Investigation of steel corrosion in cracked concrete: Evaluation of macrocell and microcell rates using Tafel polarization response

Inhomogeneous corrosion in reinforced concrete is investigated using a beam with a flexural crack intersecting the reinforcement. An Evans diagram representation of the macrocell corrosion system is developed. The relationship between the current density and the potentials relative to the crack obtained from the Tafel polarization responses of active and passive steel in concrete compares favorably with the experimental values. When both microcell and macrocell mechanisms contribute to metal loss at the crack, the Evans diagram representation indicates that an increase in the macrocell current density results in a decreasing contribution from the local microcell at the macrocell anode.     

Electrochemical corrosion parameters for active and passive reinforcing steel in carbonated and sound concrete

The electrochemical corrosion parameters, such as corrosion potential, corrosion current density, and the Tafel constants are necessary inputs for the corrosion modeling in reinforced concrete. Literature shows large variability in their values, whereas the data are scarce for the carbonated concrete. This paper presents a range of corrosion parameters for the active steel in carbonated and the passive steel in noncarbonated concrete. Forty-eight singly reinforced concrete cylinders were cast, of which 24 were carbonated and the others were sound samples. Potentiodynamic polarization curves were obtained at three different scan rates and extrapolated to extract the corrosion parameters. To validate these parameters, a macrocell corrosion system was simulated using FEM-based Comsol multiphysics® software. The numerical results were compared to two experimental studies. A natural dispersion in the values of corrosion parameters for both active and passive steels was observed. The average Stern–Geary constant was 54 and 47 mV for active and passive steels, respectively. Numerical simulations with the obtained parameters predicted the macrocell corrosion in partially carbonated concrete with a high accuracy. The presented values of corrosion parameters in this study could help researchers and engineers to simulate the corrosion phenomena in concrete accurately.     

Probing of Atmospheric Corrosion of Metals: Carbon Steel

The methods of scanning vibrating capacitor (SVC, Kelvinprobe), IR microscopy (IRM), and galvanic current sensor are used in studying the atmospheric corrosion of carbon steel with locally applied NaCl. The SVC method is shown to give insight to the electrochemical heterogeneity of the surface. The distribution of cathodic and anodic domains is found by the IRM analysis of corrosion products. The passive surface of steel around the applied chloride takes part in the cathodic reaction of the corrosion process. The currents of the corrosion macrocell constituted by a chloride-covered anode domains and surrounding it cathodic area are measured by the sensor in humid air. From the areas of cathodic and anodic domains, the partial current ratio and the limiting stage of corrosion are found. The mechanism of galvanic couple formation and inhibition is discussed.     

Galvanic corrosion of high alloyed austenitic stainless steel welds in LiBr systems

The corrosion behaviour of the austenitic stainless steel alloy 926 (UNS N08926) in its welded and non-welded condition as well as the galvanic corrosion generated by the base metal–welded metal pair have been studied by electrochemical methods. The materials have been exposed to highly concentrated LiBr solutions, which are used as absorbent in LiBr absorption refrigeration systems. The microstructure of the samples was studied by SEM and EDX analysis. The results show that galvanic corrosion between the pair is not severe in the studied conditions. The ratio between the galvanic current density of the pair and the corrosion current density of the uncoupled anode is less than 5, which implies compatibility of the members in the couple.     

Responsive behaviour of galvanic anodes in concrete and the basis for its utilisation

In this study, a unique concrete specimen was used to analyse the response of embedded sacrificial zinc and steel anodes to rainfall and fluctuations in temperature. Current from the zinc and steel anodes increased when the environment was aggressive, showing that the required protection current depends on the present level of corrosion risk. A basis for using the ‘responsive behaviour’ of galvanic anodes is provided by the protective effects of pit re-alkalisation and pH maintenance. By contrast, protection based on achieving adequate polarisation inhibits the use of responsive behaviour and galvanic anodes might only deliver adequate polarisation in aggressive environments.     

FEM‐models for the propagation period of chloride induced reinforcement corrosion

The paper reports the results of numerical simulations carried out with FEM and aimed at evaluating the corrosion conditions of steel bars in concrete elements subjected to chlorides. Two case studies were analysed: a reinforced concrete element subjected to de‐icing salt in the presence of a crack and a concrete tunnel in a chloride‐contaminated, water saturated soil. Attention was focused on the selection of proper values of concrete resistivity and of the parameters suitable to describe the electrochemical behaviour of steel in the different conditions of exposure. The results allowed to quantify the effects of the galvanic coupling between active and passive areas on the corrosion rate of steel.     

Galvanic corrosion of titanium coupled to welded titanium in LiBr solutions at different temperatures

The galvanic corrosion generated between the titanium-welded titanium pair has been studied in heavy brine LiBr solutions at 25, 50 and 100 °C under open circuit conditions using a zero-resistance ammeter (ZRA). The results showed that welded titanium was the anode of the pair, so that its corrosion resistance decreases due to the galvanic effect. However, the extremely low galvanic current densities registered by the pair reveal the poor severity of the coupling under the studied conditions. Furthermore, it was observed that the electrodes were in the passive state, increasing the probability of localized corrosion with temperature.     

Electrochemical behaviour enhancement of stainless steels by a SiO2 PACVD coating

Amorphous silica coatings have been deposited by plasma assisted chemical vapor deposition (PACVD) process on M2 steel and 304 stainless steel substrates. The chemical inertia of the coating has been established by resonant quartz crystal microbalance (RQCM), and no porosity has been observed at the AFM scale. The barrier effect has been evaluated on M2 steel by applying the concept of dipolar relaxation. The protection is very efficient, and no deleterious galvanic coupling with the uncoated metallic surface can occur.

A coated stainless steel was also investigated in NaCl solution in order to understand how the passive properties are influenced by such a barrier coating. When deposited on stainless steel, coated part keeps the beneficial passive behaviour inherent to the substrate. They evidence almost no corrosion. The silica based layer behaves as a quasi perfect dielectric. The corrosion rate is then greatly reduced, and the pitting resistance is improved. The excellent localized corrosion protection has been explained by a tiny porosity rate, and a limited access of chloride ions through open pores.     

Modelling of reinforcement corrosion – geometrical effects on macrocell corrosion

Reinforcement corrosion is still the most frequent reason for damage of concrete structures. It can be caused by carbonation or the ingress of chlorides. In cases of localized contaminations with chlorides, macrocells with very high corrosion rates can be established. Thereby the resulting macrocell current is dependent on many different boundary conditions like driving voltage, concrete resistivity and the geometrical arrangement of anode and cathode. In order to investigate macrocell corrosion, the herein presented research work was carried out by laboratory experiments and additionally by numerical analyses. First the numerical simulations were calibrated by laboratory measurements and thereafter, a numerical parameter study was carried out to increase the available database and identify the impact of changes in single parameters. As the focus is on geometrical effects, all laboratory specimens and numerical models were designed to represent practical conditions with diverse geometrical arrangements, e.g., slabs or beams with localized depassivations. In addition, parameters like concrete resistivity, driving voltage and cathode to anode surface area ratios have been varied. Thereafter, all results were used to derive cell factors for a simple macrocell current estimation. The present status of the project will be presented and discussed.     

Effect of Fe-Zn alloy layer on the corrosion resistance of galvanized steel in chloride containing environments

In this study, the effect of Fe-Zn alloy layer that is formed during galvanizing process on the corrosion behavior of galvanized steel has been investigated. The galvanostatic dissolution of galvanized steel was carried out in 0.5 M NaCl solution to obtain the Fe-Zn alloy layer on the base steel. The alloy layer was characterized to be composed of FeZn₁₃, FeZn₇ and Fe₃Zn₁₀ intermetallic phases, which constitute the zeta, delta1 and gamma layers of galvanized steel, respectively. It was observed that the alloy layer has similar cathodic polarization behavior but different anodic polarization behavior compared to galvanized steel. The anodic current plateau of alloy layer was up to 100 times lower than that of galvanized coating. Corrosion test performed in wet-dry cyclic condition has shown that the alloy layer has lower corrosion rate as compared to galvanized steel. From the results of corrosion test of alloy layer and base steel, it was concluded that Zn²⁺ has positive effect on the protectiveness of the zinc corrosion products. The measurement of surface potential over the alloy/steel galvanic couple has confirmed the galvanic ability of alloy layer to protect both the alloy layer itself and the base iron during initial stage of atmospheric corrosion.     

混凝土结构中NO_2~-的不均匀性对钢筋宏电池腐蚀行为的影响

基于宏电池腐蚀理论,将阴极区钢筋和阳极区钢筋分别浇筑成两个独立的砂浆试块,并用导线将其连接成循环电路,通过断开和连接电路来控制钢筋的微电池腐蚀和宏电池腐蚀,再将不同浓度的NO_2~-溶液分别添加到阴极试块和阳极试块中以模拟其渗透过程,通过比较NO_2~-作用前后阴阳极钢筋之间宏电池腐蚀电流的大小,来评价阴阳极钢筋表面NO_2~-的不均匀性对钢筋宏电池腐蚀行为的影响。结果表明:当NO_2~-作用于处于腐蚀状态的阳极钢筋时,能够增加阳极钢筋的极化阻抗,减少阴阳极钢筋间的宏电池腐蚀电位差,降低宏电池腐蚀电流,从而有效抑制宏电池腐蚀;当NO_2~-主要作用于处于腐蚀状态的阴极钢筋时,阴阳极钢筋间的宏电池电位差的增大易使宏电池腐蚀电流增加,存在加速钢筋宏电池腐蚀的危险。     

Corrosion macrocell behavior of reinforcing steel in partially submerged concrete columns

The corrosion behavior of reinforcing steel embedded in laboratory concrete columns partially submerged in a 5% NaCl solution, under different wetting conditions of the region above water line, has been investigated. A macrocell corrosion pattern developed in the test columns, with steel in the upper portions of the columns being mainly the site of the oxygen reduction reaction and a small passive current. When the columns were dry or partially moist, the cathodic reaction in the upper portions of the columns was primarily under activation polarization, while under full wetting nearly complete diffusional control took place.     

Galvanic corrosion of magnesium alloy AZ91D in contact with an aluminium alloy, steel and zinc

An investigation was carried out into the galvanic corrosion of magnesium alloy AZ91D in contact with zinc, aluminium alloy A380 and 4150 steel. Specially designed test panels were used to measure galvanic currents under salt spray conditions. It was found that the distributions of the galvanic current densities on AZ91D and on the cathodes were different. An insulating spacer between the AZ91D anode and the cathodes could not eliminate galvanic corrosion. Steel was the worst cathode and aluminium the least aggressive to AZ91D. Corrosion products from the anode and cathodes appeared to be able to affect the galvanic corrosion process through an “alkalisation”, “passivation”, “poisoning” effect or “shortcut” effect.     

Chloride threshold level for corrosion of steel in concrete

The steel rebar inside reinforce concrete structures is susceptible to corrosion when permeation of chloride from deicing salts or seawater results in the chloride content at the surface of the steel exceeding a chloride threshold level (CTL). The CTL is an important influence on the service life of concrete structures exposed to chloride environments. The present study discusses the state of art on the CTL for steel corrosion in concrete, concerning its measurement, representation, influencing factors and methods to enhance the CTL. As the CTL values reported in the majority of previous studies were varied with experimental conditions, corrosion initiation assessment method, the way in which the CTL was represented, direct comparison between the results from different sets and evaluation was subjected to the difficulty. As a result, total chloride by weight of cement or the ratio of [Cl⁻]:[H⁺] is the best presentation of CTL in that these include the aggressiveness of chlorides (i.e. free and bound chlorides) and inhibitive nature of cement matrix. The key factor on CTL was found to be a physical condition of the steel-concrete interface, in terms of entrapped air void content, which is more dominant in CTL rather than chloride binding, buffering capacity of cement matrix or binders. The measures to raise the CTL values using corrosion inhibitor, coating of steel, and electrochemical treatment are also studied.     

冷喷涂钛涂层与1Cr13不锈钢的电偶腐蚀行为

以氮气为载气,在压力为2.0MPa,载气温度为500℃的状态下,用冷喷涂的方法在1Cr13不锈钢基体上制备了钛金属涂层。经SEM观察,发现涂层的厚度较大,没有直接贯穿到基体的孔隙存在,距离基体越近的区域,涂层的孔隙率越小,XRD分析表明,涂层基本无氧化物相生成;从热力学和动力学两方面研究冷喷涂钛涂层与1Cr13不锈钢的电偶腐蚀行为,发现两者的电偶腐蚀倾向较小。     

Effect of humidity and chloride content on the corrosion of steel embedded in LWA concrete

Measurements have been made with mortar specimens with light aggregates (LWA) and the cements Norcem HS65 and MP30, corresponding to CEM I and CEM II. The w/b ratio was 0.30 and 0.40 and in one mix prewetted LWA was used. The silica fume addition was 8%. The specimens have two sets of electrodes consisting of a 10 mm reinforcing steel and a 10 mm rod of stainless steel. In the start of the experiment the chloride penetration was accelerated by applying a voltage between a negatively charged steel plate within the salt solution and a positively charged net of stainless steel embedded in the concrete above the reinforcing steels. This caused some problems and the set-up was changed to using a Plexiglas brim and a solution of calcium hydroxide and sodium chloride was applied. In the solution, a titanium net was placed and a positive voltage of 12 V was applied between the embedded steels and the net with the titanium net as the positive pole. Measuring galvanic currents, electrochemical potential and polarisation resistances have presented a good picture of the corrosion activity of the embedded steels. There is a distinct effect of the humidity upon the corrosion activity registered by the electrochemical measurements. There is a good relation between the electrochemical measurements and the observed corrosion attacks at the end of the experiments. The LWA concrete with water saturated aggregates showed more corrosion than the corresponding LWA concrete with dry aggregates. The rates of corrosion measured both as a galvanic current and as a function of the polarisation resistance showed that the concrete with watered aggregate concrete had a corrosion rate about twice as high as the corresponding LWA concrete with dry aggregates. There are marked differences in the chloride penetration for the different cements. Effects have been observed on the chloride content of the different concretes. Lower w/b ratio, MP30 cement compared to HS65 and no prewatering of the LWA gave less chloride penetration.     

Micro-electrochemical characterization of corrosion of pre-cracked X70 pipeline steel in a concentrated carbonate/bicarbonate solution

The electrochemical corrosion behavior of a stressed, pre-cracked X70 pipeline steel was studied in a bicarbonate/carbonate solution by electrochemical and micro-electrochemical measurements, numerical calculation and surface analysis technique. The effects of stress and potential on passivity, corrosion and electrochemical behavior of the steel at crack-tip were mechanistically determined. It was found that the passive film formed at crack-tip was less stable than that formed in the region ahead of the crack. Moreover, the crack-tip is more susceptible to pitting corrosion than other region of the specimen. The applied stress enhances the anodic dissolution of the steel. In particular, the stress concentration at crack-tip further increases the local anodic dissolution rate. The enhancement of the anodic dissolution of the steel at crack-tip is also resulted from the formation of a galvanic couple, i.e., the crack-tip as an anode and the surrounding region as a cathode.     

Monitoring of corrosion processes in chloride contaminated mortar by electrochemical measurements and X‐ray tomography

Corrosion of steel reinforcement in concrete exposed to chloride containing environments is a serious problem in civil engineering practice. Electrochemical methods, e.g., potential mapping, provide information whether the steel reinforcement is still passive or depassivation has been initiated. By applying such techniques no information on the type of corrosion, its extent and distribution of corrosion products is available. Particular the corrosion progress is a significant problem. Especially in the case of macrocell corrosion in reinforced concrete structures, the development at the anode cannot be separated into corrosion damage resulting from macrocell corrosion or self‐corrosion. Until now also in laboratory tests it is impossible to collect such information without destroying specimens after electrochemical testing was performed. To overcome this problem it was tried to study the steel surface within the mortar specimens by X‐ray tomography (CT). Within the scope of these investigations it could be shown, that X‐ray tomography is suitable to make corrosion pits and their development visible which are embedded in a mortar with a cover thickness of about 35 mm. In this publication the time‐dependent corrosion damage of reinforced steel is documented by X‐ray tomography.     

Effect of hydrochloric acid on pickling of hot-rolled 304 stainless steel in iron chloride-based electrolytes

Weight loss, corrosion potential and EIS measurements were performed to understand the role of HCl in the pickling of oxidised 304 stainless steel in iron chloride-based electrolytes. The surface finish was analysed with SEM-EDX. The oxidised 304 stainless steel is active on immersion, resulting in a low corrosion potential and a high weight loss. After certain duration the stainless steel either remains active or becomes passive depending on HCl content. At low HCl contents, an ongoing active-to-passive transition results in localised corrosion at pits, grain boundaries and honeycombed recesses. The corrosion potential becomes high and the weight loss is suppressed. The weight loss decreases in the initial stage and rises on extended pickling with adding HCl. Because of anodic brightening, the 304 stainless steel is always active as HCl is concentrated. In contrast with the material that is passivated, the charge transfer resistance is considerably low and the double layer capacitance is large during that brightening.     

Influence of diurnal and seasonal temperature variations on the detection of corrosion in reinforced concrete by acoustic emission

Chloride rich reinforced concrete prisms were coupled to chloride-free prisms and exposed to diurnal and seasonal temperature cycles typical of those found in the UK. Acoustic emissions (AE) and galvanic currents were continuously monitored and correlated with ambient temperature. AE and galvanic currents were found to emulate the evolution of temperature in the diurnal cycles, although no specific relationship between AE and galvanic current could be obtained. The influence of seasonal variations in galvanic current had no obvious influence on AE Energy per second over the range of corrosion rates studied. The findings suggest that AE is more sensitive to short term (diurnal) changes in corrosion rates than the longer (seasonal) effects. It was hypothesised that this is due to transitory changes in the internal microclimate of the concrete.