Untersuchungen über die Kathodenwirkung von Eisensulfid und Mangansulfid auf die Korrosion von Eisen in dreiprozentiger Natriumchloridlösung

Studies on the cathodic effect of iron sulfide and manganese sulfide on the corrosion of iron in a three percent sodium chloride solution In the galvanic element iron/sodium chloride solution/iron sulfide and in the element iron/sodium chloride solution/manganese sulfide, flushed with nitrogen or air, the sulfides become the cathode and the iron is anode. Upon short circuiting the electrodes a strong cathodic polarisation of the sulfides occurs. However, only if the iron is in contact with iron sulfide in the air-flushed sodium chloride solution the corrosion of the iron is enhanced because of considerable anodic polarisation. The corrosion of the iron electrode is increased corresponding to the increase in cathodic area. According to these results inclusions of iron sulfide or manganese sulfide act as local cathodes but cannot cause pitting corrosion of unalloyed steels in a neutral chloride solution.     

Zum anodischen und kathodischen Verhalten des Eisens in neutralen und alkalischen Lösungen

The anodic and cathodic behaviour of iron in neutral and alkaline solutions To investigate the behaviour of iron in the p_H range from 7 to 14 in the presence of chloride ions, the authors make use of the electrochemical affinity/overpotential diagram. At p_H =7, the activation of the iron by the destruction of the FeO primary layer is possible. With increasing alkalinity, the passive zone is reduced by the direct effect of Cl^? ions, by the oxidation of FeCl₂, and by the formation of the iron complexes FeO₄^2?. The decomposition of Fe(OH)₂ by Cl^? ions is thermodynamically possible up to p_H = 10.5 only. At p_H > 10.5, there occurs the equilibrium Fe(OH)₂ = HFeO₂^? + H⁺.     

Reactions at the CO,CO2/Ni electrode in molten sodium carbonate

Reactions at the nickel electrode in molten sodium carbonate at 1000°C under CO/CO₂ atmospheres were studied by electrode polarization. The polarization curves were analysed by means of a previously described computer model, which in turn was interpreted in terms of the absolute rate expressions for postulated reactions. In agreement with previous workers' observations for inert electrodes, the dominant anodic reactions were found to be oxidation of CO at low and intermediate anodic overpotentials, and oxidation of CO₃^2? at high anodic overpotentials. The cathodic reaction was found to be reduction of CO₃^2?. The form of the polarization curves was described by activation, concentration, and resistance polarization of these reactions; however, anomalous anodic behaviour was observed which was attributed to corrosion reactions at the electrode and accurately described through the use of additional anodic reactions displaying passivity.     

Galvanic corrosion behaviour of iron coupled to aluminium in NaCl solution by scanning eectrochemical microscopy

Scanning electrochemical microscopy (SECM) was used to sense the concentration of ions in 0.1M NaCl aqueous solution at the iron-aluminium couple. The SECM measured the concentration of ions relevant to the corrosion processes. The electrochemical behaviour of galvanic Fe/Al coupling was investigated as a function of time using SECM microelectrode tip. SECM amperometric line scan curves were obtained over the Fe/Al at a constant distance. In the first case the chemical species participating in the corrosion reactions at the sample are detected at the SECM-tip by applying appropriate potential values to the microelectrode. The release of Al³⁺ into the solution from local anodic surface, as well as the consumption of dissolved oxygen at the corresponding cathodic surface was successfully monitored. The results revealed that the galvanic couple where Fe/Al is close to each other will show lower corrosion rate due to the formation of corrosion products on the metal surface with further increase in exposure times.     

Electrochemische Untersuchungen zur Hochtemperaturkorrosion von Chromstählen in Alkalisulfatschmelzen

Electrochemical investigation into the high temperature corrosion of chromium steels in alkali sulfate melts Electrochemical and corrosion-chemical investigations have been carried out with scaling resistant chromium steels, iron, chromium and platinum in a eutectic (Li, Na, K)₂-SO₄. It has turned out that sufficiently exact data concerning corrosion reactions can be obtained only from mass losses, not, however, from current density. The corrosion behaviour depends from potential. Comparable to the conditions in aqueous solutions potential ranges exist with passive and transpassive corrosion and with a rupture potential which depends from the chromium content of a steel. Protective oxide layers exist in the passive range where the mass loss becomes almost constant after a certain in cubation period. In the transpassive range corrosion follows an almost parabolic law with formation of an inner sulfide layer and a thicker external oxide layer where chromium is enriched. These layers are largely formed by oxidizing media carried to the metal surface via sulfur oxides; during this reaction inert marks in the steel remain unchanged. Sulfur oxides may be formed as secondary consecutive products by reactions between metal ions and sulfates. The solubility of metal ions in the sulfate melt is an important parameter for corrosion rates. Oxide ions (as reduction products of O₂) act as inhibitor on the anodic partial reaction, while SO₃ and ferric ions have a large lating effect, so that the anodic dissolution is autocatalyzed. Chlorides, too, act as stimulators in the transpassive range. Corrosion at the free corrosion potential is largely controlled by ferric ions which act as anodic and cathodic stimulators in acid melts. In neutral melts under oxygen an 18% chromium steel is passive.     

Reactions at the corroding nickel electrode in molten sodium carbonate under CO/CO2 atmosphere

Anodic reactions at the CO, CO₂|Ni electrode in molten sodium carbonate at 1000°C are examined in detail. Experimental polarization curves are fitted via a computer graphics procedure to a model employing five anodic reactions with activation, concentration and resistance polarization, and passivation. The resultant empirical parameters are interpreted in terms of absolute rate expressions describing possible anodic reactions. Three of the anodic reactions are found to be in accord with the findings at inert electrodes by previous investigators, i.e. oxidation of CO by two mechanisms and oxidation of carbonate, while the cathodic reaction is consistent with reduction of CO₂. The remaining two anodic reactions were associated with the oxidation of nickel to Ni²⁺ with formation of a NiO film above a passivation potential, and, at higher anodic overpotentials, oxidation of NiO to a higher oxide, e.g. Ni₂O₃. There is an indication of the formation of a third oxide at still higher anodic overpotentials.     

Electrochemical behaviour and stress-corrosion cracking of titanium in alcoholic solutions

The electrochemical behaviour of titanium in neutral methanolic and ethanolic solutions containing chlorides, and corrosion tests performed with U-bend specimens in similar solutions, indicate that stress-corrosion phenomena occur on titanium when particular anodic and cathodic conditions are settled.In neutral aerated ethanolic solutions the oxide film is stable and its electronic properties (ionic and electronic currents) are similar to the ones observed in aqueous solutions. Exception is made only for the breakdown potential. In ethanolic solutions, stress-corrosion occurs only in the presence of depolarizing species which can set mixed potentials more noble than the one characteristic of oxygen (i.e. FeCl₃).Breakdown potential weakly depends on the chloride concentration, but the adsorption of chlorides on the oxide surface, in ethanolic solutions shifts the steady state potential of the metal to more active potentials so that, from the electrochemical point of view the more concentrated NaCl solutions, appear to be less effective in promoting stress-corrosion, the corrosion potentials settled at the higher Cl^? concentrations, being less noble than the potentials settled at the lowest Cl^? concentrations. In this case the role of Cl^? in the dissolution of titanium in ethanolic solution seems to be restricted to the formation of complex ions. The stability of titanium oxide films in methanolic solutions is very weak and, in the anodic sense, is restricted to a very narrow range of potentials. Presence of oxygen can create mixed potentials more noble than the breakdown potentials and thus the stress-corrosion occurrence.The beneficial effect of cathodic polarization in neutral alcoholic environment can be due to the establishment of corrosion potential less noble than breakdown potential of the oxide film.     

Preparation and performance of fluorescent sensing coating for monitoring corrosion of Al alloy 2024

A kind of fluorescent sensing coating was prepared for monitoring corrosion of aluminum alloys by incorporating phenylfluorone（PF） into acrylic paint as sensing material. The fluorescent dye PF reacts with aluminum ions on corroded aluminum substrate to occur fluorescence quenching observed in UV light. This paint system is sensitive tO underlying corrosion processes through reacting with the Al^3＋ produced by anodic reaction accompanying corrosion. After a certain time, when the samples of Al alloy 2024 coated with PF-acrylic paint were immersed in 1 mol/L NaCl solution, fluorescence quenching spots can be seen with unaided eyes. With the development of corrosion process, the size of fluorescence quenching spots increases. Active corrosion areas on the sample surface were found under the fluorescence quenching spots by optical microscope. The corrosion areas can be observed more clearly by SEM, and many pits are found. This suggests that the fluorescence quenching spots are the sites of produced Al^3＋ by the anodic reaction of the local attack of the coated Al alloy substrate in the chloride solution and the corrosion process of the coated AI alloy can be monitored on-line by the sensing coating. The sensitivity of this coating system for detection of anodic reaction associated with corrosion was determined by applying constant charge current and measuring the charge, at which fluorescence quenching is detected in the coating with unaided eyes. Visual observation of coated samples can detect fluorescence change resulting from a charge corresponding to an equivalent hemispherical pit with approximate depth of 50 μn.     

Corrosion Behavior of Cu40Zn in Sulfide-Polluted 3.5% NaCl Solution

The corrosion behavior of a duplex-phase brass Cu40Zn in clean and sulfide-polluted 3.5% NaCl solutions was investigated by conducting electrochemical and gravimetric measurements. The corrosion product films were analyzed by scanning electron microscopy, energy-dispersive spectroscopy and x-ray diffraction. The presence of sulfide shifted the corrosion potential of Cu40Zn toward a more negative value by 100 mV and increased the mass loss rate by a factor of 1.257 compared with the result in the clean solution. The corrosion product film in the clean solution was thin and compact; it mainly consisted of oxides, such as ZnO and Cu₂O. By contrast, the film in the sulfide-polluted solution was thick and porous. It mainly contained sulfides and zinc hydroxide chloride (i.e., Zn₅(OH)₈Cl₂·H₂O). The presence of sulfide ions accelerated the corrosion damage of Cu40Zn by hindering the formation of protective oxides and promoting the formation of a defective film which consisted of sulfides and hydroxide chlorides.     

无磷海水缓蚀剂的开发

通过失重法测定腐蚀速率,并计算缓蚀率,确定了具有较好协同缓蚀效应的四元无磷海水缓蚀剂配方,并通过电化学极化、SEM、EDS等分析手段,对该无磷海水复合缓蚀剂的缓蚀机理进行了研究.结果表明:配方以40 mg/L钨酸盐+40 mg/L葡萄糖酸盐+4 mg/L Zn2++30 mg/L三乙醇胺为宜,该缓蚀剂在海水中对碳钢的缓...     

Kinetics study of iron dissolution in highly concentrated acidic media using Hammett acidity function

This study aimed at establishing a mechanism of iron corrosion, in highly HCl concentrated medium, using Hammett acidity functions instead of conventional pH measurements. Linear relationships between the rate of Armco iron corrosion and the acidity of aqueous HCl media expressed in the form of Hammett acidity , were obtained. At high ionic strength, it was shown by electrochemical analysis, that chloride ions were adsorbed at the metal surface and decreased the corrosion process of iron. A detailed mechanism of iron corrosion involving activity of protons H⁺ as promoter of the iron dissolution, the inhibiting effect of chloride activity, and activity of water as a kinetics variable parameter. It reveals that this competitive mechanism on the anodic sites, agrees well with experimental kinetic parameters obtained in this study.     

THE SYNERGISTIC INHIBITION STUDIES OF SEVERAL AMINES AND CHLORIDE ION ON IRON IN THE ACIDIC PERCHLORIDE SOLUTIONS

１．ＩｎｔｒｏｄｕｃｔｉｏｎＭａｎｙｋｉｎｄｓｏｆｉｎｈｉｂｉｔｏｒｓｕｓｅｄｉｎｉｎｄｕｓｔｒｙａｒｅｉｎｈｉｂｉｔｏｒｍｉｘｔｕｒｅｓ〔１〕．Ｔｈｅｙｐｒｏｖｉｄｅｂｅｔｔｅｒｉｎｈｉｂｉｔｉｏｎｅｆｆｅｃｔｓｗｈｉｃｈａｒｅｋｎｏｗｎａｓｔｈｅｓｙｎｅｒｇｉｓｔｉｃｉｎｈｉｂｉｔｉｏｎｅｆｆｅｃｔｔｈａｎｔｈｅｉｎｄｉｖｉｄｕａｌｉｎｈｉｂｉｔｏｒｓ．Ａｌｏｔｏｆｓｔｕｄｉｅｓａｂｏｕｔｔｈｅｓｙｎｅｒｇｉｓｔｉｃｉｎｈｉｂｉｔｉｏｎｅｆｆｅｃｔｏｆｏｒｇａｎｉｃａｍｉｎｅｓａｎｄｃｈｌ…     

Peculiar effect of chloride ions on the anodic dissolution of iron in solutions of various acidity

Anodic behavior of iron in chloride solutions is studied in the pH range from 0 to 6 and at the concentration of chloride ions from 0.2 to 3 mol/l. Electrochemical measurements were supplemented with the raster electron microscopy studies, of electrode surfaces. The observed independence of the limiting dissolution current of pH is explained by a hypothesis that water does not participate in the anodic process at sufficiently large anodic overpotentials. Depending on the concentration of chloride ions, the continuity of the logi _a-pH dependence is first interrupted and the, restored; the reaction order in hydroxide ions changes from a positive value to zero. This is probably caused by the “dynamic passivation” of iron with the adsorbed oxygen atoms and their competitive replacement by chloride ions.     

Inhibition of corrosion of AZ91 magnesium alloy in ethylene glycol solution in presence of chloride anions

The influence of chloride ions on the corrosion of AZ91 magnesium alloy in water/ethylene glycol solutions and the inhibiting effect of lactobiono‐tallowamide (LTA) were investigated using electrochemical and surface analysis methods. Potentiodynamic polarization curves in aqueous solution of ethylene glycol (50:50w%) containing 0.1 g.L⁻¹ chloride and up to 0.5 g.L⁻¹ LBT were obtained at room and at elevated temperatures. The chloride anions showed a distinct deteriorating effect as they caused pit initiation and accelerated the dissolution of the tested alloy. The selected organic compound demonstrated good protective properties against corrosion of AZ91 magnesium alloy and behaved as inhibitor of mixed type hindering both the cathodic and the anodic partial reactions. It showed inhibition efficiency of 77% at relatively low concentration of 0.2 g.L⁻¹ and was considered as a promising corrosion inhibitor. The mechanism of inhibition was discussed on the basis of the electrochemical impedance spectroscopy (EIS) and XRD analysis of the surface.     

Einfluß der Kationenzusammensetzung von Chloridlösungen beim Modellieren von Korrosionseinflüssen der Küstenatmosphären

Influence of the cation composition of chloride solutions in modelling corrosion effects due to caoastal atmospheres Pure sodium chloride solutions cannto be used for accelerated weathering tests in artificial sea atmosphere. A spray liquid suitable for such tests is artificial seawater containing (grams/litre) 27 NaCl, 6 Mg chloride, 1 Ca chloride and 1 KCl. The corrosion behaviour of the two metals studied – iron and zinc – is characterized by typical differences, in particular variations depending on the salt quantity applied. While the corrosion of iron at first increases with the salt concentration applied and achieves a maximum at about 1000 mg/m²d, the losses of zinc increase just to a salt quantity of 200 mg/m² d and are then practically constant up to about 1000 mg/m²d; at higher concentrations there is again a slight diminuation of corrosion. The corrosion in artificial seawater is more pronounced than in sodium chloride solutions; this fact can be attributed to the hydrolysis of the chlorides in seawater, which gives rise to the formation of anodic and cathodic regions on the metal surface; as a consequence, pH differences are created and corrosion is enhanced. In order to simulate natural conditions as closely as possible the application of the salt spray should be interrupted by extended dry periods.     

碳钢在含硫化氢及高压二氧化碳饱和的NaCl溶液中的腐蚀行为

利用自制的高温、高压腐蚀试验及电化学测试装置,通过失重法、电化学极化曲线法及电子探针微观分析等方法,研究了温度、硫化氢浓度对碳钢在高压二氧化碳饱和的39％NaCl溶液中腐蚀的影响．结果表明较低温度(80℃)下,升高温度及增大硫化氢浓度均加速腐蚀反应的阴、阳极过程,失重腐蚀速率增大;高浓度的硫化氢抑制了腐蚀反应的阴极过程;120℃时碳钢CO2腐蚀产物膜对金属基体起很好的保护作用,失重腐蚀速率减小了3～4倍,随硫化氢浓度的增大,失重腐蚀速率缓慢增长,腐蚀产物FeCO3膜逐渐转变为以硫铁化合物为主的腐蚀产物膜．     

Development of prepitting procedure for turbine disc steel

Abstract

A prepitting procedure to generate a single pit of ‘controlled’ depth on a steam turbine disc steel has been developed using disodium tetraborate–sodium chloride solution. The optimal conditions for controlled pit generation and growth were obtained by varying the ratio of tetraborate to chloride ions and applying an anodic current of 10 μA. The procedure can be used to produce a single pit on a cylindrical specimen, which can be used subsequently for stress corrosion cracking or corrosion fatigue tests. The growth rate and the shape of the pits have also been investigated.     

Use of SVET and SECM to study the galvanic corrosion of an iron-zinc cell

The work makes use of the scanning vibrating electrode technique (SVET) and the scanning electrochemical microscope (SECM) to investigate microscopic aspects of the electrochemical reactions that occur in an iron-zinc galvanic couple immersed in aqueous sodium chloride solution. Detection of the corrosion processes was made by sensing the phenomena occurring in solution. The SVET provided information on the distribution of ionic currents arising from the metal surface, whereas the SECM measured the concentration of chemical species relevant to the corrosion processes. The two techniques had comparable sensitivity for the corrosion of iron but significant differences were observed concerning the detection of corrosion of zinc.     

Über den Einfluß der Strömungsgeschwindigkeit und der NaCl-Konzentration auf die Sauerstoffkorrosion unlegierten Stahls in Wässern

Influence of flow velocity and sodium chloride concentration on the oxygen corrosion of unalloyed steels in water Corrosion of mild steel in aerated water normally leads to pitting, and to an enrichment of anions within the pits. Even HCO₃^? ions can support localized corrosion. The intensity of this is remarkably increased by chloride ions. In equilibrium water with Ca²⁺ and HCO₃^? ions and about 40 ppm free CO₂ protective rust layers are formed after about 500 hs exposure not depending on flow rate and salt concentration. These layers decrease the mean corrosion rate to about 0.1 mm/a, but do not prevent localized corrosion. Protective passive layers with extremely low corrosion rates are formed in flowing water with Ca²⁺ and HCO₃^? ions and about 40 ppm free CO₂. The minimum flow rate can be estimated for a 3/4 inch pipe to lie between 0.35 and 1.5 m/s. Investigations with water containing 40 ppm free CO₂ without Ca²⁺ at pH 4.7 and with NaHCO₃ (pH 7.2) show at low pH high corrosion rates – as expected –, and at pH 7.2 formation of protective layers with poor reproducibility in comparison with the situation in equilibrium water. Thus, Ca²⁺ ions support the protective properties of rust layers. HCO₃^? ions are necessary for the formation of protective layers because these are not formed in pure NaCl solutions.     

Environmental factors affecting pitting corrosion of type 304 stainless steel investigated by electrochemical noise measurements under potentiostatic control

Electrochemical noise measurements on anodically polarised type 304 stainless steel surfaces in contact with buffer solutions of neutral pH were performed to study the effect of chloride ions in the nucleation of pitting corrosion. Passive layer stability and susceptibility to pitting corrosion after pickling and passivation at different environmental conditions were also investigated by means of electrochemical current noise measurements under cathodic and anodic polarisation. According to the obtained experimental results pits nucleate independently on the presence of chloride ions. It has been also shown that protectiveness of stainless steel surfaces after pickling strongly depends on the relative humidity of the environment in which the surface is subsequently passivated.