Spannungsrißkorrosion des Chromstahles X 210 Cr 12 in chloridund sulfathaltigen Lösungen Teil 1: SpRK-Belastungsverhalten

Stress corrosion cracking of the chromium steel X 210 Cr 12 in chloride and sulphate containing solutions Part 1: Influence of loading conditions on SCC – behaviour The stress corrosion cracking behaviour of the cold work steel X 210 Cr 12 with both martensitic and pearlitic structure has been investigated in distilled water as well as in aqueous chloride and sulphate solutions at room temperature by means of a fracture mechanics method. This steel is found to be sensitive to stress corrosion cracking and this behaviour points to strain induced SCC. The crack propagation is caused by hydrogen embrittlement by interaction of the corrosive produced hydrogen at the crack tip with material straining. Abnormal crack propagation can be caused by both large inhomogeneities of the structure and strain rates below threshold values. For realisation of a demanded strain rate it is necessary to use different loading conditions as follows: The CR-mode (Constant Displacement Rate) with a sufficiently high strain rate is necessary for the determination of K_ISCC-value as well as the region I of the crack propagation curve. On the other hand, the CL-mode (Constant Load) can lead in the region I to abnormal crack propagation with temporary crack stop (“resting effect”) because of too low strain rate. However, the CL-mode with increasing K-value is required to determine the plateau of crack propagation rate in region II as well as to the transition into the region III of instable cracking. The use of the CR-mode in region II would lead to a lower plateau of crack rate combined with discontinuous crack growth. The CD-mode (Constant Displacement) with strain rate of about zero is not applicable to determine the plateau value as well as the threshold value K_ISCC.     

Crack growth behaviour and crack tip chemistry of X70 pipeline steel in near-neutral pH environment

In this paper, crack growth behaviour and the crack tip chemistry of X70 pipeline steel in the near-neutral pH environment were investigated using the in-situ measurements. The stress corrosion crack propagated forward under the cyclic load, with a mean crack growth rate (CGR) of 4.28?×?10^–3?mm/cycle. The CGR da/dN obeys well with the stress corrosion fatigue mechanism. During the crack propagation, the crack tip environment differed significantly from the bulk solution. An aggressive electrolyte with low pH (about 4.0) and high concentration of Cl^– (about 3.8?M) was produced near the crack tip. These results provided the direct evidence for simulating the crack tip solution and investigating the crack tip anodic dissolution.     

Crack tip chemistry and electrochemistry of environmental cracks in AA 7050

The role of the crack environment in establishing environment-assisted crack (EAC) propagation in AA 7050 alloys is elucidated. A suite of mini-electrodes provided real-time in situ measurements of the crack potential, pH, and chloride concentration during stage II cracking in a chromate-chloride electrolyte under electrochemical control. For material aged to an EAC-susceptible condition, crack growth during an incubation period is characterized by tip polarization to near the applied electrode potential (E_App) and bulk-like chemistry near the crack tip. In contrast, establishment of high-rate crack growth coincided with the development of an acidic, high chloride concentration tip environment and tip depolarization. During steady state high rate crack growth, the tip potential was ∼−0.85V_SCE; near-tip potential gradients were ∼1 V/cm. Large ohmic potential drop within fast-growing cracks is indicative of net anodic current in the near tip region and increased mass transport resistance within the crack due to solid corrosion products and/or hydrogen bubble formation. Microinjection of a corrosion-inhibiting or corrosion-promoting solution at the tip suppresses or prompts, respectively, the transition from incubation to high-rate cracking, highlighting the intimate dependence of the crack growth kinetics on the local chemistry. The exceptional EAC resistance of over-aged AA 7050 is intrinsic; injection of an acidic aluminum chloride solution at the tip of a crack of this material while polarized to a high E_App failed to induce brittle crack advance.     

Untersuchungen zur transkristallinen und interkristallinen Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle in heißen Magnesiumchlorid-Lösungen

Investigations Into transgranular and intergranular stress corrosion cracking of austenitic stainless steels In hot magnesium chloride solutions The stress corrosion cracking (SCC) of austenitic stainless steels in hot magnesium chloride solutions is known to be transgranular. Therefore the slip-step-dissolution model is most favourable when explaining the failure mechanism. Constant load and constant extension rate tests (CERT) show that both methodes are almost equivalent. Moreover constant extension rate tests in more concentrated magnesium chloride solutions at 135°C reveal a small potential range of intergranular stress corrosion cracking more negative than the range of transgranular SCC. Observations of crack nucleation and crack propagation make plain that crack nucleation is a localized corrosion process. Pitting produces crack nucleis in the elastic range whereas cracks start along slip lines after plastic deformation. Fractography of specimens which failed by intergranular and transgranular SCC show macroscopically brittle fracture surfaces. Therefore a model is proposed which explains crack propagation by hydrogen-induced intermitted cracking at high-stressed sites at the crack tip.     

Über die korrosive Wirkung einiger einfacher Salze im Salzsprühtest auf Stahl

Corrosive effect on steel of some simple salts during the salt spray test The authors using a salt spray cabinet according to an ASTM standard have investigated the corrosion of steel St 37 by distilled water and 0.05 N solutions of sodium fluoride, chloride, bromide, iodide, sulfate and phosphate. According to the results obtained the corrosion rates by sodium chloride, bromide, iodide and sulfate are practically identical (within the limits of the experimental error) with a mean value of 14.0 ± 0.2 g/m² · day in the period from the third week to the end of the tests (84 days). The different overall losses found by the end of the tests can be attributed to differences in the behaviour during the initial period of the tests. From among the halides only fluoride attacks at a corrosion rate which – though also constant – is rather low (3.4 g/m² · day). The corrosion rate in distilled water is characterized by an asymptotic approach to zero. In the case of sodium phosphate the overall corrosion is negligible because of the inhibiting effect of this salt.     

Chloride induced pitting initiation on 304L stainless steel in acidic sodium sulphate solutions

Abstract

The kinetics of pit initiation on 304L austenitic stainless steel in aqueous sodium sulphate solutions containing various concentrations of sodium chloride have been investigated by induction time measurements using the potentiostatic test method. Oxide films were grown on the surface of the steel by potentiostatic oxidation in the passive region. Pitting induction time has been found to be dependent on the oxide film thickness: the thicker the oxide film, the longer the induction time. Measurements at temperatures rangingfrom 20 to 80°C gave an apparent activation energy for pitting initiation of ~ 48·1 kJ mol^?1, which suggests that the mechanism of single ion transport through the film is the rate determining step.     

Stress corrosion crack propagation: A constant charge criterion

An analysis of metallurgical and electrochemical events at the tip of a propagating stress corrosion crack is presented. The condition for crack propagation is that the creep strain-rate prevents repassivation for a period of time during which it is hypothesized that a change, Q_mln, passes which initiates a further increment of crack growth. The analysis is restricted to those alloys in which crack propagation occurs by anodic dissolution. Factors determining both regions I and II cracking are discussed, including solution viscosity, potential and strength level.     

Mechanismus des Rißwachstums bei der Spannungsrißkorrosion des austenitischen Stahls X6 CrNiTi 18 10 in wäßriger Chloridlösung bei erhöhten Temperaturen

SCC crack growth mechanism of austenitic stainless steel X6 CrNiTi 18 10 in aqueous chloride solution at elevated temperatures The SCC crack growth mechanism of steam generator heat transfer tubes from stainless steel X6 CrNiTi 18 10 under internal stress conditions at elevated temperatures is discussed. Based on crack tip characterization by means of Scanning and Transmission Electron Microscopy and the evidence of hydrogen originated throughout the corrosion process a crack propagation model is presented. The results refer to a microcrack induced gradual crack growth caused by local hydrogen embrittlement. Microcrack growth has been observed due to slip-band decohesion. The crack growth rate is mainly influenced by the stress state near the crack tip and the hydrogen evolution throughout the corrosion process.     

The effect of strain-rate upon stress corrosion crack velocity in α-brass in ammoniacal solutions

Stress corrosion crack velocity in a-brass has been measured as a function of tensile crosshead speed and solution composition in aqueous solutions, mainly at pH 6.8. At low crosshead speeds the velocity was constant but at crosshead speed values high enough to cause general yielding the velocity increased with crosshead speed in a double logarithmic relationship. The maximum velocity producing 100 % intergranular fracture was ca. 30 mm/h. Potentiostatic experiments indicated an increase in crack velocity with anodic polarization but only within the potential/pH region where Cu₂O was stable. At more noble potentials the crack velocity was lower, an effect seen also with cathodic polarization and with the addition of chloride and bromide ions, and in all cases the fracture was partly transgranular. The important factor controlling both the type and amount of stress corrosion cracking is the ratio of tarnish film formation rate/crack tip strain-rate.     

Die Ausbreitung von Spannungskorrosionsrissen in der Aluminium-Legierung AlZnMg 3

Crack propagation during stress corrosion cracking of AlZnMg 3 alloy The authors have studied crack propagation in notched specimens exposed to sodium chloride solution at 30°C under constant load. Crack propagation have been followed by microphotography which has enabled gliding phenomena within the specimen to be followed, too. Starting from the rest potential and shifting it toward more cathodic values crack propagation is at first slowed down which is a type of cathodic protection, but afterwards – in the hydrogen evolution region with simultaneous cathodic corrosion – the crack propagation velocity increases. This fact can be attributed to hydrogen embrittlement. Taking into account that the industrial alloy is more resistant than the high purity alloy one may arrive at the conclusion that the impurity content acts as a decelerating factor giving rise to an extension of the incubation period.     

Untersuchungen zum Einfluß von Sulfationen auf die chloridinduzierte lokale Korrosion am Stahl 1.4541 bei Temperaturen bis zu 280°C und gleichzeitiger Werkstoffdehnung

The influence of sulphate ions on stress corrosion cracking for stainless steel AlSl 321 with constant extension rate in aqueous chloride solution at temperatures up to 280°C CERT (Constant extension rate test: ? = 3·10 ⁶ s ¹) was used to study the inhibition effect of sulphate ions on stress corrosion cracking. Smooth tensile round specimens (AlSl 321) have been tested in solutions with sulphate (10 ²m Na₂SO₄) and chloride ions (10^?3m NaCl. 5. 10 ³m NaCl) at 150°C and 280°C. The presence of sulphate in chloride solutions increases the time to fracture and the reduction of area in comparison to pure chloride solution. The reason is the lower concentration of corrosive hydrogen near the fracture surface in comparison with tests in chloride solutions. The stress corrosion cracking is completely inhibited by the ratio of 10:1 for sulphate and chloride ions in the solution. The fracture surface investigated by Scanning Electron Microscopy has shown a cleavage type of fracture in chloride solutions and a ductile fracture in sulphate/chloride mixtures. The diffusivity of corrosive hydrogen is increased at 280°C in comparison to 150°C. Therefore at 280°C the corrosive hydrogen is able to diffuse into the inner part of the specimen and to influence the fracture mode.     

新型含钪Al-Mg-Cu合金的抗应力腐蚀开裂特性

对3.5%NaCl溶液中新型含钪Al-Mg-Cu合金的应力腐蚀开裂宏观性能进行测试,并对裂纹尖端的成分与微观形貌进行分析。根据线弹性断裂力学理论,预制疲劳裂纹试样裂纹尖端处于平面应变状态,得到裂纹匀速扩展时的扩展速率、裂纹尖端应力强度因子以及应力腐蚀开裂强度因子的门槛值。扫描电镜及EDS分析表明:应力腐蚀开裂主要是沿晶扩展,预制裂纹与腐蚀介质中的溶解氧生成Al2O3,产生楔入力促使裂纹扩展;裂纹尖端基体主要发生阳极溶解反应,腐蚀产物以氯化铝为主。     

The role of the tarnish film in the stress corrosion crack propagation process in α-brass in neutral ammoniacal solutions

In solutions of pH 6.8 containing 1.5M ammonia and 0.04M total copper, stress corrosion crack arrest can occur in α-brass specimens because of tarnish film formation. Removal of the film by inhibited acid restores crack propagation in the same solution. Alterations in the ammonia and copper content and solution pH all lead to results that emphasise the protective role of the tarnish film. Various proportions of transgranular cracking were seen in all the solutions employed. In high K regions of the fracture (111) planes were identified and in low K regions planes mutually inclined at 90° were observed. The important factor determining both crack velocity and crack arrest is the ratio R consisting of the tarnish film formation rate/crack tip strain-rate.     

不同晶粒尺寸5083铝合金晶间腐蚀和应力腐蚀开裂的差异性

通过冷轧和退火获得具有不同晶粒尺寸(8.7～79.2μm)的5083铝合金板.研究其微观结构、晶间腐蚀(IGC)、应力腐蚀开裂(SCC)和裂纹扩展行为.结果表明,粗晶粒样品表现出更好的抗IGC性能,其腐蚀深度为15μm.慢应变速率测试结果表明,细晶粒样品表现出更好的抗SCC性能,敏感性指数ISSRT为11.2％.此外,...     

About the Authors

Abstract

Anisotropy in stress corrosion cracking of lean grade UNS S32101 and standard grade UNS S32205 hot rolled duplex stainless steels was evaluated in this study. Microstructures were characterised with stereology methods in three orthogonal orientations to quantify anisotropy in phase distribution. Constant extension rate tests were conducted in an autoclave at 2×10^?6 s^?1 in chloride containing alkaline sulphide solution at 170°C. The maximum crack lengths and average crack densities were measured for the rolling longitudinal and transverse longitudinal orientations after stress corrosion cracking tests. Results indicate that stress corrosion crack initiation and growth are a strong function of the microstructure in each orientation. Maximum crack length and average crack density were maximum for the rolling longitudinal orientation. Crack initiation and growth were favoured along the transverse direction. Chlorides enhanced anisotropy in crack growth behaviour by facilitating crack initiation and crack coalescence along phase boundaries, which lead to more severe stress corrosion cracking in the alkaline sulphide environments that were studied.     

Einfluß der Mikrostruktur und des Elektroden-potentials auf die Ausbreitungsgeschwindigkeit von Spannungskorrosionsrissen einer reinen AlZnMg3-Legierung in 1 M NaCl-Lösung

Influence of microstructure and electrode potential upon the velocity of stress corrosion cracks of a pure AlZnMg3-alloy in an 1 M NaCI-solution The influence of microstructure and electrode potential upon the stress corrosion crack velocity in a fully aged pure aluminium-zincmagnesium 3-alloy with two step ageing and quench interruption treatments was investigated in airsaturated 1 M sodium chloride solution at 303 K. For this, propagation of the crack tip was observed microphotographically under potentiostatic conditions and under constant load using specimens precracked by fatigue. The crack propagation at the corrosion potential is accelerated by denser matrix-precipitates, narrower precipitate free zone, the presence of great T-grain boundary precipitates and minor sensitivity to intercrystalline corrosion and occurs to judge from the look of the fracture surfaces without considerable metal dissolution. In contrast to the alloy, which shows no intercrystalline corrosion, the crack propagation in the alloy with intercrystalline corrosion is obviously faster above the breakdown potential than at the corrosion potential. The results suggest that the crack propagates essentially mechanically. As mechanisms are discussed a crack growth by anodic dissolution of grain boundary precipitates or of zinc- and magnesium-rich grain boundary regions and mechanical seperation of the intermediate regions and propagation by local hydrogen embrittlement of the crack tip.     

Formulating stress corrosion cracking growth rates by combination of crack tip mechanics and crack tip oxidation kinetics

A theoretical equation for stress corrosion crack growth rate of austenitic alloys in high temperature water is reformulated based on crack tip asymptotic fields and crack tip transient oxidation kinetics. A general oxidation kinetic law is introduced, emphasizing the role of mass transport through solid oxide film at the crack tip. The effects of several parameters on crack growth rate are evaluated. The results are compared with available experimental data and other equations. A good prediction of the effect of K on stress corrosion cracking growth rate of typical austenitic alloys in simulated light water reactor environments has been achieved.     

Untersuchungen der Spannungsrißkorrosion austenitischer Stähle durch saure Chloridlösungen bei Niedrigen Temperaturen

Investigation of stress corrosion cracking of austenitic steels in acid chloride solutions at low temperatures Tests were carried out on materials 1.4301, 1.4571, 1.4439 and 1.4558 in cold hydrocloric acid (c(Cl^?) = 1.5 mol/L and c(H⁺) = 1 mol/L, a few tests also at 0.1 and 0.01 mol/L). Chronopotentiostatic tests yielded data on active corrosion, passivity and pitting corrosion. CERT tests (10^?6s^?1, a few tests also at 2 · 10^?7s^?1) showed superposition of general corrosion on stress corrosion under free corrosion condition, while the rest potential was relatively negative in the active range. Oxygen purging has only a minor effect. The extent of cracking decreases with decreasing c(H⁺). In the case of cathodic polarisation straining induced surface notches occur which can be attributed to hydrogen induced effects. In the case of anodic polarisation pittings are generated without any crack initiation. Characteristic features of stress corrosion increase with decreasing strain rate. It follows from the results that high acid concentrations are necessary for stress corrosion cracking in the active state to occur. CERT tests cannot be used as an accelerated test for this kind of stress corrosion cracking. Few CERT tests carried out in warm NaCl solution did not show any stress corrosion cracking though these solutions are known to cause stress corrosion cracking after long periods. Furthermore, predamage in the form of pitting does not alter the situation.     

Untersuchungen über den Ablauf der Spannungsrißkorrosion an austenitischen Chrom-Nickel-Stählen in Magnesiumchloridlösung Mitteilung aus dem Forschungsinstitut der Mannesmann AG

Investigation into the cinetics of stress corrosion cracking of austenitic chromium-nickel steels in magnesium chloride solution Experimental investigation with the steels (German designation) W.-Nr. 4301 (X5CrNi18 9), 4401 (X5CrNiMo18 10), 4449 (X5CrNiMo17 13), 4541 (X10CrNiTi18 9), 4550 (X10CrNiNb18 9) and 4580 (X10CrNiMoNb18 10) in 42% MgCl₂ solution at 144 and 130°C. The known distribution of stress corrosion cracking between an incubation and a crack propagation period has been confirmed (the first taking about 10% of the whole specimen life). The cracking rate is proportional to the potential at the base of the crack which, in turn, depends from the surface potential. Nb and Ti have no bearing on the cracking rate. Addition of Mo displaces to more noble potentials the potential limit below which there is no stress corrosion cracking and reduces cracking rates when differences between the potential limit and a given potential are equal. The influence of temperature is restricted to the crack propagation period. A co-action of increasing Ni contents in connection with the Mo addition cannot be excluded.