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.     

Einfluß des Molybdängehaltes niedriglegierter Stähle auf die Spannungsrißkorrosion in Nitratlösungen unter CERT-Belastung

Effect of molybdenum on the stress corrosion cracking behaviour of low alloy steels in nitrate solutions under slow strain rate testing conditions CERT tests ($ \mathop \varepsilon \limits^. = 10^{ - 6} {\rm s}^{ - {\rm 1}} $) were carried out in NaNO₃ solutions (0.1… 5 mol/l and 25… 90ßC) to determine the critical parameters for stress corrosion cracking. The steels tested were 15 Mo 3, LStE 36 and two laboratory heats with 0.08% C and molybdenum concentrations of 0 and 1%. All cracked specimens were examined metallographically. Different kinds of corrosion attack with and without intergranular features were observed. The critical potential range for intergranular attack is defined by two critical potentials. The negative critical potential is about U_H = ?0.2 V. It is hardly affected by the test parameters and the material. All free corrosion potentials lie in this range. All freely corroding specimens failed by intergranular cracking in 5 M NaNO₃ at 90°C. Only the coarse grained material without Mo showed intergranular cracks at even lower concentrations of NaNO₃. The positive critical potential varied widely over 0 to 0.5 V, depending on the test parameters and the material. Furthermore, in all cases a second critical potential range of intergranular attack was found at about U_H = 0.8 V. The potential range of U_H = 0.5 to 0.8 V for resistance against intergranular attack disappears with increasing temperature, as with the test conditions according to DIN 50 915 in boiling Ca(NO₃)₂ solutions. Furthermore, this was observed for the heat without Mo at already 90°C. With respect to environmental parameters, the effect of NaNO₃ concentration is very small but the effect of temperature is markedly high. The materials can be better characterized by critical temperatures rather than by critical potential ranges. Unified ranking of the materials with respect to their resistance to intergranular attack is not possible because of its dependence on the potential. However, on the basis of the extent of the domains in which the materials are resistant, it is possible to differentiate among the materials. Their resistance to intergranular attack increases in the following order: Heat without Mo → LStE 36 → 15 Mo 3 → heat with 1% Mo. The effect of Mo is explained in terms of its association with high proportions of bainite in the microstructure.     

Interkristalline Korrosionsrisse an einem Wellenkompensator aus nichtrostendem Stahl X6CrNiTi1810 bei Korrosionsbelastung durch Heißdampf/Dampfkondensat

Intergranular corrosion cracks of a X6CrNiTi1810 SS expansion bellow stressed in heated steam/steam condensate at elevated temperature Cracking of a single layer corrugated expansion bellow of titanium-stabilized austenitic chromium nickel stainless steel was caused by corrosion fatigue. The crack starts with an intergranular crack path, and the transgranular residual fracture shows striations. Similar manifestations of cracked surfaces of austenitic CrNi steel which were obtained under cyclic loading in MgCl₂ solution are described in the literature. There are, however, corrosion damages by intergranular crack formation in the non-sensitized microstructure of titanium-stabilized ferritic chromium and austenitic chromium-nickel stainless steels in water at elevated temperature, the causes of which have not been elucidated until now.     

CERT-Untersuchungen an Leitungsrohrstählen über eine Korrosionsgefährdung durch wasserstoffhaltige Gase bei hohen Drücken

CERT tests on line pipe steels for corrosion risks by hydrogen containing gases under high pressure Notched and unnotched round tensile specimens of line pipe steels were subjected to constant extension rate testing in autoclaves. The testing included pulling the specimens at a slow strain rate to fracture as well as slow strain rate cyclic loading. Besides base material, welded joints with varying hardness values ranging up to 400 HV were tested. Testing environments were inert gases (N₂, Ar, air, natural gas), 70 bar H₂ and various mixtures of H₂ and natural gas. In all gases, fracture occurred in the region of highest plastic deformation. When H₂ was present in the environment secondary cracking and fractions of brittle fracture were found on the fracture surface, their extent being reduced by the presence of O₂ and CO. No crack growth occurs in H₂ environment under cyclic loading when there is no plastic deformation. For the range investigated, there is no influence of hardness on the resistance of the materials to cracking, but surface finish and notch acuity exercise a strong influence.     

Stress corrosion cracking at constant load of 304L stainless steel in molten NaCl-CaCl2 at 570°C

The incubation and propagation times of cracks in 304L in molten NaCl-CaCl₂ at 570°C were related to the applied stress value, from creep and creep rate curves. Rest potential versus time curves were recorded simultaneously. The results showed intergranular stress corrosion cracking. When the temperature was kept at 570°C, precipitation of chromium carbide M₂₃C₆ which promoted cracking propagation, was induced. Determination of the crack rate shows that anodic dissolution at the bottom of the cracks is the main process during the stress corrosion crack propagation of 304L stainless steel in the stress range used.     

Effect of temperature on transgranular and intergranular stress corrosion crack velocity of Ag–Au alloys

The effect of temperature on both the transgranular and the intergranular stress corrosion crack velocity of silver–gold alloys in a 1 M KCl solution was studied for temperatures ranging from 25 to 80 °C by means of slow strain rate experiments. At a constant potential and a constant elongation rate, the crack propagation rate was higher the higher the temperature. Transgranular stress corrosion cracking (TGSCC) velocity was found to change with the temperature, as well as with the potential, in the same way as intergranular stress corrosion cracking (IGSCC) velocity. In the region of potentials where the crack velocity was not controlled by ion diffusion in the crack, it was concluded that TGSCC and IGSCC for Ag–Au alloys in KCl solutions were controlled by the same stress corrosion cracking mechanism.     

Inhibition von Niedertemperaturkreisläufen in Kraftwerken mit Lithiumhydroxid

Inhibition of low temperature circulating systems with lithium hydroxide in power stations Corrosion current density measurements of mild steel St 37 in deionized, air-saturated water between 20 and 60°C showed that addition of small quantities of lithium hydroxide (ca. 25 ppm) protects against corrosion attacks similar to pitting corrosion and is found adequate even up to 5 ppm chloride ion concentration. Constant strain rate tests of mild steel St 37 showed no indication of stress corrosion cracking (SCC) at 70°C, up to 500 ppm lithium hydroxide, and 100 ppm chloride ion concentration. Metallographic examination of specimens indicated intergranular stress cracking with cracks of 1–30 m?m depth in environments containing 2%, 4%, and saturated lithium hydroxide (with solid excess salt) at 70°C. Accumulation of lithium hydroxide should therefore be avoided. No SCC was observed in austenitic stainless steel X 5 CrNi 18 9 specimens in the above environments.     

Determination of corrosion‐assisted stress crack growth rate in 304L stainless steel welds

The corrosion assisted stress crack growth rate of 304L stainless steel welds, was determined by the rising displacement test method. The experiments were carried out in a Boiling Water Reactor (BWR) environment, using a refreshed autoclave loop system at 288 °C and 8 MPa of pressure. Pre‐fatigued CT specimens were used and the crack length was monitored by the potential drop technique. After testing, fractographical analysis and measurement of the initial and final crack length were made. The displacement rates of 3.6 and 2.37 μm/h allowed the initiation and propagation of cracks by stress corrosion cracking, and the test results agree with the crack growth rate values determined with another method. The fracture surfaces are evidence of stress corrosion crack propagation with a transgranular appearance in some cases and transgranular with localized areas of intergranular propagation in others.     

The effects of low frequency load cycles on crack initiation in low alloy steels

The stress corrosion behaviour of low alloy steels was investigated with low frequency cyclic loading in 2 M (NH₄)₂CO₃ at 70°C. The tests were carried out using triangular and sawtooth load cycles with a constant critical strain-rate prevailing within periods of increasing stress irrespective of frequency. Since frequency changes with stress amplitude when using equal ?-values, the effects of frequency were also determined. The experiments were concentrated on crack initiation and initial crack propagation. The results show that the threshold stress which causes stress corrosion cracking (SCC) in ammonium carbonate solution is considerably reduced for all steels tested during low-frequency load cycles. In contrast to tests with constant load, propagating stress corrosion cracks can be initiated below the lower yield strength. The effects of stress/strain amplitude and of the magnitude of deformation on crack propagation were determined.     

不同热处理态的304和321奥氏体不锈钢在氯化铵环境中的应力腐蚀行为对比研究

目的 对比研究原始、固溶和敏化态的304和321奥氏体不锈钢在模拟加氢催化氯化铵环境中的应力腐蚀（SCC）行为及机理。方法 将304和321奥氏体不锈钢经过热处理制备成固溶和敏化态试样,采用U形弯试样在模拟加氢催化氯化铵环境中浸泡的应力腐蚀试验方法对其进行研究,通过观察U形弯弧顶的腐蚀形貌和开裂时间,并结合腐蚀及裂纹的SEM照片和电化学测试结果进行分析。结果 原始和固溶状态304不锈钢U形弯试样在氯化铵溶液环境中开裂时间为25 d左右,断口形貌分别为穿晶断口和沿晶断口;敏化态试样18 d后发生开裂,断口形貌为穿晶和沿晶的混合断口。原始和固溶态321不锈钢U形弯试样在该环境中经过39 d均无应力腐蚀裂纹;敏化试样经30 d后产生宏观开裂。电化学测试结果显示,不同热处理态的304不锈钢在氯化铵溶液中均具有明显的点蚀敏感性,321不锈钢在该环境中耐点蚀和应力腐蚀的能力优于304不锈钢。结论 不同状态的304不锈钢在高温氯化铵环境中具有较强的应力腐蚀倾向,特别是敏化态试样;321不锈钢在该环境中的应力腐蚀敏感性相对较小,但敏化处理显著增加了其沿晶应力腐蚀倾向,而固溶态试样具有明显的沿晶腐蚀特征。     

Effect of Strain Rate on Stress Corrosion Cracking of 316L Austenitic Stainless Steel in Boiling MgCl2 Environment

Stress corrosion cracking behaviors of AISI 316L austenitic stainless steel at slow strain rates in two environments of air and MgCl₂ at ambient temperature and 154 °C were investigated. The results revealed that a decrease in strain rate, during testing in boiling MgCl₂ environment, led to a rigorous deterioration of the mechanical properties of the material, causing brittleness of the steel. The results obtained from fractography indicated that the samples tested in air had typical ductile fracture surface appearances, while the fracture surfaces of the samples tested in a corrosive environment showed a combination of intergranular and transgranular fracture modes, having a brittle macroscopic appearance. The transgranular mode became predominant as strain rate decreased. The results suggested that the presence of deformation bands in front of crack tips were responsible for transgranular cracking caused by stress corrosion.     

氯化物溶液中不锈钢腐蚀疲劳裂纹初始萌生的过程机理

研究了动应力（往复弯曲疲劳应力）对氯化物溶液中不锈钢点蚀敏感性的促进作用。 在固溶态３１６Ｌ不锈钢表面点蚀孔中产生了明显的非敏化态晶间腐蚀。在点蚀孔内受腐蚀晶界处孕育 产生了初态腐蚀疲劳裂纹,裂纹由沿晶转变为穿晶扩展,形成主裂纹并继续扩展直至材料失稳断裂。阐 述了在动应力和侵蚀性介质联合信息作用下ＣＦ裂纹萌生的过程机理。论述了动应力－环境介质－材料 特性在多种腐蚀类型交互作用中的贡献和相关性。     

Interkristalline Korrosion ferritischer Chromstähle mit rd. 17 Gew.-% Chrom nach Glühen im Temperaturbereich um 500 °C

Intergranular corrosion of ferritic 17% chromium stainless steels after heat-treatment in the 500 °C temperature range After stabilizing heat-treatment at 750°C, the non-stabilized, ferritic 17% chromium stainless steel Mat.-Nr. 1.4016 (X8Cr17) still contains a sufficient high concentration of carbon dissolved in solid solution, that after heat-treatment in the 500 °C temperature range carbides rich in chromium of the M₂₃C₆-type are precipitated, causing a relatively weak pronounced susceptibility of the steel to intergranular attack. The susceptibility to this type of attack can be detected by testing specimens in the sulfuric acid-copper sulfate-test with increased concentration of sulfuric acid as compared with the DIN standard 50914, followed by metallografic examination of the specimens. The susceptibility to intergranular corrosion of the material investigated occurring after heat-treatment in the low temperature range, which until now is unknown, is described in terms of a Rollason-diagram as it is commonly used for austenitic chromium-nickel stainless steels. As it is to be expected, the stabilized 17% chromium steels Mat. No. 1.4510 (X8CrTi17), 1.4511 (X8CrNb17) and 1.4523 (X8CrMoTi17) are resistant to intergranular corrosion after heat-treatment at low temperatures.     

The initiation of stress corrosion cracks and pits in austenitic Cr-Ni steel in MgCl2 solutions at 40–90°C

The processes of pit and crack initiation have been studied for 18Cr-11Ni type steel in 5 and 35%MgCl₂ solutions at 40, 60 and 90°C using both unstressed test specimens or specimens submitted to uniaxial tensile stress (constant loads equal to 50, 64 and 80% of the tensile strength measured at the test temperature).The results show proportionality between the logarithm of induction time of pitting on both stressed and unstressed specimens and the logarithm of induction time of stress corrosion cracking In a majority of cases (in 5%MgCl₂ at 60–90°C and in 35%MgCl₂ at 40–90°C) the induction time of stress corrosion cracking proved to be longer than that of pitting, but in 35%MgCl₂ at 90°C the opposite was observed. In the last case the stress corrosion cracks originated from a smooth metal surface, whereas in all other cases cracks started from pits. A possible mechanism for these processes is given.     

CORROSION FATIGUE CRACK INITIATION BEHAVIOR OF 316L STAINLESS STEEL IN HANK’S SOLUTION

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Spannungsrißkorrosion Mo-freier und Mo-haltiger Stähle in Calciumnitrat-Lösung und Natronlauge unter zeitlich konstanter Kraft und unter CERT-Bedingungen

Stress corrosion cracking of Mo-free and Mo-bearing steels in solutions of calcium nitrate and sodium hydroxide under constant load and under CERT conditions The threshold stress a which was obtained for a low-alloy steel (0.08% C, 0.01% Mo) under free corrosion conditions and chemi- cal conditions corresponding to DIN 50915 in boiling 60% Ca(NO₃)₂ solution in constant load SCC experiments was con- firmed by electrochemically controlled experiments (potential range 0 to 0.90 V). The susceptibility to SCC is increased with the potential becoming more anodic. However, the result of the corrosion test according to DIN 50 915 is valid also at more anodic potentials, i.e. under more severe corrosion stresses. For the determination of threshold stresses by constant load SCC experiments, the criterion “life time > 1000 hrs” is reliable and should be generally preferred to the criterion “crack length 1 = 0”. In boiling 60Y0 Ca(NO₃)₂ solution, also such steels are suscep- tible to SCC under CERTconditions which, under constant load conditions (σ = const.) are resistant. The mode of mechanical loading highly influences the SCC response. Critical potential ranges of SCC were evaluated by CERT experiments. The steel composition (Mo content) has no effect on the cathodic limiting potentials (EH=0 V), but on the extension of these ranges to the anodic side. Under CERT conditions, the favourable influence of molybde- num on the resistance against SCC is not noticed in boiling 60% Ca(NO₃)₂ solution but under less severe corrosion stresses (55% Ca(NO₃)₂ solution, 75 °C). With the given stress-induced SCC system “teel/Ca(NO₃)₂ solution”, threshold stresses could not be evaluated by interrupted CERT experiments, a method which otherwise was successfully used with strain-induced SCC systems. The critical potential ranges of SCC are in the transpassive ranges of the corrosion rate vs. potential curves obtained with unloaded specimens. Critical potential ranges of susceptibility to SCC were evaluated by CERTexperiments also in boiling 35% NaOH. The potential of highest susceptibility to SCC is always found between Ea = -0.80 and -0.70 V and is not affected by influencing factors of the medium (concentration, temperature), and by the steel composi- tion (Mo content).The unfavourable effect of Mo on the resistance to SCC in NaOH is reflected by the extensionof the potential range of increased susceptibility to SCC to more anodic potentials. The critical potential ranges of SCC are in the transition range from the active to the passive state of the corrosion rate vs. potential curves. The potentials of the highest susceptibility to SCC are identical with the minima of the corrosion rate vs. potential curves which occur after passing the potential range of active corrosion. The anodic metal dissolution at the crack tip which is obviously essential for the SCC system “steel/Ca(NO₃)₂ solution” is of minor importance for the system “steeVNaOH solution”. The results of metallographic investigations of the tested SCC specimens are presented. In distinct potential ranges, the Bainite phase (calcium nitrate solution), and the Bainite or Perlite phase (sodium hydroxide solution) are preferentially dissolved. These corrosion phenomena are stress induced, and are not observed with specimens free from external stresses.     

The influence of applied stress rate on the stress corrosion cracking of 4340 and 3.5NiCrMoV steels in distilled water at 30 °C

Linearly Increasing Stress Tests conducted in 30 °C aerated distilled water using as-quenched 4340 and 3.5NiCrMoV turbine rotor steels indicated that stress corrosion cracking occurred at all applied stress rates for 4340 steel, whilst only at applied stress rates less than or equal to 0.002 MPa s⁻¹ for the turbine rotor steel. The crack velocity increased with increasing applied stress rate for both steels with the maximum crack velocity for 4340 steel corresponded to v_II in fracture mechanics tests in room temperature water. The fracture surface morphology was mixed mode consisting of intergranular and transgranular fracture regions.     

Stress corrosion cracking of armco iron in phosphate solutions

The stress corrosion cracking of Armco iron was examined in 0.033 and 1.0 M sodium orthophosphate solutions of pH 4.0, 7.0, and 9.0 at 25–100°C in constant strain-rate tests. In 1 M solution of pH 4.0 there formed in Armco iron deep intergranular cracks at potentials ? 150 to ? 50 mV (SCE) and anodic currents of 0.4–8 A/m², and also shallow transgranular cracks at ca. ? 250 to ? 50 mV and 1–30 A/m². Intergranular cracking has been associated with the presence of the oxide passivating film of moderate protectiveness and it was ascribed to deterioration of this film on grain boundaries of the metal by segregated admixtures. Transgranular cracking has been associated with the presence of less protective layers and their rupture along deformation bands.     

The initiation of stress corrosion cracks and pits in austenitic CrNi steel in MgCl2 solutions at 40–90°C

The processes of pit and crack initiation have been studied for 18Cr11Ni type steel in 5 and 35%MgCl₂ solutions at 40, 60 and 90°C using both unstressed test specimens or specimens submitted to uniaxial tensile stress (constant loads equal to 50, 64 and 80% of the tensile strength measured at the test temperature).The results show proportionality between the logarithm of induction time of pitting on both stressed and unstressed specimens and the logarithm of induction time of stress corrosion cracking. In a majority of cases (in 5%MgCl₂ at 60–90°C and in 35%MgCl₂ at 40–90°C) the induction time of stress corrosion cracking proved to be longer than that of pitting, but in 35%MgCl₂ at 90°C the opposite was observed. In the last case the stress corrosion cracks originated from a smooth metal surface, whereas in all other cases cracks started from pits. A possible mechanism for these processes is given.     

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.