Selecting an adequate procedure for the electrochemical chloride removal

This paper concerns the proper selection of the electrochemical chloride removal (ECR) current and the ECR duration. Four constant ECR current densities, 100, 200, 400 and 800 μA/cm², were applied to concretes for 1 to 6 weeks. The remaining chloride contents nearby the reinforcement were measured to understand how fast the ECR process can remove chloride ions out of concrete. The open circuit potential and corrosion rate measurements were performed to examine the corrosion status after ECR. In addition, the pH value nearby the reinforcement and the apparent electrical resistivity of concretes after ECR were measured. The results showed that the remaining chloride ions decreased, the open circuit potential became nobler, the corrosion rate decreased, the pH value increased and the apparent electrical resistivity increased as the ECR parameter, defined as the product of the ECR current density and the ECR duration, increased. The ECR parameter can provide a direction in designing the ECR process.     

Electrical resistivity and corrosion potential of reinforced concrete: influencing factors and prediction models

Abstract

This paper presents two models: one for predicting the electrical resistivity of concrete and the other for the corrosion potential of reinforcing steel. The prediction models were developed based on experimental results, considering various influencing factors. The experiment approach included the concrete mix proportion, chloride content, concrete cover thickness and time of exposure as the parameters. The results revealed that fly ash concrete showed significantly high electrical resistivity even in the presence of chloride ions. The effects of fly ash became more significant when the water to binder ratio was lower. Chloride ions also decreased the corrosion potential of steel in both the OPC and fly ash concrete. The corrosion potential was found less negative for fly ash concrete due to higher electrical resistivity. The prediction results show good agreement with the experimental results of this study and some other researchers.     

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.     

Effect of CaCl2 and NaCl additions on concrete corrosivity

In addition to breaking down the passive film on embedded steel, the level of chloride content in concrete also influences the electrical resistivity of the concrete and, hence, the kinetics of the reinforcement corrosion, as long as the corrosion process is under resistance control. While there is general agreement in the literature that the binding of chlorides in concrete is higher when CaCl₂ is added to the fresh concrete, in comparison with NaCl, the effect of different chloride sources on the concrete resistivity is not so well known.To quantify the effect of different types of chloride source on the concrete corrosivity, different mortars with OPC and 0.50 w/c were prepared, and various amounts of CaCl₂, NaCl and NaOH were added to the fresh mixtures. The corrosivity was primarily tested by measurements of electrical resistivity and acid capacity.The paper summarizes the results of the study regarding the effect of both the type and amount of chloride source on concrete corrosivity.     

Chloride induced corrosion of reinforcing steel evaluated by concrete resistivity measurements

The chloride threshold (Cl_TH) concentration for rebar corrosion initiation has received extensive attention over the last years. The chloride threshold concentration depends on several factors involving concrete composition and quality, exposure conditions and rebar surface characteristics. As a consequence, many researchers have proposed Cl_TH ranges that take into account the relative influence of each of these many factors. On the other hand, the electrical resistivity of concrete has proven to be an effective parameter that can be used to estimate the risk of reinforcing steel corrosion, particularly when corrosion is induced by chloride attack. The present study is based on a correlation of electrochemical parameters such as corrosion potential (E_corr) and current density (i_corr) together with concrete resistivity (ρ) and chloride concentration data. A relationship between chloride threshold values for rebar corrosion initiation and resistivity values (indicative of concrete quality) is proposed. According to this correlation, when the electrical resistivity of concrete increases from 2 to 100 kΩ cm, the value of Cl_TH increases from 0.44 to 2.32% relative to the weight of cement.     

On the limitations of predicting the ohmic resistance in a macro-cell in mortar from bulk resistivity measurements

An experimental setup was designed to simulate the conditions for chloride-induced macro-cell corrosion, in which small anodes are located in a large network of cathodes. The overall aim of the present study was to assess whether measuring the bulk resistivity of reinforced concrete/mortar can give sufficient information about the resistance between anode and cathode inside a macro-cell. Measurements were executed in mortar specimens with high and low resistivities. Both the resistance in the simulated corrosion cells and the bulk resistivity of the mortar mixtures were determined. A comparison of the results showed no direct correlation. This indicates that the common practice of comparing bulk resistivity with corrosion rate may not be sufficient to characterize the corrosion process of chloride induced macro-cell corrosion.     

Effect of corrosion on the bond between concrete and steel rebar

The effect of corrosion on the bond between concrete and steel rebar was studied by measuring both bond strength and contact electrical resistivity. Corrosion of steel rebar in concrete immersed in saturated Ca(OH)₂ solution was found to cause the bond strength to increase, and the contact resistivity increased until 5 weeks of corrosion. Further corrosion caused the bond strength to decrease, while the contact resistivity continued to increase.     

龄期对电阻率法检测赤泥改良黄土抗剪强度的影响

随着我国氧化铝产能的不断增大,在生产时排放出的污染性废渣赤泥给环境带来多方面的影响,所以合理地利用并消纳赤泥就显得尤为重要.本文利用赤泥配制5种固化剂对黄土进行改良,将赤泥改良后的黄土用于路基材料.试验测试了改良黄土在直剪过程中电阻率的变化,分析了龄期对赤泥路基材料的电阻率和抗剪强度的影响.通过击实试验确定试样的最优含水量,在最优含水量下制样并养护3 d、7 d、14 d、28 d.结果表明:不同龄期下电阻率随剪切位移的增大呈现上升趋势;电阻率和抗剪强度分别与龄期的对数相关性较好,二者都随养护龄期的增大而增大;随着赤泥掺入量的增大,导电离子增多,电阻率减小;相同赤泥含量下,垂直压力越大,抗剪强度越大;得到抗剪强度与电阻率的线性关系式.     

A migrating corrosion inhibitor evaluated in concrete containing various contents of admixed chlorides

The performance of a surface-applied migrating corrosion inhibitor (MCI) based on an alkylaminoalcohol was evaluated on concrete specimens containing reinforcing steel bar (rebar) segments. Two water/cement ratios (w/c's), various chloride contents and two exposure conditions were investigated. The inhibiting efficiency was followed over a period of 1000 days by means of parameters such as corrosion potential, corrosion current and electrical resistance. Results show that when concrete is exposed to the marine environment, the inhibitor is able to reduce the corrosion rate (CR) only when the initial chloride content is below 0.16 wt.% relative to cement content. Efficiency increases as the w/c increases. There is no beneficial effect when the initial chloride content is greater than 0.43%. When concrete is immersed in a saline solution, no beneficial effect associated to the use of the inhibitor could be appreciated, regardless of w/c or initial chloride content in concrete.     

用电阻率法研究新拌混凝土的早期凝结和硬化

测定和分析了新拌混凝土早期水化期间电阻率发展的特性。根据电阻率曲线和其微分曲线上的特征点将混凝土的水化分为：溶解期、凝结期、硬化前期和硬化的减速期。提出从电阻率发展微分曲线第1峰值特征点查找混凝土终凝时间和推算初凝时间的方法。结果表明：电阻率法得到的初凝时间和终凝时间与贯入阻力法测试得到的凝结时间有良好的相关性。比较了纯水泥混凝土和掺粉煤灰混凝土的电阻率发展特性和强度特性。结果显示：掺粉煤灰混凝土的早期强度随粉煤灰掺量的增加呈下降趋势,同龄期样品的电阻率也有相同的下降趋势。     

The performance of a migrating corrosion inhibitor suitable for reinforced concrete

The protection provided by a migrating corrosion inhibitor (MCI) based on an alkylaminoalcohol was tested on concrete specimens containing reinforcing steel bar (rebar) segments. Two inhibitor dosages were investigated, together with two water/cement ratios and various chloride contents. The inhibition efficiency was followed over a period of 1000 days measuring electrical and electrochemical parameters such as the corrosion potential, the corrosion current density, the electrical resistance and performing electrochemical impedance spectra. The inhibitor was able to reduce the corrosion rate only when the initial chloride content was below 0.16 wt.% (percent weight relative to cement content). The efficiency increased as the water/cement ratio increased. There was no beneficial effect when the initial chloride content was greater than 0.43 wt.%. The efficiency of the product increased when the amount of inhibitor being applied doubled.     

Studies on the influence of chloride ion and pH on the electrochemical behaviour of aluminium alloys 8090 and 2014

The influence of chloride ion concentration and pH (2.0, 6.0 and 11.0) on the corrosion behaviour of 8090 (Al-Li-Cu-Mg-Zr) and 2014 (Al-4.4%Cu) alloys has been studied in NaCl solution using a potentiodynamic polarization technique. The corrosion rate for both the alloys was high at pH values of 2.0 and 11.0 as compared to that at pH 6.0, and the rate increased in chloride ion concentration at all pH levels. A similar result was found for the passive current density. Increase in pH changed the slope of the cathodic polarization curve by changing the cathodic reaction. Increasing the chloride ion concentration decreased the cathodic reaction rate. On the other hand, the anodic reaction rate increased with increase in chloride ion concentration. The open circuit corrosion potential and the pitting potential shifted in the active (negative) direction with increasing pH and chloride ion concentration. The i _p values for 8090-T851 were slightly lower than those for 2014-T6.     

海洋大气环境下粉煤灰混凝土耐久性研究

海洋大气环境中的混凝土结构不仅要遭受盐雾腐蚀,同时还要受碳化影响。在盐雾腐蚀及碳化双重作用下,混凝土结构极易发生钢筋锈蚀。本文开展了盐雾腐蚀及碳化双重作用下的氯离子侵蚀试验研究。研究结果表明:当粉煤灰掺量不超过30%时,粉煤灰混凝土的抗氯离子侵蚀能力随粉煤灰掺量的增加而提高。碳化反应不仅造成混凝土孔隙溶液pH值下降,同时粗化了混凝土的孔隙,引起混凝土微观结构重分布,破坏原有的过滤机制,使得侵蚀环境中的氯离子更容易渗入。采用修正后的Fick第二扩散定律解析解对盐雾环境下混凝土中的氯离子浓度进行非线性拟合,具有良好的相关性。     

Evaluation of bauxite residues (red muds) of different origins for environmental applications

Bauxite residue (red mud) samples from 11 Australian and overseas Bayer process refineries and variously treated red muds were investigated from the perspective of potential environmental uses for red muds. Red muds had pH (1:5 H₂O) values ranging from 8.4 to 12.6 and electrical conductivities of 0.7 to 18.2 mS/cm (1:5 extract), surface area ranged from 15 to 30 m²/g and texture from sandy clay loam to clay. Red muds are mostly composed of crystalline compounds of Fe, Al, Si and Ca with some red muds containing moderate amounts of Ti and Na. Most red muds contain hematite, goethite, quartz, calcite, desilication product (sodalite), gibbsite, boehmite and anatase. Other minerals, including muscovite, halite and gypsum occur in some red muds. The acid buffering behaviour of red muds was investigated by incubation of red muds with various amounts of hydrochloric acid. Buffering curves changed with time and each red mud gave a different buffering curve although curves could be grouped into five different shapes. Most of the buffering occurs between about pH 6 and 8 where the pH is buffered by dissolution of calcite, sodalite and tricalcium aluminate (when present). X-ray diffraction (XRD) analysis of acid treated red muds at different pH values indicate that sodalite and calcite buffer (i.e. dissolve) simultaneously. Buffering at lower pH values (< 4) is due to dissolution of Fe oxides. Each red mud has different pH buffering characteristics so that the use of red mud for liming of acid soil, water, sulphidic mining residues and manufacturing wastes will require detailed characterisation of the buffering reactions for the particular system.     

The influence of mineral admixtures on resistance to corrosion of steel bars in green high-performance concrete

In this paper, the influences of the types, amount and adding approaches of mineral admixtures on pH values, electrical resistance of concrete, anodic polarization potential and mass loss ratio of steel bars in concrete subjected to 50 immersion-drying cycles were investigated. The testing results showed that the addition of mineral admixtures reduced the pH values of the binder pastes in green high-performance concrete (GHPC), especially when two or three types of mineral admixtures were added at the same time (double- or triple-adding approaches), whereas the final pH values were still above the critical breakage pH value of passivation film on the steel bar surface (11.5). Double- and triple-adding approaches also greatly increased the electrical resistance of concrete, which led to a delay in the initial time of corrosion and a decrease in the corrosion rate of steel bars. Additionally, double- and triple-adding mineral admixtures, instead of single-adding, fly ash can reduce the corrosion of steel bars when a large amount of fly-ash replacement was used. All the details of this paper provided a method to reduce or prevent the corrosion of steel bar in concrete, especially for the application in aggressive marine environments.     

基于BP神经网络的油气长输管道土壤腐蚀性预测

基于BP神经网络原理,综合考虑六类土壤腐蚀指标(土壤电阻率、土壤含水量、氧化还原电位、氯离子含量、硫酸根离子含量和p H值),建立了一种土壤腐蚀速率的预测方法。基于这种方法,依据某油田的现场土壤数据,借助MATLAB神经网络工具箱建立了这一地区的土壤腐蚀性预测的BP神经网络模型。训练和预测结果表明:训练的腐蚀速率最大误差为-1.5%,预测的腐蚀速率最大误差为8%。由此可见,基于BP神经网络的土壤腐蚀性预测方法具有较好的预测精度,对油气管道的安全运行具有重要的意义。     

掺合料对钢渣混凝土电阻率的作用

把混凝土作为电工材料 ,对其电性能进行研究 ,尚属少见。作者通过测定混凝土的电性能 ,对其工艺参数、性能、质量进行无损检测和质量控制 ,具有实用意义。本文研究了掺合料 (硅灰、粉煤灰 )对钢渣导电砂浆电阻率的影响 ,结果表明此法能较好地反映掺合料活性与掺量的变化     

Corrosion behavior of steel in alkali-activated fly ash mortars in the light of their microstructural,mechanical and chemical characterization

This study concerns the corrosion behavior of steel in different room temperature cured alkali-activated fly ash mortars exposed to chloride solution. The corrosion process was monitored by polarization resistance and corrosion potential measurements and the results were interpreted in the light of a complete microstructural, mechanical and chemical characterization of the mortars. The most compact alkali-activated mortars have higher porosity and lower mechanical properties than a cement-based mortar (CEM), but the protectiveness afforded to the rebars is slightly higher than that obtained in CEM. The reason for this discrepancy is connected to a lower chloride content accumulated in the former mortar type and to a specific inhibition of the rebar corrosion afforded by the pore electrolyte in alkali-activated mortars.     

Relationships between anodic polarisation and corrosion of steel in concrete

Corrosion of reinforcement in concrete is a topic of concern mainly because of the high cost of repair and rehabilitation of concrete structural elements. There is as yet no method of assessment that would enable the rapid and accurate prediction of the extent of corrosion of reinforcing steel in concrete on site. Half cell potential techniques commonly used in situ give only probabilistic information on corrosion activity. Research effort is thus needed in both investigating and developing methods to assess more accurately the corrosion characteristics of steel in concrete with an ultimate view of site application. Long-term investigations on chloride induced corrosion of steel reinforcement have been conducted on a series of concrete slab specimens to establish relationships between electrochemical data and chloride induced corrosion of steel reinforcement. Potentiodynamic anodic polarisation procedures were used to monitor corrosion of steel reinforcement in concrete slab specimens over a period of four years. A statistically significant relationship between the area under the corrosion current and time relationship and the weight loss of steel reinforcement was established. Assessments of corrosion rates of steel in the concretes studied were thus verified. Reinforcement corrosion was found to be localised under the high chloride conditions occurring mainly in an area adjacent to the chloride source.     

Coagulation and dispersion of red mud suspensions

Red mud is a waste stream the Bayer process for alumina production. This paper deals mainly with the influence of pH on the coagulation and dispersion of red mud suspensions.For the purpose of understanding the coagulation and dispersion phenomena of dilute red mud suspensions, the relationship between zeta potential of red mud and pH is measured, and the phenomena in the case of basic suspensions are analyzed by using the experimental data of zeta potential and the DLVO theory. It is shown that the theory agrees fairly well with the experimental results.Furthermore, measurements of the specific volume of sediment, the zeta potential, the mass fraction of less than 2 μm in red mud and the apparent viscosity of some kinds of slurries, show that in order to entirely disperse red mud suspensions, the addition of a dispersing agent to the suspensions is necessary.