Reproducibility of corrosion parameters for the acidic dissolution of pure iron: potentiostatic polarisation

Polarisation curves were determined potentiostatically for pure polycrystalline iron corroding in oxygen-free 0.5 M H₂SO₄. Four different working electrode pre-treatments (abrasion/polishing, pre-polarisation and time to establish E_corr) were employed and the reproducibility of E_corr and calculated corrosion parameters (i_corr, Tafel slopes and i₀) for each treatment was determined. Electrode pre-treatment effects changes in working electrode catalytic activity with subsequent variation in the reproducibility of polarisation curves and measured and calculated corrosion parameters. A method incorporating abrasion/polishing followed by anodic/cathodic pre-polarisation resulted in general in improved parameter reproducibility and cathodic and anodic Tafel slopes close to those predicted by the reaction mechanisms.     

A guide to polarisation curve interpretation: deconstruction of experimental curves typical of the Fe/H2O/H/O2 corrosion system

Experimental DC polarisation curves are the sum of the cathodic and anodic components and can be difficult to interpret. Schematic representations of ‘typical’ curves (together with their anodic and cathodic components) are available in the literature for comparison purposes. A better approach to curve analysis is to generate mathematically the experimental curve which is then deconstructed into its components. Unfortunately the appropriate computer programmes are not readily available. We have considered it useful to revisit the collected curve concept replacing schematic representations with experimental curves. Using an up-dated programme we have accurately analysed curves representative of the Fe/H₂O/H⁺/O₂ corrosion system.     

Electrochemical pitting corrosion behaviour of α-brass in LiBr containing solutions

The potentiodynamic anodic polarization curve of α-brass (70% Cu-30% Zn) in 1 M LiBr solution showed an initial active region of the alloy dissolution followed by two well defined anodic current peaks then a narrow passivation region before the pitting potential (E_pit) is reached. The initial active anodic region exhibited Tafel slope with 90 mV dec⁻¹ attributed to the formation of CuBr₂⁻ complexes. The anodic current peaks were attributed to the formation of CuBr and Cu²⁺ ions, respectively. The change of pH values of LiBr solution did not affect the anodic polarization curves of α-brass. Increasing the solution temperature from 30 to 90 °C changed the corrosion type from pitting to general one. The addition of 10⁻² M benzotriazole (BTAH) to 1 M LiBr solution is completely inhibited the pitting corrosion at 30 °C while it did not inhibit the pitting at 90 °C. The inhibition effect was attributed to the adsorption of BTAH molecules on the alloy surface, which obeys Langmuir isotherm. The presence or absence of pitting corrosion was confirmed by using SEM.     

Structural Aspects of the Behavior of Lead-Free Solder in the Corrosive Solution

In oxidizing environments, most tin-based lead (Pb)-free alloys form a tin oxide that is easily eroded or mechanically damaged, affecting corrosion resistance and thus reliability of the soldered joints. In this study, the effect of microstructure heterogeneity on corrosion behavior of Pb-free solder candidate systems has been investigated on the example of as-cast and heat-treated alloys. The research was focused on a comparison between the corrosion resistance of binary Sn-Zn and ternary Sn-Zn-Cu alloys. Accelerated corrosion tests were performed by means of electrochemical methods in the sodium sulfate solution (VI), Na₂SO₄, of about 0.5 M concentration, pH adjusted to 2 by means of concentrated H₂SO₄ acid. In these tests, the corrosion potentials as well as polarization curves were determined for the selected alloys in as-cast state and after their heat treatment using different combinations of processing parameters. The measurements of basic electrochemical characteristics were made, i.e., the corrosion current (i _corr μA/cm²) and Tafel coefficients, both cathodic (b _c V/dec) and anodic (b _a V/dec) ones. Detailed structural characterization of as-cast and heat-treated alloys before and after accelerated corrosion tests has been made under a wide range of magnifications using light microscopy and scanning electron microscopy observations. The results showed that structural heterogeneity of the examined alloys, attributed to the presence of secondary phases, and affected by their size and distribution, significantly influences the behavior of the examined Pb-free Sn-Zn-based alloys in the corrosive environment.     

Über den Einfluß von Schwefelwasserstoff auf die Korrosion des Eisens in sauren Lösungen

The influence of hydrogen sulphide on the corrosion of iron in acid solutions A considerable change of the polarisation behaviour of iron electrodes in acidic perchlorate and sulphate solutions was observed if they where saturated with hydrogen sulfide gas. H₂S markedly increased the anodic current densities and increases the corrosion rates by a factor of ten. The anodic Tafel slopes d log i_A/dE are only about half as steep as those in the absence of hydrogen sulphide. It is probable that the SH^?-ions behave as catalysts of the iron dissolution reaction in a similar manner as the OH^?-ions, but are adsorbed almost to saturation.     

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 H₂SO₄ 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/H₂SO₄ 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.     

Investigation of corrosion behavior of silver coated brass in acidic solutions

The corrosion behavior of uncoated brass and electrolytically silver coated brass in cyanide solutions such as 0.1 M HCl, 0.1 M H₂SO₄ and 0.1 M H₃PO₄ was investigated by Tafel polarization curves and cyclic voltammetry curves at 1., 24., 48., 72., 96. and 168. hours. The coating efficiency was calculated by current density of corrosion determined from Tafel polarization curves. Surface analysis of coated brass immersed into acidic solution by 168 hours was done. It is concluded that silver coating is very effective to protect the corrosion of brass for a long time.     

Plug-In Specimens for Measurement of the Corrosion Rate of Mg Alloys

Magnesium alloy corrosion is often nonlinear. The corrosion rate accelerates to steady state after an initial period of low corrosion. Plug-in specimens permit simultaneous measurement of the corrosion rate using hydrogen evolution, P _H, and Tafel extrapolation of cathodic polarization curves, P _i. Moreover, weight loss allows independent verification. P _H is consistently greater than P _i. The data, for short exposure periods up to 10?days, are consistent with the unipositive Mg⁺ ion being a short-lived intermediate. Tafel extrapolation needs to be used with caution for estimation of Mg corrosion, as the measured corrosion rate can have a significant contribution from crevice corrosion. Furthermore, measurements made at short immersion times may not reflect the steady-state corrosion rate, and the corrosion reaction at the Mg surface may be decoupled from the electrochemical measurement.     

Study of inhibition mechanism and efficiency of indole-5-carboxylic acid on corrosion of copper in aerated 0·5M H2 SO4

Abstract

Inhibition of the corrosion of copper in aerated 0·5M sulphuric acid solutions containing various concentrations of indole-5-carboxylic acid was studied in the temperature range 25–55°C using potentiodynamic curves (Tafel lines), weight loss, analytical methods, and determination of double layer capacitance. The corrosion rates reveal good corrosion inhibition, up to 95% in the concentration range of 1 × 10^-4–4 × 10^-3M. Tafel anodic slopes in inhibited acid solutions are considerably higher than those in uninhibited acid solutions. This points to a change in the corrosion mechanism of copper in the presence of indole-5-carboxylic acid. Under these conditions, copper could electro-oxidise primarily to Cu⁺ rather than to Cu²⁺, forming slightly soluble [Cu–(indole-5-carboxylic)_n ]⁺_(ads) complexes. Corrosion rates determined by the weight loss method in both the absence and presence of inhibitor are much higher (on average by a factor of about 6) than those obtained with the potentiodynamic method. This points to a limitation of the Tafel line extrapolation method in corrosion rate determination. The double layer capacitance–potential curves indicate considerable adsorption of the inhibitor over a wide potential range (-600 to +200 mV with regard to E_corr ).     

Gel-like layer development during formation of thin anodic films on titanium in phosphoric acid solutions

Anodising of commercial pure titanium in phosphoric acid solutions at different concentrations (0.5-4 M) have been investigated using galvanostatic and potentiodynamic polarisation techniques. Under galvanostatic conditions at low current densities (0.1-0.6 A m⁻²), the chrono-potentiometric curves always show a linear section in the early stage of the process. The slope of the linear section, dE/dt, decreases initially and then increases with increasing H₃PO₄ concentration, with the minimum slope at a concentration, ∼2 M. Under potentiodynamic conditions at this concentration the samples exhibit a different anodic behaviour. The development of gel-like layer during formation of thin anodic films on titanium in H₃PO₄ solutions is proposed to account for experimental observations.     

Electrochemical characterisation of the porosity and corrosion resistance of electrochemically deposited metal coatings

Electrochemical techniques for the assessment of porosity in electrodeposited metal coatings are reviewed. The determination of porosity and corrosion, resistance is illustrated by electrochemical data from three coating/substrate systems namely: electroless nickel on aluminium and steel and immersed gold coatings on an electroless copper-plated ABS polymer. Nickel coatings were up to 24 μm thick while gold deposits had thickness between 75 and 190 nm. Tafel extrapolation and linear polarisation resistance methods were used to determine the corrosion rate of the coated substrates. The aluminium samples were tested in 5% w/v (0.85 mol dm^− 3) NaCl, while coated steel and ABS samples were immersed in 0.125 mol dm^− 3 H₂SO₄ and 0.1 mol dm^− 3 NaBH₄, respectively, at 295 K. Current vs. time curves and anodic polarisation behaviour have also been considered.     

Computational modeling of cathodic limitations on localized corrosion of wetted SS 316L at room temperature

The ability of a SS 316L surface wetted with a thin electrolyte layer to serve as an effective cathode for an active localized corrosion site was studied computationally. The dependence of the total net cathodic current, I_net, supplied at the repassivation potential E_rp (of the anodic crevice) on relevant physical parameters including water layer thickness (WL), chloride concentration ([Cl⁻]) and length of cathode (Lc) were investigated using a three-level, full factorial design. The effects of kinetic parameters including the exchange current density (i_o,c) and Tafel slope (β_c) of oxygen reduction, the anodic passive current density (i_p) (on the cathodic surface), and E_rp were studied as well using three-level full factorial designs of [Cl⁻] and Lc with a fixed WL of 25 μm. The study found that all the three parameters WL, [Cl⁻] and Lc as well as the interactions of Lc × WL and Lc × [Cl⁻] had significant impact on I_net. A five-factor regression equation was obtained which fits the computation results reasonably well, but demonstrated that interactions are more complicated than can be explained with a simple linear model. Significant effects on I_net were found upon varying either i_o,c, β_c, or E_rp, whereas i_p in the studied range was found to have little impact. It was observed that I_net asymptotically approached maximum values (I_max) when Lc increased to critical minimum values. I_max can be used to determine the stability of coupled localized corrosion and the critical Lc provides important information for experimental design and corrosion protection.     

Reactions at the CO,CO2/Ni electrode in molten sodium carbonate

Reactions at the nickel electrode in molten sodium carbonate at 1000°C under CO/CO₂ atmospheres were studied by electrode polarization. The polarization curves were analysed by means of a previously described computer model, which in turn was interpreted in terms of the absolute rate expressions for postulated reactions. In agreement with previous workers' observations for inert electrodes, the dominant anodic reactions were found to be oxidation of CO at low and intermediate anodic overpotentials, and oxidation of CO₃^2? at high anodic overpotentials. The cathodic reaction was found to be reduction of CO₃^2?. The form of the polarization curves was described by activation, concentration, and resistance polarization of these reactions; however, anomalous anodic behaviour was observed which was attributed to corrosion reactions at the electrode and accurately described through the use of additional anodic reactions displaying passivity.     

Evaluation of corrosion rates by the faradaic rectification method

Accurate evaluation of corrosion currents from polarization data has been the aim of many workers in the recent past. This, however, requires both polarization resistance and Tafel slope measurements. Although many accurate methods have been developed for polarization resistance measurements a certain amount of ambiguity still exists regarding the measurement and the validity of Tafel slopes at corrosion potential. A method for evaluation of corrosion currents is presented in this paper based on the faradaic rectification technique. According to this method the corrosion current (ICorr is given by

where R_p is the polarization resistance and (δE/V²)_ω→O and (δE/V²)_ω→∞ are the rectification ratios at low and high frequency a.c. signals. The method does not require Tafel slope measurements, hence the controversy generated as to the validity of Tafel slopes at E_Corr is completely eliminated.     

Electrochemical corrosion behaviour of Incoloy 800 in sulphate solutions containing hydrogen peroxide

The aim of this study is to evaluate the electrochemical corrosion behaviour of Incoloy 800 in sulphate solutions containing H₂O₂ in the temperature range of 25–80 °C. The open circuit potential measurements, cathodic and anodic polarization and electrochemical impedance spectroscopy (EIS) were used to characterize the corrosion behaviour. The results provide kinetic data for reduction of hydrogen peroxide on Pt surface. The anodic polarization curves for Incoloy at different pH, temperature and H₂O₂ concentration are presented. EIS data generally confirm the polarization interpretations about the effects of various parameters. An equivalent circuit was used to fit all the acquired data.     

Calculation of Uniform Corrosion Current Density of Iron in Hydrochloric Acid Solutions based on the Principle of Maximum Entropy Production Rate Applied to Literature Data

In this paper a new procedure for the calculation of uniform corrosion current density is presented. The procedure is based on the following criterion: corrosion current and extent of anodic and cathodic areas are such that the entropy production rate is maximum. Experimental data for the reaction Fe(s) + HCl(aq) = FeCl₂(aq) + H₂(g), taken from literature, are used to test the new model. A comparison between corrosion current density calculated from weight loss measurements and current density calculated from polarization curves by means of the proposed model is carried out. The proposed model provides a better agreement, between Tafel line extrapolation and gravimetric measurement, in comparison to the mixed potential theory usually adopted.     

Anodic Behaviour of 17–7 Precipitation Hardening Stainless Steel in Acid Media

Abstract

The anodic behaviour of 17–7 precipitation hardening stainless steel in 0.5 M H₂SO₄ and 0.5 M H₂SO₄ + Cl^? solutions was investigated in detail. The coulometric areas of the two anodic peaks in the potentiodynamic curve in the chloride-free solution are dependent upon the duration of the free corrosion between the preliminary cathodic activation and the starting of the potentiodynamic scan. When the free corrosion time exceeds 6 minutes the peak at the more active potential disappears completely.

The influence of the free corrosion time was much less evident in the presence of chloride ions; in these conditions anodic currents were greater.

It was also established that the two anodic peaks correspond to the active-passive transitions relative to the two phases composing the steel: austenite and δ-ferrite. This was seen by potentiostatically etching the steel in the active dissolution ranges of both anodic peaks.

Finally it was shown, both micrographically and by comparing the anodic curves of pickled and polished specimens, that ordinary nitric–hydrofluoric pickling solutions selectively attack the austenite.     

Determination of biocorrosion of low alloy steel by sulfate‐reducing Desulfotomaculum sp. isolated from crude oil field

In this study corrosion behavior of low alloy steel, in the presence of anaerobic sulfate‐reducing Desulfotomaculum sp. which was isolated from an oil production well, was investigated. In order to determine corrosion rates and mechanisms, mass loss measurements and electrochemical polarization studies were performed without and with bacteria in the culture medium. Scanning electron microscopic observations and energy dispersive X‐ray spectra (EDS) analysis were made on steel coupons. The effect of iron concentration on corrosion behavior was determined by Tafel extrapolation method. In a sterile culture medium, as the FeSO₄ · 7H₂O concentration increased, corrosion potential (E_cor) values shifted towards more anodic potentials and corrosion current density (I_cor) values increased considerably. After inoculation of sulfate‐reducing bacteria (SRB), E_cor shifted towards cathodic values. I_cor values increased with increasing incubation time for 10 and 100 mg/L concentrations of FeSO₄ · 7H₂O. Results have shown that the corrosion activity changed due to several factors such as bacterial metabolites, ferrous sulfide, hydrogen sulfide, iron phosphide, and cathodic depolarization effect.     

Prediction of pathways for the dissolution of iron

Previous mechanisms for the anodic dissolution of iron have been reviewed, and experimental evidence to distinguish between them is given. Of the various values found in the literature for the anodic Tafel slope, the values of 30 and 40 mV at (30°C) seem to be the most accurate. These two values have been taken as criteria for the possible mechanisms. New reaction pathways have been suggested which also involve the formation of Fe(OH)₂, FeOH⁺, and Fe₂OH⁺ as intermediates, and both the Tafel slope and the reaction order with respect to OH^? for these pathways have been deduced from steady state kinetics. The data thus obtained show that many of the proposed rate-determining steps give rise to the same results derived for previous mechanisms, thus indicating that such previous mechanisms are not the only possible ones. Difficulties associated with the work on iron, and with the present state of steady state kinetics are also outlined.     

A study on cobalt electrodeposition onto a carbon steel electrode

Abstract

The electrodeposition of cobalt on to a carbon steel working electrode was studied through the comparison between theoretical and experimental polarisation curves. The former was obtained using the Butler–Volmer equation with electrochemical parameters, such as Tafel slope values, exchange current densities, cathodic limit current densities and equilibrium potential values. The following results were obtained: the anodic region refers to the iron oxidation with a Tafel slope of about 0·055 V decade^?1, which is near the theoretical value (0·059 V decade^?1); the experimental cathodic region represents the cobalt and hydrogen ion reduction; the limiting current density (?1 × 10^?3 to ?1 × 10^?2 A cm^?2) obtained was near the calculated value, ?3·7 × 10^?3 A cm^?2.