Self Linear Polarization Resistance-Theory and Examples

Measuring the corrosion rate of a corroding metal is of interest in many situations, including monitoring industrial processes and undertaking fundamental research. The corrosion rate of a metal can be measured electrochemically by determining its polarization resistance, which is inversely proportional to the corrosion rate. What is described in this article is a technique for mathematically extracting from electrochemical noise (EN) data the polarization resistance as well as a measure of the frequency of anodic and cathodic transients. The theoretical framework for self-linear polarization resistance is based on a time-domain analysis of an electrical circuit model of an EN experiment. The analysis indicates that the polarization resistance for one electrode can be interpreted only if the second electrode alone is generating current transients during a given time record. One advantage of this approach, compared with other techniques for obtaining a polarization resistance from EN data, is that short time records, i.e. less than one minute, can be assessed. The self-consistency of the polarization resistance can be assessed with a correlation coefficient. Another advantage is that the nature of localized corrosion events can be attributed to either anodic or cathodic current transients from one of the electrodes.     

A rapid screening multi-electrode method for the evaluation of corrosion inhibitors

A method is described for the rapid screening of water-soluble corrosion inhibitors, which involves a fixed potential being applied between identical electrodes while immersed in an inhibitor solution. The current flowing between the electrodes is measured and the electrochemical response of the anodic and cathodic reactions, with and without the inhibitor, is then compared. The incorporation of nine pairs of metallic wires (Al, AA2024, AA7075, Fe, mild steel, SS316, Mg, Mg-AZ31 and Zn) into a single assembly enables the effect of inhibitors on different metals to be assessed rapidly. The technique is sensitive to both anodic and cathodic inhibition mechanisms and can provide information on the concentration dependency of inhibitors.This paper provides an electrochemical analysis of the method and demonstrates its application using several soluble chemical species (potassium dichromate, cerium nitrate, sodium vanadate, sodium diphosphate, sodium octanoate) on AA2024-T3 and mild steel at 10⁻² and 10⁻⁴ M and correlates the results with those obtained by conventional potentiodynamic polarization analysis. The errors in the approach were higher for poor inhibitors, however, efficient corrosion inhibitors have decreased experimental errors, thus, ensuring that promising inhibitor candidates are not overlooked.     

Modelling of three-dimensional bipolar electrodes with irreversible reactions

The behaviour of an electrochemical reactor with three-dimensional bipolar electrodes for irreversible reactions is analysed. Copper deposition at the cathodic side and oxygen evolution at the anodic one were adopted as test reactions at the bipolar electrode, from an electrolyte solution with a copper concentration lower than 1000 mg dm⁻³, pH 2 and 1 M Na₂SO₄ as supporting electrolyte. A mathematical model considering the leakage current is proposed, which can represent the tendency observed in the experimental data related to cathodic thickness and potential at both ends of the bipolar electrode. High values of leakage current were determined, which restricts the faradaic processes to small thicknesses at both ends of the bipolar electrode. Likewise, the performance of the bipolar electrochemical reactor for the treatment of effluents is experimentally and theoretically examined. In this case, the conversion for copper removal was 90.1% after 480 min of operation with one bipolar electrode and 94.8% after 300 min of operation with two bipolar electrodes at a total current of 3 A.     

Evaluation of the influence of impurities on the oxygen sensitivity of monocrystalline antimony electrodes

Measurements are presented which show that the oxygen sensitivity of ultra pure (6N) monocrystalline antimony electrodes presents an improved reproducibility and constancy as compared with monocrystalline electrodes containing impurity inclusions in the electrode surface. The former electrodes also present a higher potential stability. The time to reach a pseudo-stable potential level (drift <0.6 mV/h) is also faster for this type of electrodes. The improved characteristics of the ultra pure electrodes are thought to be due to a uniform distribution of the anodic and the cathodic areas over the electrode surfaces. On the electrodes with impurity inclusions exposed in the surfaces, a high degree of local corrosion was noted. The oxygen sensitivity of the ultra pure electrodes, dE/d(log p_O2), was found to be 15.3 ± 0.7 and 15.7 ± 0.8 mV, 5 and 25 h after immersion of the electrodes in the electrolyte respectively.     

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.     

Reconsidering differential aeration cells

The differential aeration cells are responsible in practice for multiple corrosion forms. The established tension gradients between the cathodic and anodic areas are quite reduced, and consequently both electrodes stay submitted also to corrosion phenomena due to local-action cells. By analysing the theoretical current-potential curves it is shown that differential aeration cells are not responsible for an increase of the electrode corrosion in the oxygen-starved electrolyte in the absence of secondary effects.Differential aeration cells developed on normal carbon steel were investigated in H₂CO₃/HCO₃⁻/CO₃²⁻ solutions of different pH values. The results from polarization data and immersion corrosion tests of coupling indicated have shown the following: (1) the electrode in the aerated zone, for each one of the chosen pH, tends to attain the same potential of the electrode in the deaerated zone; (2) an alkalization of the electrolyte in the aerated and deaerated zones for initial pH 5 and 7.5 in the system; (3) the current density that cross the macrocell is little, of the same order for all pH, and does not justify the observed corrosion rate in both electrodes; (4) there is a polarity inversion of the electrodes for the pH 5 and 7.5; (4) the observed corrosion rate in each one of the tested solutions was practically the same for the aerated and deaerated electrodes. An explanation for this behaviour is given.     

Electrochemical and oxygen reduction behaviour of solid silver-bismuth/antimony electrodes in KOH solutions

Oxygen reduction behaviour at silver-low bismuth/antimony electrodes was investigated by electrochemical methods in KOH solutions. The electrochemical redox characteristics such as peak multiplicity, peak currents and peak potentials on these electrodes under both potentiostatic steady state and cyclic voltammetric conditions were obtained. The anomalies associated with the redox peak potential values both in the anodic and cathodic branches of the cyclic voltammograms were attributed to the distinct electrochemical behaviour of bismuth/antimony on the electrode surface along with silver. The behaviour of these electrodes for the electrocatalytic cathodic reduction of oxygen was also investigated both by potentiostatic steady state and cyclic voltammetric techniques under rotating conditions. The kinetic parameters corresponding to the electrochemical reduction of oxygen on these electrodes were obtained. These electrodes were ranked with respect to their electrocatalytic activity both for the oxygen reduction and evolution reactions.     

Anodic, cathodic and cyclic voltammetric deposition of ruthenium oxides from aqueous RuCl3 solutions

Anodic, cathodic and cyclic voltammetric (CV) deposition of ruthenium oxides from aqueous RuCl₃ solutions have been investigated using stationary and rotating disk electrodes (RDE) in this work. The CV deposition behavior was examined using a RDE to differentiate the contribution of current from the reactions of ruthenium ions in the electrolyte and ruthenium oxides already adsorbed on the electrode. The results indicate that the CV growth of ruthenium oxides within the potential range of aqueous electrolyte decomposition is attributed to the anodic oxidation of ruthenium ions in the electrolyte. Cathodic deposition occurs only at potential negative than −0.30 V versus saturated calomel electrode (SCE) when H₂ evolves on the electrodes. Anodic deposition of ruthenium oxides can be obtained effectively in the potential range of ca. 0.9-1.1 V versus SCE, depending on the pH value of the electrolyte. The optimum anodic and cathodic deposition potential for maximum deposition efficiency is 1.0 and −0.9 V versus SCE, respectively, in the electrolyte solution of pH 2.     

Measurement of corrosion rate using two electrodes

Instruments to measure corrosion rates in aqueous media employ probes with two-electrode and three-electrode configurations. The two-electrode method generally exhibits an extended range of linearity compared to the three-electrode method. The basis of this extended linearity is analyzed with the help of general two-electrode equations. Existing methods are discussed for evaluating the corrosion rate of two electrodes corroding at the same rate and the mean corrosion rate of two electrodes corroding at different rates. A new method is described which allows separation of two-electrode polarization into individual anodic and cathodic polarizations of each electrode. This allows one to obtain the polarization curves, and hence the polarization resistance and corrosion rate of each electrode, with good accuracy.     

In Situ Soft X‐ray Microscopy Study of Fe Interconnect Corrosion in Ionic Liquid‐Based Nano‐PEMFC Half‐Cells

This in situ soft X‐ray scanning microscopy electrochemical study of model proton exchange cathodic and anodic nano‐fuel cells is exploring the evolving structure and chemical composition of key cell components represented by Au and Fe electrodes in contact with Nafion‐ionic liquid composite electrolyte containing Pt black catalyst particles. Morphological and chemical changes of the electrodes as well as the chemical state and fate of the Fe species released into the electrolyte are monitored in short circuit and with applied cathodic or anodic polarization. The in situ X‐ray absorption images of the cathodic cell fed with 2.5 × 10^–5 mbar O₂ have revealed corrosion‐induced morphology changes in the Fe electrode, being more pronounced in the vicinity of Pt‐black particles, and deposition of the Fe species released into the electrolyte, onto the intact Au counter electrode upon cathodic polarization. The Fe electrodes of the anodic cell containing NaBH₄ in the electrolyte appear relatively more corrosion resistant. The Fe L₃ absorption spectra taken in different locations within the Fe electrode have shown lateral variations in the relative ratio between Fe²⁺ and Fe^3&4+ oxidation states, whereas the Fe species released into the RTIL electrolyte are only in the high Fe^3&4+ oxidation states.     

Corrosion under a porous layer: A porous electrode model and its implications for self-repair

A one-dimensional mathematical model for the corrosion of a metal surface under a porous layer (often an oxide layer) is presented using porous electrode theory. The model allows one to predict the rates of corrosion in a metallic system where oxygen reduction is the only cathodic reaction which occurs on the surfaces of both the metal and the porous oxide layer albeit at different rates. Thus the model simulates the scenario where the porous layer residing on the metal surface competes with or complements the oxygen reduction reaction happening on the metal surface. The study finds that a large thickness, low porosity, low oxide electrical conductivity, poor oxygen reduction on the porous layer, and high specific contact resistivity of the metal–oxide lead to poor spatial separation of the anodic and cathodic reactions and hence encourage self-repair. In particular the metal–oxide contact potential and the oxygen reduction rates on the oxide surface are shown to have profound implications for self-repair strategies for corrosion mitigation.     

Chloride induced reinforcement corrosion: Rate limiting step of early pitting corrosion

Transition from passive state to stable localised corrosion of reinforcement steel in concrete owing to chloride ingress takes place over a period of time rather than being a one-step-occurrence. The depassivation process was characterised by frequent measurements of corrosion potential, polarisation resistance, and macro-cell currents when short-circuiting the working electrode with additional cathode surface. In addition, the concrete resistivity was continuously monitored and cathodic and anodic polarisation curves were measured. The results are consistent and imply that the localised corrosion process is in the initial phase of pit growth under mixed anodic/ohmic control. With time, the anodic reaction kinetics become more limited and the corrosion rate gets almost entirely determined by anodic control. The observed relationship between achieved maximum corrosion current and concrete resistivity indicates that the extent to which the anodic reaction kinetics are restricted is determined by the concrete microstructure and its ability to retain ionic movement.     

A model for the corrosion of steel subjected to synthetic produced water containing sulfate, chloride and hydrogen sulfide

A model was developed for the prediction of corrosion rates associated with steel subjected to synthetic produced water. The corrosive species included in the model, identified through water analysis conducted in the field, are sulfate, chloride and hydrogen sulfide. The effect on corrosion of these species was examined through polarization experimentation using a three electrode glass corrosion cell and potentiostat. Samples of carbon steel, used in sub-sea pipeline systems, were used at the working electrode and the experiments were carried out at similar physicochemical conditions observed in pipeline systems in the field. The model was based on heterogeneous reactions at the metal surface, with electrochemical parameters determined through experimentation employed in the model to describe the anodic and cathodic processes involved in the corrosion of steel. The model consists of a system of equations with Butler–Volmer kinetics describing the charge transfer and the Nernst diffusion model the mass transfer processes occurring in the corrosion system. The solution is based on a charge balance between the reduction and the oxidation processes which occur at the steel surface. Current density convergence criteria were used in the model to solve the system of equations for corrosion potential, surface species concentration and component current densities. The corrosion rate is determined as the rate of oxidation of iron at the surface and model results have been validated using experimental data. The model demonstrates a reasonable qualitative match with corrosion data collected in the potential region close to the corrosion potential in general, with good qualitative match in the anodic region near the corrosion potential. Some deviation occurs between model and experimental values where overpotentials become large but the model is shown to respond well to changes in input parameter values and predicts the corrosion potential and corrosion rate for each system within experimental variability and the accepted standards of accuracy.     

钢样在磷酸钠溶液中氯离子存在的极化曲线与腐蚀曲线

<正> 分析极化曲线的特微成功地探测金属在溶液中进行的腐蚀过程.把阳极极化曲线的直线部分延长到对应静电位的电流密度以代表自发腐蚀的阳极电流密度,同样得到阴级电流密度.认为阴极极化曲线和阳极极化曲线的直线部分的延是线交点,在近似情况,可代表自发腐蚀的溶解初速度.     

Mechanism of steel corrosion in concentrated NaOH solutions

A study has been made of the mechanism of corrosion of carbon steel in 1–19 N NaOH at 25–80°C. The polarization curves were obtained with a rotating steel disk electrode, and the rotating ring-hemisphere electrode technique was used to identify soluble corrosion products. It was found that at the active anodic dissolution potentials, steel dissolves to form HFeO⁻₂ ions. Both the anodic and cathodic polarization curves exhibited a well defined Tafel regime, and the electrokinetic parameters were obtained for the corrosion of steel. The results were interpreted with a corrosion mechanism based on the decomposition of molecular water to hydrogen at the cathodic sites, and the active dissolution of iron to HFeO⁻₂ ions via two adsorbed intermediates, FeOH_ads and Fe(OH)_2,ads, at the anodic sites. The theoretically derived anodic Tafel slope, anodic reaction order, corrosion potential, and corrosion current density agreed quantitatively with the experimental values. At the potentials above the anodic Tafel regime and in the neighborhood of the active—passive transition potential, steel dissolved to form both the bi-valent and tri-valent iron species, HFeO⁻₂ and FeO⁻₂ ions. The thermodynamic considerations revealed that FeO⁻₂ ion was formed from the oxidation of Fe(OH)_2,ads intermediate, rather than from the oxidation of HFeO⁻₂ ion on steel surface.     

Electrochemical behaviour of aluminium in sodiumpolysulphide melts at 330°C

An experimental set-up was developed which allows the investigation of electrodes in pure sodiumpolysulphide melts. It could be shown that aluminium forms a passivating layer of aluminiumsulphide at an anodic potential of about 1 V vs a sodium reference electrode. Below this value, aluminium is active. With a copper containing aluminium alloy the passivating potential is increased to about 1.2 V. Pitting corrosion was observed after operating the aluminium electrode at current densities of several hundred mA cm^?2. Reaction mechanisms which could explain the active, passive and transpassive regions observed, are discussed.     

A hybrid method through Faradaic rectification and a.c. impedance measurements for the corrosion rate determination of metal/oxide/electrolyte systems. II. Experimental results for anodized aluminium

An experimental verification of a method for the determination of the corrosion rate of metal/oxide/ electrolyte systems, using both faradaic rectification and a.c. impedance measurements, is reported using zone refined aluminium electrodes in 1 mol dm^–3 aqueous sodium sulphate at pH 1.9. The impedance and the rectification parameters due to the anodic oxide on the electrode have also been computed and used to determine instantaneous corrosion rate of the same system at different values of the pH. The direction and magnitude of rectification by this system are found to be sensitive to pH as well as dissolved oxygen in the electrolyte. They are explained by a decrease in equilibrium exchange current for the hydrogen evolution reaction with increase in pH and an inhibitive effect of oxygen on the former reaction.     

Electrochemistry of non-aged 90-10 copper-nickel alloy (UNS C70610) as a function of fluid flow: Part 1: Cathodic and anodic characteristics

The polarisation behaviour of freshly polished UNS C70610 (CN 102) 90-10 copper-nickel has been examined in fully characterised seawaters using the rotating disc electrode (RDE) and the rotating cylinder electrode (RCE) geometries. The charge and mass transport controlled responses of both the cathodic and anodic reactions are presented as a function of both laminar and turbulent undisturbed fluid flow. At low values of polarisation applied over short exposure periods (<1 h), the anodic behaviour of non-aged material is controlled by the selective dissolution of the copper component of the alloy. Under conditions of complete mass transport control, oxygen reduction proceeds via the irreversible direct four-electron reduction to the hydroxide ion. Above a critical range of Reynolds numbers the rate of both the reduction and oxidation reactions tended towards a more reversible character. This change in mechanism, however, was not observed for unalloyed copper and was attributed to a convective-diffusion-based modification of the corrosion product film.     

The influence of pH on corrosion inhibitor selection for 2024-T3 aluminium alloy assessed by high-throughput multielectrode and potentiodynamic testing

This paper complements our recently published work on the development of a high-throughput scanning technique for corrosion inhibitors named multielectrode. The multielectrode set-up, fully described in the previous work, combines nine pairs of different metals in a single multi-metal electrode configuration. In the working procedure a fixed potential is applied between identical metallic electrodes while immersed in an inhibitor solution, without the use of a reference electrode. The current flowing between the electrodes is measured and the combined electrochemical response of the anodic and cathodic reactions, with and without inhibitor, is then characterized. The methodology allows approximately 30 electrochemical experiments to be performed per hour, the rapid identification of inhibitors able to protect different metals, and the rapid detection of the optimal inhibitor concentration.The present work evaluates the influence of solution pH on the anticorrosion performance of three different inhibitors (CeCl₃, Ce(dbp)₃ and K₂Cr₂O₇) on 2024-T3 aluminium alloy (AA2024-T3) by means of conventional potentiodynamic polarization analysis and the multielectrode technique reported previously. The usefulness of the high-throughput multielectrode technique to detect the influence of the solution pH on the anticorrosive performance of different inhibitors has been studied and its correlation with traditional potentiodynamic polarization testing evaluated. The possible reasons for non-correlation between techniques are studied, and those considered important, highlighted. The combined dependence of corrosion inhibition on factors including pH, inhibitor type, and metal cross-contamination was also investigated showing that the multielectrode is a robust technique to detect efficiency changes due to pH variations.     

Electrochemical technique for investigating temporarily protective oil coatings

A wire-beam electrode using mild steel wires has been developed and used to study temporarily preventive oil coatings in terms of the heterogeneous electrochemical states after immersion in a salt solution. It has been found that the electrochemical states on the surface of the wire-beam electrode are homogeneous compared with an oil-painted electrode. The distribution of corrosion potential on oil-painted wire-beam electrodes is heterogeneous and follows a discontinuous binomial probability distribution. Polarization resistance of each oil-coated wire sensor is randomly distributed over a given range of resistance and follows a log-normal probability distribution. The distribution of corrosion potential over a high potential range can be increased, distribution of polarization resistance over a low resistance range can be decreased and eliminated by adding oil-soluble inhibitor. With increasing concentration of oil-soluble inhibitor, the distribution of anodic polarization resistance of oil coatings may change from discontinuous binomial distribution to exponential distribution and to log-normal probability distribution, the distribution of cathodic polarization resistance of oil coatings will transform from discontinuous binomial distribution to log-normal probability distribution.