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.     

钢筋混凝土中防腐添加剂的研究进展

在钢筋混凝土结构服役过程中,由环境侵蚀因素导致的耐久性问题是制约结构长效服役的重要原因,其中,氯离子侵蚀导致的钢筋锈蚀是引起钢筋混凝土耐久性劣化的主要因素之一。在混凝土内掺加防腐添加剂,可从促进钢筋表面成膜和改善混凝土自身性能(孔隙结构、水化过程、Friedel’s盐生成)等方面提升阻锈能力,近年来逐渐成为研究热点。本文将防腐添加剂分为传统阻锈剂、绿色植物提取物阻锈剂、纳米材料以及矿物掺合料等四类,从材料开发、作用机制、阻锈影响因素等方面对防腐添加剂的研究进行了综述。最后,探讨了防腐添加剂研究中存在的问题,并对未来的研究提出了建议,以期为防腐添加剂能够更好地防治混凝土中的钢筋锈蚀问题提供参考。     

Effect of calcium nitrite-based corrosion inhibitor in preventing corrosion of embedded steel in concrete

Due to the corrosion inhibition effect and compatibility with concrete properties, calcium nitrite solution has been widely used, in North America and Asia,. However, investigation has often been restricted to tests using a macrocell corrosion monitoring or measuring corrosion rate in an aqueous condition. This study concerns the assessment of the inhibition effect of calcium nitrite-based corrosion inhibitor using a polarisation method and its influence on the chloride transport, compressive strength and setting time of concrete.The calcium nitrite-based corrosion inhibitor significantly reduced the corrosion rate of steel in chloride contaminated mortar and raised the chloride threshold level, ranging from 0.22% to 1.95% by weight of cement, while nitrite-free specimen produced the threshold level ranging from 0.18% to 0.33%. It was observed that concrete specimens containing the corrosion inhibitor produced the higher total charge passed in a rapid test for chloride ion permeability. An increase in the dosage of corrosion inhibitor resulted in a decrease in the concrete setting time. In addition, the compressive strength at early ages was increased by corrosion inhibitor, but in a long term (900 days), decreased to the level for 28 days.     

有机防护型阻锈剂对混凝土耐久性的影响

钢筋混凝土结构耐久性不足的问题日益严重,而导致该结果的主要原因是氯离子引起的钢筋锈蚀。在提高混凝土结构耐久性的措施中,钢筋阻锈剂因其施工简易、经济、高效等优点而得到了广泛的使用。钢筋阻锈剂的使用目的是保护钢筋防止锈蚀。目前研究和应用较多的多为醇胺类阻锈剂,该类阻锈剂分子结构有其一定的优势,同时也存在一些不足,其中大多具有单个N原子的吸附中心,形成阻挡层的链较短,因此其在钢筋表面形成的吸附膜不够完整和稳定。本文介绍了一种含氨基酮分子的新型有机防护型阻锈剂。该阻锈剂可以在钢筋表面形成完整而稳定的保护膜保护钢筋免受侵蚀,但是其得以应用的前提是与混凝土有较好的相容性,对混凝土耐久性无负面影响。为了研究该阻锈剂对混凝土耐久性的影响,在混凝土试块内掺入不同量的阻锈剂,对其分别进行力学试验、毛细吸水试验,氯离子渗透试验和快速碳化试验,从而评价该阻锈剂对混凝土力学性能、抗渗性能、抗侵蚀性能以及抗碳化性能的影响。研究表明,阻锈剂的应用略微提高了混凝土的抗压强度,改善了混凝土的抗渗性、抗氯离子侵蚀性和抗碳化性,从而混凝土的耐久性得到了一定程度的提高。     

Corrosion inhibition of carbon steel in alkaline chloride media by Na3PO4

The decrease of the pH of the concrete due to the interaction with the CO₂ contained in the air creates an electrolyte in the pores with a pH < 8 that destroys the passive film. Also the presence of chloride ions at the rebar surface in alkaline media making susceptible to the aggression of the chlorides as pollutant in marine environment. The inhibiting efficiency of Na₃PO₄ against corrosion of carbon steel in alkaline medium simulating the interstitial solution of the concrete (pH 12.5) polluted with chloride ions were carried out. Two manners for adding the Na₃PO₄ were examined: one where the inhibitor was added into the pore solution also containing the aggressive solution, and other where the rebar was first pre-treated by immersion in the inhibitor solution. The protective effectiveness of these two procedures for the inhibitor addition was evaluated by means of several electrochemical techniques, EIS, polarisation curves and microbalance.The results allow concluding that:

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The pre-passivation procedure is more effective for the rebar protection.

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The impregnation duration of steel in phosphate solution and the electrode rotation speed influence considerably the phosphate-based film resistance against corrosion.

     

海洋环境混凝土功能组分优化设计与性能研究

针对海洋环境高性能铁路桥混凝土的耐久性和水化放热问题,通过轴心抗压、电通量、水化热等试验,对级配矿物外加剂与迁移型阻锈剂复合作用的高性能混凝土进行了研究.结果表明:掺入级配矿物外加剂、阻锈剂等功能组分后,通过配合比优化设计,混凝土试件28 d强度比基准组有所提高,钢筋表面的电化学性能得到进一步提升.同时,抗氯离子渗透性能比基准混凝土最大降低了39％,胶凝材料浆体水化热值比基准组降低了37％.当辅助胶凝材料粉煤灰和矿粉掺入量提高时,混凝土力学性能、抗氯离子性能得到进一步改善,水化热中的峰值会降低.复合矿物外加剂的掺量达到22％左右时,混凝土设计的综合性能达到较佳状态.掺入级配矿物外加剂和阻锈剂可以使电通量显著下降,并对混凝土耐久性的提升和水化热的控制具有协同作用.     

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.     

Electrochemical behaviour of steel in mortar and in simulated pore solutions: Analogies and differences

On evaluating the corrosion resistance of concrete, it is frequent to perform electrochemical tests in the so called simulated pore solutions (SPS) to replace tests performed in concrete specimens. Besides, to study the effect of the chloride content in concrete, chloride ions are added to the SPS. However, it is not obvious whether the SPS simulate the electrochemical behaviour of steel in concrete. Another concern is related to the relationship between the chloride content in concrete and the chloride content of a SPS. To investigate this issue a comparison between the polarization curves of steel in mortar and in SPS was performed. It was found that the SPS is not fully representative of the corrosion behaviour of steel in mortar but it yields conservative results. The relationship between the chloride content in mortar and in SPS that yields similar behaviour is not straightforward but depends on the electrochemical parameter considered.     

Concentration monitoring and performance of a migratory corrosion inhibitor in steel-reinforced concrete

Inhibitor concentration depth profiles for concrete samples treated with a proprietary migratory corrosion inhibitor (of the Cortec MCI range) are presented. The treated concrete was cored and these cores were then sectioned and crushed before being immersed in distilled water to extract the available inhibitor. The amine concentrations were quantified using an ammonium-sensing electrode and were then related to the inhibitor concentration present. The inhibitor examined, reported to contain a combination of volatile amines and amino carboxylate compounds, was found to readily diffuse through concrete. The inhibitor was subjected to a 5-year trial and found to be effective in suppressing corrosion of steel reinforcement in the presence of high chloride concentrations. The concentration profiles indicate that only relatively low concentrations of inhibitor were required to achieve inhibition in this case.     

An assessment on the mill scale effects on the electrochemical characteristics of steel bars in concrete under DC-polarization

The effects of mill scale on formation and stability of passive film on rebars in chloride-bearing concrete is investigated. Studies were carried out on some electrochemical parameters such as repassivation potential, E_rep, and passive current density, i_pass, using cyclic potentiodynamic polarization. The superficial mill scale on the rebars was naturally produced during hot-working of steel. The experiments were performed in the concrete samples with a cement/sand/water ratio 1:3:0.6. The results demonstrated that the presence of mill scale on the rebars in chloride-free concrete causes a decrease in electrical resistance of the passive film and its reparability. However in presence of chloride ions because of the severity of corrosion the effect of mill scale on corrosion of rebars is less manifested.     

Nickel-free manganese bearing stainless steel in alkaline media—Electrochemistry and surface chemistry

The use of austenitic nickel-containing stainless steels as concrete reinforcement offers excellent corrosion protection for concrete structures in harsh chloride bearing environments but is often limited due to the very high costs of these materials. Manganese bearing nickel-free stainless steels can be a cost-effective alternative for corrosion resistant reinforcements. Little, however, is known about the electrochemistry and even less on surface chemistry of these materials in alkaline media simulating concrete pore solutions. In this work a combined electrochemical (ocp = open circuit potential) and XPS (X-ray photoelectron spectroscopy) surface analytical investigation on the austenitic manganese bearing DIN 1.4456 (X8CrMnMoN18-18-2) stainless steel immersed into 0.1 M NaOH and more complex alkaline concrete pore solutions was performed. The results show that the passive film composition changes with immersion time, being progressively enriched in chromium oxy-hydroxide becoming similar to the conventional nickel-containing stainless steels. The composition of the metal interface beneath the passive film is strongly depleted in manganese and enriched in iron; chromium has nearly the nominal composition. The results are discussed regarding the film growth mechanism (ageing) of the new nickel-free stainless steel in alkaline solutions compared to traditional austenitic steels. Combining the results from pitting potential measurements with the composition of the passive film and the underlying metal interface, it can be concluded that the resistance against localized corrosion of the new nickel-free stainless steel relies on the strong chromium(III) and molybdenum (VI) oxy-hydroxide enrichment in the passive film.     

Uses of scanning electrochemical microscopy for the characterization of thin inhibitor films on reactive metals: The protection of copper surfaces by benzotriazole

Scanning electrochemical microscopy (SECM) was used to study the film formation of benzotriazole towards corrosion of copper. SECM was operated in the feedback mode by using ferrocene-methanol as redox mediator, and the sample was left unbiased at all times to freely attain its open circuit potential in the test environment. Following exposure to aggressive electrolytes the anticorrosion abilities of the layers were characterized by image analysis and by an electrochemical method derived from the experimental approach curves. Changes in the shape of the approach curves were clearly observed during the inhibitor film formation process. They showed the transition from an active conducting behaviour towards ferrocinium reoxidation typical of unprotected copper, to a surface exhibiting insulating characteristics when the metal was covered by a surface film containing the inhibitor. This supports that SECM is a practical technique in the investigation of corrosion inhibitor performance. However, a consistent tendency for the characterization of inhibitor film formation using SECM measurements in the positive feedback mode for the copper-benzotriazole system was only found if the experiments were conducted when the inhibitor molecule was not present in the test solution. That is, inhibitor molecules were found to interact not only with the copper surface during the monitoring process, but with the SECM tip as well, this effect being significantly enhanced when chloride ions were present in the electrolyte. Finally, a procedure to image the chemical activity of copper surfaces partially covered with the inhibitor film with SECM is proposed.     

养护措施和湿养护时间对掺与未掺矿渣混凝土性能的影响

研究了采用湿棉絮覆盖、喷养护剂、塑料薄膜密封3种养护措施对混凝土强度、收缩、中心温升、氯离子渗透性的影响,并通过不同龄期强度、氯离子渗透深度(5%NaCl溶液介质,质量分数)、钢筋腐蚀电位、加速腐蚀保护层开裂时间研究湿养护时间(1,7,14d和28d)对未掺和掺矿渣(等量取代水泥40%,质量分数)混凝土强度、抗渗性、护筋性的影响.结果表明:合适的养护措施能有效提高混凝土强度和抗渗性,降低水化温升和早期收缩.对未掺与掺矿渣的混凝土,更长的湿养护有助发展更高强度、耐久性和护筋性;而不充分养护导致混凝土较差的抗渗性和护筋性,该影响对掺矿渣混凝土尤为明显.7d湿养护对掺矿渣混凝土发展更高的潜在抗渗性是不够的.     

Corrosion effects on pullout behavior of hooked steel fibers in self-compacting concrete

Fiber reinforced concrete structures are subjected to chloride and carbonation penetration that could initiate corrosion of steel fibers, with eventual pernicious consequences in terms of structural and durability performance. Cracks in concrete are known to hasten initiation of steel corrosion in reinforced concrete structures. The investigation of the impact of cracks on the corrosion initiation and the associated interfacial damage between concrete and steel fibers is important for understanding the mechanical behavior of steel fiber reinforced concrete.In the present work, with the aim of studying the corrosion action on the mechanical behavior of cracked Steel Fiber Reinforced Self-Compacting Concrete (SFRSCC), an experimental program was performed to characterize the corrosion of hooked-end steel fibers and to assess the fiber pullout behavior in cracked concrete, previously subject to the action of corrosion by exposure to aggressive chloride environment.     

氨基醇类阻锈剂的阻锈机理

在分析氨基醇类阻锈剂对混凝土和钢筋作用行为的基础上,给出了氨基醇类阻锈剂的阻锈机理,即一方面阻锈剂通过在钢筋表面形成保护膜,隔离侵蚀性离子到达钢筋表面,另一方面通过和混凝土中的水化产物反应生成沉淀,提高混凝土密实度。两者相辅相成,共同起到延缓钢筋腐蚀或降低钢筋腐蚀速率的作用。氨基醇类阻锈剂在碳化混凝土中的阻锈效果微乎其微,在氯盐环境中依然有效,但当混凝土中氯离子浓度或腐蚀速率较高时,其阻锈作用大幅降低。由于阻锈剂的成分较为复杂,其成膜机制等还需进一步的研究。     

Effects of some ions on ion-selectivity of ferrous sulfide film

The ion-selectivity of ferrous sulfide film formed on a cellulose sheet was determined by measuring the film potentials in neutral chloride solution. The results show that ferrous sulfide film is a bipolar film consisting of an inner cation-selective layer and an outer anion-selective layer. This kind of film structure can accelerate the localized corrosion of steel. The influence of some anions, cations and imidazoline corrosion inhibitor on ion-selectivity of the film was also investigated. The ion-selectivity of the film is altered from bipolar to anion-selective with the adsorption of Ca²⁺, Mg²⁺ and Ba²⁺ ions, or to cation-selective with the adsorption of MoO₄^2-, PO ₄^3-and imidazoline inhibitor. Potentiodynamic polarization, AC impedance technology, and the weight-loss method were used to understand the relationship between the ion-selectivity and corrosion behaviors of steel in sulfide-containing solution.     

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.     

海水海砂混凝土力学性能与耐久性研究综述

远海工程建设面临钢筋混凝土易腐蚀、河砂和淡水匮乏等难题。国内外学者选择资源丰富的海水海砂代替淡水河砂制备混凝土,并研究其工作性能、力学性能及耐久性能。海水海砂中高含量的氯盐会加快水泥水化和凝结,导致早凝和早期强度提高,但后期增长变缓,最终强度与淡水河砂混凝土相近。海砂中少量的贝壳对混凝土工作性能和力学性能影响不大。海水海砂混凝土中的氯离子传输及结合方式更为复杂,其不同于内掺型氯离子,由此导致海水海砂混凝土中的钢筋锈蚀机理改变。辅助胶凝材料、复合型阻锈剂及纤维增强复合筋等为海水海砂混凝土结构应用提供了保障。     

A synergistic effect between zinc salt and phosphonic acid for corrosion inhibition of a carbon steel

This work is devoted to the corrosion inhibition of a carbon steel by a zinc salt/phosphonic acid association. Steady-state current-voltage curves and electrochemical impedance measurements carried out in the presence of each compound and for the mixture show a synergistic effect between the two molecules. The concentrations of the compounds in the mixture were lower than the concentrations used for each compound separately. Phosphonic acid was observed to act as an anodic inhibitor whereas cathodic action was shown for the zinc chloride. Electrochemical measurements and surface analysis (XPS and reflection-adsorption spectroscopy at grazing incidence) showed that the synergistic effect afforded by the mixture was attributable to the reaction of the phosphonic acid with the zinc salt. The inhibitor film acts as a protective layer impermeable to ionic or molecular diffusion. The film is very thin and homogeneous in composition. A chemical structure of the film is proposed.     

Corrosion process and structural performance of a 17 year old reinforced concrete beam stored in chloride environment

The long-term corrosion process of reinforced concrete beams is studied in this paper. The reinforced concrete elements were stored in a chloride environment for 17years under service loading in order to be representative of real structural conditions. At different stages, cracking maps were drawn, total chloride contents were measured and mechanical tests were performed. Results show that the bending cracks and their width do not influence significantly the service life of the structure. The chloride threshold at the reinforcement depth, used by standards as a single parameter to predict the end of the initiation period, is a necessary but not a sufficient parameter to define service life. The steel-concrete interface condition is also a determinant parameter. The bleeding of concrete is an important cause of interface de-bonding which leads to an early corrosion propagation of the reinforcements. The structural performance under service load (i.e.: stiffness in flexure) is mostly affected by the corrosion of the tension reinforcement (steel cross-section and the steel-concrete bond reduction). Limit-state service life design based on structural performance reduction in terms of serviceability shows that the propagation period of the corrosion process is an important part of the reinforced concrete service life.