Geary Equation Research Articles

Experimental Study and Mathematical Modeling for Corrosion of Amalgam at Different Periods

This work involves experimental study for corrosion of amalgam as filling material after different period time (0, 7, 14, 21, 35, 42 days) in artificial sa liva by electrochemical method to estimate corrosion parameters such as corrosion potential Ecorr, corrosion current density icorr, and Tafel slopes bc & ba. The results of polarization resistance which calculated according to Stern- Geary equation indicates that the resistance of amalgam was increased during the experimental period due to stable phases which produced between mercury and other powders in amalgam such as Sn–Hg and Ag–Hg, in addition to Ag–Cu and Ag–Sn phases. Cyclic polarization t est shows that the reverse scan curves meet the forward scan curve along the passive range.Numerical modeling was achieved to estimate the corrosion behavior of amalgam in artificial saliva through longer period time, the results of this modeling shows that values of polarization resistance for amalgam at long time of filling decreases with time, but after 2200 days of filling, i.e. after a t least 6 years.

A new perspective on measuring the corrosion rate of localized corrosion

AbstractMany corrosion phenomena are nonuniform, which means that anodic and cathodic locations are spatially separated. An example is macrocell corrosion of steel in concrete. Under these conditions, determining the corrosion rate from polarization resistance measurements and using the Stern–Geary equation is fundamentally not possible. We present a novel theoretical approach for the interpretation of galvanostatic pulse measurements, to make them applicable as a method for corrosion rate measurements in situations of localized corrosion. Experiments show that it is important to consider that (a) only a fraction of the applied current flows through the anode of the macrocell, and (b) this current is not constant over time. We propose an approach to quantify and consider these two effects, based on information generally accessible in condition assessment of concrete structures. Our results show that galvanostatic pulse measurements are a robust method to determine the corrosion current. With the traditional empirical approach, the measurement error was generally below factor 3, and occasionally up to factor 10. With the novel approach, this error could be reduced to a factor of maximum 2 in all cases.

A comparative study of mild steel passivation embedded in Belite-Ye'elimite-Ferrite and Porland cement mortars

An extensive experimental program was designed to investigate the passivation of reinforced low carbon-release Belite-Ye'elimite-Ferrite (BYF) cement mortars compared to the passivation of reinforced conventional Portland equivalents (OPC). The influence of formulation parameters such as the water-to-cement ratio and the presence of fly ash on the steel rebar passivation was evaluated by conventional corrosion measurements such as corrosion potential readings and linear polarization resistance. Visual observations were carried out as a complement. A negligible corrosion rate was reached after 28 days with BYF cement against 14 days with OPC equivalent. This difference is attributed to the pH of the medium in the BYF mortars, which is lower than in the OPC mortars at a very early age, before becoming strongly basic. The exploitation of the Tafel slopes showed that the corrosion current can be evaluated by the Stern and Geary equation with an identical B coefficient for the samples of BYF and OPC basis. Despite some differences compared to the passivation in Portland mortars, the present study clearly points to the BYF mortars intrinsically passivating nature regarding mild steel reinforcing bars.

Respuesta a pulsos galvanostáticos del acero embebido en concretos de escoria activada alcalinamente

<span>Presenta la aplicación de la técnica de pulsos galvanostáticos en el concreto de escoria activada alcalinamente, <span>sometido a procesos acelerados de carbonatación. Como material de referencia se utilizó el concreto Portland <span>ordinario. La utilización de dicha técnica permitió calcular el valor de las resistencias involucradas en el proceso <span>de forma separada, permitiendo hallar la velocidad de corrosión mediante la ecuación de Stern y Geary. Los <span>productos de corrosión fueron determinados con la técnica de Espectrometría Mössbauer, correspondientes a <span>magnetita, wuestite y goethita.</span></span></span></span></span><br /><br class="Apple-interchange-newline" /></span>

Correlation between electrochemical impedance measurements and corrosion rate of magnesium investigated by real-time hydrogen measurement and optical imaging

The corrosion behaviour of magnesium in chloride-containing aqueous environment was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) performed simultaneously with real-time hydrogen evolution measurements and optical imaging of the corroding surface. The potentiodynamic investigation revealed substantial deviations from linearity in close proximity of the corrosion potential. In particular, differences in the slope of the current/potential curves were observed for small polarizations above or below the corrosion potential. These observations, suggest that the usual method based on the use of the Stern–Geary equation to convert a value of resistance into a value of corrosion current is inadequate. Nonetheless, a very good correlation between values of resistances estimated by EIS and corrosion currents obtained from real-time hydrogen measurement was found. Real-time hydrogen measurement also enabled, for the first time, direct measurement of an ‘apparent’ Stern–Geary coefficient for magnesium. In order to rationalize the complex behaviours experimentally observed, an electrical model for the corroding magnesium surface is presented.

On the applicability of the Stern–Geary relationship to determine instantaneous corrosion rates in macro‐cell corrosion

The well‐known Stern–Geary equation was derived on the basis of the theory by Wagner and Traud and relies on the so‐called mixed potential of an electrode subject to uniform corrosion. Nevertheless, it is common practice to apply the linear polarization method to determine instantaneous corrosion rates in situations of galvanic corrosion. In the present work, the applicability of the Stern–Geary equation to galvanic corrosion was in detail discussed both with help of theoretical considerations and experimental measurements. The primary reason why the application of the Stern–Geary equation to galvanic corrosion is fundamentally incorrect is the fact that a well‐defined mixed potential does for galvanic elements not exist. In this work, equations were derived that allow a correct calculation of the corrosion rate for galvanic cells. However, the required variables can, in usual practical situations, not be measured. Application of the Stern–Geary equation to galvanic cells is in the literature claimed (or assumed) to yield adequate results. An explanation for this apparent applicability was identified in the fact that two major errors have a compensating effect so that they tend to cancel each other out. The extent to which this is possible depends strongly on the actual conditions such as the geometry of the galvanic cell as well as on the position of the counter and the reference electrode (RE). Nevertheless, since this apparent applicability is owing to specific conditions rather than a fundamental applicability of the method, further research is clearly needed to characterize these conditions and their limits in detail.

Corrosion study of SS-316L exposed to LiBr–H2O solution applying electrochemical techniques and weight loss method

Corrosion evaluation of SS-316L exposed to LiBr–H2O solution of 50 wt-% was investigated applying the electrochemical noise technique (ENT), potentiodynamic polarisation curves, and the weight loss method (WLM) at 25, 50, 60, 70 and 80°C. The current and potential noise signals were obtained each four hours during 15 days. These data were analysed in the time and frequency domains. A statistical analysis was made to obtain the localisation index and the resistance noise. Also, the Stern–Geary equation and the Faraday's law were used to determine the corrosion rates. A comparison of the corrosion rates obtained from ENT and WLM showed the same tendency with temperature, but a difference of one order of magnitude was seen. The results of the corrosion rates from ENT and WLM presented a good agreement with the inverse noise resistance results. SEM analyses of the corroded samples together with atomic absorption analysis of corrosion products are also presented.

Corrosion behavior of ferritic stainless steel alloyed with different amounts of niobium in hydrochloric acid solution

The corrosion behavior of stainless steel alloys containing corrosion-resistant elements was investigated. Ferritic stainless steel (FSSs) electrodes were synthesized by applying a scan rate of 1 mV s−1. Stainless steels were used unalloyed and alloyed with about 0.5, 1, and 3 wt% elemental Nb. The samples were obtained from casting and forging. The samples were classified into three groups. In the first group, samples were unhomogenized and remained in production condition. In the second and third groups, samples were exposed to homogenization at 1,100 °C for 30 min or 180 min, respectively, and then quenched. The corrosion performance of the FSSs was investigated in 0.3 M HCl acid solution using electrochemical impedance spectroscopy (EIS). Corrosion resistance was calculated using the Stearn–Geary equation. SEM investigations of samples immersed in 0.3 M HCl acid solution for 60 and 360 min were performed. SEM micrographs showed generalized pitting. Consequently, it was determined that niobium has a beneficial effect on the corrosion resistance of FSS since niobium reacts with carbon to form stable carbides.

Electrochemical noise analysis in the frequency domain and determination of corrosion rates for SS-304 stainless steel

Studies of the corrosion evaluation of SS-304 stainless steel exposed in lithium bromide aqueous solution have been carried out applying the electrochemical noise technique and polarization curves, as well as the weight loss method. The test temperatures were ambient to 80 °C, and the exposure time was during 15 days. The main objective was to determine the corrosion rates and the type of corrosion that SS-304 suffers under the mentioned conditions, with the purpose of evaluating its application to heat pumps/transformers. Polarization curves revealed the formation of a passive film, which under increased temperature changed its protective properties. The electrochemical noise results suggested that at the test temperatures the type of corrosion was mixed or pitting corrosion. The noise resistance was calculated through statistical analysis, and then, the Stern–Geary equation and Faraday law were applied to determine mass loss, which was compared to that obtained from weight loss method. Two localized corrosion indicators were also obtained: pitting index (PI) and the coefficient of variation of current (CV), which were compared each other and interrelated with the noise signals and visual observations. To support the type of corrosion process, scanning electron microscopy (SEM) coupled with energy dispersive X-ray analyzer (EDX) was used to study the surface morphology of the corroded specimens and for determining qualitative analyses.

Corrosion of galvanised steel under an electrolytic drop

Described here for the first time is an investigation of the local corrosion behaviour of galvanized steel using an electrolytic droplet comprising chloride and sulphate ions. The maximal height of the electrolytic layer and the diameter varied between 500–1100 μm with a total diameter between 1.47–3.35 mm, respectively. These measurements were possible by using microelectrodes as reference and counter electrodes, which could be inserted into the electrolyte droplet thus forming an electrochemical microcell. Polarization curves as well as electrochemical impedance spectroscopy plots were recorded under the electrolyte droplet configuration allowing the determination of the polarisation resistance R p and the proportionality constant k of the Stern and Geary equation. The influence of temperature, chloride concentration, pH and the thickness of the electrolytic layer on the corrosion rate of galvanised steel were studied.

Investigation of atmospheric corrosion of Zn using ac impedance and differential pressure meter

The proportional constant k of the Stern and Geary equation, for zinc corrosion in near-neutral chloride solution, has been experimentally estimated using i corr values determined from differential pressure and weight loss methods. In the initial stage, the k value has been found to be 13 mV. However, when the surface was covered with a thick corrosion product, the k value decreased down to 4 mV, and then maintained a constant value. The k value estimated from the weight loss method agreed with the one estimated from the differential pressure meter (DPM). Experimentally, it was revealed that a 10 mHz impedance could not be taken as the polarization resistance if the surface is covered with a thick corrosion product. However, a proportional relation between corrosion rate and reciprocal of impedance at 10 mHz ( Z 10 mHz −1 ) still exists, even though the surface is covered with a thick corrosion product.

EIS study of the corrosion behaviour of zinc-based coatings on steel in quiescent 3% NaCl solution. Part 1: directly exposed coatings

The possibilities offered by the EIS technique for studying the progress of corrosion in several types of galvanised coatings were investigated. Corrosion tests were performed in a 3 wt.% NaCl aqueous solution, at approximately neutral pH, without stirring and in contact with the air. A tendency was seen towards an electrochemical overestimation of the corrosion values of essentially pure zinc and Zn–5%Al (Galfan) coatings compared with the gravimetric reference values. In contrast, the opposite tendence was revealed with the Zn–10%Fe (Galvanneal) coating. These deviations have made it necessary to use empirical values of the constant B of the Stern–Geary equation. The approximately linear progress of the attack with immersion time suggests a slight barrier effect of the corrosion products layer.

EIS study of the corrosion behaviour of zinc-based coatings on steel in quiescent 3% NaCl solution. Part 2: coatings covered with an inhibitor-containing lacquer

An EIS study has been made of the behaviour of several lacquered and inhibited zinc-based coatings on steel exposed to a 3% NaCl solution in quiescent conditions. Under the hypothesis that the HF–LF arc supplied the R t value of the corrosion reaction, the possibility of obtaining quantitative information about the effect of the lacquer film and the inhibitor on the progress of attack has been demonstrated. Different values of the constant B in the Stern–Geary equation must be used depending on the specific metallic coating tested. The relatively high interfacial capacitance values of the lacquered coating suggest a highly defective lacquer film which leave a considerable fraction of the surface of the metallic substrate exposed to the corrosive medium.

Theoretical analysis of the measurement of polarisation resistance in reinforced concrete

Polarisation resistance ( R p) technique based on Stern–Geary equation is one of the most widely used methods of measuring corrosion rate of reinforcement in the field. With the aid of a “sensorised guard ring”, this electrochemical technique is claimed to be able to determine corrosion rate ( I corr) within a given measuring area. However, there are three theoretical problems in the application of this technique: (1) the original Stern–Geary equation is applicable in a uniform corrosion system at its corrosion potential, whereas the reinforced concrete structure may be subjected to non-uniform corrosion or strong polarisation by macro-cell galvanic effects or imposed currents; (2) the value of the parameter B in the original Stern–Geary equation has been estimated to fall within the range 25–52 mV. This may not be suitable for all the corrosion cases of reinforced concrete structures; (3) The polarised surface area of steel may theoretically not always be fully confined by the sensorised guard ring when the cover concrete is too thick. This paper aims at discussing the theoretical problems. A general relationship between the dissolution rate of steel reinforcement and the measured polarisation resistance is deduced under general conditions. The range of B value is also analysed based on all the possible corrosion situations of reinforced concrete. Furthermore, unsatisfactory confinement by sensorised guard ring on a thick cover concrete is demonstrated.

Electrochemical corrosion monitoring of galvanized steel under cyclic wet–dry conditions

The corrosion behavior of a galvanized steel was investigated under cyclic wet–dry environments using electrochemical techniques. The wet–dry cyclic was conducted by exposure to alternate conditions of 1 h-immersion in a 0.05 M NaCl solution (or Na 2SO 4 solution) and 7 h-drying at 60% RH. The polarization resistance R p of the galvanized steel was monitored during the wet–dry cycles by AC impedance method. Simultaneously, the corrosion potential E corr was measured only when the specimen was immersed in the solution. The corrosion current density i corr of the zinc coating was estimated from the monitored R p using Stern–Geary equation. The corrosion mass loss Δ M, which was obtained from the i corr vs time curve, was plotted vs the wet–dry cycle number. The average corrosion rate per cycle starts to decrease and the E corr shifts in the noble direction, immediately before the red rust (FeOOH) appears on the coating surface. It seems that the underlying steel corrosion commences when the zinc coating does not act as sacrificial anode due to accumulation of zinc corrosion products on the coating surface. The electrochemical methods, such as impedance and corrosion potential measurements, are an effective tool for monitoring the degradation of metallic coating under atmospheric environments.

Equivalent circuit for modelling the steel-concrete interface. II. Complications in applying the stern-geary equation to corrosion rate determinations

The application of the Stern–Geary equation in the determination of steel corrosion rates in reinforced concrete often implies the acceptance of certain assumptions. When a steady state is not reached in D.C. measurements it is accepted that the determinations made after a certain polarization time provide a very approximate value for polarization resistance. In A.C. measurements it is not unusual to resort to hazardous extrapolations of the impedance data. This paper examines the potential errors introduced by such practices.

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.

Naval Brass Condenser Tube Corrosion

Abstract Results of corrosion rates in brass tubes calculated from weight loss data obtained on equipment which simulates the operating conditions of a condenser are presented. These values are compared with electrochemical polarization measurements, and the applicability of the Stern and Geary equation is discussed.

A modification of the Stern—Geary linear polarization equation

An exact equation for the slope of the current—voltage curve at the corrosion—potential is derived. While the Stern—Geary equation is a valid approximation only if the corrosion—potential lies far from the reversible potentials of the oxidation and reduction processes which determine the corrosion process, two new formulae proposed here are valid approximations under all conditions. For given Tafel-constants these approximations introduce no error greater than 30 per cent, while the error accompanying the Stern—Geary method increases without limit as the corrosion—potential approaches either of the reversible potentials. The proposed formulae include the cases of the corroding Zn electrode in strong alkaline solution and the Fe 2+/Fe 3+-couple on stainless steel. Large discrepancies between corrosion rates determined by electrochemical and weight loss techniques reported recently are analyzed.

The Use Of Pulsed Applied Voltage in the Derivation of Corrosion Rates From Polarisation Resistance Measurement

Abstract An experimental technique has been developed for the rapid determination of changes in corrosion rates using the well established Stern–Geary equation. The value of the technique was demonstrated for the corrosion rates of aluminium brass in seawater and in seawater containing 0·01% sodium dimethyl dithiocarbamate for which the corrosion characteristics had been established previously. An instrument based on these principles could be developed for a variety of corrosion measurement application.