Stress corrosion cracking of amorphous iron base alloys

The stress corrosion cracking behaviour at room temperature of amorphous Fe-Cr-Ni-P-C alloys subjected to constant strain rates was studied in some acidic solutions containing Cl⁻ ions. Hydrogen embrittlement of the alloys occurred in the potential region lower than −300 mV relative to the corrosion potential in acidic solutions regardless of Cl⁻ concentration. In the passive potential region no embrittlement was observed during tests in neutral NaCl solutions and in acidic solutions with low concentrations of Cl⁻ ions. Only when tensile stress was applied to the specimen in relatively strong acidic solutions containing a certain amount of Cl⁻ ions, fracture stress decreased in this potential region. The lowering of the fracture stress can also be attributed to hydrogen embrittlement.     

Stress corrosion cracking of aluminium brass in acidic chloride solutions

Stress corrosion cracking of aluminium brass in non-ammoniacal acidic chloride solutions has been studied. The most intense transgranular penetration was found in the range of active potentials in synthetic sea water containing 1.5 g/l. of CuCl at pH 1.5, in 1 M sodium chloride solution containing 1.5 g/l. of CuCl at pH 2.0 and in 1 M sodium chloride + 0.25M tri-sodium citrate solution containing 1.5 g/l. of CuCl at pH 4.0. Experimental evidence that high As and high P contents in the alloy can play an important role on the morphology and on the intensity of the corrosion process is presented.     

Stress corrosion cracking mechanism of prestressing steels in bicarbonate solutions

Prestressing steels occasionally fail by a process named “stress corrosion cracking”. This process has not been fully elucidated and several theories exists in order to explain the cases in which real structures have collapsed. This paper briefly mentions the different theories and identifies the progress in understanding whether it is necessary to use a testing method, which is able to separate the different steps and mechanisms contributing to the failures.This paper presents the methodology used for inducing controlled localized attack to study the susceptibility of the high strength steels resistance to stress corrosion cracking (SCC). The method is designed to study the growth of cracks initiated from a mechanical notch; the crack is not produced by fatigue.It consists of several stages: coating of the bar with epoxy resin, generation of a small notch, constant load and controlled potential test in the media, mechanical test in air and fractographic study. It allows us to calculate the crack propagation rate and the fracture toughness in the same test.Finally, it has been possible to apply the surface mobility mechanism (SMM) in order to identify the SCC mechanism that operates.     

Stress corrosion cracking of pure copper in dilute ammoniacal solutions

Studies of the stress corrosion cracking (SCC) of 99.999% copper and Cu-Zn alloys containing up to 10 wt%Zn in NH₄OH solution were made with varying concentrations (0.03–0.07 M) and temperatures (40–70°C). Stress corrosion cracking occurs on pure copper and all of the alloys under the condition in which thick tarnish film (Cu₂O oxide film) forms. The path of cracking is transgranular for pure copper and alloys containing < 1.3 wt%Zn, but intergranular for alloys containing > 1.3 wt%Zn. Crack propagation rates and times-to-failure estimated by the tarnish rupture theory, utilizing experimentally determined values of the fracture strain of film and the creep rate of specimens during SCC tests, are in good agreement with those observed under constant load.     

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.     

Stress corrosion cracking of zirconium and Zircaloy-4 in halide aqueous solutions

Zirconium and Zircaloy-4 in 1 M NaCl, 1 M KBr and 1 M KI aqueous solutions were found to be susceptible to stress corrosion cracking (SCC) only at potentials above the pitting potential. In all the tested systems the following steps were found: first electrochemical breakdown of the passive film, followed by intergranular attack due to anodic dissolution assisted by stresses; and finally a fast transgranular propagation. This last step was identified as the “true” SCC process. The analysis of the possible mechanisms involved during this process led to the conclusion that the surface-mobility SCC mechanism can be used to explain the experimental results found in the present work.     

Stress corrosion cracking of bone implants

A diagnostic investigation has been carried out on some AISI type 316 stainless steel fixation nails that failed during service in the recovery treatment of femoral fractures. The association of pitting and cracking at the nail edges where the cracking process appears to start together with fractographic features of the ruptured surfaces has led to the belief that pitting in crevice-induced stress corrosion cracking from acid chlorides is the cause of failure. In this respect, consideration should be given to the new high-Cr ferritic stainless steels, extra-low in interstitial elements, which are much more resistant than ordinary austenitic grades to pitting and s.c.c. in chloride media.     

Effect of carbon on stress corrosion cracking and anodic oxidation of iron in NaOH solutions

Anodic behaviour of decarburised iron and of quenched Fe-C materials with up to 0.875 wt% C was examined in 8.5 M NaOH at 100 °C to explain the role of carbon in caustic stress corrosion cracking (SCC) of plain steels. Removal of carbon from Armco iron strongly reduced its intergranular SCC. Slip steps on grains did not initiate cracks. It has been shown that carbon at low contents deteriorates the passivation of iron, whereas at high contents it promotes the formation of magnetite. High resistance to SCC of high carbon steels can be explained by an intense formation of magnetite on these steels.     

Stress corrosion cracking of brass in ammonia solution

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Stress corrosion cracking of 18 carat gold

Due to the so-called “parting limit” in gold alloys there is a general belief that copper-gold alloys with a gold content above 40-45 a/o (atomic %) are immune to stress corrosion cracking (SCC) in aqueous solutions. In the present work it is reported that, by an adequate choice of the corrosive environment, it is possible to produce intergranular SCC in Au-Cu alloy of 18 carat gold, equivalent to Au 50 a/o, in aqueous solutions.     

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.     

Intergranular stress corrosion cracking of copper in nitrite solutions

Semi-hard tubes of deoxidized high phosphorous copper with different levels of tangential residual stresses have been exposed to nitrite solutions in a laboratory heating circuit. After characterization of investigated materials influence of temperature, location of heating, concentration of solution, electrochemical potential, and atmosphere on stress corrosion cracking susceptibility of those copper tubes has been investigated. Threshold stress for crack initiation has been determined. Maximum duration of experiments was 1 month. Breakthrough time of tubes has been measured as criterion for susceptibility to SCC.To vary stress level in a wider range constant load tests on tubes with different degrees of cold working (annealed, semi-hard, hard) were done by mounting them in a steel frame.Stress corrosion cracks were always intergranular. A tenorite layer covered surface of cracked copper tubes. A reaction scheme for ammonia formation is presented. Necessary conditions for formation of stress corrosion are shown and critically discussed.Results show that intergranular cracking takes place at much lower stress levels below yield strength when compared to literature data on transgranular cracking above yield strength. For transgranular cracking cross slipping and cleavage formation as cracking mechanism is confirmed while for intergranular cracking chemical dissolution of grain boundaries plays a more important role.     

Stress corrosion cracking of cold-worked austenitic stainless steels

Stress corrosion cracking (SCC) of AISI 304L and AISI 316L stainless steels, cold-worked under various conditions (i.e. at different degrees of deformation obtained by drawing and rolling at room temperature and at liquid nitrogen temperature) has been carried out in H₂O containing 1000 ppm Cl^? at 250°C and in a boiling MgCl₂ solution. The effect of heat treatments at 400 and 900°C on the SCC of previously cold-worked steels has also been studied. Particular attention was directed towards heat treatment at 400°C. In steels deformed at room temperature, it increases the SCC resistance. By contrast, for steels deformed at liquid nitrogen temperature, heat treatment at 400°C reduces the SCC resistance if carried out for short periods of time (1–6 h). Hardness measurements, structural analyses via X-rays, scanning and transmission electron microscopy (SEM and TEM), as well as modified Strauss tests, seem to prove that reduced stress corrosion resistance is not to be related to the chromium-rich carbides precipitation which could have been accelerated by the presence of α′-martensite. Instead, they tend to suggest that perhaps this phenomenon is connected to an increase in the level of internal micro-stresses which are generated by a reciprocal re-ordering of the α′ and γ structural phases.     

Stress corrosion cracking and hydrogen embrittlement of sensitized austenitic stainless steels in boiling saturated magnesium chloride solutions

Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of the sensitized stainless steels of type 304, 310 and 316 were investigated as a function of test temperature in boiling saturated magnesium chloride (MgCl₂) solutions under a constant applied stress condition. The test temperature dependence of fracture appearance and three parameters of time to failure (t_f), steady-state elongation rate (l_ss) and transition time to time to failure ratio (t_ss/t_f) suggests that type 304 suffered SCC and HE, while type 316 suffered only HE and type 310 SCC. It was also found that the test temperature dependence of three parameters for the sensitized type 304 and 310 was almost similar to that of the solution annealed stainless steels, whereas that of type 316 showed a clear difference between sensitized and solution annealed specimens. The relationships between the logarithms of the time to failure and the steady-state elongation rate became a straight line for all stainless steels. However, its slope depended upon the fracture mode; −2.0 for SCC and −1.5 for HE. This showed that the steady-state elongation rate was the parameter for predicting the time to failure for the stainless steels in the MgCl₂ solutions. The results obtained were explained in terms of martensite transformation, hydrogen entry site, sensitization, and so on.     

Stress corrosion cracking detection using non-contact ultrasonic techniques

In this work a method is presented for detecting and locating stress corrosion cracking (SCC) in stainless steel pipe samples. The method combines laser generation and either laser or electromagnetic acoustic transducer (EMAT) detection, scanning the generation point across the sample surface. Using laser-generated ultrasonic waves that interact with the cracks, and performing time–frequency analysis techniques to examine changes in the generated wavemodes, surface plots that clearly resolve the spatial extent and geometric alignment of the cracks are created and presented here. The method is demonstrated using components removed from service after exhibiting SCC.     

The stress corrosion cracking of admiralty brass in sulphate solutions

The field of occurrence, the severity and the morphology of the stress corrosion cracking (s.c.c.) of Admiralty brass exposed, under open circuit conditions, to sulphate solutions have been determined, as a function of acidity and of Cu²⁺ content, by means of ‘U’-bend and of constant load tests. Depending upon the solution pH, cracking exhibits two different modes: intergranular in strongly acidic solutions and transgranular in a wide range of moderately acidic and in alkaline solutions. The transition in the cracking mode occurs near the borderline conditions of Cu₂O stability; in the same conditions the maximum susceptibility to s.c.c. has been observed. Different roles played by Cu₂O in the nucleation and in the propagation stages of transgranular cracking are discussed.     

Stress corrosion cracking of 13% Cr martensitic steels in sodium chloride solutions in the presence of thiosulphate

The stress corrosion cracking (SCC) susceptibility of 13% Cr martensitic (UNS S42000) and supermartensitic (UNS S41125) steels in sodium chloride solutions in the presence of thiosulphate was evaluated by slow strain rate tests (SSRT). The tests were performed in 5% sodium chloride solutions buffered at pH 2.7, 3.5, 4.5 and 6.0 in the absence and presence of thiosulphate in a concentration range between 10⁻⁶ and 10⁻³ M, at 25 ± 0.1 °C. The electrochemical behaviour of the two steels in the different solutions was determined by recording the anodic and cathodic polarisation curves. 13% Cr martensitic steel showed SCC in 5% sodium chloride solutions with pH ≤ 4.5 in the presence of 3 × 10⁻⁶ M thiosulphate. Decreasing the chloride ion concentration from 50 to 10 g/l, the critical concentration of thiosulphate to provoke SCC susceptibility increased from 3 × 10⁻⁶ to 1 × 10⁻⁵ M. The resistance to SCC of the supermartensitic steel was higher than that of the martensitic steel. The critical concentration of thiosulphate to induce SCC on the supermartensitic steel were 1 × 10⁻⁵ M at pH 2.7 and 1 × 10⁻⁴ M at pH 3.5. At pH ≥ 4.5 the supermartensitic steel did not crack. The anodic and cathodic polarisation curves evidenced the influence of the thiosulphate on the corrosion and the activation effect on the steels. The SCC of the two steels was attributed to hydrogen embrittlement produced by sulphur and hydrogen sulphide formed by dismutation and reduction of thiosulphate.     

Technical note Stress corrosion cracking of ammonia receiver tank

Abstract

A 900 t/day capacity installation, used for recycling of ammonia to a urea plant, with two ammonia streams and four receiver tanks, failed after 3 years in service. The vessels were of the cylindrical bullet type, made of SA 516 grade 70 carbon steel. Of the four receiver tanks, only one suffered stress corrosion cracking, caused by incomplete mixing of the 0·2% water required by the process parameters. The vessel was operated at ambient temperature and a pressure of 1·765 x 10⁶ Pa; the design pressure of the vessel was 2·256 × 10⁶ Pa.