Acceleration and quantitative evaluation of degradation for corrosion protective coatings on buried pipeline: Part II. Application to the evaluation of polyethylene and coal-tar enamel coatings

The pulsed potentiostatic polarization technique proposed previously has been applied to evaluate the electrochemical degradation of polyethylene and coal-tar enamel coated SS400 for buried pipeline. The degradation of coated systems was accelerated and evaluated using electrochemical techniques (pulsed potentiostatic polarization test, electrochemical impedance spectroscopy) and surface analyses (scanning electron microscopy and energy dispersive X-ray spectroscopy). It was found that polyethylene coating had better protective performance than coal-tar enamel coating due to its low porosity. It was confirmed that the electrochemically accelerated test is an effective technique in the evaluation of coating performance.     

The effect of heat treatment and carbon content on the corrosion behaviour of iron—chromium steels in sulphuric acid solutions

In this study the corrosion behaviour of iron—chromium steels was investigated. The samples were subjected to heat treatment. The current density—potential curves were obtained in 1 N H₂SO₄ by electrochemical, potentiodynamic (1 mV min^?1), potentiostatic and galvanostatic methods. Corrosion rates were obtained from the extrapolation of anodic and cathodic Tafel lines and using potentiodynamic, potentiostatic and galvanostatic linear polarization techniques with the Stern—Geary equation. It was seen that the effect of chromium on the rate of corrosion varies with the heat treatment method applied and the amount of carbon content.     

Behaviour of Al–Ga alloy during cathodic polarization

The effect of the addition of small quantities of gallium to high-purity aluminium (99.999 wt%) on its electrochemical behaviour at high cathodic potentials (up to −2.0 V versus SCE), has been investigated using the potentiostatic pulse method. After cathodic polarization, anodic current was traced versus time to determine the quantity of charge necessary for oxidation of substances formed. Anodic current responses to the return to the E _OCP were also recorded in the period of 1 s. Time responses of the cathodic and anodic currents were analyzed. The cyclic voltammetry method was used to determine the hydration potential. The range of low and high cathodic potentials (LCP, HCP) was defined for all the samples. It has been established that the oxide film retains its properties in the LCP range, while in the HCP range cathodic breakdown and hydration of the oxide take place. Electrochemical methods complemented the SEM and EDAX analysis before and after the cathode pulse of −1.9 V versus SCE.     

Effects of pH and temperature on the deposition properties of stannate chemical conversion coatings formed by the potentiostatic technique on AZ91 D magnesium alloy

The effects of pH and temperature of a stannate bath on the quality of stannate chemical conversion coatings formed on AZ91 D magnesium alloy by using the potentiostatic polarization technique at E = −1.1 V were investigated in order to improve uniformity and corrosion protection performance of the coating films. It was found that the uniformity and corrosion resistance of coating films deposited by potentiostatic polarization were closely associated with pH and temperature of the coating bath. The pH and temperature to obtain the best coating film were investigated as a function of corrosion protection performance evaluated by curves of potentiodynamic anodic polarization conducted in borate buffer solution. Scanning electron microscope observation and electrochemical corrosion tests of the stannate-coated samples confirmed significant improvement in uniformity and corrosion resistivity of coating films deposited by the potentiostatic technique by modifying the pH and temperature of the coating bath. It was also found that uniformity and corrosion resistivity of the coating films deposited by the potentiostatic technique were considerably improved compared to those of coatings deposited by the simple immersion method at the best conditions of pH and temperature of the coating bath.     

Electrochemical behavior of La0.8Sr0.2FeO3 electrode with different porosities under cathodic and anodic polarization

Electrochemical behavior of La_0.8Sr_0.2FeO₃ (LSF) electrode with different porosities under cathodic and anodic current polarization has been investigated by electrochemical impedance spectroscopy and the galvanostatic method. The activation and degradation behavior of the LSF electrode may be related to the partial reduction and oxidation of the Fe ions under cathodic and anodic polarization, especially in the LSF electrodes with high porosities. The performance of the LSF electrode has been found to depend on the oxygen vacancies at the LSF surface, which would promote the transport of oxygen intermediate species at the LSF surface close to the triple-phase boundary (TPB) region. Results show that the polarization resistance (Rp) of the LSF electrode decreases at the beginning with the increase of cathodic polarization time, while Rp always increases with the increase of anodic polarization time. The effect of cathodic and anodic polarization becomes predominant with the increasing of the porosities of the LSF electrode, which is ascribed to the decrease of the interface area between electrode and electrolyte.     

Dodigen 213-N as corrosion inhibitor for ASTM 1010 mild steel in 10% HCL

This article describes a study of the behavior of a mixture of amines and amides, commercially known as Dodigen 213-N (D-213 N), as a corrosion inhibitor for ASTM 1010 mild steel in 10% w/w HCl solution. The concentration range used was 1 × 10⁻⁵ M to 8 × 10⁻⁴ M. The weight loss and electrochemical techniques used were corrosion potential measurement, anodic and cathodic polarization curves, and electrochemical impedance spectroscopy (EIS). The solution temperature was 50 ± 1 °C and it was naturally aerated. The corrosion potential values shifted to slightly more positive values, thus indicating mixed inhibitor behavior. The anodic and cathodic polarization curves showed that D-213 N is an effective corrosion inhibitor, since both the anodic and the cathodic reactions were polarized in comparison with those obtained without inhibitor. For all concentrations the cathodic polarization curves were more polarized than the anodic ones. The inhibition efficiency was in the range 75–98%, calculated from values of weight loss and corrosion current density, i _corr, obtained by extrapolation of Tafel cathodic linear region.     

Inhibition of copper corrosion in acidic pickling solutions by N-phenyl-1,4-phenylenediamine

Inhibition of copper corrosion by N-phenyl-1,4-phenylenediamine (NPPD) has been investigated in de-aerated, aerated, and oxygenated aqueous 0.50 M HCl solutions by using potentiodynamic polarization, potentiostatic current-time, electrochemical impedance spectroscopy, and weight-loss measurements, along with scanning electron microscopic (SEM) and energy dispersive X-ray (EDX) experiments. Potentiodynamic polarization measurements showed that the NPPD molecules significantly decrease cathodic, anodic, and corrosion currents in all these solutions. Potentiostatic current-time measurements as well as SEM and EDX investigations of the copper surface revealed that NPPD suppresses the copper dissolution current due to its adsorption on the copper surface as a Cu(I)-NPPD complex. Impedance measurements also supported the results obtained from both the potentiodynamic and potentiostatic experiments. The inhibition efficiencies measured from polarization, electrochemical impedance spectroscopy (EIS), and weight-loss experiments are all internally consistent with each other. These results together showed that NPPD is a good mixed-type inhibitor for copper corrosion in all solutions studied.     

Fabricated super-hydrophobic film with potentiostatic electrolysis method on copper for corrosion protection

A novel one-step potentiostatic electrolysis method was proposed to fabricate super-hydrophobic film on copper surface. The resulted film was characterized by contact angle tests, Fourier transform infrared spectra (FT-IR), X-ray photoelectron spectroscopy (XPS), Field emission scanning electron microscopy (FE-SEM) and electrochemical measurements. It could be inferred that the super-hydrophobic property resulted from the flower-like structure of copper tetradecanoate film. In the presence of super-hydrophobic film, the anodic and cathodic polarization current densities are reduced for more than five and four orders of magnitude, respectively. The air trapped in the film is the essential contributor of the anticorrosion property of film for its insulation, the copper tetradecanoate film itself acts as a “frame” to trap air as well as a coating with inhibition effect. The super-hydrophobic film presents excellent inhibition effect to the copper corrosion and stability in water containing Cl⁻.     

Optimization of pulsed electrodeposition of aluminum from AlCl3-1-ethyl-3-methylimidazolium chloride ionic liquid

In this study, Al was electrodeposited on a platinum substrate at room temperature from an ionic liquid bath of EMIC containing AlCl₃ using potentiostatic polarization (PP), galvanostatic polarization (GP), monopolar current pulse polarization (MCP) and bipolar current pulse polarization (BCP). Transition of current or potential during galvanostatic or pulse polarization revealed that the initial stage of the deposition process was controlled by a nucleation process depending on the polarization condition. For example, the average size of Al deposits decreased with increasing current density in the case of GP. FE-SEM observation showed that dense and compact Al deposits with a smooth surface were obtained by the current pulse method. Roughness factor evaluated from electrochemical impedance measurement confirmed the smooth surface of these deposits. Adhesion strength of Al deposits was greatly improved using BCP in which an anodic pulse was combined with a cathodic pulse for electrodeposition. In this study, the optimal parameters for BCP were found to be I_C = −16.0 mA cm⁻², I_A = 1.0 mA cm⁻², r_C (duty ratio) = 0.5, and f = 2 Hz. The mechanisms of electrodeposition by these three methods are discussed.     

Influence of electrochemical potential on the tribocorrosion behaviour of high carbon CoCrMo biomedical alloy in simulated body fluids by electrochemical impedance spectroscopy

The corrosion and tribocorrosion behaviour of a high carbon CoCrMo alloy sliding against alumina in simulated body fluids under potentiostatic conditions was investigated. The electrochemical behaviour of the sample in two electrolytes at different potentials (−1 V_Ag/AgCl, −0.5 V_Ag/AgCl, +0.05 V_Ag/AgCl, +0.5 V_Ag/AgCl and +0.75 V_Ag/AgCl) was studied by means of electrochemical impedance spectroscopy (EIS). The effects of solution chemistry and applied potential on the wear volume and anodic current were determined. Result shows that wear of CoCrMo alloy is negligible under cathodic and in the cathodic-anodic transition and considerably increases in the passive domain. Third body behaviour depends on surface chemistry which also varied depending on solution chemistry and electrochemically applied potential thus, modifies the tribocorrosion rate of CoCrMo alloy.     

Potential and morphological transitions during bipolar plasma electrolytic oxidation of tantalum in silicate electrolyte

Surges in the cell potential, due to an increased overpotential for hydrogen evolution, and transitions in ceramic oxide coating morphology during plasma electrolytic oxidation (PEO) of tantalum under a pulsed bipolar current regime at 1000 Hz in a silicate electrolyte are investigated using real-time imaging of gas evolution, analytical scanning electron microscopy, X-ray photoelectron spectroscopy and supplementary potential-controlled electrochemical measurements. The coatings, which contained Ta₂O₅, TaO and incorporated silicon species, revealed a nodular morphology that transformed with treatment time to a “pancake” type and then a “coral reef” type. The first potential surge occurred only in the cathodic potential, coinciding with an increased spark intensity, more vigorous gas evolution, emergence of “pancake” structures and a reduction in the coating porosity. The later increases in both the anodic and cathodic potential, coincided with intensification of the sparking, the establishment of silicon-rich “coral reef” structures, and formation of a comparatively thick coating. The kinetics of coating growth differed significantly between the three morphological stages. Electrochemical measurements showed that anodic discharges increased the overpotential for hydrogen evolution in the subsequent cathodic pulse, which is proposed to be due to gas impeding the coating and at and near the coating surface increasing the resistance to ionic transport.     

Effect of aluminium microstructure on corrosion and inhibiting effect of polyacrylic acid in H<Subscript>2</Subscript>SO<Subscript>4</Subscript> solution

The corrosion behaviour of a microcrystalline aluminium (mc-Al) coating fabricated from pure cast aluminium substrate by magnetron sputtering and the pure polycrystalline aluminium (pc-Al) were studied using electrochemical impedance spectroscopy and potentiodynamic polarization techniques in aerated 0.5 M H₂SO₄ at 30 ± 1 °C. The corrosion inhibiting effect of polyacrylic acid (PAA) and synergistic effect of iodide ions was also investigated. Results obtained show that surface microcrystallization increases the corrosion susceptibility of pure cast aluminium leading to decrease in interfacial impedance and an increase in the kinetics of the anodic dissolution. Introduction of PAA into the corrosive medium was observed to retard the corrosion rates of both specimens. The inhibition mechanism was affected by the microstructure of the sample. PAA functions as a mixed inhibitor but under cathodic control for pc-Al and under anodic control for mc-Al. Addition of iodide ions synergistically increased the inhibition efficiency of PAA and this effect was more pronounced with mc-Al.     

Iron carburization in a calcium carbide molten salt solution

The kinetics of iron carburization by both chemical and electrochemical treatments were determined at 920° C in a solution of CaC₂ in molten CaCl₂.In order to study the mechanism of carburization, anodic and cathodic galvanostatic experiments were performed.The iron carburization rate by chemical or anodic treatments was found to be controlled by the diffusion of carbon into the iron sample. Carburization rates under cathodic polarization are lower, and carburization may be prevented at the higher cathodic current densities. A mixed electrode model is proposed to explain the behaviour of the iron electrode in calcium carbide solutions.     

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.     

Ni-Mo alloying of nickel surface by alternating pulsed electrolysis using molybdenum(VI) baths

Electrochemical Ni-Mo alloying of the surface of a nickel substrate was investigated using alternating pulsed electrolysis in an aqueous solution containing only molybdate ions (MoO₄²⁻) as a metal ion component. In this electrochemical process, the nickel substrate was slightly dissolved during the anodic pulses, providing nickel ions into the solution in the vicinity of the substrate, while Ni and Mo were both electrodeposited on the substrate surface during the subsequent cathodic pulses. Through the optimization of anodic and cathodic conditions independently based on a set of direct-current electrolysis data, amorphous Ni-Mo alloy layers were found to be formed at the surface of the nickel substrate by the alternating pulsed electrolysis using the MoO₄²⁻ solution of pH 3.0-5.0. The conditions for Ni-Mo alloy formation were discussed in terms of the dissolving regime of ionic species in the electrolytes determined by an equilibrium calculation.     

Role of oxygen in the galvanic interaction between polypyrrole and aluminum alloy

Conjugated polymers continue to be of interest as possible corrosion-control coatings for metal alloys. In this work, electrochemical interactions between polypyrrole (PPy) films and the aluminum alloy 2024-T3 (AA) were investigated by electrochemical polarization and galvanic coupling techniques in dilute Harrison's solution (0.35 wt% (NH₄)₂SO₄, 0.05 wt% NaCl). A two-compartment electrochemical cell was used for the galvanic coupling measurements which permitted assessment of the role of dissolved oxygen by segregating the PPy film and AA in individual compartments. Electroactive 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt (BDA or Tiron) and electroinactive benzene-1,3-disulfonic acid, disodium salt (BDS) were used as dopants to study the mediating effect (if any) of BDA on the coupling behavior. The results indicate that oxygen reduction mediated by the PPy coating dominates the coupling interaction. The electrochemical polarization state of AA 2024-T3 in a pinhole simulating a PPy coating defect was characterized by the potential measured immediately after decoupling, by the anodic hydrogen evolution, and by galvanodynamic polarization. The results support the conclusion that the alloy in the coating defect was electrochemically active, not passive, during coupling with PPy. The anodic protection model commonly accepted for ferrous metals does not appear to apply to AA 2024-T3.     

The anodic behaviour of iron in ethanol—water solutions in the presence and absence of NaClO4 as the supporting electrolyte

The anodic behaviour of iron in ethanol—water solutions and the effect of NaClO₄ have been investigated on the basis of potentiostatic polarization curves and electrochemical impedance plots. The influence of the water content in ethanol (6–80 vol.%) on the anodic polarization curves without a supporting electrolyte is to increase the anodic current density monotonically with potential. Furthermore, for a given potential, the current density is higher when the water content is increased. In the presence of NaClO₄ the polarization curves shift towards more high current in the low anodic potential range. Thus, addition of NaClO₄ not only increases the conductivity of ethanol solutions in its role as a supporting electrolyte, but it also modifies the electrochemical process significantly.     

Electrochimie du polynitrure de soufre dans les solvants non aqueux—I. Comportement dans l'acetonitrile

The electrochemical behaviour of polysulphur nitride was studied by cyclic voltametry in alkaline and silver salt acetonitrile solutions. In an alkaline salt solution during the cathodic polarization, superficial degradation of (SN)_x probably gives S₇N^? ions. In a silver perchlorate solution silver deposition was observed. It can be dissolved anodically. During the first anodic polarization, oxidation pic occurs. The quantity of electricity involved during this reaction is a function of the association constant of the electrolyte.     

The role of electrical current mode in calcium carbonate deposition on conductive carbon nanofiber–epoxy coating

Calcium carbonate is one of the most common scaling minerals. In this paper we have used different electrical current modes (direct current [DC], pulsed DC, and alternating current [AC]) to control the amount, morphology, and distribution of calcium carbonate deposit on electroconductive epoxy/carbon nanofiber (CNF) coating. The effect of different current modes on surface scaling was visualized using scanning electron microscopy. It has been shown that both AC and DC anodic polarization limited scale deposition on epoxy/CNF coated surfaces, although the mechanisms of scale inhibition during AC and DC polarization were different. DC polarization of the coating at +2 V resulted in the smallest scale buildup without leading to coating degradation, while DC polarization at potentials as high as +5 V caused the coating to degrade. Interestingly, application of pulsed DC with high pulse frequency (50 Hz) inhibited the degradation. The type of current applied affected also the morphology of the precipitate at the cathode. The results presented in this work show, for the first time, how different modes of electrical current applied to electroconductive composite coatings can be used to control the morphology and distribution of calcium carbonate scale, and how the organic coating degradation at high polarization potentials can be avoided.     

Electrochemical investigations on the corrosion inhibition of aluminum by 3-amino-1,2,4-triazole-5-thiol in naturally aerated stagnant seawater

The corrosion behavior of aluminum and its inhibition by 3-amino-1,2,4-triazole-5-thiol (ATAT) in naturally aerated stagnant Arabian Gulf water was reported. The work was carried out using electrochemical impedance spectroscopy (EIS), cyclic potentiodynamic polarization (CPP), and potentiostatic current–time (CCT) measurements. EIS data indicated that ATAT decreased the corrosion of Al by increasing the solution and polarization resistances, the impedance of the interface and the maximum degree of phase angle. ATAT was also found to decrease the cathodic, anodic, and corrosion currents and corrosion rate as well as increase the polarization resistance of Al as confirmed by the CPP and CCT measurements.