Untersuchungen zum Einfluß von Sulfid- und Sulfationen auf die Korrosion des Eisens in alkalischer Lösung

Investigation into the influence of sulfide and sulfate ions on the corrosion of iron in alcaline solution The corrosion behaviour of Armco iron as well as that of a technical high-strength steel was investigated in sulphide - and sulphate-containing electrolytes of pH ～ 12.6. Current density - potential curves show that iron, after successful passivation, remains passive and protected against corrosion in a solution saturated with CaS, CaSO₄ and Ca(OH)₂ at potentials up to 800 mV(EH). Passivation experiments conducted by changing from the active region to various anodic potentials yielded a critical potential region around 300 mV(EH), above which corrosion appeared. Up to E_H = 200 mV complete passivation was observed. The same critical potential region was observed in experiments of repassivation after mechanical damaging of a protective passive layer. Additional tensile stresses of 85%· σ_0,2 caused no stress corrosion cracking at a potential of 500 mV(EH). After damaging the passive layer, through scratching of the stressed wires fracture occured at and above potentials of 300 mV(EH). At 200 mV(EH) repassivation was observed even under tensile load. In a sulphate-free Na₂S/Ca(OH)₂ solution of the same sulphide ion concentration no stress corrosion cracking of the samples occured at any potential up to 800 mV(EH) even after scratching the steel samples. The observed stress corrosion in sulphide - and sulphate-containing electrolytes is to be ascribed to the action of sulphate and not to sulphide ions.     

Stress corrosion cracking study of microalloyed pipeline steels in dilute NaHCO3 solutions

Studies were carried out to evaluate the stress corrosion cracking (SCC) behavior of a X-70 microalloyed pipeline steel, with different microstructures by using the slow strain rate testing (SSRT) technique at 50 °C, in NaHCO₃ solutions. Both anodic and cathodic potentials were applied. Additionally, experiments using the SSRT technique but with pre-charged hydrogen samples and potentiodynamic polarization curves at different sweep rates were also carried out to elucidate hydrogen effects. The results showed that the different microstructures in conjunction with the anodic applied potentials shift the cracking susceptibility of the steel. In diluted NaHCO₃ solutions cathodic potentials close to their rest potential values decreased the SCC susceptibility regardless the microstructure, whereas higher cathodic potentials promote SCC in all steel conditions. Certain microstructures are more susceptible to present anodic dissolution corrosion mechanism. Meanwhile concentrated solution did not promotes brittle fracture.     

Relationships between the critical potential for stress corrosion cracking of stainless steels and the chemical composition of the films formed in boiling MgCl2 solutions

Activation analysis and Auger electron spectrometry have been used to study the surface film formed on 304L Type stainless steel in boiling concentrated MgCl₂ solutions. Analytical study shows that the complex oxide of chromium and nickel formed at potentials where the steel is susceptible to stress corrosion cracking contains more chromium than nickel. Conversely, it is richer in nickel than chromium if it develops at potentials where the steel is protected. The critical potential for stress corrosion cracking deals with the change in the chemical composition of the oxide. Aggressive ion effects can be related to the same phenomenon.     

硼酸缓冲溶液中pH值和腐蚀产物对低碳钢活化/钝化敏感性的影响

研究了在pH值分别为8,9和10的除氧硼酸缓冲溶液中,低碳钢腐蚀产物对其活化/钝化敏感性的影响.实验结果表明,在pH值为8时,低碳钢一直处于活性溶解状态,不受腐蚀产物影响;在pH值为9和10时,表面腐蚀产物使低碳钢钝化,其腐蚀电位最后稳定于钝化区间.XPS和XRD等分析结果表明,腐蚀产物由FeB(OH)12B4O7和γ...     

Factors affecting the potential of galvanostatically polarised pipeline steel in relation to scc in co2−3-hco-3 solutions

The relationship between potential and applied cathodic current density for steel in CO^2-₃-HCO^-₃ solutions is affected by the nature of the oxides on the steel, temperature and solution composition. The presence of Fe₃O₄, a major constituent of mill scale, on the surface can promote potentials within the range where stress corrosion cracking is possible, under galvanostatic conditions simulating the application of cathodic protection to a pipeline. The combinations of potential and pH under which form the various solid reaction products, identified by X-ray diffraction and optical microscopy, can be related thermodynamically to the relevent electrochemical reactions.     

Synopses of Papers in this Issue

Abstract

A variety of techniques involving electrochemical, hydrogen permeation and acoustic emission measurements have been used in studying the stress corrosion cracking of initially smooth and pre-cracked specimens of a maraging steel in different chloride-containing solutions. In solutions having pH's in excess of 2, cracking of smooth specimens occurred in two regimes of potential separated by a region in which cracking did not occur, although cracking was induced in this latter region if the specimens were precracked or pre-pitted, or pitting was facilitated by non-metallic inclusions emerging at the test specimen surfaces by stressing the specimens transversely to the rolling direction. It is considered that such geometrical discontinuities are more important in facilitating cracking by fostering local changes in solution chemistry than because of their effect in terms of stress intensification. However, the evidence, viewed in its entirety, does not support the hypothesis that failure invariably results from the ingress of hydrogen into the steel following the creation of acidic conditions within an initiating pit or pre-crack. Rather does it support crack growth by dissolution at high potentials, and by a hydrogen induced process at low potentials, with possibly both processes involved at some intermediate potentials, including the free corrosion potential. This conclusion is supported by various electrochemical measurements, which show good correlation with the potential dependence of cracking and with the effects upon cracking of additions of chloroplatinic acid, sodium arsenate or thiourea to the chlQride solutions, with or without applied polarisation. Thus, the effects of these additives upon the hydrogen and dissolution reactions are in agreement with their influence upon stress corrosion cracking, as are the effects of applied polarisation. Hydrogen permeation measurements under conditions of cathodic polarisation confirmed the effects of these additions upon the uptake of hydrogen by the steel, but when permeation membranes were subjected to anodic polarisation they were perforated by dissolution and the apparent hydrogen permeation was due to the passage of solution through the membrane. Further support for hydrogen induced cracking dominating at lower potentials and dissolution being controlling at higher potentials was derived from acoustic emission experiments, which showed emissions to be enhanced by cathodic but not by anodic polarisation as crack growth occurred, implying different mechanisms of growth in the different potential regimes.     

45MnSiV预应力钢筋的应力腐蚀

用慢应变速率、动电位扫描和断口分析等试验方法,研究了45MnSiV 预应力高强度钢筋的应力腐蚀行为。结果表明,在未受侵蚀的混凝土中.45MnSiV 预应力钢筋不会发生应力腐蚀开裂;若 HCl,NaCl 或 SO_2等侵入混凝土,使其 pH 值下降,则会出现阳极溶解型的应力腐蚀。45MnSiV 预应力钢筋的最佳阴极保护电位为-800mV(SCE)左右。如果外加电位偏低,则不论混凝土受侵蚀与否,均将引起45MnSiV 预应力钢筋的氢致开裂。     

Stress—corrosion cracking of austenitic stainless steel in hydrochloric acid media at room temperature

Stress—corrosion cracking of solution quenched, type 304, stainless steel can occur at room temperature in HCl solutions ranging between 5·10^?1M and 1M HCl. The cracking observed in HCl solutions is similar to that previously observed in H₂SO₄ + NaCl and HClO₄ + NaCl solutions. Cracking occurs at ? 0·200 V (NHE), in the active potential region, it is under cathodic control, and it develops in conditions under which the corrosion rate of the external surface area is more or less constant and independent of the HCl concentration, in the range 10^?1 M?1 M HCl. At higher HCl concentrations, corrosion rates increase and uneven, general corrosion occurs instead of cracking. The development of pitting and stress—corrosion cracking under active conditions precludes the conclusion that active—passive cells always play a role in localized corrosion and, in particular, in stress—corrosion cracking. Under these conditions, it has been shown that sensitized and non-sensitized specimens behave similarly (giving rise in both cases to transgranular cracking); active—passive cells, due to chromium depletion at the grain boundaries, are not involved. Active—passive corrosion mechanisms can however arise at more noble potentials (0·100?0·200 V NHE), as in the case of HClH₂O₂ solutions of specific concentration, producing intergranular corrosion of the stainless steel in the sensitized condition.     

Stress corrosion cracking of α–β brass in chloride containing waters

Abstract

Slow straining of an α–β brass in aqueous solutions with various concentrations of chloride ion has been used to ascertain the effects of potential, pH, and chloride content on the susceptibility to stress corrosion cracking. The ductility is at a minimum with 55 ppm Cl present and increases at higher concentrations, where dezincification effects become more significant. Anodic polarization increases the ductility loss but the effects are again complicated at high potentials by greater dezincification. The strqjn rate dependence of the embrittlement results in a ductility minimum at 2 × 10^?6. The results are assessed in terms of various cracking mechanisms but seem to be most consistent with a mechanism involving the rupture of a surfacefilm of CU₂O that forms on the alloy in the critical potential range.     

Lateral and radial corrosion propagation behavior of 9–21% Cr and 18% Cr + 2.8% Mo stainless steel reinforcing materials in simulated concrete environments

The life of a concrete structure exposed to deicing compounds or seawater is often been limited by chloride induced corrosion of the steel reinforcement. A complete assessment of the potential benefits afforded by new candidate rebar alloys must address both the lateral and radial corrosion propagation behavior in comparison to conventional steel as well as other factors that might affect the risk of corrosion‐induced concrete cracking. The radial (depth) and lateral (length) corrosion propagation behavior of 18% Cr + 2.8% Mo (S31653) stainless steel, 21% Cr (S32101) duplex stainless steel, and 9% Cr steel compared to plain ASTM A615 carbon steel were characterized in saturated Ca(OH)₂ solution. Radial pit growth was found to be Ohmically controlled for all materials but repassivation occurred more readily at high applied potentials for 18% Cr + 2.8% Mo and 21% Cr stainless steels. Conversely, pit growth on plain steel propagated at all applied anodic potentials and did not repassivate until deactivation by cathodic polarization. Stainless steel also showed the highest resistance to lateral corrosion propagation from an active site during microelectrode array testing. 21% Cr duplex stainless and 9% Cr steel showed similar radial propagation behavior and corrosion morphology, which was intermediate to that of plain steel and S31653 stainless steel. Based on an existing concrete cracking model, it is expected that 9–21% Cr and 18% Cr + 2.8% Mo corrosion resistant rebar materials would require a greater depth of corrosion attack than carbon steel before damaging concrete via corrosion product formation.     

A Study on Stress Corrosion Cracking of X70 Pipeline Steel in Carbonate Solution by EIS

In this study, electrochemical impedance spectroscopy (EIS) simultaneously with the slow strain rate testing were used to investigate the stress corrosion cracking (SCC) behavior of X70 pipeline steel in high pH bicarbonate solution at different applied potentials. Potentiostatic EIS tests were also conducted at certain times to determine the changes associated with the SCC. Circuit models for the cracking were proposed by the use of the potentiostatic EIS measurements at different applied potentials. Finally, the results of the potentiostatic EIS tests and the SSR tests showed the decline of the circuit element resistance by increasing the stress which was related to the cracking. It was also observed that the X70 pipeline steel was most susceptible to SCC at potential of ?650 mV versus SCE.     

X70管线钢在不同温度近中性pH溶液中的应力腐蚀破裂行为

采用慢应变速率实验(SSRT)研究了不同温度和电位下X70管线钢在近中性pH溶液中的应力腐蚀破裂(SCC)行为．结果表明，不同温度下，X70管线钢在近中性pH溶液中的开裂方式都是穿晶型的，具有准解理特征，并且随着外加阴极电位的降低，SCC敏感性增加，氢致开裂占主导．随温度的下降，溶液pH值略有降低，SCC敏感性增加。     

Role of imposed potentials in threshold for caustic cracking susceptibility (KISCC): Investigations using circumferential notch tensile (CNT) testing

A fracture mechanics-based novel approach, i.e. circumferential notch tensile (CNT) testing has been employed for determination of threshold stress intensity factor for susceptibility of engineering materials to stress corrosion cracking (K_ISCC) using small specimens. Using CNT technique, K_ISCC of a carbon steel at an open circuit potential (E_corr) in 500 g L⁻¹ NaOH at 100 °C was determined to be 42.9 MPa m^1/2. In order to establish the application of the CNT technique in understanding the mechanistic aspects of caustic cracking as well as for developing guidelines for mitigation, tests have also been performed under the imposed electrochemical potentials. An imposed potential in the active–passive potential regime (E_a–p) caused an extremely rapid failure (than observed at E_corr) whereas, at an imposed potential in the passive region (E_p), the specimen did not fail even after relatively very long exposure time. The fractography of the CNT specimens tested at E_corr and E_a–p presented evidence of SCC. The study has established the use of experimental CNT testing as a simple, relatively fast and cost-advantageous approach for generating the K_ISCC data, which are also consistent with the electrochemical mechanism for caustic cracking.     

Stress corrosion cracking susceptibilities of Fe3Al-based iron aluminides

The susceptibilities to stress corrosion cracking (SCC) were investigated using the constant-potential U-bend SCC test. U-bend SCC evaluations were conducted on two iron aluminide compositions based on Fe₃Al and containing 2 and 5 at.% Cr in acid-chloride (pH = 4200 ppm Cl), thiosulfate, and tetrathionate solutions at the freely-corroding conditions. Cracking failures occurred in the thiosulfate and tetrathionate solutions, but not in the acid-chloride solution. The iron aluminides were very susceptible to the sulfur-bearing environments in terms of SCC and aqueous corrosion characteristics. To investigate the effect of applied potential on the cracking behavior, U-bend tests were conducted in the acid-chloride solution at an anodic pitting potential and at cathodic hydrogen-evolution potentials. Cracking occurred within 200 h only at the highly negative cathodic potentials and only for the lower Cr composition. These results indicated that two iron aluminides investigated are susceptible to SCC in acid-chloride solution if the corrosion potential is sufficiently active to generate hydrogen, that the cracking mechanism was related to hydrogen embrittlement. The resistance to hydrogen embrittlement cracking increased with increasing Cr content, i.e., higher Cr levels were beneficial in minimizing this form of cracking. Increased resistance to cracking for the U-bend specimens is influenced by the chemical composition of the passive film. Metallographic examinations by scanning electron microscopy revealed that increasing Cr content decreased the proportion of transgranular cleavage cracking and increased the proportion of intergranular cracking.     

The predictability of stress corrosion cracking susceptibility of steels in acetate solutions from potentiodynamic polarization curves

Stress corrosion cracking susceptibilities of mild and low alloy steels in acetate solutions, assessed using slow strain-rate testing techniques, correlate well with those predicted on the basis of data from fast and slow sweep rate potentiodynamic polarization curves. Accurate prediction of cracking susceptibility necessitates allowances being made for changes of solution pH which occur during stress corrosion and polarization tests. A predictive technique which allows for these pH changes has been developed, and when implemented yields polarization data that indicates the influence of potential, steel composition and solution temperature on cracking susceptibility. The stress corrosion cracking of steels in acetate solutions is intergranular and occurs only when the electrochemical conditions correspond to those of a predicted potential-pH domain that is associated with the Fe²⁺/Fe₃O₄ transition.     

Untersuchungen über die transkristalline Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle in heißen Chloridlösungen

Investigations into the transcrystalline stress corrosion cracking of austenitic chrome-nickel steel in hot chloride solutions To obtain a further clarification of various details in respect of the transcrystalline stress corrosion cracking of austenitic Cr-Ni steels, tensile strength tests were carried out with X 5 CrNi 18 9 in hot, highly concentrated MgCl₂, solutions. Tests without extraneous currents showed that, depending on the test conditions, the relationship between stress and service life can be formally described by an exponential function. Down to stresses of 2 kg/mm², there was no discernible threshold below which stress corrosion cracking does not occur at all. By means of potentiostatic holding tests at different stresses, it was possible to plot a potential/tensile stress/service life diagram. The occurrence of stress corrosion cracking shows a distinct threshold on the side of negative potentials; on the side of positive potentials, however, the occurrence of stress corrosion cracking is increasingly accompanied by pit corrosion. Strain measurements carried out during the tests yielded, in a non-destructive way, a sufficiently exact criterion for the division of the total test period into incubation period and tearing period. The length of the incubation period depends on the tensile stress as well as on the specimen potential. As regards the tearing period it was found that the growth rate of the cracks is formally proportional to the stress at the bottom of the crack. The proportionality constant, in its turn, is a linear function of the test potential. By means of extrapolation, it is possible to deter-mine the minimum potential below which stress corrosion cracking can no longer occur. Electro-chemical measurements showed that, in the tested solutions, it is the hydrogen reduction which represents the vital catholic part-process. The tested material is in the state of passive stability; its rest potential in the non-stressed condition lies at the lower threshold of the pit corrosion range. Tensile stresses, or the deformations caused by them, have the effect of shifting the minimum potential for heterogeneous corrosion into the negative range.     

Stress corrosion cracking of weldments of AISI Type 316 stainless steel

The weldments of AISI Type 316 stainless steel prepared by the tungsten inert gas (TIG) and manual metal arc (MMA) welding processes, were tested in a boiling solution of 5 M NaCl + 0.15 M Na₂SO₄ (pH = 1.3) at an initial stress level of 200 MPa under the open circuit potential (OCP) and the impressed potential (IP) conditions using the constant load technique. Anodic polarisation above the critical cracking potential (CCP) which was determined by the OCP test, accelerated the stress corrosion cracking of both the weldments whereas slight cathodic polarisation below the CCP prevented it. The cracks initiated through pits in both the types of weldments under open circuit as well as impressed potential conditions. The TIG weldments cracked in the heat affected zone whereas the MMA weldments failed in the fusion zone at all potentials. At open circuit potential, the MMA weldments failed by the stress assisted dissolution of delta ferrite, while the TIG weldments failed by the TGSCC of austenite. On anodic polarisation, the MMA weldment failed by the dissolution of delta ferrite and intergranular SCC of austenite, while in the TIG weldments the cracks initiated and propagated in the intergranular mode in the austenite.     

Elektrochemische Meßmethoden zur Untersuchung der Spannungs- und Schwingungsrißkorrosion

Electrochemical measuring methods for studying stress-corrosion cracking and corrosion fatigue An apparatus is described for the investigation of local corrosion phenomena with simultaneous static and dynamic loading. The apparatus enables measurements to be made of potential variations in time, interdependance of current density and potential, and current density variation at constant potential. It is shown that stress corrosion cracking of carbon steel in boiling Ca(NO₃)₂, solution and of soft iron in NaOH is a clear local corrosion process. Analogous results are obtained with austenitic stainless steel in an activating medium. Corrosion fatigue of passive austenitic steels can be produced in SO, saturated water or in ferroammonium sulphate solution. Between corrosion fatigue and stress corrosion cracking there is a number of analogies but there are also differences. Stress corrosion cracking strongly depends from material composition but may be produced by almost any medium, while corrosion fatigue is possible in certain media only, but is not related with a particular metal composition.     

外加电位对X70管线钢在近中性pH溶液中的应力腐蚀破裂的影响

采用慢应变速率试验(SSRT)研究了不同电位下X70管线钢在近中性pH溶液中的应力腐蚀破裂(SCC)行为．同时研究丁溶液中通入不同含量CO2对SCC的影响．结果表明．X70管线钢在近中性pH溶液中的开裂方式是穿晶型的．具有准解理特征．并且随着外加阴极电位的降低，SCC敏感性增加；随CO2含量的增加，pH值降低，SCC敏感性增加．均表现为氢致开裂占主导．     

外加电位对X70管线钢在近中性PH溶液中的应力腐蚀破损的影响

采用慢应变速率试验（SSRT）研究了不同电位下X70管线钢在近中性pH溶液中的应力腐蚀破裂（SCC）行为,同时研究了溶液中通往不同含量CO2对SCC的影响。结果表明,X70管线钢在近中性pH溶液中的开裂方式是穿晶型的,具有准解理特征,并且随着外加阴极电位的降低,SCC敏感性增加,随CO2含量的增加,pH值降低,SCC敏感性增加,均表现为氢致开裂占主导。