsequently control the quantity of active grain boundaries in a constant extension rate (CER) test. A significant decrease in resistance to IGSCC was observed when greater than 23 pct of the grain boundaries were active during the CER test, regardless of the distribution of Cr-depletion levels among the individual grain boundaries. Such an abrupt decrease in IGSCC resistance at greater than 23 pct active grain boundaries has been interpreted within the context of bond percolation theory. This critical value corresponds to a one-dimensional bond percolation threshold for a three-dimensional array of tetrakaidecahedra and confirms the results of Wells and co-workers. The implications of a possible threshold in the case of IGSCC in sensitized 304 stainless steel (304SS) exposed to oxygenated water at 288 °C are examined. Here it is believed that the 23 pct criterion is a necessary but not sufficient criterion for IGSCC. Other considerations are discussed.

Stress Corrosion Cracking of Dual-Phase Steel in Carbonate/Bicarbonate Solutions

Abstract The stress corrosion cracking (SCC) susceptibility of an experimental, as-rolled, dual-phase steel was investigated in 0.5 M Na2CO3 − 0.5 M NaHCO2 solution at various potentials and temperatures by constant extension rate tensile (CERT) tests. A commercial API X-65 pipeline steel with similar mechanical properties was tested in the same way and the results compared. It was found that the dual-phase steel is less susceptible to SCC than the X-65 pipeline steel in the critical potential range above the primary passivation potential. Briefly-exposed, metallographically-polished dual-phase steel samples were observed to be selectively attacked at the martensite phase in the critical potential range, but since the martensite phase is not aligned in any way, there is no susceptible cracking path. For both steels, a considerable amount of deformation is required before hydrogen embrittlement occurs.

The corrosion behavior of AISI 304L stainless steel in 0.1 M HCl at room temperature—II. The effect of cold working

This paper describes the results of a study concerning the influence of cold working upon the corrosion behaviour of an AISI 304L in HCl 0.1 M at room temperature. Potentiostatic tests, with test times of ≥ 150 h, followed by SEM observations, revealed that even though cold working lowers the critical pitting potential, it does not impair the susceptibility to localized corrosion. Evidence is provided for (1) pitting activity below the primary passivation potential, and (2) preferential attack on either the austenitic matrix or the martensitic phase, depending upon the corrosion potential below the protection field. It can be hypothesized that the above-mentioned behaviour is a consequence of the formation of corrosion products favoured by the presence of two metallic phases. Such products have inhibiting properties which determine pitting corrosion below the primary passivation potential and, in addition, they hinder the formation of passivity films and facilitate their local destruction.

The Effect of Arsenic on the Active-Passive Transition of Titanium

Abstract Anodic polarization experiments on titanium showed that the presence of arsenic markedly decreased the critical current and the primary passivation potential. These results indicate that arsenic acts as an anticorrosive agent.

Interpretation of electrochemical behavior of nickel-implanted type-430 stainless steel using x-ray photoelectron spectroscopy and transmission electron microscopy

Multiple implantations of Ni + ions into annealed type-430 stainless steel were carried out at energies of 25 and 75 keV in order to produce a surface alloy. X-ray photoelectron spectroscopy, coupled with argon ion etching, was used to characterize the surface composition and chemical depth profiles of the implanted species prior to electrochemical studies. Radiation damage and alloy phase structure were evaluated using transmission electron microscopy. Potentio-kinetic studies were carried out in 1 N H 2 SO 4 deaerated by hydrogen. Nickel implantations reduced both the critical and passive current densities. The open-circuit potential was not altered and a change was observed in the primary passivation potential only at the highest concentration of nickel. Potentio-kinetic studies in hydrogen-saturated 1 M NaCl revealed a general enhancement of pitting resistance due to nickel implantation.

Effect of Tempering on the Anodic Polarization Behavior of an 11 Cr‐12 Ni Martensitic Stainless Steel

Anodic polarization measurements in and in were made on an 11 Cr‐12 Ni martensitic stainless steel in the as‐rolled, as‐quenched, and quenched and tempered conditions. Structural changes on tempering between 100° and 600°C were evaluated by hardness measurements, light metallography, and transmission electron microscopy. The basic shape of the anodic polarization curves was not significantly affected by cold work (75% reduction) or by the carbide precipitation, recovery and recrystallization which occurred on tempering. The critical potential for pitting was unaffected by tempering and chloride addition. The primary passivation potential became more active on tempering above 300°C for 1 hr. A 600°C temper, accompanied by significant recrystallization, resulted in severe transpassive region pitting when the steel was polarized in the presence of Cl−.

The Electrochemical Behavior of Titanium: Effect of pH and Chloride Ions

The anodic and cathodic behavior of titanium in sulfuric acid in the absence and presence of chloride ions have been investigated. Two stable corrosion potentials were observed. One was in the active region, the other in the passive region. Neither corrosion potential was appreciably affected by chloride ions or pH changes between pH 0.25 –2.00. The corrosion current of active titanium was not affected significantly by chloride ions up to 3.5% but decreased with increase in pH. Passivation parameters decreased with increase in pH but either remained unchanged or decreased only slightly with addition of chloride ions. The rate of the h.e.r. in the Tafel region was not appreciably affected by chloride ions. The differential capacitance of titanium in solutions of pH 0.25–2.00 ranged from 150 to 200 µf/cm2 at the active corrosion potential and was a reasonably constant value, 25 /µf/cm2, at the passive corrosion potential. For the negative potential range a maximum capacitance, 222 µf/cm2 at which was approximately the primary passivation potential, and a minimum capacitance, 73 µf/cm2 at , were observed. At potentials more positive than the primary passivation potential, the capacitance decreased monotonically with increase in potential and reached a value of 7 µf/cm2 at 3.2V.

Corrosion Behavior of Nickel and Inconel in Acidic Chloride Solutions

Abstract The corrosion behavior of nickel and Inconel in acidic solutions has been investigated by polarization methods. The cathodic Tafel slope for the h.e.r. of nickel in de-aerated 1N H2SO4 and the corresponding exchange current density were 135 mv and 3.4 μa/cm2. Two cathodic Tafel slopes and exchange currents for the h.e.r. were obtained for Inconel in the same solution: 50 mv and 0.22 /μa/cm2; 115 mv and 7.2 μa/cm2. With increase in concentration of chloride ions, the corrosion potential of nickel decreased and the open circuit differential capacitance increased. Each parameter approached a constant value at the higher chloride ion concentrations. Anodic polarization of both metals showed that the primary passivation potential, the critical and passive current densities increased with increase in chloride ion concentration. The passivity of Inconel was shown to be superior to that of nickel in acidic solutions in the absence and presence of chloride ions.

Mechanism of the Corrosion Inhibition of Stainless Steel in Sulfuric Acid by Sodium Molybdophosphate

Sodium 12‐molybdophosphate inhibits the corrosion of an actively corroding type 430 stainless steel sample in sulfuric acid by first polarizing the sample to the primary passivation potential. After this potential is reached, the stainless steel sample passivates spontaneously. From the potentiodynamic polarization curves it was deduced that the addition of sodium molybdophosphate to the sulfuric acid does not significantly alter the anodic characteristics of the type 430 steel in sulfuric acid. The experimental results were in agreement with the electrochemical theory for the passivator‐type inhibitors.

Effect of Halide Additions on Anodic Behavior of Nickel in Sulfuric Acid Solutions

Abstract A potentiokinetic study, dE/dt = 950 mV/hr, has been made of the effects of sodium chloride, bromide and iodide additions, in the range 0.001 to 0.1 M, on the anodic behavior of pure nickel in deaerated N/10 sulfuric acid. Each experiment comprised a forward and reverse sweep in the potential range (relative to the standard hydrogen electrode) −0.5 to +1.2 V. Halide additions affected the anodic behavior of the nickel in both active and passive regions. All halides studied lowered the corrosion potential, raised the primary passivation potential and increased the critical passivation current density. Additions of 0.01 M and greater resulted in elimination of the normal passive region. Reactivation was much greater on reverse sweeps with chloride additions than in either plain acid or with other halides. Negative slope marking the active-passive transition on forward sweep sometimes was eliminated but was evident always on the reverse sweep showing that even when attacks in active and passive regions merge there is a change of electrode process at the normal active passive transition.

The effect of organic acids on the anodic dissolution of nickel alloys and stainless steels

The anodic dissolution rate of some stainless steels, nickel and nickel alloys has been studied by anodic polarization curves in 5N solutions of formic, acetic, lactic and citric acids. The anodic behaviour has been evaluated by the shape of the anodic polarization curves, by the determination of the critical anodic current density, the primary passivation potential and the passive current density. In all cases, the formic acid solutions are the most aggressive for the anodic dissolution process of the materials quoted. A series of spectrophotometric and coulometric measurements has been carried out for the interpretation of the action mechanism of this acid.

THE EFFECTS OF POLARIZATION UPON THE STEEL WIRE-NITRIC ACID MODEL OF NERVE ACTIVITY

The active process in a short length of steel wire passivated by 65 per cent nitric acid has been observed under the influence of a polarizing current, and the form of the potential recorded by the cathode ray oscillograph. In the passive wire, 80 per cent of the total potential drop takes place at the anode, 20 per cent at the cathode. The change from active to passive states, as measured by the potential change, is very abrupt compared to the duration of activity and the potential curve at a point on the wire is probably almost rectangular. The duration of the refractory state is decreased at the anode and increased at the cathode, as in nerve. This fact is against the idea that reactivity after passivation results from a partial reduction of an oxide layer. Soft iron wire passivated by anodal polarization repassivates after activation in acid of a dilution that fails to passivate it initially. It soon becomes rhythmic with a very short refractory phase, and then reacts continuously. Such a wire exhibits a very sharp alternation between a dark brown oxide coat during activity, and a bright clean surface during passivation. A passive steel wire in nitric acid shows many of the characteristics of an inert electrode such as platinum, and it may be inferred that, superposed upon the primary passivation potential, there exists an electrode or oxidation-reduction potential equilibrium between the effects of the various constituents of the solution. It is suggested that the phenomena of nerve-like reactivity in this system may involve an alternation between two protective coatings of the steel wire. During activity, the surface becomes mechanically coated with a brown oxide. If this coating does not adhere, due to gas convection or to rapid solution of the oxide, passivation does not result. Under sufficiently intense oxidizing conditions, a second oxide coat may form in the interstices of the first, and cover the surface as the first coating dissolves off. This furnishes the electrochemical protection of passivation, which is followed by the gradual attainment of electrode equilibrium with the solution.