Crevice corrosion monitoring of titanium and its alloys using microelectrodes

Crevice corrosion of titanium and its alloys in 10% sodium chloride was investigated at 100°C with the aid of microelectrodes. Potential, pH and chloride ion concentration inside the crevice were monitored using an Ag/AgCl electrode, a tungsten microelectrode and a Ag/AgCl chloride ion selective microelectrode, respectively. The pH and Cl^? concentrations within the crevice were calculated from the standard potential‐pH and potential‐log[Cl^?] calibration curves. The effect of Mo on the crevice corrosion of titanium was also studied. The passivation behavior on the titanium and Ti‐15%Mo alloy was studied using electrochemical impedance studies. There was no apparent change in pH and Cl^? ion activity inside the crevice for the alloy at 100°C, whereas a marginal decrease in pH and increase in Cl^? ion concentration were observed for pure titanium. Thus pure titanium is susceptible to crevice corrosion in hot 10% NaCl solutions at 100°C. The chloride ion activity was found to be reduced for the alloy so that the pH inside the crevice increased. The corrosion reaction resistance (R_t) was found to increase with the addition of Mo as an alloying element. It also increases with externally applied anodic potential. Hence, Mo is an effective alloying element, which enhances the crevice corrosion resistance of titanium.     

Corrosion behaviour of zirconium electrode in sodium azide electrolyte as compared to the alkali halide solutions

The kinetics of open circuit growth of an oxide film on zirconium electrode in NaN₃ solution of various concentrations was investigated using potential and capacitance measurements. The protective oxide film formed in azide solutions was found to thicken with time in two steps, the rate of oxide growth was found to decrease as the azide concentration increased. The corrosion behaviour of the electrode was characterized by ac impedance measurements to obtain detailed information about the effect of azide ion concentration on the electrical properties of the Zr electrode surface. The impedance response for the metal‐metal oxide‐electrolyte system was modeled with a transfer function. The potentiodynamic oxidation of Zr was also studied as a function of the azide ion concentration. The polarization curves showed the peak‐shaped active‐to‐passive transition and the corrosion rate was found to increase with increase of azide ion concentration. The activation energy of corrosion was calculated according to Arrhenius plot and found to be 14.5 kJ mol^?1. The polarization results in azide solutions were compared to those obtained in solutions of different halide ions, namely, F^?, Cl^?, Br^? and I^? ions. The rate of corrosion was found to decrease in the order Br^? > Cl^? > F^?, I^? > azide. Opposite to the behaviour in azide solutions, the halide ions do not show the active‐passive transition.     

Effects of coolant chemistry on corrosion of 3003 aluminum alloy in automotive cooling system

In this work, effects of coolant chemistry, including concentrations of chloride ions and ethylene glycol and addition of various ions, on corrosion of 3003 Al alloy were investigated by electrochemical impedance spectroscopy measurements and scanning electron microscopy characterization. In chloride‐free, ethylene glycol–water solution, a layer of Al‐alcohol film is proposed to form on the electrode surface. With the increase of ethylene glycol concentration, more Al‐alcohol film is formed, resulting in the increase in film resistance and charge‐transfer resistance. In the presence of Cl^? ions, they would be involved in the film formation, decreasing the stability of the film. In 50% ethylene glycol–water solution, the threshold value of Cl^? concentration for pitting initiation is within the range of 100 ppm to 0.01 M. When the ethylene glycol concentration increases to 70%, the threshold Cl^? concentration for pitting is from 0.01 to 0.1 M. In 100% ethylene glycol, there is no pitting of 3003 Al alloy even at 0.1 M of Cl^?. Even a trace amount of impurity cation could affect significantly the corrosion behavior of 3003 Al alloy in ethylene glycol–water solution. Addition of Zn²⁺ is capable of increasing the corrosion resistance of Al alloy electrode, while Cu²⁺ ions containing in the solution would enhance corrosion, especially pitting corrosion, of Al alloy. The effect of Mg²⁺ on Al alloy corrosion is only slight.     

Effect of the applied potential on the potential and current distributions within crevices in pure nickel

The electrode potential profile within a crevice was measured in situ during crevice corrosion of nickel in 1N H₂SO₄ under conditions of effective convective mixing of the crevice and bulk electrolytes. Oxidizing power was investigated by applying potentials in the passive region. A steep potential gradient and constant pH were always measured during crevice corrosion. The magnitude of the potential drop and the distance into the crevice of the passive-to-active transition, X_pass,both increased linearly with increasing applied potential, in accordance with the IR>ΔΦ1 criterion. The in situ measured potential E_passatX_pass was, at all times, constant and in the range of the passive-to-active transition for the polarization curve of the bulk solution.     

Galvanic cells encountered in the corrosion of steel reinforcement — III. Differential surface condition cells

Galvanic cells resulting from coupling passive/active or rusted/bright electrodes have been investigated in alkaline solutions, in absence or presence of Cl^? ions. The results indicated that in absence of Cl^? ions, the pre-passivated electrodes were protected from corrosion in solutions of pH 10 and 10·5 when coupled with normal steel electrodes. The steady-state galvanic current density increased with the decrease of pH, accompanied by a slight increase of cell e.m.f., a negative shift in the mixed potential and depolarization of the cathodic process. In presence of Cl^? ions, the corrosion rate was determined by the kinetic retardation of the corrosion system and the cell e.m.f. Computation of the ohmic drop was found to be highest in NaOH of pH 10 (ohmic control ? 19·4%). The passive film formed on a bright electrode had better passivating properties than that formed on an electrode having originally mill-scale on its surface. Breakdown of passivity of the non-rusted electrode occurred when coupled with rusted steel in just inhibiting solutions. In corrosive solutions, there was an initial enhancement of corrosion of the bright electrode which decreased greatly with time accompanied by activation of the rusted electrode.     

Initiation and repassivation of crevice corrosion of type 444 stainless steel in chloride solution

Effects of chloride ion concentrations, solution temperature, and crevice-forming materials on the crevice corrosion of type 444 stainless steel were investigated using a potentiostatic method. Critical crevice potential (E_crev) and repassivation potential (E_r) of the creviced alloy decreased with an increase in chloride concentration [Cl⁻], satisfying the logarithmic relationship between E and [Cl⁻]. In addition, E_crev and E_r of the alloy with silicone crevice former were measured to be higher than those of the alloy with an EPDM (Ethylene Propylene Diene Monomer) crevice former, suggesting that silicone was more effective in preventing water from penetrating crevices between a stainless steel sheet and the crevice former. In electrochemical current transient measurements with an applied potential, the intensity of current transients corresponding to the initiation of metastable pits increased abruptly near the E_r of the alloy, indicating that the stability of crevice corrosion is associated with the initiation of metastable pits.     

The effect of molybdate anion on the ion-selectivity of hydrous ferric oxide films in chloride solutions

The ion-selective property of hydrous ferric oxide precipitate films has been investigated by measuring membrane potentials which arise across precipitate membranes of hydrous ferric oxide with and without adsorbed MoO₄^2? ions and of ferric molybdate in solutions of NaCl, KCl, MgCl₂, CaCl₂, BaCl₂, AlCl₃, and FeCl₃. The hydrous ferric oxide membrane was only permeable to Cl^? ions in chloride solutions, whereas the membrane with adsorbed MoO₄^2? ions was permeable to cations in NaCl and KCl solutions, and to both Cl^? and cations in the presence of multivalent cations. The ferric molybdate membrane was permeable to Cl^? and cations in NaCl and KCl solutions, and only to Cl^? ions in the presence of multivalent cations. It is suggested that in chloride solutions, the corrosion of iron covered with a precipitate film of hydrous ferric oxide is accelerated by enrichment of Cl^? ions under the film, which may decrease the local pH and introduce a positive diffusion potential in the film. The adsorption of MoO₄^2? ions on the oxide changes the ion-selectivity of the precipitate film from the anion-selective to the cation-selective in solutions of NaCl and KCl. This cation-selectivity of the film may inhibit the corrosion of iron, because of H⁺ ions diffusing out of the film. The inhibitive effect of MoO₄^2? ions would be reduced in the presence of multivalent cations.     

Effects of Halides on the Corrosion of Mild Steel in Molten NaNO3-KNO3 Eutectic

Abstract

The effect of alkali halides on the corrosion behaviour of mild steel in, (Na,K)NO₃ eutectic has been studied at temperatures ranging from 300°–450°c. Steady-state potentials of the steel electrode vary with the concentration of Cl^?, Br^?, and I^? ions according to: E = a + b log C. Gravimetric; measurements show that the corrosion process of steel in the melt without and with halide ions, at concentrations ≤0·l M, is essentially the same, the weight gain being parabolic with time. At higher halide concentrations the corrosion rate is linear with time. The corrosion rate also increases with increase in temperature. Both potential and weight gain measurements show that aggressiveness increases in the order: Cl^?<Br^?<I^?. It is suggested that in this system two opposing forces compete on the steel surface. The oxide ions help in building or repairing a passivating film of Fe₃O₄, whilst the aggressive halide ions attack weak areas in the film and assist the corrosion process. The process which prevails depends on the relative concentrations of the two species.

In the case of fluoride, both the potential and weight gain of steel vary with time in an irregular manner, and no definite relation could be obtained. This has been attributed to the low solubility of fluoride in the melt and to the properties of the corrosion products, in that the film formed does not adhere to the metal surface.     

Electrochemical and SEM study on Type 254 SMO stainless steel in chloride solutions

An investigation was conducted to examine the critical crevice potential (E_crev) and the critical protection potential (E_prot) for Type 254 SMO stainless steel in 4% NaCl solution by using potentiodynamic cyclic anodic polarization (PCAP) technique at temperature ranging from 30 to 90 °C. The critical crevice temperature (CCT) and the critical crevice protection temperature (T_prot) were determined by plotting the values of breakdown potential and E_prot versus solution temperature, respectively. The values of CCT and T_prot were recorded at the abrupt transition with increasing the temperature from transpassive corrosion to crevice corrosion and were found to be at 55 and 52 °C, respectively. Above CCT (70 °C) the following points were recorded. The E_crev and E_prot decreased linearly with log [Cl⁻]. The addition of bromide ions to chloride ions at a fixed halide content of 4% increased both E_crev and E_prot. The E_crev value in 4% NaCl increased linearly with increasing pH in the range 1-10. The addition of 0.5 M bicarbonate ions to 4% NaCl completely removed the crevices effect while increasing the addition of sulphate ions to 4% NaCl increased both of E_crev and E_prot. The morphology of the crevice corrosion produced on the steel surface was examined by scanning electron microscope (SEM) after PCAP treatment under different test conditions.     

Corrosion behavior of hafnium in anhydrous isopropanol and acetonitrile solutions containing bromide ions

The corrosion behaviors of hafnium in Et₄NBr isopropanol and acetonitrile(ACN) solutions were investigated using electrochemical measurements, ICP-AES and SEM techniques. Results revealed that the open circuit potential gets more positive due to the increased passivity of the surface oxide film with increasing immersion time until it reaches a steady state value. The potentiodynamic anodic polarization curves did not exhibit an active dissolution region near corrosion potential due to the presence of an oxide film on the electrode surface, which was followed by pitting corrosion. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allowed the pitting potential (?_pit) and the repassivation potential (?_p) to be determined. ?_pit increased with increasing potential scanning rate but decreased with increasing temperature, Br^? concentration and ACN concentration. The impedance spectra showed that the resistances of the solution and charge transfer decreased with the increase of ACN concentration.     

On the pitting corrosion of tin in aqueous solutions

Four independent techniques were employed to prove that Sn undergoes pitting corrosion. The first was based on the measurement of the variation of the open circuit potential of the Sn electrode in aerated Cl^? solutions of various concentrations. Steady-state potentials were attained slowly and erratically from negative values, and were more positive the higher the dilution. Sn electrodes prepassivated in CrO₄^2? solutions responded readily to additions of Cl^? ion. The potentials developed were more noble than those measured in presence of the passivator alone, and changed to positive values with the increase of the concentration of the pitting corrosion agent. Attack was under cathodic control.Galvanostatic polarization of the Sri electrode was carried out in 0.005 to 0.1M NaOH, in the presence of various additions of Cl^?. Above a certain Cl^? content, contingent upon the alkali concentration, the aggressive anion prevented the evolution of Oa on the electrode, and oscillations in the E?t curves were recorded. Competitive adsorption of Cl^? and OH^? is assumed to occur, which affected both the quantity of electricity, Q_p, consumed along the oxide formation steps, and the rate of potential increase, dE/dt, following oxidation. Plots of the two variables as a function of the Cl^? ion concentration exhibited a definite break at the value characteristic for the initiation of pitting attack. Potentiodynamic polarization showed that the pitting corrosion potential progressively shifts towards negative values as the concentration of the aggressive agent in solution was increased. The pitting corrosion currents resulting from the addition of Cl^? to Sn electrodes prepassivated in CrO₄^2? solutions were measured. The dependence of the maximum currents on solution composition is explained on the basis of competitive adsorption.     

Corrosion behavior of Cu/Al casting-rolled clad plates in different alkaline solution

The effect of pH value and different kinds of anions on the corrosion behavior of Cu/Al casting-rolled clad plates in the alkaline solution was evaluated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), weight loss analysis, 3D confocal laser scanning microscopy (CLSM) and electrochemical test. Results show the corrosion mainly occurs on the aluminum side. The corrosion resistance of the Cu/Al decreases as the pH value increases. When pH≥12, the dissolution of the film layer is faster than the passivation process. The addition of Cl⁻ ions reduces the corrosion resistance of the Cu/Al clad plates, which leads to pitting corrosion. The higher the concentration of Cl⁻ ions, the more prone the pitting to occur. The addition of SO₄²⁻ ions causes the denudation of the samples. The corrosion resistance of the Cu/Al is better in the alkaline solution containing NO₃⁻ ions than that in the solution containing Cl⁻ ions or SO₄²⁻ ions. When adding SO₄²⁻, NO₃⁻ and Cl⁻ to the pure alkaline solution, the corrosion resistance of the Cu/Al clad plates decreases.

     

Determination of the zero-charge ph value of a passive iron surface in aqueous solution

Two techniques of measurement of pH₀, which represents the pH of solution at which the total charge of passive metal electrode surface is zero have been developed. The procedures are based on the use of acid-base equilibria between oxide on electrode surface and solution. In the former technique, the pH of the drop of solution that is applied onto the electrode surface is measured. At pH₀, the pH of the drop does not change in time. In the latter procedure, the potential of a passive electrode that is immersed into solutions with various pH values is measured. pH₀ has been measured on a passive iron electrode. The pH₀ values measured by the two procedures are similar. Comparison of pH₀ of passive electrode and various oxides one to determine the composition of a passive film. The introduction of surface-active ions Ba²⁺ and Cl^? to the solution leads to a shift of pH₀ values and electrode potential in opposite directions.     

New insights into copper corrosion in presence of benzotriazole and chloride ions

The effect of Cl^– concentration and temperature on the copper corrosion in NaCl solution in presence of 40?mg?L^–1 benzotriazole was investigated using cyclic voltammetry, electrochemical impedance spectroscopy, electrochemical noise (EN) and scanning electron microscopy (SEM). Results revealed that the charge transfer resistance decreased with increasing the Cl^– concentration and temperature. The EN generated at low Cl^– concentration and low temperature had small noise amplitude, while the EN showed large potential oscillation amplitude in the case of high Cl^– concentration and high temperature. A parameter, named corrosion active energy C_AE, was proposed to further study the relationship between the EN feature and corrosion rate and corrosion severity (SEM). The variation trend of corrosion active energy C_AE with Cl^– concentration and temperature was found to be opposite with energy gap E_g and charge transfer resistance. Thus, C_AE shows a promising potential to in situ evaluate the corrosion rate and corrosion severity.     

Synergistic or additive corrosion inhibition of mild steel by a mixture of HEDP and metasilicate at pH 7 and 11

Electrochemical measurements (steady‐state current‐voltage curves and AC impedance) were coupled with mass‐loss measurements, SEM examinations, and EDSX analyses to investigate the inhibition of corrosion of a carbon steel by a mixture of phosphonic acid HEDP (acid 1, hydroxyethylene, 1‐1 diphosphonic) and sodium metasilicate pentahydrate Na₂SiO₃. 5H₂O in an industrial hard water containing 3.10^?3 M Ca²⁺ ions. At pH 7, HEDP and Ca²⁺ act in a synergistic manner, by formation of a HEDP and calcium containing layer. Addition of silicate at this pH value, allows to reach an efficiency of 94% due to an additive inhibition effect. At pH 11, metasilicate, HEDP, and Ca²⁺ ions reinforce the passive layer in a synergistic way. The mixture (1.7 · 10^?5 M HEDP + 2.6 · 10^‐3 M SiO^2?₃) in the Ca²⁺ containing electrolyte is shown to be able to inhibit efficaciously the corrosion of iron at room temperature, considering uniform corrosion at pH 7 or pitting corrosion at pH 11.     

Galvanic cells encountered in the corrosion of steel reinforcement — IV. Differential aeration cells

Differential aeration cells were investigated in alkaline solutions of different pH values in absence or presence of Cl^? ions. In nitrogen-saturated solutions and in oxygen-saturated solutions of pH < 11, the uncoupled steel electrodes corroded.The results of coupling indicated that steel electrodes immersed in aerated solutions of pH 10 and 10·5 and in nitrogen-saturated solution of pH 12 were completely protected from corrosion. In chloride-free galvanic cells, corrosion was of the general type, which increased with the decrease of pH and was mainly determined by the kinetics of the cathodic reaction and the cell e.m.f. However, in presence of Cl^? ions, corrosion was of the pitting type and was controlled by the cell e.m.f. and the kinetic retardation of the corrosion process.     

Effect of molybdate ions as corrosion inhibitors of iron in neutral aqueous solutions

Abstract

The effect of molybdate ions on the corrosion of Fe in neutral solutions was investigated by electrochemical measurements (dc polarisation and impedance spectroscopy) together with gravimetric determinations. Studies were conducted in solutions containing sodium hydrogen/sodium sulphate salts with molybdate concentrations ranging between 10^?4 and 10^?2M at pH 8 and 9. Mass loss measurements indicated that about 10^?3M of molybdate was necessary in order to inhibit completely the corrosion of Fe at room temperature. The potentiodynamic polarisation and electrochemical impedance studies gave indications about the mechanism of action of the MoO^2?₄ ion. They also showed that the inhibiting effect of the oxyanion is increased in the presence of dissolved oxygen. Spontaneous passivation of the corroding Fe electrode could happen only in the presence of dissolved oxygen at concentrations greater than 10^?4M. However, passivity was also obtained under potentiodynamic polarisation conditions in deaerated solutions. The effect of the oxygen was attributed to displacement of the corrosion potential into the region of selfpassivation of the steel. Finally, the results indicated that the corrosion inhibition of Fe in neutral solutions by molybdate ions was largely insensitive to pH over the range from 8 to 9.     

Eine charakterisierung der lochfraßkorrosion des nickels—I. Die bestimmung der lochfraßpotentiale in neutralen und alkalischen lösungen

Quasistationary values for the characteristic pitting potentials for nickel were determined by means of potentiokinetic polarization measurements and their dependence on chloride and hydroxide ion concentration was investigated. The pit nucleation potential U_np is a linear function of the logarithm of the anion concentration. The value being determined by the adsorption equilibrium of the anions on the passivated metal surface.The critical pitting potential U_cp depends on the pH of the solution only. Above pH 6-5 U_cp decreases with increasing pH. On the basis of the mechanism for crevice corrosion this behaviour may be explained by the influence of OH^?-ions being known to take part in the ionization of the metal atoms.     

Miniaturisierte elektrochemische Sensoren zur in-situ-Untersuchung der Spaltkorrosion in Modellsystemen

In-situ-investigation of crevice corrosion in model systems using miniaturized electrochemical sensors Measuring characteristics of miniaturized sensors for the determination of the parameters pH value, oxygen content, concentration of chloride ions and redox potential was tested and optimized with regard to the application in model crevices. The sensors were used for examinations of crevice corrosion in 0.1 molar and 1 molar solution of NaCl at the following alloys: X 20 Cr 13, X 2 CrNi 18 10, X 2 CrNiMoN 17 13 5, X 2 CrNiMoN 17 13 5*, X 2 CrNiMo CuN 20 25 6. Corrosion characteristic of the alloys and the change of crevice electrolyte without external polarization and with galvanostatic polarization (i = 10 μA/cm², i = 200 μA/cm²) were tested and discussed by the hand of the model concepts from Oldfield and Sutton [1]. In experiments with galvanostatic polarization the steels with increasing content of chromium or molybdenum showed a growing stability against crevice corrosion. The results showed that simultaneous measurements of the above mentioned chemical parameters of crevice electrolyte in connection with the corrosion potential can supply additional information about causes and mechanism of crevice corrosion.     

Predicting the chemistry,corrosion potential and corrosion rate in a crevice formed between substrate steel and a disbonded permeable coating with a mouth

A model developed in an earlier work was used in this work to investigate the effect of coating permeability on the evolution of solution chemistry, corrosion potential, and rate in a crevice formed between a steel surface and a coating disbonded from it. The crevice gap varies along distance from the mouth, and the coating is permeable to ions and/or oxygen (O₂). The earlier work focused specifically on modeling the effect of variable gap (on crevice corrosion) with the coating impermeable to either ions or O₂. In both works, the crevice chemistry was an aerated, diluted sodium chloride solution, which at the mouth was set to be different from that initially in the crevice. The results of this work show that a permeable coating behaves like a membrane, which, under a cathodic polarization at the crevice mouth, tends to raise the in-crevice sodium ion concentration and pH more rapidly relative to an impermeable coating. Later, as the sodium ion concentration and pH in the crevice become greater than at the mouth, the permeable coating tends to reverse the transport direction for ions. At a mouth potential of ?0.900 V vs. saturated Cu/CuSO₄, the cathodic current is sufficient to suppress all O₂ penetrating the crevice both from the mouth and through the coating. The practical implication is that in the presence of sufficient cathodic polarization, a permeable coating, when disbonded, can still be capable of protecting the substrate steel from corrosion attack.