Cathodic Tafel Slopes Research Articles

Electrochemical corrosion behaviour of lead-free Sn–8.5 Zn– X Ag–0.1 Al–0.5 Ga solder in 3.5% NaCl solution

The electrochemical corrosion behaviour of Pb-free Sn–8.5 Zn– X Ag–0.1 Al–0.5 Ga solder in 3.5% NaCl solution was investigated by using potentiodynamic polarization methods, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analysis. The results obtained from polarization studies showed that an increase in the Ag content from 0.1 to 1.5 wt% decreased the corrosion current density ( I corr) and shifted the corrosion potential ( E corr) towards more noble values. These changes were also reflected in the linear polarization resistance (LPR), corrosion rate, anodic Tafel slope ( b A) and the cathodic Tafel slope ( b c) values, respectively. Passivation behaviour was noted in the Sn–Zn– X Ag–Al–Ga solders with Ag content > 0.1 wt%. The oxides and hydroxides of zinc were responsible for the formation of passive film. Presence of Ag atoms in the oxide layer also improved the passivation behaviour of solders to a certain extent. X-ray photoelectron spectroscopy revealed that two different oxygen species were formed on the surface films, one was assigned to OH − in Zn(OH) 2 and the other to O 2 − in ZnO. XPS depth profile results revealed that the two species had different depth distribution in the films. SEM and EDX analyses confirmed SnCl 2 as the major corrosion product formed after the electrochemical experiments.

Some observations concerning the Tafel equation and its relevance to charge transfer in corrosion

Following some biographic notes about Tafel, his famous equation is discussed. Particular attention is given to the limitations of applicability of this equation, the distinction between the symmetry factor and the transfer coefficient, and the theoretical basis for the former. The possible dependence of the symmetry factor on potential and temperature are discussed, and some highly accurate experimental examples are shown. The mechanism of charge transfer in metal dissolution, which can be one of the elementary steps in corrosion, is discussed. It is argued that in this case, as well as in metal deposition, charge is transferred across the interface by ions rather than by electrons. This could have significant ramifications concerning the mechanism. It is proposed that the often observed difference between anodic and cathodic Tafel slopes could result from values of the symmetry factor that are far from 0.5, rather than from a stepwise mechanism that is commonly assumed. Assuming that charge is carried across the interface by ions rather than by electrons eliminates the need to postulate the formation of unstable intermediates.

An evaluation of the electrochemical frequency modulation (EFM) technique

The EFM technique has been used to calculate anodic and cathodic Tafel slopes as well as corrosion current densities for the systems Cu/NaCl, Al 5083/NaCl, stainless steel/NaCl and mild steel in H 2SO 4 and NaCl. These values have been compared with those obtained from the analysis of polarization curves determined in the vicinity of the corrosion potential. In addition, polarization resistance values calculated based on the analysis of EFM data have been compared with values obtained from analysis of impedance data collected for the same systems. In most cases the corrosion rates determined with the EFM technique were much higher than the values determined with the other two techniques. This result is mainly due to the fact that for systems with low corrosion rates the modulation frequencies that are used in the EFM technique are in the capacitive region of the impedance spectra. Good agreement of corrosion rates obtained with the EFM and the polarization techniques was only observed for the mild steel/H 2SO 4 system that has very high corrosion rates. Analysis of EFM data assuming charge transfer or diffusion control produced different Tafel slopes and corrosion rates, but the same polarization resistance ( R p) values. When the EFM technique was applied to a resistor, the calculated R p value equaled the value of the resistance which is further proof that the EFM technique measures the impedance of a corroding system in a narrow frequency band. The difference to the traditional EIS technique is that the amplitude of the ac signal can be different at different frequencies.

An interesting and efficient organic corrosion inhibitor for mild steel in acidic medium

The effect of nicotinic acid (pyridine 3-carboxylic acid) on mild steel dissolution in H2SO4 is investigated by electrochemical methods: steady state galvanostatic and potentiostatic polarisation measurements. The dissolution parameters, such as corrosion currents, passive current, flade potential, open circuit potential, cathodic and anodic Tafel slopes and inhibition efficiencies, are determined. The adsorption and corrosion rates based on heat of adsorption and activation energies are calculated from Langmuir’s adsorption isotherm. The inhibition efficiency range of 94–99% is obtained for a concentration range of 10−1 to 10−7 mol L−1. Passivating parameters revealed that these are strongly passivating additives. Synergistic effects of I— have a strong impact on the performance of these additives. These results are verified by the weight loss method. The surface analysis studies carried out by SEM supplement the galvanostatic results.

Adsorption kinetics of benzyl triphenyl phosphonium chloride on mild steel in 1N sulphuric acid

The inhibitive effect of benzyl triphenyl phosphonium chloride (BTPPC) on the corrosion of mild steel in aerated 1N sulphuric acid at different temperatures was investigated using galvanostatic polarisation, potentiostatic polarisation, infrared spectroscopy, scanning electron microscopy and quantum chemical studies. Corrosion potential, corrosion currents, cathodic and anodic Tafel slopes, heat of adsorption and effective activation energies were calculated in the presence and absence of the BTPPC. Inhibition efficiencies of BTPPC at different concentrations and temperatures were studied. The nature of the adsorption of BTPPC on metal surface was also examined. It is concluded that the passivation parameters depend on the presence of the already adsorbed anions. BTPPC is one of the rare inhibitors that only passivate the metal at lower concentrations while passivation at higher concentrations is not observed. The electrochemical data fit Temkin’s isotherm. The results of surface morphology and quantum chemical analysis are in agreement with the electrochemical results.

Electrochemical thermodynamic and kinetic studies of the behaviour of aluminium in hydrochloric acid containing various benzotriazole derivatives

In this study the inhibitive behaviour of benzotriazole, 5-methyl benzotriazole, 5-chloro benzotriazole and 5-nitro benzotriazole on the corrosion of pure aluminium in 0.1M HCl has been investigated by the potentiostatic polarisation technique. For this purpose, corrosion potentials Ecorr, corrosion rates icorr, anodic and cathodic Tafel slopes βa, βc, inhibition efficiencies IE, and surface coverages θ, were determined in 0.1M HCl solutions containing various concentrations (i.e. 1×10-5 M, 5×10-5 M, 1×10-4 M, 5×10-4 M, 1×10-3 M) of benzotriazole derivatives. The efficiency of the studied benzotriazole derivatives at a concentration of 1×10-4 M was found be in the following order: 5-nitrobenzotriazole > 5-methyl benzotriazole > 5-chloro benzotriazole > benzotriazole. It was observed that the benzotriazole derivatives were behaving as cathodic inhibitors and the inhibiting action of these heterocyclic compounds was mainly due to adsorption on the metal surface, which parallels the total negative charge of each molecule. From the adsorption isotherms, values of the free energy of adsorption ΔGads and the equilibrium constant Kads for the adsorption processes were calculated. Corrosion rates were determined electrochemically at 20°C, 30°C, 40°C, 50°C and 60°C with 1×10-4 M or 1×10-3 M of these compounds in 0.1M HCl. From the corrosion rates obtained at different temperatures, activation parameters for the corrosion processes such as activation energies Ea, activation enthalpies ΔH* and activation entropies ΔS* were determined and possible mechanisms for the corrosion and inhibition processes have been proposed.

Effect of some thiophene derivatives on the electrochemical behavior of AISI 316 austenitic stainless steel in acidic solutions containing chloride ions: I. Molecular structure and inhibition efficiency relationship

The electrochemical behavior of austenitic stainless steel (AISI 316) was studied in an acid medium that contains chloride ions in absence and presence of some thiophene derivatives. The corrosion rate as well as pit growth of the steel was inhibited variably with each of the sulfur-containing compound present in the corroding medium. The thiophene derivatives studied were 2-acetyl thiophene (AcT), 2-thiophene carboxylic acid (TCA), 3-thiophene carboxyaldehyde (TCAL) and 2-thiophene carboxylic hydrazide (TCH). Electrochemical techniques such as potentiodynamic polarization, Tafel experiments, polarization resistance and electrochemical impedance spectroscopy were used to investigate the electrochemical behavior of the steel. The results showed distinct effects for the different thiophene derivatives used that depended on the molecular structure and the electron density of the sulfur atom of the thiophene ring. The order of inhibition efficiency was TCH > TCA > TCAL > AcT. It was also concluded that the inhibitors studied were of the mixed type. This conclusion was realized by comparing the change in the values of the anodic and cathodic Tafel slopes. Inhibition efficiencies of ca. 97% were achieved using these inhibitors in 0.5 M H 2SO 4.

Inhibition of the corrosion of low carbon steel in acidic solution by selected polyelectrolytes and polymers

The inhibiting effects of 2,6‐ionen and 2,10‐ionen type polyvinylbenzyltrimethylammonium chloride, and latex, on low carbon steel in hydrochloric acid solution was investigated by potentiodynamic polarisation measurements and impedance measurement techniques over the temperature range of 20‐60°C at different inhibitor concentrations. It was found that the inhibition efficiencies increased with increasing inhibitor concentration. The degree of shift in Ecorr values, together with the change in anodic and cathodic Tafel slopes (βa, βc), revealed that the studied inhibitors behaved as anodic inhibitors. Thermodynamic and kinetic parameters for dissolution and adsorption were also calculated.

Aluminium corrosion in hydrochloric acid solutions and the effect of some organic inhibitors

Electrochemical behaviour of pure aluminium in HCl solutions of different concentration, in presence and absence of organic inhibitors has been studied by potentiostatic and potentiodynamic methods. The evolution of the electrode surface was examined by optical microscope. The resistance of aluminium against corrosion in solutions with low concentrations of HCl can be attributed to a rapidly formed surface oxide. The presence of the chloride ions creates extensive localized attack. In strong acidic solutions no passive–active transition up to high overvoltages was observed. The increase of HCl concentration led, in all the cases, to the increase of the dissolution rate of aluminium in according to the dissolution proposed mechanism and Pourbaix diagram. The existence of an oxide layer on the metal at an appreciable coverage and thickness leads to anomalously high anodic and cathodic Tafel slopes (at high overpotential), which is not the case for potentials in the range of about 70mV around the stationary corrosion potential. The addition of surfactants leads in all cases to the inhibition of the corrosion process. The results obtained indicate that, Tween 20 and 81 are more effective to inhibit the corrosion of pure aluminium than hexadecylpyridinium bromide (HDPB). Under the critical micelle concentration (CMC), the inhibition of used surfactants is negligible. At a concentration higher than the CMC, the inhibiting action increases rapidly. The inhibition process was attributed to the formation of adsorbed film on the metal surface that protects the metal against corrosive agents. The sigmoidal shape of the adsorption isotherm confirm the applicability of Frumkin’s equation to describe the adsorption process of the three surfactants tested in 0.5M HCl solution on an aluminium electrode.

Inhibition of the corrosion of low carbon steel in acidic solution by selected quaternary ammonium compounds

Potentiodynamic polarisation studies were carried out on the inhibition of low carbon steel in 0.1M hydrochloric acid solution over the temperature range 20‐60°C at different inhibitor concentrations by various quaternary ammonium salts and cationic surfactants. The inhibitors examined were tetraethyl ammonium chloride, tetrabutyl ammonium chloride, benzyltrimethyl ammonium chloride, benzyltriethyl ammonium chloride, benzyltributyl ammonium chloride, phenyltrimethyl ammonium chloride, alkylbenzyldimethyl ammonium chloride, tetradecyltrimethyl ammonium bromide and cetyltrimethyl ammonium bromide. Maximum inhibition efficiencies of cationic surfactants were observed around and above critical micelle concentration (cmc), while the inhibition efficiencies of the quaternary ammonium salts were found to increase with the increase in their concentrations. The degree of shift in Ecorr value, together with change in anodic and cathodic Tafel slopes (ba, bc), revealed that cationic surfactants behave as an anodic inhibitor, while quaternary ammonium salts behave as mixed type inhibitors. Inhibition efficiencies of studied inhibitors seem to be closely related with the chain length of the alkyl group as well as the presence of benzene ring in quaternary ammonium compounds. Thermodynamic and kinetic parameters for dissolution and adsorption were also calculated.

Rhodanine azosulpha drugs as corrosion inhibitors for corrosion of 304 stainless steel in hydrochloric acid solution

The effect of rhodanine azosulpha drugs on the corrosion behaviour of 304 stainless steel in 1.0 M hydrochloric acid solution as corrosive medium has been investigated using weight loss and potentiostatic polarization techniques. Some corrosion parameters such as anodic and cathodic Tafel slope, corrosion potential, corrosion current, exchange current densities, surface coverage and inhibition efficiency were calculated. The polarization measurements indicated that the inhibitors are of mixed type and inhibit corrosion by parallel adsorption on the surface of steel due to the presence of more than one active centre in the inhibitor molecule. The adsorption is obeyed Langmuir adsorption isotherm. The activation energy and thermodynamic parameters were calculated at different temperature.

Oxygen reduction reaction at rust free iron surface in neutral unbuffered chloride solutions

Cathodic reduction of oxygen on an iron in neutral unbuffered sodium chloride solutions was studied by a linear potential sweep technique. The experiment was performed before iron lost its luster in order to avoid the complication resulting from the corrosion products on the surface. The steady-state current was observed when the potential sweep rate was less than 2 mV s −1 at angular frequencies of rotation of 5–262 s −1. The steady state polarization curves showed no significant hysteresis. The concentration of Cl − and pH did not affect the cathodic current. The cathodic Tafel slope was about −130 mV per decade and the reaction order was 0.5 with respect to the partial pressure of oxygen. The reaction scheme: O 2=2 O ads O ads + H 2 O+ e→ OH ads + OH − ( rate-determining) OH ads + e= OH − explained these results.

Electrochemical and spectroscopic study of arsenate removal from water using zero-valent iron media.

This study investigated the mechanisms involved in removing arsenate from drinking water supplies using zero-valent iron media. Batch experiments utilizing iron wires suspended in anaerobic arsenate solutions were performed to determine arsenate removal rates as a function of the arsenate solution concentration. Corrosion rates of the iron wires were determined as a function of elapsed time using Tafel analysis. The removal kinetics in the batch reactors were best described by a dual-rate model in which arsenate removal was pseudo-first-order at low concentrations and approached zero-order in the limit of high arsenate concentrations. The presence of arsenate decreased iron corrosion rates as compared to those in blank 3 mM CaSO4 background electrolyte solutions. However, constant corrosion rates were attained after approximately 10 days elapsed, indicating that the passivation processes had reached steady state. The cathodic Tafel slopes were the same in the arsenate and the blank electrolyte solutions. This indicates that water was the primary oxidant for iron corrosion and that arsenate did not directly oxidize the iron wires. The anodic Tafel slopes were greater in the arsenate solutions, indicating that arsenate formed complexes with iron corrosion products released at anodic sites on the iron surfaces. Ion chromatography analyses indicated that there was no measurable reduction of As(V) to As(III). X-ray absorption spectroscopy analyses indicated that all arsenic associated with the zero-valent iron surfaces was in the oxidation state. Interatomic arsenic-iron distances determined from EXAFS analyses were consistent with bidentate corner-sharing among arsenate tetrahedra and iron octahedra. Results from this study show that under conditions applicable to drinking water treatment, arsenate removal by zero-valent iron media involves surface complexation only and does not involve reduction to metallic arsenic.

The effect of pH on the corrosion of dental metal alloys.

The aim of this study was to determine the effects of the oral environment's pH on the corrosion of dental metals and alloys that have different compositions, using electrochemical methods. The corrosion rates and the cathodic Tafel slopes were obtained from the current-potential curves. The effect of pH on the corrosion of dental metals and alloys was dependent on their composition. Dissolution of the ions occurred in all of the tested pH states. The dissolution was moderately low for samples containing titanium because its surface was covered with a protective layer, whereas the dissolution was maximal for the samples containing tin and copper. Addition of cobalt and molybdenum to the alloys improved their corrosion resistance; these cobalt and molybdenum alloys were not effected by changes in the pH. Dissolution of the precious metal alloys increased as the percentage of noble metals increased. The corrosion characteristics of dental metals and alloys are important because the corrosion tendencies of dental alloys in the mouth may cause health hazards, weakening and the aesthetic loss of dental restorations.

A correlation between phosphorous impurity in stainless steel and a second anodic current maximum in H 2SO 4

Phosphorous as a minor element (∼0.03%) in AISI 304 austenitic stainless steel greatly affects the polarization and corrosion behavior in sulfuric acid solution. The presence of P in stainless steel created a second current maximum in the anodic polarization curve and the current increased with increasing aging in the solution. An adhesive corrosion surface layer, rich with phosphate, formed on the surface of a P-containing steel during active dissolution. The layer lowered the cathodic Tafel slope at low current densities, and is likely, due to a change in hydrogen evolution mechanism. Phosphorous increases the H-adsorbed (and/or absorbed) atoms on the surface, leading to the appearance of a second anodic current peak that is interpreted as re-oxidation of hydrogen atoms. Also, P shifted the corrosion potential to the noble side, decreased effectively the active anodic dissolution, and lowered the corrosion rate.

Corrosion behaviour of dental metals and alloys in different media.

The corrosion tendencies of metals are related with their position in the electromotive series. These electrode potential degrees may change due to the compositions of the alloys, the surrounding media, or due to alterations in the composition because of recurrent casting. Therefore in this research, the electrode potentials and their changes over a period of time were measured in different pH media simulating the oral electrochemical conditions in vitro. The surface structure of the first and second castings of 29 different dental metals and alloys were examined under a scanning electron microscope and their composition in percentage weight was calculated by the Energy-dispersive X-ray Analysor system. Further the current-potential curves of the dental alloys were found by the potentiodynamic method in three different solutions and, in addition, the changes of corrosion potentials over time were also determined. The corrosion rates, corrosion potentials, their changes over time and their cathodic Tafel slopes were determined. All alloys tested showed ion leakage in corrosive media. Titanium exhibited the least, but alloys with tin and cobalt content displayed the greatest corrosion tendencies. Alloys with iron and copper corroded in the acid media, conversely alloys containing chromium, nichel and molybdenum proved to be resistant to corrosion. The recurrent castings were also corrosion resistant.

The behaviour of cadmium monocrystals and polycrystals in acid chloride solution

The polarisation characteristics of Cd polycrystals and monocrystals, with (0001), (112̄0), and (101̄0) surfaces, were studied in 0.1 M NaCl + 0.04 M HCl (pH 1.3) and compared with thermodynamic equilibria. Anodic dissolution rates were independent of surface orientation, exhibiting Tafel slopes ( b a ) near 27 mV that were reasonably consistent with a rate controlled process based on mass transport of CdCl + from the Outer Helmholtz Plane (OHP). Other processes are likely to have played a contributory role in the anodic behaviour, because crystallographic faceting accompanied dissolution. Cathodic evolution of hydrogen exhibited mixed rate control, with a cathodic Tafel slope ( b c ) near −81 mV. Hydrogen evolution appeared to be sensitive to surface orientation, with the (101̄0) surface showing the largest deviations in behaviour.

Effect of adsorbed S‐benzylisothiouronium cation on hydrogen evolution reaction at mild steel electrode

AbstractS‐benzylisothiouronium (SBTU) chloride and S‐methylisothiouronium (SMTU) sulphate retard the hydrogen evolution reaction (HER) on mild steel in HCl solutions, and increase the cathodic Tafel slope (‐bc) of HER to different extent depending on their concentration. The cause for the occurrence of high ‐bc values, and the dependence on additive concentration (cA), of the two parameters ‐bc and the electrochemical reaction order with respect to additive concentration (zA), have been explained in terms of a mechanism based on blocking of the electrode surface by the additive and its dependence on cA and the electrode potential. According to this treatment ‐bc of HER is a function of the interaction parameter rA‐A of the adsorbate and y, the parameter which measures the change of free energy of adsorption of the adsorbate with potential. Basing on the two sets of values for rA‐A and y by SBTU at lower and higher concentrations, effects by SBTU on zSBTU and ‐bc have been suggested as a result of different orientations the molecule adopts at two concentration regions, y and rA‐A have been interpreted in terms of adsorbate‐electric field and adsorbate‐solvent interactions at the interphase. The general applicability of the equation for calculating surface coverage (θHA) from polarisation measurements has been discussed.

The evaluation of corrosion rate through an inversion method

The basic ideas of the inversion method for computing the corrosion rate of a given metal or alloy in an aggressive environment are expounded and the main mathematical equations are given. It is also shown that the correct application of this numerical technique needs, in principle, a preliminary knowledge of the values of the anodic and cathodic Tafel slopes, B a and B c . It has been proved that I c does not seem to depend critically on the random choice of the values of B a and B c when they are rather close to the true ones. Examination of some artificial data showed that the approximate polynomial of the fourth degree gives very accurate values of ΔE as a function of w over the interval [− 0.5,0.5]. The inversion method has been used to compute I c relating to Armco iron in H 2SO 4 solutions at various pH values and at 25 °C, and in 1 N H 2SO 4 and HCl solutions containing suitable corrosion inhibitors at 75 °C. Polarization measurements, performed using current transients and experimental data corrected to account for the contribution of the ohmic drop to the electrode potential, were generally best-fitted over the ΔE interval [− 50, 50] mV using the INTER1 program. The actual values of B a and B c were obtained by analysing the polarization curves examined by the NOLI method. The analysis of the behaviour of these electrochemical systems has given satisfactory results compared with those obtained by the NOLI and linear polarization methods.

The effect of varying ruthenium content on the corrosion behaviour of two cathodically modified superferritic stainless steels

The corrosion behaviour of two superferritic stainless steels, both containing varying amounts of ruthenium, was evaluated and compared in a 10% sulphuric acid solution. It was found that small additions of ruthenium increased the corrosion resistance in both types of alloys dramatically. Apart from raising the corrosion potential of the base alloys, the cathodic Tafel slopes were also decreased with an increasing amount of ruthenium in each alloy type. The amount of ruthenium for an optimum increase in the corrosion resistance was found to be 0.16% for both types of alloy.