Korrosionsverhalten von Stahl in Zementmörtel bei kathodischer Polarisation in Meerwasser und 0,5 M NaCl-Lösung

Corrosion characteristics of steel in cement mortar under cathodic polarization in seawater and 0.5 M NaCl solution Steel specimens in cement mortar were exposed to seawater on the VDEh testing stand at Helgoland and to 0.5 M NaCl solutions in the laboratory up to three years. In seawater specimens covered with portland cement were cathodically protected by use of zinc and magnesium anodes. In the laboratory specimens in portland cement and in blast furnace cement were exposed to free corrosion and to potentials ranging from U_H = ?0.53 to ?0.98 V. In all cases small pittings were observed. Those developed in seawater were significantly dependent on potential and accompanied by a slight increase of chloride concentration. Here a normal anodic pitting corrosion must be assumed that cannot be totally prevented by zinc. The process slows down with increasing time. The pitting observed in the laboratory tests showed no systematic dependence on potential and the pits were substantially smaller. In the case of the strongest negative potential U_H = ?0.98 V the specimen in portland cement displayed pits as deep as 0.5 mm. These are assumed to be of cathodic origin. They were full of magnetite and virtually free of chloride. Because that far negative potentials, which lie in the region of hydrogen development, do almost not occur this type of corrosion contrary to literature has no technical bearing.     

Aeration cell corrosion of carbon steel in soil: In situ monitoring cell current and potential

Aeration cell corrosion in soil has been studied in situ in the soil at a test site in Göteborg, Sweden. The aeration cell consisted of two carbon steel sheets, the anodic, buried in a lump of clay, and the cathodic, buried in the surrounding filling material. Two aeration cells were used, one with a cathodic sheet of the same size as the anodic sheets, and an other with a cathodic sheet 10 times larger. The anode-cathode area ratio of the cells were 1:1 and 1:10. Both corrosion current and potential was measured during the 2.67 y test period. As a reference, the corrosion potential of sheets with no connection to any aeration cells were measured. The corrosion of the anodic sheets in clay can be calculated from the cell current to 31.7 μm/y (2.59 μA/cm²) for the cell with area ratio 10 and 5.0 um/y (0.47 μA/cm²) with area ratio 1. The difference in cell current between the cells increased with time; from about three times larger in the beginning to about eight times larger. This can be explained by deposition of corrosion products on the cathodic sheet, causing a larger part of the anodic dissolution to be transferred to the anodic sheet. The difference in cell current will be 10 with no corrosion of the cathodic sheets and 1.82 with the same corrosion rates on both anodic and cathodic sheets.     

Einflußgrößen der Fremdkathoden aus Bewehrungsstahl in Beton auf die Erdbodenkorrosion von Stahl

Factors influencing the activity of reinforcing steel as cathode on the soil corrosion of steel Factors influencing the corrosion cell established between steel/soil (anode) and reinforcing steel in concrete (cathode) were investigated in field tests extending up to 3 years. In all cases, the differences between the corrosion rates measured in the cell and under free corrosion conditions were higher than the equivalent cell current. This effect means an increase of the cathodic partial reaction by cell action (negative difference effect), and is related to better protecting layers in the case of free corrosion. As to the cathodes, the parameters of the concrete have nearly no effect. But galvanizing of the reinforcing steel has a strong influence on the cell action. Since the cell resistance is mainly controlled by the electrolyte resistance (ground resistance of the anodes) the changes in the polarization resistance of the cathodes can hardly influence the cell action. A decrease of cell currents due to galvanizing the reinforced steel is no doubt present, but the effect is too poor for proper protection against cell activities. Therefore, only conventional electrochemical methods or organic coatings on the reinforcing steel can be applied as protective measures.     

Effects of Tafel slope,exchange current density and electrode potential on the corrosion of steel in concrete

A parametric study is carried out to investigate the effect of variations in anodic and cathodic Tafel slopes, exchange current densities and electrode potentials on the rate of steel corrosion in concrete. The main goal of this investigation is to identify the parameters that have significant influence on steel corrosion rate. Since there is a degree of uncertainty associated with the selection of these parameters, particularly during modelling exercises, it is intended that the results of this study will provide valuable information to engineers and researchers who simulate steel corrosion in concrete. To achieve this goal, the effect of a parameter on the corrosion rate of steel is studied while all other parameters are kept constant at a predefined base case. For each parameter, two extreme cases of anode‐to‐cathode ratio are studied. The investigation revealed that the variations in the anodic electrode potential have the greatest impact on the corrosion rate, followed by the variations in the cathodic Tafel slope.     

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 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.     

Observation and micro-electrochemical characterisation for micro-droplets in initial marine atmospheric corrosion

Micro-droplets formed around NaCl droplet on carbon steel surface were observed with a confocal laser scanning microscope. Micro-electrochemical characterisation of the micro-droplets zone was also performed by a scanning Kelvin probe (SKP) and the scanning vibrating electrode technique (SVET). Results show that the electric current density and potential distribution of carbon steel under the NaCl droplet are asymmetrical; the peripheral regions of the droplet are cathodic, whilst its centre is anodic. The potential difference between the anode and cathode is 0.36?V, and the cathodic current density reaches 2.02?μA?cm^?2. This kind of asymmetrical distribution of electrochemical characterisation results in cathodic polarisation at the fringe of the NaCl droplet, thus inducing the formation of OH^–, which could promote water adsorption and subsequent formation of micro-droplets around the NaCl droplet. The electrochemical potential difference of the oxygen concentration cell formed between the central and peripheral regions of the NaCl droplet is the main driving force for micro-droplet formation on metal surfaces.     

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.     

外加电位下X80双相管线钢在模拟沿海土壤环境中的应力腐蚀行为

采用交流阻抗谱、极化试验、慢应变拉伸试验研究了不同外加电位下在模拟沿海土壤环境中X80双相管线钢的应力腐蚀行为,对拉伸断口和极化后试样进行SEM表面形貌及能谱分析。结果表明,与慢扫极化(模拟的非裂尖区域)相比,X80双相管线钢快扫极化模拟的裂尖腐蚀电位较负且腐蚀电流较大。-750 mV外加阴极电位处于裂尖自腐蚀电位范围,不足以起到阴极保护的作用,对应力腐蚀仍十分敏感。外加电位为-1050 mV时,阴极反应速率显著大于阳极反应,阴极反应产生的氢被金属吸收且扩散,慢应变拉伸未经颈缩即发生断裂,为准解理断裂。外加阴极电位为-900 mV,阴极电流有效抑制了阳极溶解反应,因此管线钢在模拟沿海土壤溶液中慢应变拉伸抗拉强度和断面收缩率都最高,断口表现为韧性断裂,侧面裂纹细小,阻抗模值最大,应力腐蚀敏感性最小。     

Determination of electrochemical parameters and corrosion rate for carbon steel in un-buffered sodium chloride solutions using a superposition model

Corrosion of carbon steel in un-buffered NaCl solutions was studied applying linear potential sweep technique to a rotating disk electrode. Current-potential curves were obtained from linear potential sweep at a rate of 1 mV s⁻¹ in solution with concentrations in the range 0.02-1 M NaCl and rotation rates in the range 170-370 rad s⁻¹, at 22 °C. Potential sweeps, which were conducted in the potential range −700 to −100 mV/SHE, were started from the cathodic limit in order to approach the measurement of corrosion under rust-free conditions. Polarization curves were analyzed with a superimposition model developed ad hoc and implemented in a computer program, which enabled determining the corrosion rate and kinetics parameters of the underlying anodic and cathodic sub-processes. The anodic sub-process, dissolution of iron, was well described in terms of a pure charge transfer controlled reaction, while the cathodic sub-process, oxygen reduction on iron, was well described in terms of mixed mass transfer and charge transfer control. Increase of electrode rotation rate increases the limiting current of oxygen reduction, which results in an enhanced corrosion rate of carbon steel. Increase of NaCl concentration has a dual effect: the limiting current of oxygen reduction decreases as a result of the influence of NaCl concentration on solution viscosity and the anodic dissolution of iron increases due to the influence of NaCl on pitting formation. However, this last mechanism predominates and a net increase in carbon steel corrosion rate is observed in this case.     

Corrosion resistance of carbon steel in simulated concrete pore solution in presence of 1-dihydroxyethylamino-3-dipropylamino-2-propanol as corrosion inhibitor

The inhibition behaviour of 1-dihydroxyethylamino-3-dipropylamino-2-propanol (HPP) as an environment friendly corrosion inhibitor for reinforcing steel was investigated in simulated concrete pore solution contaminated by 0·1 mol L^?1 Cl^? by means of linear polarisation resistance, electrochemical impedance spectroscopy and cyclic voltammetry (CV). The surface morphology and corrosion products were also examined by scanning electron microscopy and X-ray diffraction (XRD). The results show that HPP can effectively inhibit the corrosion of reinforcing steel. The CV interpreted the corrosion inhibitor by restraining the reaction of cathodic reduction and anodic oxide. The XRD shows that the corrosion inhibitor reduces Cl^? adsorption on oxide film and reduces the production of corrosion products. In the conditions of this investigation, HPP behaves as an anodic corrosion inhibitor, protecting steel against corrosion in chloride contaminated environments.     

Effects of Microstructure on Corrosion of X70 Pipe Steel in an Alkaline Soil

Corrosion of X70 steel with different heat treatments (quenching, air cooling, and furnace cooling) in an alkaline soil was investigated by weight-loss, surface characterization and electrochemical measurements. The cathodic/anodic reactions of X70 steel in alkaline soil are dominated by the oxygen reduction and formation of iron oxides that deposit on the steel surface. The protection of the oxide deposit is through a physical block effect. The deposit layer formed on as-received steel has a compact, complete structure and thus, provides an effective protection over the underneath steel. However, the deposit layers on the heat-treated steels are generally loose, porous and defective, and provide minor protectiveness. Corrosion of steel is affected by its microstructure. Generally, steels with heat treatments have a higher corrosion rate than the as-received steel. The presence of more pearlite enhances the corrosion rate of ferrite by a galvanic effect. When the steel contains bainite and martenite phases, the activity of the steel is further increased.     

X100管线钢在酸性土壤模拟溶液中的应力腐蚀行为

采用慢拉伸(SSRT)、动电位极化和SEM观察等方法,研究了在不同的阴极保护电位条件下X100钢在酸性土壤模拟溶液中的应力腐蚀行为.结果表明,X100钢发生穿晶裂纹的应力腐蚀,裂纹的萌生和发展与阴极保护电位有关.完全阳极过程控制时,X100钢无裂纹出现,但出现晶间腐蚀;在混合过程控制时,应力腐蚀敏感性较低,裂纹发展缓慢;在完全阴极过程控制时,氢脆机制起主要作用,裂纹扩展迅速.     

SECM Study of Effect of Chromium Content on the Localized Corrosion Behavior of Low-Alloy Steels in Chloride Environment

This paper investigates the effect of chromium (Cr) content (0, 1, 3 and 5% Cr) in epoxy-coated alloy steel against corrosion using in situ electrochemical techniques such as EIS and SECM in a 3% NaCl solution. The EIS results revealed that the epoxy-coated Cr steel exhibited higher impedance values than carbon steel, which is attributed to the greater resistance of Cr steel toward corrosion. Based on the cyclic voltammogram results, the tip potentials were set at ?0.7, 0.04 and 0.60 V for determining the concentration of dissolved oxygen at cathodic region, and oxidation of Cr²⁺ and Fe²⁺ at anodic region, respectively. The SECM measurements showed that, the tip current in the anodic region has decreased with increase in Cr content of the sample, which indicates that the oxidation of Fe²⁺ and Cr²⁺ decreases (corrosion is reduced) with the increase in Cr content of the steel. Besides, 5% Cr steel can maintain the highest corrosion resistance, and 1 and 3% Cr steels have higher corrosion resistance than the 0% Cr steel. This higher corrosion resistance of Cr steel samples could be due to the formation of Cr-rich hydro-oxide layers [Cr(OH)₃ as a corrosion product] on the surface of the samples. Thus, the epoxy-coated Cr alloy steel has greater corrosion resistance in a chloride-containing environment than the carbon steel. Hence, epoxy-coated Cr alloy steel can be successfully used as a construction material in structures.     

Insights into the cut edge corrosion of 55% Al–Zn metal coating on steel from simultaneous electrochemical polarization and localised pH sensing experiments

Band microelectrode arrays were used to model the cut edge corrosion behaviour of 55% Al–Zn (Galvalume/Zincalume) and Zn coated steels in chloride and sulfate electrolytes. Simultaneous electrochemical polarization experiments revealed increased cathodic current on the steel during anodic dissolution of neighbouring Al–Zn alloy electrodes. The increased cathodic current on the steel was shown to be a result of pH buffering by Al³⁺, enhancing the rate of hydrogen evolution. A large negative shift of the corrosion potential of the Al–Zn alloy electrodes was observed during cathodic polarization on neighbouring steel electrodes and was attributed to alkaline pH generated from cathodic processes.     

Electrochemical corrosion of X65 pipe steel in oil/water emulsion

The electrochemical corrosion behavior of X65 pipeline steel in the simulated oil/water emulsion was investigated under controlled hydrodynamic and electrochemical conditions by rotating disk electrode technique. Results demonstrated that mass-transfer of oxygen plays a significant role in the cathodic process of steel in both oil-free and oil-containing solutions. Electrode rotation accelerates the oxygen diffusion and thus the cathodic reduction. The higher limiting diffusive current density measured in oil-containing solution is due to the elevated solubility of oxygen in oil/water emulsion. The anodic current density decreases with the increase of electrode rotating speed, which is attributed to the accelerated oxygen diffusion and reduction, enhancing the steel oxidation. Addition of oil decreases the anodic dissolution of steel due to the formation of a layer of oily phase on steel surface, increasing the reaction activation energy. The steel electrode becomes more active at the elevated temperature, indicating that the enhanced formation of oxide scale is not sufficiently enough to offset the effect resulting from the enhanced anodic dissolution reaction kinetics. The corrosion reaction mechanism is changed upon oil addition, and the interfacial reaction is activation-controlled, rather than mass-transfer controlled. When sand particles are added in oil/water emulsion, there is a significant increase of corrosion of the steel. The presence of sands in the flowing slurry would impact and damage the oxide film and oily film formed on the steel surface, exposing the bare steel to the corrosive solution.     

Galvanic Corrosion of a Carbon Steel-Stainless Steel Couple in Sulfide Solutions

The galvanic corrosion behavior of carbon steel-stainless steel couples with various cathode/anode area ratios was investigated in S ²⁻-containing solutions, which were in equilibrium with air, by electrochemical measurements, immersion test, and surface characterization. It is found that the galvanic corrosion effect on carbon steel anode increases with the cathode/anode area ratios, and decreases with the increasing concentration of S²⁻ in the solution. A layer of sulfide film is formed on carbon steel surface, which protects it from corrosion. When the cathode/anode area ratio is 1:1, the potentiodynamic polarization curve measurement and the weight-loss determination give the identical measurement of the galvanic corrosion effect. With the increase of the cathode/anode area ratio, the electrochemical method may not be accurate to determine the galvanic effect. The anodic dissolution current density of carbon steel cannot be approximated simply with the galvanic current density.     

Influence of pH on corrosion behavior of carbon steel in simulated cooling water containing scale and corrosion inhibitors

In this paper, the influence of pH on the corrosion behavior of AISI 1020 carbon steel in simulated cooling water was investigated by using electrochemical and surface analysis methods. The results of polarization showed that the corrosion resistance of carbon steel increased with an increase in pH of the simulated water, and the corrosion control process changed from cathodic polarization to anode polarization control. The scale and corrosion inhibitor 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) had a certain anodic corrosion inhibition effect on carbon steel, whereas Zn²⁺ acted as a cathodic inhibitor for carbon steel in simulated water with pH 7–9. In simulated water containing both PBTCA and Zn²⁺, a good synergistic corrosion inhibition was found between PBTCA and Zn²⁺, and their corrosion inhibition effect on carbon steel was the best at pH 8. This was attributed to the formation of Zn(OH)₂ precipitate film in the cathode region and the formation of Zn–PBTCA complex film in the anode region at this pH.     

Inhibitory effects of 5-aminouracil on cathodic reactions of steels in saturated Ca(OH)2 solutions

The inhibitory effects of 5-aminouracil on cathodic reactions of steels were investigated using saturated Ca(OH)₂ solution to simulate the environment in pores in concrete. Polarization measurements showed that 5-aminouracil effectively inhibited these cathodic reactions, indicating the potential of 5-aminouracil as an inhibitor for macro-cell corrosion in concrete/soil systems, which are caused by cathodic reactions in concrete. Cathodic reactions were inhibited in accordance with the Langmuir adsorption isotherm. FT-IR and XPS measurements suggested that 5-aminouracil was adsorbed onto cathodic portions of the steel through the formation of a coordination bond with the amino group, mainly in the form of dimer.     

Korrosionsverhalten von feuerverzinktem Stahl in erwärmten Wasser1

Corrosion behaviour of hot galvanized steel in warm water Pipe specimens made of commercial grade galvanized steel and zinc have been tested in comparison to pipe specimens having modified zinc coatings. The specimens were corroded in 11 test lines with warm water at approx. 60 °C and cold water with continuous and intermittent flow. Dortmund town water with polyphosphate and orthophosphate additions in individual test lines was used for the tests. One test line was operated with warm water from a water heater with cathodic vessel protection according to the Guldager method. The investigation included visual assessment, determination of mass loss, and electrochemical measurements. The main results were as follows: 1. The localised corrosion tendency is not determined by the amount of potential ennoblement, but rather by the inhibition of the cathode reaction on the surface layer formed, which can be read off the cathodic current density vs. potential curve. The inhibition of the cathode reaction can be affected both by variations of the material and by variation of the water quality. 2. The tested materials with modified coatings featured throughout a poorer corrosion behaviour in warm water than commercial zinc coatings according to DIN 2444. 3. The phosphates used for the investigation induce an inhibition of the cathode reaction in zinc. Zinc-iron alloy phases in warm water fail to ensure an appropriate effect. A treatment of the water according to the Guldager process ensures a strong inhibition of the cathode reaction both with zinc and especially with zinc-iron alloy phases and induces an important improvement of the corrosion behaviour of galvanized steel in warm water.