Carbon steel passivity examined in alkaline solutions: The effect of chloride and nitrite ions

The effect of chloride and nitrite ions on the passivity of steel in alkaline solutions was investigated. Four nitrite dosages were tested, resulting in various nitrite/chloride ratios. The behavior of steel was evaluated on electrodes aged during 1 and 90 days, measuring electrochemical parameters such as the corrosion, pitting and repassivation potentials, the corrosion current density, the weight loss and performing electrochemical impedance spectra. The presence of 0.8 mol l⁻¹ of chloride induced pitting only under polarization and when the chloride/hydroxyl ratio was not less than one. Nitrite ions behave as effective inhibitors of pit propagation for all the concentrations tested (0.2-0.8 mol l⁻¹). A nitrite/chloride ratio of 0.25 induces complete surface repassivation. Nevertheless, at open circuit potential, the high alkalinity guarantees passivation even in the presence of chlorides. In the event that the passive layer is damaged and pitting can be initiated, nitrite ions are effective in inhibiting pit propagation.     

电化学法研究钼系缓蚀剂

用动电位慢扫描法研究了钼系缓蚀剂在去离子水中和含５ｐｐｍＣ１＾－的去离了 对Ａ３钢的缓蚀性能,并与２００ｐｐｍ的Ｋ２ＣｒＯ４在同样条件下对Ａ３钢的缓蚀性能作了比较。在５ｐｐｍ Ｃｌ＾－的去离子水的敝开体系中,１００ｐｐｍ Ｎａ２ＭｏＯ４的缓蚀性能与２００ｐｐｍ的Ｋ２ＣｒＯ４相当,复合缓蚀剂Ｎａ２ＮｏＯ４（１２ｐｐｍ）＋ＮａＮＯ２（２３ｐｐｍ）＋ＺｎＳＯ４（５ｐｐｍ）＋十二酰基肌拟定酸钠（１００ｐｐ     

Corrosion resistance of aluminum oxide film and electrolytically coloured film in sodium chloride solution

The main interest of this study is to compare the corrosion resistance of aluminum anodic oxide film anodized in sulfuric acid bath with that of preanodized and electrolytically coloured film.Examinations have been carried out referring to (1) potentiostatted polarization curves shown on the specimens in 0.5 M sodium chloride solution, (2) impedance diagrams of the specimens and barrier thickness by Hunter's method and (3) optical metallographical observations of the aspects of pitting corrosion which took place on these specimens, and dipping test in corrosive solution.The results obtained are summarized as follows: (1) Typical difference was observed in regard to the potentiostatted polarization curves (i-t curves) between the two types of specimens in 0.5 M sodium chloride solution. The corrosion current caused by pitting corrosion of the electrolytically coloured specimens was smaller than that of anodized ones on the measurement by i-t curves. (2) The survey through the interfacial impedance diagrams of the specimens made clear that the impedance of the electrolytically coloured film was larger than that of uncoloured anodic oxide film, especially this greater increasing part of impedance was achieved in 15 s. This tendency was similar for the measurement of barrier thickness by Hunter's method. (3) The optical metallographical observations have demonstrated that there were far more advanced pitting corrosion on the anodic oxide film than on the electrolytically coloured film. In the dipping test, uncoloured anodized aluminum showed high extent of pitting corrosion, while electrolytically coloured aluminum performed well during colouring. So the total result of above examinations reveals that electrolytically coloured films are definitely more resistive to corrosion than anodic oxide films and this is due to the thick barrier layer of the former films.     

氯离子对催化裂化分馏塔顶冷却系统腐蚀的研究

针对延安炼油厂年产量200万t催化裂化装置分馏塔顶空冷器内侧腐蚀问题,进行现场取样检测,配制模拟溶液,研究氯离子单一因素的变化对腐蚀结果的影响,选用了空冷管束常用的10钢、20钢和304不锈钢三种材料进行腐蚀试验。通过对试验的极化曲线和电化学阻抗的分析,得到氯离子对10#钢和20#钢的腐蚀机理相同;三种材料都会随着氯离子的加入,出现局部腐蚀点蚀,且随着氯离子浓度的增大都有腐蚀加剧的趋势;304钢随着氯离子浓度的变大,点蚀电位下降不明显,钝化膜变的不稳定。综合对比,304钢抗腐蚀性表现优于10#钢,10#钢优于20#钢,但10#钢对于氯离子变化更敏感。     

Corrosion behaviour of electrogalvanized steel in sodium chloride and ammonium sulphate solutions; a study by E.I.S.

Zinc and zinc-nickel (13% Ni) electrodeposits were passivated by dipping in chromate baths and characterized by scanning electron microscopy. The corrosion behaviour was studied using a.c. electrochemical techniques; electrochemical impedance spectroscopy (EIS) measurements were performed at open circuit and under galvanostatic control during the 24 h immersion time. In sodium chloride solution the zinc-nickel electrodeposits show a better corrosion resistance compared to the pure zinc coatings. During the immersion time, a surface nickel enrichment was observed which, together with the zinc corrosion products, acts as a barrier layer reducing the total corrosion rate. In the same solution the passivation treatment improves the corrosion resistance of the electrodeposits; nevertheless, on zinc substrates, the protection exerted by the chromate film is not, always effective during the immersion time. On the contrary the chromate coating on zinc-nickel substrates induces a remarkable and durable improvement of the corrosion resistance reducing the zinc dissolution almost completely. In the ammonium sulphate solution, the corrosion mechanism is significantly influenced by hydrogen reduction on the zinc-nickel surfaces, and by the production of a local surface acidity which is aggressive for the chromate coatings.     

Impedance study on calcium nitrite as a penetrating corrosion inhibitor for steel in concrete

Penetrating corrosion inhibitors are thought to be able to penetrate through the capillary structure of concrete to reinforcing steel and to reduce the already initiated corrosion of steel. In this work the ability of calcium nitrite to inhibit the chloride induced corrosion of steel was studied. The test protocol was adjusted to simulate the performance of penetrating corrosion inhibitors in concrete structures. Steel samples were first prepassivated in saturated solution of Ca(OH)₂ and then exposed to the same solution with 1% NaCl addition, simulating pore liquid in chloride contaminated concrete. After the initiation of steel corrosion, the first dose of calcium nitrite was added, and then its concentration was gradually increased and the inhibition effect was related to the molar ratio of chloride to nitrite ions [Cl⁻]/[NO₂⁻]. Different rates of the increase in the inhibitor concentration were applied.Electrochemical impedance spectroscopy was used to follow the behaviour of steel at different stages of the corrosion process. The evolution of acquired spectra reflected the initiation of localized corrosion of steel and then the gradual inhibition with increasing concentration of the inhibitor. It was found, that calcium nitrite is able to inhibit the initiated corrosion (pitting) of steel and the optimum inhibitor efficiency was observed for the [Cl⁻]/[NO₂⁻] ratio below 1. The inhibition efficiency was larger, when this value of the [Cl⁻]/[NO₂⁻] ratio was reached in early stages of the corrosion development. Calcium nitrite can be effective as a penetrating corrosion inhibitor for steel in concrete, if it will be present in the sufficient concentration at the steel surface in early stages of the corrosion development.     

电化学聚合导电聚苯胺膜的研究

分析了恒电位法在酸性和碱性溶液中于不锈钢基体上电聚合聚苯胺的过程及其影响因素,测定了聚苯胺膜与基体的附着力、膜的导电性和防腐性.结果表明:该膜在空气中呈绿色,稳定、完整致密,导电性好,与基体的结合情况较好.阳极极化曲线测定表明,在碱性溶液中预镀后酸性溶液中聚合聚苯胺膜,其点蚀电位比酸性溶液中聚合聚苯胺膜升高1 V左右,表明前者的导电聚苯胺膜可显著提高不锈钢的抗点蚀性能,具有良好的点腐蚀防护效果.     

Corrosion inhibition of steel in chloride-containing alkaline solutions

Some organic inhibitors of steel corrosion in saturated calcium hydroxide solutions containing 0.1 M chlorides were investigated to elucidate their interactions with the steel surface. The compounds studied were dicyclohexylammonium nitrite (DCHAMN), dicyclohexylamine (DCHA), sodium -glycerophosphate (GPH) and 5-hexyl-benzotriazole (C6BTA). Sodium nitrite (SN) was also studied as a reference. The techniques applied were electrochemical impedance spectroscopy (EIS), polarization curves, cyclic voltammetry (CV) and Fourier transform infrared (FTIR) spectroscopy. FTIR spectra showed that DCHAMN, DCHA, GPH and C6BTA interact with the steel surface by chemisorption and some information about the mechanism also emerged. From DCHAMN solution, DCHA chemisorption is induced by slow salt hydrolysis. CV tests show that, among chemisorbed substances, only GPH avoids chloride penetration on cycling, most likely due to a quick chemisorption, while DCHAMN can only enlarge the passive potential range. Addition of SN also prevents chloride attack on cycling. Twenty days of immersion in inhibited solutions revealed that, besides SN, GPH and DCHAMN also form an impervious surface film on steel, which blocks any localized corrosion attack, whereas in the case of DCHA and C6BTA solutions, pitting corrosion is slowed down, but not avoided. DCHAMN exhibits the highest inhibiting efficiency at long immersion times, as a result of a synergetic inhibitive action which develops between nitrite and chemisorbed DCHA.     

Passivation and pitting corrosion of nanostructured Sn-Ni alloy in NaCl solutions

The passivation and pitting corrosion of tin-nickel alloy (34% Ni-66% Sn) in NaCl solution was studied using potentiodynamic, cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques complemented by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of concentration of the chloride ion, the switching potential, scans rate and pH on the electrochemical behavior of Sn-Ni alloy is discussed. The data indicate that the corrosion rate and the pitting corrosion of Sn-Ni alloy increases by the increasing of chloride ion concentration. The observed corrosion resistance of electrodeposited Sn-Ni alloy is due to the formation of a thin passive film from tin and nickel oxides.     

Cr23Mo1N奥氏体不锈钢的耐点蚀性能研究

通过Mott-Schottky曲线、动电位极化、循环极化以及电化学阻抗谱等方法研究了温度对Cr23Mo1N奥氏体不锈钢(HNSS)在3.5%Na Cl溶液中耐点蚀性能的影响。结果表明:随着温度的不断升高,高氮钢的自腐蚀电位和点蚀电位呈下降趋势,腐蚀电流密度逐渐增大,钝化膜阻抗降低,高氮钢钝化膜的半导体性质在不同温度下发生改变,高氮钢随温度的升高点蚀敏感性增大,已发生点蚀的试样不能自修复。并与普通316L不锈钢进行对比,高氮钢表现出更加优越的耐蚀性。     

Electrodeposition of bilayered polypyrrole on 316 L stainless steel for corrosion prevention

The electropolymerization of pyrrole in aqueous solutions of salicylate leads to the formation of hollow rectangular-sectioned microtubes. With the aim to develop a coating with this morphology but with better anticorrosive properties we synthesized a bilayer system by depositing a polypyrrole underlayer electropolymerized in the presence of molybdate and nitrate and a polypyrrole film formed by the microtubes as a top layer. The corrosion performance of the coatings was monitored by following the open circuit potentials, polarization curves and electrochemical impedance spectroscopy in acid and neutral chloride solutions. The bilayers have the capacity to protect the steel against uniform as well as against pitting corrosion during long exposure times. The system with the inner layer formed in alkaline solution was the most protective coating in this study. The results are discussed in terms of the galvanic interaction between the polymer and the substrate and the role played by the dopant anions.     

循环水中304不锈钢发生点蚀的Cl~-浓度阈值研究

采用电化学法研究了循环水中304不锈钢点蚀电位(Eb)、蚀孔深度、蚀孔数量与氯离子浓度的关系。结果表明,常温下304不锈钢的Eb总是随着氯离子浓度的增大而减小,当氯离子质量浓度小于800 mg/L时,增大氯离子浓度易导致点蚀敏感,而大于该值时增大氯离子浓度不会明显增大点蚀倾向;从蚀孔深度和表面蚀孔数量得出的304不锈钢点蚀倾向由大变小的氯离子质量浓度阈值为770 mg/L。     

The Influence of Halide Anions on the Anodic Behavior of Nickel in Borate Solutions

The anodic behavior of nickel in Na₂B₄O₇ solutions containing various concentrations of NaCl, NaBr, or NaI as pitting corrosion agents was studied using the potentiodynamic technique. In absence of the halide anions, the E/i curves exhibit active and passive regions prior to oxygen evolution. The passivity is due to the formation of nickel oxides on the electrode surface. The presence of low concentrations of the halide anions has no effect on the mechanism of nickel passivity. High aggressive anion concentrations stimulate the active region and tend to break down the passive film, leading to pitting corrosion. The susceptibility of the nickel anode to pitting corrosion is enhanced with increasing halide anion concentration and is decreased with increasing pH of solution. The aggressiveness of the halide anions towards the stability of the passive film decreases in the order: Cl^– > Br^– > I^–. The addition of increasing concentrations of tungstate, chromate, or molybdate anions causes a shift of the breakdown potential in the noble direction, indicating the inhibitive action of the added anions. The inhibiting tendency of these anions decreases in the order: WO₄^2– > CrO₄^2– > MoO₄^2–.     

Initiation and propagation of pitting and crevice corrosion of hydrogen-containing passive films on X70 micro-alloyed steel

Hydrogen promoted initiation and propagation of pitting and crevice corrosion of X70 micro-alloyed steel were characterized by potential dynamic measurements, the scanning reference electrode technique (SRET) and electrochemical impedance spectra (EIS). At open circuit potential, in situ SRET results show that hydrogen accelerates the nucleation and propagation of pitting of X70 steel. The pitting potential E_p of X70 steel gradually decreases with an increase of chloride ion concentration in NaHCO₃ solution. Pre-charged hydrogen does not have a significant effect on the pitting potential E_p and open circuit potential E_corr of the steel in 0.5 M NaHCO₃ solution. However, a synergistic effect of hydrogen and Cl⁻ on the anodic dissolution and pitting potential of X70 steel is observed in 0.5 M NaHCO₃ solution containing chloride ions. When crevices are present in X70 steel, hydrogen accelerates the initiation and progress of crevice corrosion. The mechanisms by which hydrogen promotes the initiation and propagation of pitting and crevice corrosion are proposed and discussed.     

Electrochemical impedance studies of sol-gel based ceramic coatings systems in 3.5% NaCl solution

Ceramic coatings based on salts like vanadia, ceria, silica and molybdate prepared by sol-gel route are proposed as alternatives to the process involving toxic hexavalent chromate. The surface of the specimens were modified under different conditions and then dipped in a corresponding salt solution prepared via sol-gel method. Electrochemical impedance spectroscopy (EIS) and polarization measurements have been used to evaluate the coating performance in 3.5% NaCl. The optimum conditions under which such treatments can provide good corrosion protection to the aluminum substrate were determined. The surface morphologies of the treated samples were investigated using SEM and EDS. Optical microscope was also used to investigate the occurrence of pitting corrosion. The surface preparation prior to sol-gel treatment was found to have a marked effect on the corrosion protection of AA6061 T6.     

2205双相不锈钢在醋酸环境下的点蚀行为研究

2205双相不锈钢由于其具有良好的力学性能,耐腐蚀性能被广泛地应用石油化工设备和管道用材料和选材设计中。本文主要利用电化学极化曲线及交流阻抗技术研究了22Cr双相不锈钢在醋酸以及氯离子条件下耐点蚀行为。结果表明,22Cr双相不锈钢在醋酸环境下有良好的耐腐蚀性能,由于氯离子的存在,增大了该材料的腐蚀倾向,同时也进一步加大了发生点蚀的几率。     

Inhibiting effect of citric acid on the pitting corrosion of tin

Potentiostatic and potentiodynamic studies were carried out to establish the inhibiting effects of citric acid on the pitting corrosion of tin. The critical potential (Ecrit), which leads to pitting or general corrosion, was determined in sodium perchlorate solution in the pH range 1.0 to 4.0. Pit nucleation and growth, at pH4.0, can be described by instantaneous nucleation followed by progressive nucleation. The results show that the minimum acid concentration needed to inhibit pitting of tin is 10–2m. Pitting occurrence by direct interaction between metal and perchlorate anions was observed.     

Corrosion monitoring of reinforcing steel in cement mortar by EIS and ENA

Health degradation by corrosion of steel in civil engineering, especially in rough environment, is a persistent problem. Environment pollution and global warming will exacerbate this problem. The assessment of whole-life costing and residual service life prediction of structures is very important. Pitting corrosion is the most important factor which influences the service life of the reinforced concrete structures in many chloride included environments. Electrochemical impedance spectroscopy (EIS) method is used to study the corrosion process of reinforcing steel in cement mortar. According to the results of the experiments, dispersion and diffusing effect control the electrochemical process of carbon steel corrosion in the cement mortar. By fitting the results with EC, the parameters about CPE and Warburg impedance are calculated. The pitting corrosion behavior of reinforcing steel in cement mortar has been studied by electrochemical noise analysis (ENA) method, the wavelet transform has been employed to analyze the EN data of reinforcing steel in mortar, and the energy distribution plot (EDP) is plotted. The experimental results show that the change of EDP during the corrosion process can qualitatively reveal useful information on corrosion mechanisms.     

Effect of Ni content on the corrosion behavior of Cu-Ni alloys in neutral chloride solutions

The effect of systematic increase of Ni content on the electrochemical behavior of the Cu-Ni alloys in neutral chloride solutions was investigated. The pitting corrosion behavior of Cu-Ni alloys with different Ni contents, namely, 5, 10, 30 and 65 mass% Ni, in a stagnant 0.6 mol dm⁻³ NaCl solution of pH 7.0 was studied. The effect of chloride ion concentration on the electrochemical behavior of these alloys was also investigated. The results show that the increase in nickel content decreases the corrosion rate of the alloys in the neutral chloride solution. The increase of chloride concentration up to 0.3 mol dm⁻³ increases the corrosion rate. At higher concentrations ([Cl⁻] > 0.3 mol dm⁻³) the corrosion rate decreases due to the hydrolysis of Cu(I) chloride to form the passive Cu(I) oxide film. The breakdown potential depends on the chloride ion concentration and the nickel content of the alloy. For these investigations conventional electrochemical techniques and electrochemical impedance spectroscopy (EIS) were used. The impedance measurements have shown that the increase of the Ni content and the immersion time of the alloys in the chloride solution increase the corrosion resistance of the alloys. The experimental impedance data were fitted to theoretical values according to a proposed equivalent circuit model.     

阳极极化法分析缓蚀剂对Al-Mn-Mg合金的缓蚀作用

阳极极化法是评价缓蚀剂缓蚀作用的有效方法,本文采用此技术对缓蚀剂在3%NaCl溶液中对Al-Mn-Mg合金的缓蚀作用进行了研究.结果表明:无机缓蚀剂中,钼酸钠提高了Al-Mn-Mg合金抗点蚀的能力,而且维钝电流很小;交流阻抗试验也也证实其阻抗提高,说明钼酸钠起到了缓蚀作用.有机缓蚀剂中,硫脲的极化电阻率最大,有腐蚀电位与点蚀电位分离的现象,并且极化电流较其它有机缓蚀剂的小,因此其抗蚀能力最好.