Corrosion of Carbon Steel and Corrosion-Resistant Rebars in Concrete Structures Under Chloride Ion Attack

Corrosion of reinforced concrete is the most challenging durability problem that threatens reinforced concrete structures, especially structures that are subject to severe environmental conditions (i.e., highway bridges, marine structures, etc.). Corrosion of reinforcing steel leads to cracking and spalling of the concrete cover and billions of dollars are spent every year on repairing such damaged structures. New types of reinforcements have been developed to avoid these high-cost repairs. Thus, it is important to study the corrosion behavior of these new types of reinforcements and compare them to the traditional carbon steel reinforcements. This study aimed at characterizing the corrosion behavior of three competing reinforcing steels; conventional carbon steel, micro-composite steel (MMFX-2) and 316LN stainless steel, through experiments in carbonated and non-carbonated concrete exposed to chloride-laden environments. Synthetic pore water solutions have been used to simulate both cases of sound and carbonated concrete under chloride ions attack. A three-electrode corrosion cell is used for determining the corrosion characteristics and rates. Multiple electrochemical techniques were applied using a Gamry PC4? potentiostat manufactured by Gamry Instruments (Warminster, PA). DC corrosion measurements were applied on samples subjected to fixed chloride concentration in the solution.     

Einflußeiner Verzinkung auf die Korrosion von mit Zementmörtel beschichtetem Stahl in NaCl-Lösung

Effect of galvanizing on the corrosion of steel in concrete immersed in NaCl solution Galvanized or pickled steel sheet specimens were embedded in portland cement mortar of various water cement ratios and curing conditions and then wholly or partially immersed in 0.5 M NaCl solution for 1 to 5 yrs. Free corrosion potentials and electrical resistances have been measured. Immersion conditions and the presence of zinc have a significant effect on the corrosion resistance of the embedded steel sheets. The potentials of the wholly immersed specimens are very negative. Thus, these specimens cannot act as cathodes in corrosion cells, and the steel sheets within the mortar do not corrode. The partially immersed specimens, on the other hand, show very noble potentials. Also in the case of galvanized steel sheets the potentials are shifted to the same positive values in the course of exposure time. Thus, all these specimens can act as cathodes in corrosion cells. Localized corrosion generally occurs at the water/air line. In the case of pickled specimens the mortar is cracked due to growing corrosion products. In the case of galvanized steels the corrosion is retarded significantly. The test results are discussed in detail with respect to practical problems of cell formation, internal and external protection of pipes as well as the corrosion resistance of reinforced concrete.     

Incipient processes in the corrosion of mild steel in 90°C hypersaline geothermal brine

Mild steel (AISI 1009 and 1018) specimens were exposed for ca. 20 h to Salton Sea, hypersaline, geothermal brine at ca. 90°C. The incipient growth of the precipitated scale and iron corrosion products were analyzed by scanning and transmission electron microscopy, X-ray fluorescence, X-ray diffraction, electron diffraction and electron microprobe techniques. In brine at its natural pH of 5.7, several distinct phases appear on the steel surfaces, the most important being iron-rich amorphous silica. The silica deposit grows in irregular patches, and corrosion of the substrate occurs between the patches. Subsequent acidification of the brine for short periods does not disturb the protective layers of silica but greatly increases the rate of corrosion of surface areas not covered by silica. Oxidation of mild steel in acidified brine is quite different. Rather than iron compounds, deposits of both metallic lead and PbCO₃, the latter in unusual crystalline needles, and an unidentified copper compound dominate the surface coverage at low pH.     

Incipient processes in the corrosion of mild steel in 90°C hypersaline geothermal brine

Mild steel (AISI 1009 and 1018) specimens were exposed for ca. 20 h to Salton Sea, hypersaline, geothermal brine at ca. 90°C. The incipient growth of the precipitated scale and iron corrosion products were analyzed by scanning and transmission electron microscopy, X-ray fluorescence, X-ray diffraction, electron diffraction and electron microprobe techniques. In brine at its natural pH of 5.7, several distinct phases appear on the steel surfaces, the most important being iron-rich amorphous silica. The silica deposit grows in irregular patches, and corrosion of the substrate occurs between the patches. Subsequent acidification of the brine for short periods does not disturb the protective layers of silica but greatly increases the rate of corrosion of surface areas not covered by silica. Oxidation of mild steel in acidified brine is quite different. Rather than iron compounds, deposits of both metallic lead and PhCO₃, the latter in unusual crystalline needles, and an unidentified copper compound dominate the surface coverage at low pH.     

The use of pumice as a coating for the reinforcement of steel against corrosion and concrete abrasions

This study presents the results of corrosion resistance for pumice collected from pyroclastic exposure in the Van, Kayseri, Nevsehir and Osmaniye regions of Turkey. The corrosion resistance of reinforced steel (RS) and mass losses of concrete specimens were investigated. The specimens were exposed to 5 and 10% sodium sulphate solutions. Reinforced steel mass losses and compressive strengths of concrete were measured. Improvements of the compressive strength and corrosion resistance were observed for specimens with the increasing layers of coating. The corrosion rates of pumice coated specimens were lower than the control specimens. There was a close relationship between type of pumice coating and reinforcement corrosion. Corrosion ratio decreased with increasing amounts of reactive SiO₂. The alkalinity of the concrete, permeability of the concrete cover, the quality of the concrete and the corrosion environment were crucial factors in influencing the effectiveness of the concrete cover for corrosion protection of reinforced steel. All pumice concretes offered excellent resistance to corrosion, where Osmaniye pumice (OP) best increased corrosion resistance of reinforced steel.     

架空导线不同防腐措施的耐腐蚀性能比较

为探究常见架空导线耐腐蚀性能,对普通钢芯铝绞线、铝包钢芯铝绞线、防腐型钢芯铝绞线和碳纤维复合材料芯架空导线进行中性盐雾腐蚀实验。结果表明：普通钢芯铝绞线的腐蚀情况最为严重;而铝包钢芯铝绞线因避免了异种金属接触,内层铝线和铝包钢线表面腐蚀程度较低,表明铝包钢芯铝绞线耐蚀性优于普通钢芯铝绞线。防腐型钢芯铝绞线由于防腐脂的物理隔绝作用,受到防腐脂保护的铝线和钢芯在实验周期内均未见明显腐蚀迹象。而碳纤维复合材料芯导线,因碳纤维复合材料芯化学性质稳定,耐腐蚀性能优异,其紧密绞合的型线阻止了腐蚀介质向绞线内部渗透,绞线的耐腐蚀性能显著优于普通导线。     

Nonmetallic conducting coatings: Properties and behaviour of iron and iron-copper phosphate coatings

Surface conversion of steel by means of conducting iron phosphate coatings protects mechanical parts blanked from steel sheets, even in a very high-humidity environment, and also allows their assembly through electrical resistance welding. Corrosion tests carried out by exposing the materials to alternate humid/dry cycles, in a humidostatic chamber at 40°C, indicate that the welding process gives rise to corrosion phenomena. These can be strongly mitigated by using conversion coatings of mixed iron/copper phosphates. Conditions for obtaining optimum corrosion resistance for the final conversion coatings are therefore defined. Measurements of the related corrosion potentials made at 25 °C in a sodium hydroxide solution (pH 12, saturated in oxygen) indicate the existence of threshold values above which corrosion resistance of the phosphatized specimens is acceptable, and suggest a method for performing an industrial control of phosphating efficiency. The performance of converted steel surface has been related to the porosity of phosphate coatings.     

Effect of heat treatments on Cu-Ni 70/30 alloy pitting susceptibility in sea water at different temperatures

A series of experiments was carried out on Cu-Ni 70/30 commercial alloy specimens presenting different microstructures. The microstructure of homogeneized specimens was modified with annealing treatments by which grain size significantly increased and intermetallic compounds precipitated, thus creating some dishomogeneity in the alloy. Free corrosion and electrochemical tests were carried out at 20, 40, 60, 80°C in quiescent sea water at pH 8.2 with dissolved oxygen (D.O) content ranging from 6.5 ppm (at 20°C) to 3.0 ppm (at 80°C). The corrosion products adherent to the metallic surface were analyzed with chemical and XPS methods. The following could be observed:

• with increasing temperature, a decrease in corrosion rate and selective copper dissolution was observed in homogeneized and annealed specimens
• the annealed specimens have the highest corrosion rate in the whole temperature range and undergo pitting corrosion at low temperature.

     

Industral News

Abstract

The validity of the ac impedance technique has been tested using polished steel samples cast in concrete cubes, followed by immersion in oxygen-saturated sea water and distilled water. In order to accelerate the corrosion processes the concrete used was very porous with a low cover thickness and comparisons were made with samples of pre-rusted steel specimens.

The impedance results show that, as well as estimating the corrosion rates, information related to the surface film on the steel may also be obtained. The data are in accord with corrosion potential measurements; The results highlight a problem inherent in dc polarisation techniques when applied to the study of corrosion of reinforcing steel in concrete, namely, that steady state data are only obtained many hours after the initial measuring perturbation.     

硫脲-二乙烯三胺缩聚物对混凝土中钢筋的缓蚀作用

用动电位极化曲线及电化学阻抗谱研究了硫脲－二乙烯三胺缩聚物在浊凝土孔隙模拟液及混凝土中对钢筋腐蚀的抑制作用。结果表明它是一种混合型缓蚀风筋的点蚀有较好的抑制作用。在模拟液中添加１％的该缓蚀剂就可以使对氯离子的容忍度从０．０２ｍｏｌ／Ｌ提高到０．１ｍｏｌ／Ｌ。它与ＮａＮｏ３有交好的协同作用。它妈能吸附于钢筋表面,又能提高混凝土密实程度,减缓腐蚀介质向混凝土内的渗透。能谱分析的结果表明它能够富集于钢筋     

Electrochemical chloride removal from reinforced concrete structures and its ability to repassivate prerusted steel surfaces

Gravimetric and electrochemical measurements were used to expose the relationship between the degree of prerusting of reinforcements embedded in chloride‐contaminated mortar or concrete and the efficiency with which they are repassivated once depassivating ions are removed. The results show that, above a certain, ill‐defined rusting threshold, the electrochemical chloride removal does not ensure effective repassivation of reinforcements in the steel/concrete system. Once the passivity of reinforcements embedded in reinforced concrete structures (RCS) is overcome, the typical corrosion rates of the active state seemingly suffice to maintain an acid pH at the metal/rust interface in the bulk of such a strongly alkaline material as concrete. After this local acidification, the metal inside pits probably behaves similarly as it would in an acid solution. As a result, it does not suffice to remove the rusting agent in RCS in order to stop corrosion; rather, the agent must be removed before the first visible signs of deterioration appear.     

Corrosion behavior of reinforced no-fines concrete

The CO₂ induced corrosion behavior of no-fines concrete manufactured with three different strength classes and reinforcements is compared. The main results showed that black steel corrodes with rates three times higher with respect to those monitored in the other reinforcements, with higher corrosion rates in lower strength class concretes. The corrosion rates of steel covered by cement grout and galvanized reinforcements are not affected by concrete strength class since they protect themselves with the cement grout coating or zinc passivation, respectively. Among the reinforcements considered in this work, galvanized steel shows the best corrosion behavior.     

Effect of curing temperature on corrosion of steel bars embedded in calcium aluminate mortars exposed to chloride solutions

Important corrosion problems have been detected in reinforced concretes made of calcium aluminate cement (CAC). However, very little research has been performed to evaluate the incidence of the use of such cement on the corrosion process. Reinforced mortar specimens, pre-mixed with or without 4% NaCl of weight of cement, cured at 20° C, 40° C and 60° C for two weeks and immersed in either a 0.5 M or a 1.5 M NaCl solution, or kept in plastic bags for 255 days, have been monitored electrochemically. Corrosion potential, corrosion intensity and ohmic drop were recorded over time. The appearance with time of chloroaluminates has been monitored by X-ray diffraction, and its microstructure has been observed by means of back-scattering electron (BSE) microscopy. The reaction of chloroaluminates formation in reinforced concrete deals with the process of immobilising chloride ions which penetrate through the concrete up to the reinforcement. Therefore, the enhancement of such a reaction would be a way to reduce chloride ions in the pore solution contained in the concrete pores. Furthermore, stable chloroaluminates formation could mean a decrease in the risk of corrosion. Thus, corrosion rate measurements were carried out throughout the testing period to evaluate this point. Aluminous hydrates showed a high capacity to react with chloride ions to form chloroaluminates; however, the remaining chloride ions in the pore solution were still in an amount enough to promote reinforcement corrosion over time. Corrosion rate was found to be directly influenced by the curing temperature and, therefore, by the degree of conversion from hexagonal to cubic phases.     

Corrosion inhibitor systems for remedial treatment of reinforced concrete. Part 2: sodium monofluorophosphate

Sodium monofluorophosphate (MFP) has been applied in the form of concentrated aqueous solutions to the surfaces of concrete structures with the aim of inhibiting corrosion of embedded reinforcing steel which has become depassivated as a consequence of carbonation and/or chloride contamination. To evaluate the effectiveness of such treatments, a series of laboratory investigations was undertaken with reinforced concrete specimens that were chloride-contaminated to varying extents in the presence or absence of carbonation. The corrosion responses of embedded steel bars at various depths of cover were monitored electrochemically during a controlled programme of cyclic wetting and drying undertaken for several months prior to the inhibitor treatment and for approximately 18 months thereafter. Gravimetric measurements of the quantities and distribution of corrosion on the steel were also made on completion of the exposure tests. It has been found that there were no marked reductions in the corrosion rates of the steel under the conditions investigated. Analysis of aqueous extracts from the treated concrete specimens by means of ion chromatography revealed that negligible penetration of soluble MFP ions had occurred into any of the specimens. Hydrolysis products of MFP (phosphate and fluoride) were present at significant depths in aqueous extracts of the carbonated concrete specimens but only fluoride was detectable in similarly obtained aqueous extracts of non-carbonated specimens.     

Corrosion of Reinforcing Bars in Carbonated Concrete

Abstract

In all published studies of cement carbonation there has been general agreement that it must be considered as a cause of generalised corrosion in reinforcements, but quantitative data have been provided in very few cases. In the present work the intensity of attack has been modified by means of the accelerated carbonation of mortar specimens without additives, with 2% CaCl², and with 3% NaNO² It has been shown that a critical level of atmospheric moisture must also be present for considerable attack to occur. The measurement of polarisation resistance (Rp) has been used to evaluate the intensity of corrosion of the reinforcements. This method has already been usefully applied by the authors to investigations of corrosion of steel bars in hardened concrete specimens.     

Einfluß steigender Phosphat-Silikatgehalte auf die Loch- und Kontaktkorrosion verzinkter Stahlrohre sowie die Korrosion von Kupfer in Warmwasser-Mischinstallationen aus beiden Werkstoffen

Influence of increasing phosphate/silikate contents on the pitting and general corrosion of galvanized steel tubing and the corrosion of copper in warm water mixed installation systems In hot tap water (65°C) the influence of a mixture of phosphate/silicate inhibitor on the general, the pitting and the galvanic corrosion of galvanized steel tubes and the general corrosion of copper in mixed installations of both metals was investigated. Increasing concentration of inhibitors decreases the general corrosion rate of galvanized steel and copper. A worth mentioning reduction of pitting and galvanic corrosion of steel could be reached only with high concentrations of 5 mg/l P₂O₅ and 30 mg/l SiO₂. Galvannealed tubes are much more sensitive to pitting corrosion than galvanized ones. Referring to this they could not be inhibited.     

Corrosion and Corrosion Inhibition of Reinforcing Steel: II. Embedded In Concrete

Abstract

Reinforcing steel, whether embedded in Portland concrete or Portland cement-blast furnace slag concrete and mixed with distilled or tap water, becomes passivated. On the other hand, when the concrete is mixed with sea water, the steel corrodes severely, and when Helwan mineral water is used for mixing, a borderline condition is obtained. Portland cement-blast furnace slag concrete had poorer corrosion resistance compared with ordinary Portland cement concrete.

Steel passivity is not impaired when Portland cement concrete or Portland cement-blast furnace slag concrete is admixed with up to 8% of sulphates. However, when cement extract is used, the passivity is impaired when as little as 0·2% of sodium sulphate is added.

Additions of some anodic inhibitors such as benzoate, chromate, nitrite, phosphate and stearate to a corrosive concrete medium are efficient in preventing the corrosion of reinforcing steel. The critical concentration for inhibition is higher in the case of Portland cement-blast furnace slag concrete than that of Portland cement concrete. Coating the steel with Portland cement slurry alone or Portland cement slurry containing inhibitors improves the corrosion resistance of steel to some extent but does not inhibit the corrosion in strong attacking media.     

Considerations on reproducibility of potential and corrosion rate measurements in reinforced concrete

This communication analyses information supplied by the electrochemical parameters related to rebar corrosion in reinforced concrete structures (RCS). Corrosion potential and corrosion current density are determined for different sets of conditions. Tests are performed to gather data on conflictive aspects of the two electrochemical parameters, with regard to evaluating the results of RCS inspections. Consideration is made of the limitations of potential measurements if not accompanied by complementary indications such as concrete resistivity, rebar surface area involved in the measurements, or the instrumentally determined corrosion current. The capacity of galvanostatic pulses applied directly on large RCS to offer a reliable guide to corrosion rate of steel reinforcements is discussed.     

Aluminium-Thiourea Mixtures as Corrosion Inhibitors for Mild Steel in 1·0% NaCl

Abstract

The inhibitive properties of mixtures containing aluminium salts and thiourea (so called aluminium-thiourea inhibitors) have already been investigated in relation to their inhibitive properties towards mild steel corrosion in distilled water and in water of high specific salinity (1·0% Na₂SO₄. This paper reports laboratory test data obtained during further investigations of aluminium-thiourea inhibitors in another corrosive medium, 1·0% NaCl at 20°c and under stagnant conditions. It was found that the corrosion rate of mild steel in sodium chloride solutions can be decreased markedly (by ~ 80%) if aluminium ions are used in the form of a mixture of sulphates and oxychlorides and in conjunction with thiourea, cationic surfactants and reducing agents (SnCl₂). Any change in the composition of the aluminium salts in the mixture, keeping a constant Al³⁺ concentration (for instance a substitution of sulphates/oxychlorides by sulphates/chlorides or chlorides/oxychlorides) reduces the inhibitive efficiency of the formulations tested. The author fails to find a satisfactory explanation of the very specific inhibitive behaviour of aluminium-thiourea mixtures in 1·0% NaCl.     

Reinforced concrete structures – shall concrete remain the dominating means of corrosion prevention?

The acknowledged serious deterioration of reinforced concrete structures due to chloride induced corrosion has been the main fuel for research and development of very dense and impermeable concrete, so‐called high performance concrete (HPC). This development has dominated concrete research up through the 80'ies and 90'ies. The results have technically been successful. However, the practical use of such concretes on site have often posed serious difficulties, resulting in at times very low performance concrete structures although HPC was specified. The discrepancy between concrete quality reached in the laboratory, what is being specified in the design and what can realistically be achieved on site is seldom in balance. Alternative means of more or less reliable means of corrosion prevention, often based on organic materials, have during recent years been developed to protect our inorganic concrete and reinforcement. However, a highly reliable means of corrosion prevention has been the introduction of stainless steel reinforcement, which is available with dimensions and strengths directly interchangeable with ordinary carbon steel reinforcement. It has been proven that stainless steel and carbon steel can be in metallic contact when cast into concrete, without causing galvanic corrosion. This seems, for the present, to be like an unexpectedly simple and highly reliable solution to the corrosion problems. As exemplified, this technology is rapidly gaining momentum in highly corrosive environments – and concretes being much more robust to execution can now take over from HPC.