敏化处理温度对高氮奥氏体不锈钢显微组织和腐蚀性能的影响

利用OM、SEM及Cu-H_2SO_4-CuSO_4腐蚀试验方法、双环电化学动电位活化法和电化学阻抗谱,研究了不同敏化温度对高氮奥氏体不锈钢显微组织特征和晶间腐蚀的影响。结果表明:敏化处理后试样的显微组织为奥氏体,晶界上有析出物;不同敏化温度奥氏体不锈钢的腐蚀敏感性不同,经过相同条件的固溶处理,800℃敏化处理6 h的试样耐腐蚀性能较差,此高氮奥氏体不锈钢晶间腐蚀敏感温度约为800℃。     

超纯奥氏体不锈钢00Cr25Ni22Mo_2N非敏化态晶间腐蚀与磷(硅)的晶界偏聚

运用金相、电子显微镜观察及表面分析技术,研究了超纯奥氏体不锈钢00Cr25Ni22Mo2N制造的汽提管在尿素生产环境下的腐蚀形态和机理。结果表明,该钢种在生产条件下产生非敏化态晶间腐蚀,其特征是腐蚀介质不仅沿晶界向钢的内部渗入,而且同时向晶界的两侧扩展;晶界区磷的含量(约25%)比晶内高约三个数量级,由此造成的晶界与晶内在腐蚀电解质溶液中的电位差,是导致该钢种非敏化态晶间腐蚀的原因。     

硝酸重沸器的腐蚀失效分析和防护对策

运用金相显微镜、扫描电子显微镜、光电子能谱仪等测试分析手段,研究了304L奥氏体不锈钢制造的硝酸重沸器在生产环境下的腐蚀形态,探讨了腐蚀机理.结果表明:该设备在89℃,55%浓硝酸生产条件下产生了非敏化态晶间腐蚀,其特征是腐蚀介质不但沿晶界向钢的内部渗入,而且同时向晶界的两侧扩展,晶界区磷的含量(约2.6%)比晶内高约...     

热处理温度对晶间腐蚀的影响

晶间腐蚀是沿着晶粒边界发生的选择性腐蚀,也是一种最易受热处理影响的腐蚀类型。发生晶间腐蚀的主要原因是由于晶界贫铬。例如,奥氏体不锈钢在焊接或受到其他热影响而被加热到400～850℃时,铬的碳化物易沿晶界析出,结果造成晶界贫铬。     

1Cr18Ni9Ti薄壁不锈钢管断裂失效分析

对1Cr18Ni9Ti奥氏体不锈钢焊接接头进行SEM、显微组织观察分析和晶间腐蚀倾向分析,研究焊接工艺对奥氏体不锈钢的影响及其发生腐蚀的原因.试验结果表明:断口为脆性断口,主要由"氢脆"和 "疲劳"引起;焊接过程中1Cr18Ni9Ti奥氏体不锈钢由于经受高温,在晶界组织中形成了贫铬区,使得焊接接头的耐蚀性大大下降,产生晶间腐蚀.     

敏化处理对含氮奥氏体不锈钢抗腐蚀性能的影响

通过对含氮奥氏体不锈钢进行晶间腐蚀和硫酸-硫酸铁法腐蚀试验,对不同温度敏化处理和不同含氮量试样进行了耐腐蚀性能研究。结果显示:750℃敏化处理使低氮奥氏体不锈钢中有Cr(C,N)相析出,导致奥氏体稳定性降低,从而诱导δ相的析出。900℃敏化处理时,低氮和高氮奥氏体不锈钢中的析出物比例急剧增加,低氮奥氏体不锈钢析出物呈片状Cr_2N析出,导致晶间惰性元素贫化,使表面组织破坏最严重。     

奥氏体不锈钢敏化动力学过程模拟

奥氏体不锈钢的敏化过程实质上是过饱和奥氏体沿晶界析出碳合化物(Fe、Cr)(23)_Co、同时溶质Cr、Ni和C在基体中重新分布的过程。本文作者对此进行了计算机模拟。模拟中认为;碳化物的析出过程受代位溶质的扩散控制,碳化物和奥氏体的界面达到动态的热力学平衡。在动态提供Fc-Cr-Ni-C多元系相平衡信息的条件下,确定扩散过程的边值;求解溶质在基体中的分布曲线;进而可以预测不锈钢的温度-时间-敏化(TTS)曲线.这一模型在奥氏体不锈钢中进行了具体应用和分析讨论,为不锈钢晶间腐蚀的贫铬理论提供了定量的理论依据。     

尿素级316L不锈钢的非敏化态晶间腐蚀与电位的关系

用恒电位浸蚀方法在高压釜中研究了供货态316 L钢在尿素介质中发生非敏化态晶间腐蚀与电位区段的关系,并在常温硫酸溶液中研究了316 L钢非敏化态晶间腐蚀的发展过程及其与电位值的关系。结果表明,固溶态316 L钢在活化或活化—钝化过渡电位区仅产生全面腐蚀。而在过钝化电位区,既产生全面腐蚀,也产生晶间腐蚀。在过钝化电位区进行恒电位浸蚀时,在等电量条件下,随着浸蚀电位值升高,晶间腐蚀所占比例逐渐减小;在非等电量条件下,随浸蚀时间推移(即随浸蚀电量增加),晶间腐蚀深度和宽度都增加,腐蚀断面形态呈“V”字形。使用高纯不锈钢可以减轻非敏化态晶间腐蚀。     

晶界工程对于改善304奥氏体不锈钢焊接热影响区耐晶间腐蚀性能的影响

通过拉伸变形5%及1100℃退火30 min的晶界工程(GBE)处理工艺,将304奥氏体不锈钢低Σ重合位置点阵(CSL)晶界比例提高到75%(Palumbo-Aust标准)以上,形成大尺寸的"互有Σ3n取向关系晶粒的团簇"显微组织.采用钨极气体保护焊焊接样品,对焊接后样品的HAZ区域进行显微组织表征和耐腐蚀性能测试.结果表明,GBE处理过的304奥氏体不锈钢具有较好的晶界网络稳定性,HAZ区域内仍具有高比例低ΣCSL晶界,并且晶粒尺寸并未明显变大.在晶间腐蚀浸泡实验和电化学动电位再活化法(EPR)测试中,GBE处理的样品HAZ敏化区都表现出了更好的耐腐蚀性能,表明晶界工程可以有效改善304奥氏体不锈钢焊接热影响区耐晶间腐蚀性能.     

固溶处理温度对高硅锰铬镍奥氏体不锈钢晶间腐蚀的影响

利用光学显微镜(OM)、扫描电镜(SEM)及能谱仪(EDS)等对不同温度下固溶处理高硅锰奥氏体不锈钢(UNS S21800钢)的晶间腐蚀进行了研究,分析了固溶处理和固溶+敏化处理两种工艺下进行晶间腐蚀试验后试样的组织形貌及晶间腐蚀倾向。研究结果表明:固溶处理后进行晶间腐蚀的试样中,表面均未发现裂纹,900℃固溶处理试样其晶界处存在较多的第二相,在950℃时第二相颗粒数量明显减少,当温度达到1200℃时,晶界附近已难以观测到第二相颗粒;经固溶处理+675℃敏化处理后进行晶间腐蚀试验的试样中,900℃固溶处理试样表面观测到有晶间裂纹存在,而固溶温度在950℃及其以上温度的试样未出现晶间裂纹。形成晶间裂纹原因是由于900℃固溶处理试样经过敏化处理后晶界处析出了更多的球状和长条状的富Cr碳化物,使得晶界附近区域形成了贫铬区,发生了晶间腐蚀;而950～1200℃固溶处理+敏化处理试样由于在固溶过程中第二相已大量溶入基体,虽然在敏化过程有部分析出,但不足以形成贫铬区,因此在该温度区间内难以发生晶间腐蚀行为。     

固溶态不锈钢在尿素生产介质中的晶间腐蚀与磷(硅)的晶界富集

用扫描电镜、透射电镜能谱和俄歇能谱等研究了尿素厂汽提塔内实际腐蚀部件的腐蚀形态和机理。结果发现:固溶态的316L 不锈钢发生了晶间腐蚀,并伴生全面腐蚀和晶粒脱落,因而晶间腐蚀不深;其晶界区并不贫铬但富集磷、硅,晶界磷含量比钢中平均含量高二个数量级;而同样介质条件下未遭受晶间腐蚀的固溶态00Cr25Ni22Mo2N 高纯不锈钢却未见磷、硅的晶界富集。故认为磷(硅)的晶界富集是固溶态316L 不锈钢在尿素生产介质中产生晶间腐蚀的重要原因之一.     

Surface spreading of intergranular corrosion on stainless steels

The spreading of intergranular corrosion was investigated using micrometer scale simulations and experimental verifications on sensitized stainless steel [UNS S30400]. The degree of sensitization, presence of a pit, and applied potential all affected spreading. The inputs used in the simulation were obtained from Fe-XCr(X = 10, 12, 14, 16 wt.%)-Mo-Ni alloys representing various grain boundary Cr depletion levels. Corroding grain boundaries and pits triggered corrosion of nearby sensitized boundaries due to Ohmic potential drop. Large connected clusters of corroding grain boundaries formed at high fractions of Cr-depleted grain boundaries. The metallurgical, electrochemical and geometric conditions for this behavior could be forecasted.     

EBSD investigation of intergranular corrosion attack on low interstitial stainless steel

Grain boundary effect on corrosion behaviour of low interstitial AISI 316LN austenitic stainless steel after homogenization and heat treatment at 700 and 800°C was investigated by means of Electron Backscattered Diffraction (EBSD) technique. Rapid oxalic acid etch test (ASTM A262‐ practice A) was used to determine steel susceptibility to intergranular corrosion. An attempt to quantify oxalic acid etch test results was also made. Beneficial effect of low carbon and nitrogen content on grain boundary precipitation and corrosion was observed. No changes neither in grain orientations nor in grain boundary types between sensitised and non‐sensitised steel states using orientation measurements were recorded. Twin boundaries were found in all samples with highest amount and only small occurrence changes. CSL categorizations showed only little increase of special boundary fractions with increasing time and temperature of heat treatment. Irrespective to annealing conditions a continued network of random boundaries was retained. Additionally, based on misorientation measurement of 50 grooved grain boundaries, a tendency for preferred attack of high angle boundaries (30–55°) was noted.     

Influence of shot peening and thermal ageing treatment on resistance to intergranular corrosion in shielded metal arc weld metal for type 600 nickel base alloy

Shielded metal arc weld metal for type 600 nickel base alloy (alloy 182) is used for weld components in nuclear power plants. To evaluate the intergranular corrosion resistance of alloy 182 after application of shot peening and subsequent thermal ageing treatment at 593–793 K, we conducted the corrosion test (immersed in boiled 16% sulphuric acid +5.7% copper sulphate aqueous solution at 57.6 ks) using specimens of alloy 182. The results show that the intergranular corrosion resistance of alloy 182 subjected to heat treatment at 893 K for 72 ks was improved by shot peening. Also, the intergranular corrosion resistance was not changed by thermal ageing treatment at 593–793 K subsequent to shot peening. Because remaining chromium depletion layers along grain boundaries were still observed by transmission electron microscope (TEM) after shot peening, disappearance of chromium depletion layers cannot be a factor in the improvement of the intergranular corrosion resistance. The results of measurement of surface residual stress by the X-ray diffraction method show that the compressive residual stress introduced by shot peening still remained on the surface of the specimens. Based on these observations, we assumed that chromium depletion layers along grain boundaries near the surface were dissolved by the environment of the corrosion test, the dissolved regions were closed by the compressive residual stress on the surface, and then the remaining chromium depletion layers were protected from the corrosive environment. This assumption explains why the intergranular corrosion resistance was improved although chromium depletion layers remained.     

Investigation of intergranular corrosion of 316L stainless steel diffusion bonded joint by electrochemical potentiokinetic reactivation

Electrochemical potentiokinetic reactivation technique (EPR) was employed to assess degree of sensitization in 316L stainless steel diffusion bonded joint (DBJ). The result showed the degree of sensitization of DBJ was much smaller than that of base material (BM). No chromium carbides precipitated at grain boundaries in DBJ after 100 h treatment at 650 °C, while chromium carbides could be seen clearly in the BM after 8 h treatment, indicating that DBJ has better intergranular corrosion resistance than BM. Diffusion bonding technique will not increase intergranular corrosion susceptibility of 316L DBJ. Reactivation potential has the biggest effect on sensitization.     

Korrosionselemente durch Gefügeheterogenität in nichtrostenden Chrom-Nickel-Stählen

Corrosion elements set up by structure heterogeneity in stainless chromium-nickel steels Transformation processes and precipitation of carbides could be the reason for corrosion elements in austenitic stainless steels. Examples are the intergranular corrosion and the selective martensite corrosion. The intergranular corrosion is based on a change of the electrochemical properties of the grain boundary regions by impoverishment of chromium. This “classical intergranular corrosion” takes place at all potentials. Resulting from the impoverishment of chromium after colling to very low temperatures, or by cold deformation, an α′-phase can be produced. This phase is only attacked in the active region of the current density-potential-curve. Both the intergranular corrosion and the corrosion of martensite are caused by local anodes. A selective attack of grain boundaries is also observed in the transpassive region. The intensity of this attack increase with precipitation of carbides. During the sensitivisation cathodes are produced, which cause a polarisation in the intergranular corrosive regions. Therefore, this attack of grain boundaries is not only connected with the formation of local anodes but also influenced by the formation of local cathodes.     

晶粒尺寸对奥氏体不锈钢晶间腐蚀敏感性的影响

采用微观形貌观察、电化学方法以及晶界分布特征分析研究316L不锈钢扩散连接接头的晶间腐蚀敏感性,探讨晶粒尺寸和微观结构对不锈钢扩散连接接头敏化的影响。结果表明,扩散连接接头在650 ℃下敏化100 h后晶界处几乎没有发现络合物的析出,而母材经过8 h敏化则析出大量络合物。随着晶粒的增大,316LSS晶间腐蚀敏感性降低。晶粒粗化和低能晶界比例的提高导致扩散连接接头的抗晶间腐蚀能力提高,但晶粒粗化是主要因素。     

Die Bedeutung des Oxalsäure-Tests für die Prüfung der Korrosionsbeständigkeit nichtrostender Stähle

The meaning of the oxalic acid etch test for testing the corrosion resistance of stainless steels In the oxalic acid etch test according to ASTM A 262 practice A, precipitations of phases rich in chromium and molybdenum which can occur in stainless steels, are preferentially dissoved. The behaviour of such phases in the oxalic acid etch test was investigated taking precipitations of carbide M₂₃C₆, s?-phase, χ-phase and Laves-phase in stainless steels AISI 304 L and 316 L as examples. The chemical composition of these was evaluated with a scanning transmission electron microscope (STEM) by EDS. With coarser precipitations, it was possible to support this analytical method by EDS of metallographic cross sections in a scanning electron microscope (SEM). In oxalic acid, critical threshold potentials exist above which the above mentioned phases are preferably attacked, furthermore critical pH values, below which no selective attack of the precipitated carbides and intermetallic phases occurs. The numerical values of the threshold potentials as well as the critical pH values were evaluated. When testing stainless steels in the oxalic acid etch test, the steel specimens are polarized to a highly positive potential in the very trans passive range. In this potential range the corrosion rate of stainless steels increases with increasing chromium content, while in the active and passive range the corrosion rate decreases with increasing chromium content. Other than the nitric-hydrofluoric acid test, the copper-copper sulfate-sulfuric acid test, and the ferric sulfate-sulfuric acid test, the oxalic acid etch test does therefore not indicate any chromium depletion. Hence, an intergranular attack also occurs when precipitations of carbides rich in chromium are present at the grain boundaries of austenitic stainless steels with the carbides being precipitated without any chromium depletion of the areas adjacent to the grain boundaries. Sensitized austenitic stainless steels which are susceptible to intergranular corrosion due to the precipitation of chromium rich carbides and chromium depletion of the areas adjacent to the grain boundaries, can suffer intergranular SCC in high temperature aqueous environments when additionally critical conditions with respect to the mechanical stress level and the oxygen concentration in the environment are given. For the detection of sensitized microstructures, the oxalic acid etch test must be valued critically due to the dependence of the corrosion rate on the chromium content mentioned above, and is obviously by far less suited than the conventional tests for establishing resistance to intergranular corrosion in sulfuric acid-copper sulfate solutions with additions of metallic copper (Strauß test, severe Strauß test).     

元素V对奥氏体焊缝金属晶间腐蚀敏感性的影响

采用透射电镜(TEM)分析技术研究了1Cr19Ni23N焊条熔敷金属中钒含量对组织及对熔敷金属抗晶间腐蚀性能的影响.结果表明,低钒含量焊条熔敷金属组织中沿奥氏体晶界析出M23C6型富铬碳化物,在晶界附近形成贫铬层,导致晶间腐蚀的发生;高钒含量焊条熔敷金属组织中,V优先于Cr与C形成细小弥散的碳化钒分布在晶粒内部,使得该焊条在保持原较高碳含量的基础上,通过改变碳化物形态和分布达到了保持高强度和改善晶间腐蚀敏感性的目的.