Sulfide stress cracking susceptibility of nickel containing steels

A systematic evaluation of the sulfide-stress cracking (SSC) behavior of five steels with nickel contents ranging from 0 to 3 pct was conducted in an acidified chloride solution saturated with H₂S at room temperature (NACE solution). All of the steels were low-alloy, structural, or pressure vessel steels that are heat treatable to high strength levels with high toughness. All of the steels were heat treated to yield strength of approximately 690 MPa (100 ksi) and evaluated by identical test methods. The relative cracking susceptibility of the steels was determined from threshold stresses in constant-load tension tests and threshold stress intensities shown by precracked double-cantilever-beam specimens. Tempering treatment was a decisive factor in SSC susceptibility of low-nickel steels. When double tempered, low-Ni steels with greater than 1 pct Ni can be equivalent in SSC resistance to nominally nickel-free 4130 steel.     

Effect of single aging on stress corrosion cracking susceptibility of INCONEL X-750 under PWR conditions

Unfavorable morphology of precipitates and inclusions has been thought to be the cause of severe intergranular stress corrosion cracking (IGSCC) in double aged INCONEL* X-750 alloy used in reactor water environments. A single step aging treatment of 200 hours at 811 °C followed by furnace cooling after solution treating for 2 hours at 1075 °C has been found to provide an improved combination of strength, ductility, and resistance to SCC under simulated PWR test conditions. In this single aged condition a reprecipitated secondary carbide, together with γ′ was produced at the grain boundary which resulted in a mixed fracture mode comprising dimple rupture and microvoid coalescence compared with a predominantly intergranular mode for the fully age hardened specimens. This improvement has been explained in terms of the morphology of the second phase precipitates which are produced in these heat treatment regimes. INCONEL is a trademark of the INCO family of companies.     

The effect of microstructure on the hot salt stress corrosion susceptibility of titanium alloys

It was the purpose of this study to identify what metallurgical processes could be applied to commercial structural titanium alloys to increase the hot salt stress corrosion (HSSC) threshold stress and therefore increase their range of application. Toward this purpose Ti-6A1 and Ti-6A1-4V were evaluated as a function of microstructural variables. Specifically, it was shown that both increasing amounts of cold work and increasing grain size decrease HSSC resistance of Ti-6A1. Also, for Ti-6A1-4V it was shown that preferred orientation can have a profound effect on the HSSC resistance. Crack initiation time, crack growth rate, and stress rupture life were evaluated in Ti-6A1-4V as a function of applied stress at 727 K. These results indicate that HSSC cracking can be described by a critical resolved shear stress criterion, and that increased high temperature creep resistance and decreased room temperature notch rupture strength combine to increase HSSC susceptibility and embrittlement.     

The effect of microstructure on the hot salt stress corrosion susceptibility of titanium alloys

It was the purpose of this study to identify what metallurgical processes could be applied to commercial structural titanium alloys to increase the hot salt stress corrosion (HSSC) threshold stress and therefore increase their range of application. Toward this purpose Ti-6A1 and Ti-6A1-4V were evaluated as a function of microstructural variables. Specifically, it was shown that both increasing amounts of cold work and increasing grain size decrease HSSC resistance of Ti-6A1. Also, for Ti-6A1-4V it was shown that preferred orientation can have a profound effect on the HSSC resistance. Crack initiation time, crack growth rate, and stress rupture life were evaluated in Ti-6A1-4V as a function of applied stress at 727 K. These results indicate that HSSC cracking can be described by a critical resolved shear stress criterion, and that increased high temperature creep resistance and decreased room temperature notch rupture strength combine to increase HSSC susceptibility and embrittlement.     

Effect of methane on sulfur dioxide and sodium sulfate-lnduced corrosion of nickel-base alloys

The corrosion of nickel-base binary alloys, with up to 20 wt pct Cr or Al, by sulfur dioxide contained in a nitrogen gas stream, has been found to be considerably enhanced between 800 and 1000°C in the presence of low concentrations of gaseous methane. The magnitude of the effect, which is related to the ability of the alloy surface to catalyze the decomposition of the hydrocarbon to solid carbon, decreases with increasing Cr and Al content of the alloy. Ni-Al alloys containing less than 10 wt pct Al are more readily attacked by SO₂ alone than are the corresponding Ni-Cr alloys, although the deleterious effects of added methane are more marked in the Ni-Cr series. The extent of high temperature corrosion of these alloys by deposited sodium sulfate at 900 to 1000°C is also generally increased when small amounts of methane are introduced, although in this case the addition of Cr to the alloy confers greater corrosion resistance than does the addition of an equivalent concentration of Al. In the presence of either sodium sulfate or SO₂, the addition of methane to the ambient nitrogen atmosphere results in localized reducing conditions and increased sulfur potentials near the alloy surface.     

Diffusion of sulfur into thoriated nickel

The diffusion of sulfur-35 into thoriated nickel has been studied by means of a surface intensity decrease technique. We have found that the kinetics of diffusion in the temperature range 925° to 1150°C can be expressed in terms of an apparent activation energy of about 38,000 cal/mole; this is markedly lower than the activation energy for lattice diffusion of sulfur-35 into unalloyed nickel, which is about 90,000 cal/mole. Our results support the conclusion that more than one diffusion mechanism is involved, most probably lattice and grain boundary diffusion. R. K. HOTZLER, formerly Instructor, Polytechnic Institute of Brooklyn, Brooklyn, N. Y.     

残余应力对金属材料局部腐蚀行为的影响

基于残余应力测试新方法与先进电化学测试技术的进展,围绕残余应力类型和大小对金属材料点蚀以及应力腐蚀行为的作用机理进行了总结和归纳.研究发现,尽管残余压应力对腐蚀行为的抑制作用得到了大量实验的证实,但是在不同条件下其作用方式以及机理不尽相同,并且与材料的结构特点以及腐蚀产物等密切相关.同时,残余拉应力的作用尚不明确,受到材料类型和其他因素耦合的严重影响.另外,在某些环境下,影响腐蚀行为的关键是残余应力梯度或残余应力的某个临界值.但是对有色金属的研究表明残余拉应力和压应力均会导致基体中位错和微应变等结构缺陷增加,进而促进点蚀敏感性,降低材料服役性能.最后,对目前研究存在的局限进行了讨论和展望.     

残余应力对金属材料局部腐蚀行为的影响

基于残余应力测试新方法与先进电化学测试技术的进展, 围绕残余应力类型和大小对金属材料点蚀以及应力腐蚀行为的作用机理进行了总结和归纳. 研究发现, 尽管残余压应力对腐蚀行为的抑制作用得到了大量实验的证实, 但是在不同条件下其作用方式以及机理不尽相同, 并且与材料的结构特点以及腐蚀产物等密切相关. 同时, 残余拉应力的作用尚不明确, 受到材料类型和其他因素耦合的严重影响. 另外, 在某些环境下, 影响腐蚀行为的关键是残余应力梯度或残余应力的某个临界值. 但是对有色金属的研究表明残余拉应力和压应力均会导致基体中位错和微应变等结构缺陷增加, 进而促进点蚀敏感性, 降低材料服役性能. 最后, 对目前研究存在的局限进行了讨论和展望.      

加载方向对Al—Zn—Mg合金型材应力腐蚀开裂行为的影响

采用恒载荷拉伸应力腐蚀试验和电化学试验研究取向对Al-Zn-Mg合金型材的应力腐蚀(SCC)开裂的影响,腐蚀介质采用质量分数3. 5%的Na Cl溶液,容器温度维持在50±2℃,并通过光学金相显微镜(OM)、扫描电子显微镜(SEM)、电子背散射衍射(EBSD)等研究不同取向试样应力腐蚀前、后的微观形貌.结果表明横向试样在315 h时断裂,而纵向试样在整个加载过程中未发生断裂,纵向试样有更好的抗应力腐蚀开裂性能;纵截面(L-S面)的腐蚀电流密度为0. 980 m A·cm-2,约为横截面(T-S面)的5倍,腐蚀倾向于沿挤压方向发展;相比T-S面,L-S面晶粒间取向差较大,大角度晶界多,容易被腐蚀产生裂纹;在应力腐蚀加载过程中,试样先发生阳极溶解,形成腐蚀坑,聚集的腐蚀产物所产生的楔入力和恒定载荷的共同作用促使裂纹在腐蚀介质中加速扩展,两种取向试样均发生了明显的晶间腐蚀,存在应力腐蚀开裂的倾向.     

加载方向对Al—Zn—Mg合金型材应力腐蚀开裂行为的影响

采用恒载荷拉伸应力腐蚀试验和电化学试验研究取向对Al-Zn-Mg合金型材的应力腐蚀(SCC) 开裂的影响, 腐蚀介质采用质量分数3. 5%的Na Cl溶液, 容器温度维持在50±2℃, 并通过光学金相显微镜(OM)、扫描电子显微镜(SEM)、电子背散射衍射(EBSD) 等研究不同取向试样应力腐蚀前、后的微观形貌.结果表明横向试样在315 h时断裂, 而纵向试样在整个加载过程中未发生断裂, 纵向试样有更好的抗应力腐蚀开裂性能; 纵截面(L-S面) 的腐蚀电流密度为0. 980 m A·cm-2, 约为横截面(T-S面) 的5倍, 腐蚀倾向于沿挤压方向发展; 相比T-S面, L-S面晶粒间取向差较大, 大角度晶界多, 容易被腐蚀产生裂纹; 在应力腐蚀加载过程中, 试样先发生阳极溶解, 形成腐蚀坑, 聚集的腐蚀产物所产生的楔入力和恒定载荷的共同作用促使裂纹在腐蚀介质中加速扩展, 两种取向试样均发生了明显的晶间腐蚀, 存在应力腐蚀开裂的倾向.      

The influence of temper embrittlement on the stress corrosion susceptibility of Fe-3 wt.% Ni alloys

The influence of a temper embrittling heat treatment on the intergranular stress corrosion susceptibility of an Fe-3 wt.% Ni alloy containing minor additions of tin or phosphorus has been investigated. Microanalysis of the grain boundary regions has been carried out using a scanning transmission electron microscope (STEM) fitted with an energy dispersive X-ray spectrometer. Measured changes in stress corrosion cracking behaviour, resulting from embrittlement at 823 K, have been correlated with changes in the electrochemical properties of the grain boundary regions. Crack growth has been interpreted in terms of an anodic dissolution process.     

双级时效对7075合金应力腐蚀性能影响

双级时效处理虽能有效提高7075铝合金抗应力腐蚀开裂(SCC)性能,但同时会导致合金力学性能降低。为了同时提高7075铝合金的拉伸性能和抗SCC性能,并优化双级时效参数,对双级时效处理7075合金进行了正交试验。通过扫描电镜和透射电镜在慢应变速率实验中研究7075合金的SCC行为。结果发现,在130 ℃条件下保温4 h后,在170 ℃条件下保温8 h,合金抗拉强度、伸长率和应力腐蚀指数I_SSRT分别为488 MPa、10.8%和0.095。      

碳含量对C-276合金组织及晶间腐蚀敏感性的影响

选取不同碳含量的C-276合金板材样品,先对样品进行固溶处理,随后在900 ℃下进行不同时间的敏化处理。采用ASTM G28中A法,对固溶及敏化后的C-276样品进行晶间腐蚀敏感性试验,得出C-276材料的晶间腐蚀年腐蚀率随敏化时间的增加而增加,即延长敏化时间会增加C-276合金的晶间腐蚀敏感性。固溶状态下,高碳型C-276合金晶间腐蚀敏感性高于低碳型C-276合金;而在900 ℃敏化30 min以后,低碳型C-276合金呈现出更高的晶间腐蚀敏感性。采用金相显微镜和扫描电镜（SEM）研究碳含量对C-276合金显微组织的影响,固溶态下,低碳型合金晶界和晶粒内部均未发现有明显析出相,而高碳型C-276合金在部分晶界和晶粒内部分布着颗粒状富含Mo的析出相μ相。敏化过程中,无论低碳还是高碳型C-276合金,其析出相均为富含Mo的M6C型碳化物。高碳型C-276合金中,由于μ相对Mo元素起到钉扎作用,导致富含Mo的碳化物析出速度减缓,因此在敏化后,高碳型C-276合金具有更低的晶间腐蚀敏感性。      

Gas phase embrittlement of nickel by sulfur

Nickel and nickel-base alloys are extremely sensitive to embrittlement and enhanced crack propagation due to sulfur, which may contaminate during melting, processing, or service. This paper reports the results of a systematic study of the effect of sulfur partial pressure and exposure temperature on intergranular penetration and tensile embrittlement in Ni270. For exposures between 450 °C and 900 °C, a maximum embrittlement was observed at about 700 °C. Partial pressures of sulfur exceeding about 5 × 10^-8 atm were embrittling. Kinetic measurements, based on the depth of postexposure intergranular fracture, gave an activation energy of 74 kJ/mol in the temperature range 450 °C to 800 °C. This value is considerably less than the value reported for matrix sulfur diffusion and also less than that reported for intergranular oxygen diffusion. Auger spectroscopy confirmed high local concentrations of sulfur at grain boundaries. In only one case, under the most severe sulfur exposure, was indirect evidence of sulfide formation observed; clearly, the embrittlement was a consequence of intergranular diffusion and segregation of elemental sulfur. A simple composite model for the tensile strength and ductility of partially embrittled specimens, in terms of sulfur penetration distance, satisfactorily accounted for the experimental measurements. Formerly with the Materials Engineering Department, Rensselaer Polytechnic Institute.     

The effect of potential and aging on the Pb-assisted stress corrosion cracking susceptibility of alloy 22 gas tungsten arc-welded weldments

The susceptibility of as-received, solutionized, and short-term thermally aged mill-annealed (MA) and gas tungsten arc-welded (GTAW) alloy 22 to Pb-assisted stress corrosion cracking (PbSCC) was evaluated in supersaturated, deaerated, acidic PbCl₂ solutions at 95 °C. Anodic polarization tests in acidic PbCl₂ solutions showed that 16,000 ppm of Pb produced a strong anodic peak and an order of magnitude greater passive current density for both MA and GTAW alloy 22 as compared to pure NaCl solutions. Current spikes were also observed in the anodic polarization plots for the PbCl₂ solutions, suggesting periodic events of passivity breakdown and repassivation. Constant deformation SCC tests were conducted using double U-bend samples of as-received, solutionized, and thermally aged MA and double U-groove welded alloy 22 plates. The results indicate that as-received, solutionized, and thermally aged MA and GTAW alloy 22 were resistant to PbSCC in supersaturated PbCl₂ solutions at 95 °C, pH 0.5, and applied potentials near the anodic peak ranging from −100 to 50 mV_SCE. Enhanced dissolution of alloy 22 was also observed in the crevice region of the double U-bend samples tested in the 16,000 ppm PbCl₂ solutions. This Pb concentration is seven orders of magnitude greater than that found in the anticipated repository environments, and chemical speciation modeling showed that Pb²⁺ is strongly immobilized in J-13 Yucca Mountain waters through the precipitation of PbCO₃ solids. Therefore, although enhanced dissolution of the inner U-bend did occur in our tests, the overall results from this PbSCC investigation suggest that as-fabricated, solutionized, and aged MA and GTAW alloy 22 are resistant to SCC in extremely aggressive, acidic, and supersaturated PbCl₂ solutions at 95 °C. Provided that these high Pb concentrations are not attainable in the anticipated repository environments, alloy 22 is unlikely to be susceptible to SCC, localized corrosion, and enhanced dissolution by the presence of Pb. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

Effect of strain-heat treatment on the tendency for corrosion cracking of pipe steel under stress

Strain-heat treatment conditions were found to affect the overall properties and structural state of 06G1NDB steel, which in turn affect resistance of the steel to stress corrosion cracking. The technique used in this paper enables strain-heat treatment conditions to be ranked by their effect on stress corrosion cracking of type 06G1NDB steel. Type 06G1NDB steel was found to have relatively high resistance to stress corrosion cracking for all strain-heat treatment conditions studied. This arises out of the fact that the chemical composition of the steel and all strain-heat treatment conditions used led to the formation of steel structure with highly disperse grains, high homogeneity, and a lack of banding or well-defined structurally inhomogeneous zones (axial liquation).     

The stress corrosion susceptibility of a quenched and tempered 12 pct crmov martensitic stainless steel

The stress corrosion susceptibility of a martensitic 12 pct Cr 1 pct MoV stainless steel in alkaline chloride solution has been measured as a function of tempering heat treatment. The microstructures produced during tempering have been characterized by transmission electron microscopy and related to measured hardness values. In addition, scanning transmission electron microscopy combined with energy dispersive X-ray microanalysis has allowed the distribution of alloying elements within the microstructure to be examined. Electron energy loss spectroscopy was used to establish fully precipitate compositions, and the microanalysis results have been explained in terms of a diffusion controlled growth of grain boundary precipitates. The overall stress corrosion cracking susceptibility has been correlated with the development of chromium solute depletion profiles about prior austenite grain boundaries.     

pH值对Q235碳钢与304L不锈钢在典型含硫环境中电偶腐蚀行为的影响

采用电化学法和浸泡法研究了Q235碳钢与304L不锈钢在典型含硫环境中的电偶腐蚀行为。采用SEM观察试样表面形貌。结果表明：在实验体系中304L的阴阳极过程均为电化学活化步骤控制;在pH为4和7的实验溶液中,Q235钢的阴阳极过程均受电化学活化控制;而在pH=13.3的实验溶液中,Q235阴极过程受电化学过程控制,阳极过程受离子扩散控制。在三种实验溶液中的电偶腐蚀效应随阴阳极面积比的增大而增大,但电偶电流随阴阳极面积比的变化并不呈现出对数正比规律。随着溶液pH值的升高,Q235钢的电偶腐蚀速率明显减小,但电偶腐蚀效应变化不明显。