Effect of bromide ions on the corrosion behavior of tantalum in anhydrous ethanol

The electrochemical behaviors of Ta in Et₄NBr ethanol solutions were investigated using potentiodynamic polarization, cyclic voltammetry, potentiostatic current-time transient and impedance techniques. The potentiodynamic anodic polarization curves did not exhibit active dissolution region due to the presence of thin oxide film on the electrode surface, which was followed by pitting corrosion as a result of passivity breakdown by the aggressive attack of Br⁻ anions. The pitting potential (E_b) decreased with the increase of solution temperature and Br⁻ concentration, but increased with increasing potential scan rate and water concentration. The incubation time derived from potentiostatic current-time transients decreased with increasing potentials. The impedance spectra exhibited two time constants for all the potentials and the resistance of passive layer decreased with increasing potential.     

Stress corrosion in brass considered against the background of potential/pH diagrams

The paper describes a study of stress corrosion in brass (62·8 per cent Cu, 37·2 per cent Zn) exposed to ammoniacal copper sulphate solution. The relation between the stress corrosion behaviour and the composition of the corrosive medium is discussed against the background of potential/ pH diagrams for the corroding system.

The time to cracking was found to be very dependent on the pH. At pH 2·0 cracking did not take place within 1000 h of exposure. At about pH 7·3 the time to cracking was at its minimum value, 1–2 h. The results indicated that the process is governed by the formation of a black surface coating of copper oxide on the metal, but counteracted by the precipitation of basic copper sulphate. The cracks were generally transgranular in the absence of black oxide but intergranular in the presence of such a coating. It appeared that the shape of the cracks was influenced by the precipitation of basic copper sulphate. Reduction reactions likely to cause metal dissolution during stress corrosion are discussed for different pH ranges. According to the potential/pH diagram the formation of Cu(NH₃)₂⁺ ions is in general important.     

水泥硬化体在(NH4)2SO4溶液中的溶解动力学

水泥基材料在服役过程中会受到来自外界的腐蚀,其中(NH4)2 SO4腐蚀是一种较为严重的硫酸盐腐蚀.通过测定水溶液中的pH值的变化分析水泥硬化体在(NH4)2 SO4溶液中的溶解过程.结果表明,水泥硬化体粉末在(NH4)2 SO4溶液中的pH值的变化趋势符合Avrami溶解模型,即pH值随时间的增加而增加,从而得出水泥硬化体粉末的溶解速率与(NH4)2 SO4溶液的浓度之间的关系.     

The corrosion of copper in ethylene glycol-water mixtures containing chloride ions

The anodic oxidation of copper at 25°C in 50% (w/w) ethylene glycol-water and in aqueous solutions has been studied by linear sweep voltammetry. The effect of chloride concentration at pH 0 and 3 has been explored. The results in both solvents follow a similar pattern. At pH 0 and in the absence of chloride, only one anodic peak is observed corresponding to the dissolution of copper metal as copper(II) ions. At intermediate chloride concentrations (0.01–0.03 M), two additional peaks are detected which have been attributed to the following reactions: $$\begin{gathered} Cu + Cl^ - \to CuCl + e^ - \hfill \\ CuCl \to Cu^{2 + } + Cl^ - + e^ - \hfill \\ \end{gathered} $$ When the chloride concentration is increased further, the three peaks gradually collapse back into one, corresponding to the dissolution of copper as a copper(I) chloro-complex. An additional peak appears at pH 3 which has been ascribed to the formation of copper(I) oxide. The results have been interpreted usingE-pCl diagrams determined for the copper-chloride system in both 50% ethylene glycol-water and aqueous solutions. Further information has been obtained from rotating disc measurements and from microscopy. The relevance of these results to corrosion in automotive cooling systems is discussed.     

Investigation of copper corrosion inhibition by STM and EQCM techniques

The mechanism of copper corrosion and its inhibition were studied using electrochemical techniques, in situ scanning tunneling microscopy (STM) and electrochemical quartz crystal microbalance (EQCM) methods. The morphological changes of Cu (111) were followed in 0.1 M Na₂SO₄ solution at pH 2.95. The adsorption of 5-mercapto-1-phenyl-tetrazole (5-McPhTT) at different concentrations was studied. The anodic dissolution of copper in 0.1 M Na₂SO₄ was clearly modified and hindered by the addition of 5-McPhTT. The anodic current density was reduced sharply due to the adsorption of the inhibitor on the metal surface thus providing protection. EQCM data revealed that the addition of the inhibitor to the aggressive solution did not cause a continuous increase in the electrode mass. This fact indicates that the inhibitor effect in hindering copper corrosion was due to the adsorption of a monolayer or even a submonolayer.     

Investigations of nonlinear processes by transients: kinetics of dissolution of inhibitor layers on copper

The dissolution of inhibitor layers of AHT (3-amino-5-heptyl-1,2,4-triazole) on copper wires in 0.5m H₂SO₄ was studied by potentiostatic current and capacity transients in a time range from 10 µs to 1000 s. The potential dependent transients show three characteristic regions corresponding to different transfer controlled processes. For short periods an almost constant current of inhibited corrosion through a metastable inhibitor layer is observed. In the second region the dissolution of this layer causes a strong increase in copper corrosion, which was taken as a direct measure of the desorption process. The electrode capacity,C, increases correspondingly. The dissolution itself is not accompanied by a notable charge transfer and, hence, cannot directly be monitored. The last region is dominated by an almost constant active copper dissolution current. The layer dissolution in the second region starts with an inhomogeneous nucleation process. Initially, holes are generated stochastically in the AHT layer and form nuclei of corrosion pits on the copper surface. Based on this model the complete transients can be calculated.Notation b _free Tafel slope of an uncovered copper electrode - b _nucl potential dependence of pit formation (Equation 23) - b ₃ b ₃ = dU/d log (di _pits/dt ³) - C electrode capacity - D dielectric constant of the layer - D ₀ dielectric constant of the vacuum - d layer thickness - F Faraday constant - I ₁ time dependent corrosion current of a single pit - i total current density - i _free corrosion current density of an uncovered copper electrode - i _inh current density of an inhibited copper electrode - i _pits corrosion current density of n pits - i _0v extrapolated value ofI _free at 0 V vs SHE - n time dependent number of pits - n _max maximum number of pits - r time dependent radius of a single pit - S total area of all pits t time - U potential against SHE - U _nucl potential of hole formation - V molar volume - z charge number - degree of coverage of inhibitor This paper is dedicated to Professor Brian E. Conway on the occasion of his 65th birthday, and in recognition of his outstanding contribution to electrochemistry.     

Surface protection of copper in aerated 3.5% sodium chloride solutions by 3-amino-5-mercapto-1,2,4-triazole as a copper corrosion inhibitor

Surface protection of copper in aerated 3.5% NaCl solutions by 3-amino-5-mercapto-1,2,4-triazole (AMTA) has been reported. The study has been carried out using weight loss, pH, potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and chronoamperometric (CA) measurements along with scanning electron microscopy (SEM) and energy dispersive X-ray (EDS) investigations. Weight-loss data indicated that the dissolution rate and the pH of the solution decreased to a minimum after 24 days of copper immersion due to the inhibitive action of AMTA. PDP, CA, and EIS measurements showed that AMTA decreased the corrosion rates and increased the polarization resistance and inhibition efficiency. This effect was increased with increasing AMTA concentration as well as the immersion time of the copper electrode to 50 h before measurements. SEM micrograph and EDS analysis proved that the inhibition of copper corrosion takes place due to adsorption of AMTA onto the surface. These results together confirmed that AMTA is a good mixed-type inhibitor and the inhibition of copper corrosion is achieved by strong adsorption of AMTA molecules.     

Selection of an oxidant for copper chemical mechanical polishing: Copper-iodate system

Diagnostic criteria were developed to elucidate the reduction mechanism of an oxidant on a copper (Cu) surface at the corrosion potential. The corrosion potential of Cu tu was measured for various pH and iodate (IO₃^–) concentrations using the rotating disk electrode technique. According to the measured corrosion potentials, IO₃^– was an effective CMP oxidant only below pH Application of the diagnostic criteria on the Cu – IO₃^– system showed that the reduction of IO₃^– on Cu was under the mixed kinetic and diffusion control at the corrosion potentia l below pH 3. Above pH 3, however, the anodic process dominated over the cathodic process.     

Atomic emission spectroelectrochemistry applied to dealloying phenomena: I. The formation and dissolution of residual copper films on stainless steel

The rates of elemental dissolution were measured for a 304 stainless steel containing 0.19% Cu in real time during linear sweep voltammetry experiments using the atomic emission spectroelectrochemistry (AESEC) method. The results demonstrate that Fe, Cr, Ni, Mn, and Mo dissolve simultaneously leaving a residual copper film that inhibits the steel dissolution. The formation and dissolution of the copper film leads to the appearance of two peaks in the anodic dissolution transient, one due to inhibition of steel by residual copper and the other due to the formation of the passive film. The addition of NaCl to the electrolyte results in a marked increase in the intensity of the second dissolution peak, while hardly affecting the first peak. This is interpreted in terms of the lowering of copper dissolution potential by chloride ions due to the stabilization of CuCl₂⁻. A simple phenomenological kinetic model is used to simulate the variation of dissolution rate with potential.     

pH-dependent anodic reaction behavior of tungsten in acidic phosphate solutions

Potentiodynamic and potentiostatic polarization experiments, and the electrochemical impedance spectroscopy technique were used to study the pH dependent anodic behavior of tungsten (W) in acidic phosphate solution. At very low pH values (pH < 2.6) the dissolution of tungsten was H⁺-assisted and as the pzc (pH 2.6) was approached H₂O-assisted dissolution became main dissolution pathway. Above pH 2.6, however, tungsten dissolution was OH⁻-assisted. The thickness and dielectric properties of the W-oxide barrier layer were observed almost pH independent at corrosion potential. The oxygen vacancy transport across the oxide film caused a capacitive response at very acidic solutions (pH ≤ 3.5) and as the dissolution rate increased (pH > 4.5) the capacitive response turned into the inductive one due to the accelerating effect of negative surface charge in Tafel region. The inductive response in the tungsten impedance spectra shifted to a very low frequency range as the tungsten dissolution rate decreased in the pseudo-plateau and potential independent regions. Fitting of the tungsten impedance data according to the surface charge approach showed that the resistance to the defect migration increased as the pzc was approached and the film capacitance decreased above pH 3.5 due to the accelerated formation rate of the non-protective loosely bound hydrated layer on the metal oxide surface.     

Accelerated degradation method for cement under CO2-rich environment: The LIFTCO2 procedure (leaching induced by forced transport in CO2 fluids)

For emerging applications such as long-term CO₂ storage, establishing and maintaining well integrity during the life of the well pose significant challenges. Particularly, well cement will need to be durable under corrosive environments, such as exposure to CO₂-rich fluids, over a long period of time from 25 years to 100-1000 years. To model long-term cement durability in such severe environments, accelerated degradation methods must be considered and developed. This study presents experimental equipment and a procedure showing that it is possible to accelerate Portland cement degradation in the presence of CO₂ fluids by applying a potential gradient through a set cement sample in a CO₂ environment (the LIFTCO₂ procedure). The degradation pattern consists of a series of successive fronts penetrating the cement sample: a dissolution front, a carbonation front, and a gel layer. Such degradation is accelerated when increasing the potential gradient, the temperature, and the CO₂ concentration in the electrolyte.     

Evaluation of mechanically treated cerium (IV) oxides as corrosion inhibitors for galvanized steel

The use of cerium salts as corrosion inhibitors for hot dip galvanized steel has been object of a numerous studies in the last few years. The role of cerium ions as corrosion inhibitors was proved: cerium is able to block the cathodic sites of the metal, forming insoluble hydroxides and oxides on the zinc surface. This fact leads to a dramatic decrease of the cathodic current densities and, therefore, to a reduction the overall corrosion processes. On the other hand, the potential of cerium oxides as corrosion inhibitors was also proposed. However, the real effectiveness of this kind of anticorrosive pigments has not been clarified yet.In this work cerium (IV) oxides are considered as corrosion inhibitors for galvanized steel. The corrosion inhibition mechanism of mechanically treated (milled) CeO₂ alone and in combination with milled SiO₂ nanoparticles was investigated. For this purpose milled CeO₂, CeO₂ and SiO₂ milled together and milled SiO₂ particles were studied as corrosion inhibitors in water solution. Therefore, the different mechanically treated particles were dispersed in 0.1 M NaCl solution to test their effectiveness as corrosion inhibitors for galvanized steel. The galvanized steel was immersed in the different solutions and the corrosion inhibition efficiency of the different particles was measured by means of electrochemical techniques. For this purpose, electrochemical impedance spectroscopy (EIS) measurements were carried out, monitoring the evolution of the corrosion processes occurring at the metal surface with the immersion time in the solution. The effect of the different pigments was also investigated by carrying out anodic and cathodic polarization measurements. The polarization curves were acquired under conditions of varied pH. The experimental measurements suggest that the mechanical treatment performed on the SiO₂ and CeO₂ particles promote the formation of an effective corrosion pigment. The tests evidence also the beneficial effect of the CeO₂ milled particles when used in combination with the mechanically treated SiO₂ particles. It was proved that in alkaline conditions the effect of the mechanically treated CeO₂ and SiO₂ particles is dramatically increased.     

Electrochemical impedance studies of modified Ni-P and Ni-Cu-P deposits in alkaline medium

Ni-P and Ni-Cu-P deposits were supported over the commercial carbon using the electroless plating technique. The formed samples were characterized by applying SEM, XRD and EDX analyses. An amorphous Ni-P surface was obtained with 73.70 wt% Ni and 11.45 wt% P. The addition of copper to the plating bath reduces the deposited amount of nickel and phosphorus. The electrochemical performance of these deposits has been investigated in 0.1 M KOH solution using electrochemical impedance spectroscopy (EIS) measurements. The effect of pH, deposition time and temperature of the plating bath on the impedance characteristics of the two deposits was studied. It was found that the resistance (R_T) and relative thickness (1/C_T) of the two coatings in 0.1 M KOH solution increase with increasing either pH or deposition time or temperature of the plating bath. Our results indicate that Ni-Cu-P deposit has more corrosion resistance and lower corrosion current density (i_corr) value than Ni-P deposit under different conditions. EIS results were well confirmed by potentiodynamic polarization and cyclic voltammetry techniques.     

Corrosion behavior of silicon nitride bonded silicon carbide refractory material by molten copper and copper slag

Silicon Carbide refractories may be used in contact with copper and copper slags. The object for the investigation of corrosion by the melt of copper and the copper slag was SiC siphon block of the slag collector in the runner of the cathode shaft furnace. It is exposed to the permanent flow of copper with small amount of slag. Slag on the surface of the melt is stopped by siphon block and it suffers the most extensive corrosion wear, but the same time it is a good object for investigation, because different parts of this plate are exposed to intensive corrosion by different corrosive agents.General observations show, that the corrosion of Si₃N₄-SiC by slag is sufficiently more extensive and the wear is about 3 mm per month, while the wear by the flowing copper is below 1 mm per month.The observation of the cross section of the exposed Si₃N₄-SiC plate on macro level shows 4–5 zones of different color. Microstructural observations show almost no changes of the material in direct contact with molten copper without exposure of air, that suggests slow dissolution of Silicon Carbide and Silicon Nitride in the flowing melt of copper (physical dissolution in case of permanent removal of reactants). The most severe wear of Si₃N₄-SiC refractory is by slag, because the chemical erosion takes place.     

Electrochemical and surface properties of aluminium in citric acid solutions

The electrochemical and surface properties of aluminium in 0.05 M citric acid solutions of pH 2–8 were studied by means of open circuit potential (OCP), potentiodynamic polarization and potentiostatic current–time transient measurements. The OCP reached a steady-state value very slowly, probably due to the slow rate of detachment of surface complexes into the solution. Aluminium exhibits passive behaviour in citric acid solutions of pH 4–8. Tafel slopes b _c were characteristic for hydrogen evolution on aluminium covered by an oxide film. The corrosion kinetic parameters E _corr, i _corr and b _a suggest that surface processes are involved in the dissolution kinetics, especially in the pH range 3–6. Current–time transient measurements confirm that, in citric acid solutions of pH 3–6, the dissolution is controlled by surface processes, that is, by the rate of detachment of surface complexes, while in solutions of pH 2, 7 and 8 dissolution is under mass-transport control. The addition of fluoride ions to citric acid changes the controlling steps of the dissolution process. Citrate and fluoride ions compete for adsorption sites at the oxide surface, and adsorption of these ions is a competitive and reversible adsorption.     

The roles of macrosegregation and of dendritic array spacings on the electrochemical behavior of an Al-4.5 wt.% Cu alloy

The microstructural pattern and the solute redistribution of as-cast aluminum alloys play an important role on the resulting corrosion behavior. However, of the main aluminum alloys, Al-Cu alloys have the lowest negative corrosion potential. The purpose of this work was to evaluate the electrochemical behavior of an Al-4.5 wt.% Cu alloy solidified under unsteady-state heat flow conditions. This evaluation was carried out through the analysis of both potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) tests in a 0.5 M NaCl solution at 25 °C. The experimental segregation profile obtained in the solidification experiment was characterized by positive and negative copper content regions at the bottom and the top of the casting, respectively. Likewise, in conventional foundry practice, in a same casting both positive and negative copper segregation regions may occur. Such casting can exhibit different corrosion responses at different locations. The influences of solute redistribution during solidification, the magnitude of dendritic spacing and hence of the Al-rich phase and of Al₂Cu particles distribution along the casting on the corrosion resistance, were examined in samples collected along the casting length. The corrosion rate and impedance parameters (obtained from an equivalent circuit analysis) are also discussed.     

A novel hybrid composite coating of poly-3-amino-5-mercapto-1,2,4-triazole/TiO<Subscript>2</Subscript> on copper for corrosion protection

Poly-3-amino-5-mercapto-1,2,4-triazole/TiO₂ (p-AMTA/TiO₂) composite was effectively synthesized over the copper surface by cyclic voltammetric technique and used as a protective coating against corrosion. The resulting polymeric composite was characterized using Fourier transform infrared spectroscopy. The presence of TiO₂ particles in the polymer matrix was substantiated from X-ray diffraction pattern and energy-dispersive X-ray spectrum. The uniform dispersion of TiO₂ particles in the polymeric matrix was confirmed by the scanning electron microscope images. The protective effect of composite coating was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization methods in 3.5 % NaCl medium. Impedance measurements showed that charge transfer resistance (R _ct) values increased for polymeric composites which suggested the enhanced corrosion protection of copper. Further, the decrease in corrosion current density (i _corr) values and shifting of corrosion potential (E_corr) toward the cathodic direction confirmed the anticorrosive behavior of the polymeric composite. The reason for the higher protection of polymeric composite may be due to the well-dispersed TiO₂ particles in the polymer matrix exhibiting the enhanced barrier properties to protect copper surface from corrosion. The defects in the coatings can be reduced by embedded TiO₂ particles in the pores of the polymeric films to enhance the corrosion protection, consequently.     

Mechanistic investigation of hydrogen-enhanced anodic dissolution of X-70 pipe steel and its implication on near-neutral pH SCC of pipelines

The effects of hydrogen-charging on anodic dissolution of pipe steel under near-neutral pH condition were studied by electrochemical techniques. Hydrogen-charging enhances the anodic dissolution rate of the steel. The hydrogen-enhanced dissolution increases with increasing charging current density. The hydrogen effect is attributed to the alteration of chemical potential and exchange current density of steel. Hydrogen-charging affects the corrosion process of the steel. In particular, at a high charging current density, a layer of corrosion product forms on the electrode surface to change corrosion potential and interfacial double-charge layer capacitance as well as charge-transfer resistance. The hydrogen effect factor for enhanced anodic dissolution of steel at an anodic potential of −0.4 V (SCE) is 1.53 only. Hydrogen-enhanced anodic dissolution of steel by itself may not be the major factor contributing to the high rate of crack growth in pipe steel in near-neutral pH electrolyte. A further investigation of the synergistic effect of hydrogen and stress on dissolution at the crack-tip is essential to determine the mechanism of near-neutral pH stress corrosion cracking of pipelines.     

Effect of H2S on the CO2 corrosion of carbon steel in acidic solutions

The objective of this study is to evaluate the effect of low-level hydrogen sulfide (H₂S) on carbon dioxide (CO₂) corrosion of carbon steel in acidic solutions, and to investigate the mechanism of iron sulfide scale formation in CO₂/H₂S environments. Corrosion tests were conducted using 1018 carbon steel in 1 wt.% NaCl solution (25 °C) at pH of 3 and 4, and under atmospheric pressure. The test solution was saturated with flowing gases that change with increasing time from CO₂ (stage 1) to CO₂/100 ppm H₂S (stage 2) and back to CO₂ (stage 3). Corrosion rate and behavior were investigated using linear polarization resistance (LPR) technique. Electrochemical impedance spectroscopy (EIS) and potentiodynamic tests were performed at the end of each stage. The morphology and compositions of surface corrosion products were analyzed using scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that the addition of 100 ppm H₂S to CO₂ induced rapid reduction in the corrosion rate at both pHs 3 and 4. This H₂S inhibition effect is attributed to the formation of thin FeS film (tarnish) on the steel surface that suppressed the anodic dissolution reaction. The study results suggested that the precipitation of iron sulfide as well as iron carbonate film is possible in the acidic solutions due to the local supersaturation in regions immediately above the steel surface, and these films provide corrosion protection in the acidic solutions.     

Effect of chromate additive on the kinetics and mechanism of skeletal copper formation

The addition of chromate to the leach liquor for the preparation of skeletal copper increases and stabilizes the copper surface area and slows the leaching rate. The kinetics have been studied using a rotating disc electrode (RDE) at 269–293 K in 2–8 M NaOH and 0–0.1 M Na₂CrO₄. The rate of leaching was found to be constant with time, with an activation energy of 74 ± 7 kJ mol^–1. By monitoring the kinetics and free (mixed) corrosion potential, it was possible to elucidate the mechanistic effect of chromate causing the increased surface area. Chromate was found to deposit on the copper surface as chromium(III) oxide, hindering the leaching reaction as well as the dissolution/redeposition of copper, the main mechanism of structural formation/rearrangement for skeletal copper. This blocking of the surface resulted in a finer structure, with a corresponding larger surface area. It also stabilised the surface area by minimizing the rearrangement. The effect of chromate was found to reach a limit at around 0.03 M Na₂CrO₄.