Improvements in corrosion resistance,mechanical properties,and weldability of duplex austenitic/ferritic steels

The corrosion behaviour, toughness and weldability of duplex stainless steel can be improved by controlling the composition and austenite: ferrite ratio in the base metal and in the weld deposits. Addition of nitrogen is beneficial to ductility and restistance to pitting corrosion; the latter is also improved by increased chromium contents which stabilize the passive film. This effect is further enhanced by the addition of molybdenum which, however, should not exceed sigma phase at high temperatures. Copper in amounts up to 1.5% improves the resistance to marine environments; however, a certain upper limit should not be exceeded because of adverse effects on hot ductility. In view of the function of nickel concerning the austenite: ferrite ratio (which should be about 50:50) the nickel content should be appropriately selected and should be higher in the filter metal.     

Stainless steel reinforcing bars – reason for their high pitting corrosion resistance

In harsh chloride bearing environments stainless steel reinforcing bars offer excellent corrosion resistance and very long service life for concrete structures, but the high costs limit a more widespread use. Manganese bearing nickel‐free stainless steels could be a cost‐effective alternative. Whereas the corrosion behavior of stainless steels in alkaline solutions, mortar and concrete is quite well established, only little information on the reasons for the high pitting resistance are available. This work reports the results of pitting potential measurements in solutions simulating alkaline and carbonated concrete on black steel, stainless steel DIN 1.4301, duplex steel DIN 1.4462, and nickel‐free stainless steel DIN 1.4456. Duplex and nickel‐free stainless steels are fully resistant even in 4 M NaCl solutions with pH 13 or higher, the lower grade DIN 1.4301 shows a wide scatter between fully resistant and pitting potentials as low as +0.2 V SCE. In carbonated solutions with pH 9 the nickel‐free DIN 1.4456 shows pitting corrosion at chloride concentrations ≥3 M. This ranking of the pitting resistance can be rationalized based on XPS surface analysis results: both the increase of the Cr(III)oxy‐hydroxide and Mo(VI) contents in the passive film and a marked nickel enrichment beneath the film improve the pitting resistance. The duplex DIN 1.4462 shows the highest pitting resistance, which can be attributed to the very high Cr(III)oxy‐hydroxide, to a medium Mo(VI) content in the film and to a nickel enrichment beneath the film. Upon time, the protective properties of the surface film improve. This beneficial effect of ageing (transformation of the passive film to a less Fe²⁺ containing, more hydrated film) will lead to higher pitting potentials. It can be concluded that short‐term solution experiments give conservative results in terms of resistance to chloride‐induced corrosion in reinforced concrete structures.     

The Microstructure and Pitting Resistance of Weld Joints of 2205 Duplex Stainless Steel

2205 duplex stainless steel (DSS) was welded by submerged arc welding. The effects of both heat input and groove type on the ferrite/austenite ratio and elemental diffusion of weld joints were investigated. The relationships among welding joint preparation, ferrite/austenite ratio, elemental diffusion, and pitting corrosion resistance of weld joints were analyzed. When the Ni content of the weld wire deposit was at minimum 2–4% higher than that of 2205 DSS base metal, the desired ratio of ferrite/austenite and elemental partitioning between the austenite and ferrite phases were obtained. While the pitting sensitivity of weld metal was higher than that of base metal, the self-healing capability of the passive film of weld metal was better than that of the base metal when a single V-type groove was used. Furthermore, the heat input should be carefully controlled since pitting corrosion occurred readily in the coarse-grained heat-affected zone near the fusion line of welded joints.     

温度对Cr26Mo1超纯高铬铁素体不锈钢在3.5%NaCl溶液中耐点蚀性能的影响

通过循环极化曲线、Mott-Schottky曲线以及电化学阻抗谱等方法研究了温度对Cr26Mol超纯高铬铁素体不锈钢在3.5%NaCl溶液中耐点蚀性能的影响.结果表明:随着温度升高,Cr26Mol超纯高铬铁素体不锈钢的自腐蚀电位降低,腐蚀电流密度增大,点蚀电位下降,钝化膜阻抗降低.Cr26Mol不锈钢钝化膜的半导体类型和性质在不同温度下发生改变.Cr26Mol不锈钢发生点蚀的孕育期随着温度的升高而缩短,点蚀敏感性增加,已发生点蚀的试样不能够自修复.     

温度对Cr26Mol超纯高铬铁素体不锈钢在3.5%NaCl溶液中耐点蚀性能的影响

通过循环极化曲线、Mott-Schottky曲线以及电化学阻抗谱等方法研究了温度对Cr26Mo1超纯高铬铁素体不锈钢在3.5%NaCl溶液中耐点蚀性能的影响.结果表明:随着温度升高,Cr26Mo1超纯高铬铁素体不锈钢的自腐蚀电位降低,腐蚀电流密度增大,点蚀电位下降,钝化膜阻抗降低.Cr26Mo1不锈钢钝化膜的半导体类型和性质在不同温度下发生改变.Cr26Mo1不锈钢发生点蚀的孕育期随着温度的升高而缩短,点蚀敏感性增加,已发生点蚀的试样不能够自修复.     

真空度对2205双相钢在热浓缩海水中点蚀行为的影响

目的研究真空度对2205双相不锈钢在海水淡化环境中耐点蚀性能的影响。方法在1.5倍人工浓缩海水中,采用循环阳极极化曲线与电化学阻抗谱等电化学方法,研究了2205双相不锈钢的点蚀和再钝化行为,并通过扫描电子显微镜对极化后试样的腐蚀形貌进行分析。结果测试了七种不同真空状态下2205双相不锈钢的循环阳极极化曲线和电化学阻抗谱,发现随着真空度的升高,试样的自腐蚀电位和点蚀电位均不断降低,分别约从-256 m V和605 m V下降到-485 m V和363 m V(均vs.SCE),点蚀倾向明显增大。同时,Nyquist曲线中的半圆弧逐渐变得扁平,Bode图中的相位角约从80°下降到77°,但是点蚀电位与再钝化电位之差逐渐升高。不同真空度下循环阳极极化后,试样表面的点蚀坑形貌不完全相同,蚀坑数量随着真空度的升高而明显减少,当真空度升高为0.72时,点蚀坑尺寸明显减小。结论随着真空度的逐渐升高,不锈钢钝化膜的致密性和保护性降低,电化学阻抗值逐渐减小,耐点蚀性能变差,但是再钝化性能却有所增强。循环阳极极化后试样的腐蚀程度减小。     

Corrosion behaviour of duplex stainless steels sintered in nitrogen

Duplex stainless steels obtained through powder metallurgy (PM) technology from austenitic AISI 316L and ferritic AISI 430L powders were mixed on different amounts to obtain biphasic structures with austenite/ferrite ratio of 50/50, 65/35 and 85/15. Prepared mixes of powders have been compacted at 750 MPa and sintered in N₂-H₂ (95% and 5%) at 1250 °C for 1 h. Corrosion behaviour, using electrochemical techniques such as anodic polarization measurement, cyclic anodic polarization scan and electrochemical potentio-kinetic reactivation test and double loop electrochemical potentio-kinetic reactivation double loop test were evaluated. For duplex stainless steels, when austenite/ferrite ratio increases the corrosion potential shifts to more noble potential and passive current density decreases. The beneficial effect of annealing solution heat treatment on corrosion behaviour was established and was compared with corrosion behaviour of vacuum sintered duplex stainless steels. The results were correlated with the microstructural features.     

Effect of p H on the Electrochemical Behaviour and Passive FilmComposition of 316L Stainless Steel

The effect of pH on the electrochemical behaviour and passive film composition of 316 L stainless steel in alkaline solutions was studied using electrochemical measurements and a surface analysis method. The critical pH of 12.5 was found for the conversion from pitting corrosion to the oxygen evolution reaction(OER). OER was kinetically faster than pitting corrosion when both reactions could occur, and OER could postpone pitting corrosion. This resulted in pitting being initiated during the reversing scan in the cyclic polarization at the critical pH. According to the X-ray photoelectron spectroscopy analysis, the content of Cr and Mo decreased with pH, while Fe content increased. This induced the degradation of the passive film, which resulted in the higher passive current densities under more alkaline conditions. The selective dissolution of Mo at high p H was found, which demonstrated that the addition of Mo in austenitic stainless steels might not be beneficial to the corrosion resistance of 316L in strong alkaline solutions.     

高锰氮双相不锈钢25Cr-2Ni-10Mn-xW-N的组织和性能研究

研究了钨含量对新型高锰氮双相不锈钢25Cr-2Ni-xW-10Mn-N(x=1.5,3.0,4.5)的显微组织、力学性能以及耐腐蚀性能的影响。结果表明:该系不锈钢固溶处理后具有典型的铁素体+奥氏体双相组织,随着固溶温度的增加,铁素体含量上升。随着钨含量的增加,σ相析出增加,铁素体体积分数增大,耐点蚀性增强,屈服强度上升,断裂延伸率和冲击韧性降低。此类钢作为结构材料具有广阔的应用前景。     

Influence of chemical composition on the pitting corrosion resistance of non-standard low-Ni high-Mn-N duplex stainless steels

The pitting corrosion resistance of a new family of duplex stainless steels has been evaluated. These non-standard duplex stainless steels are characterised by low Ni content and high N and Mn levels. Potentiodynamic polarisation scans in NaCl solution have been carried out to determine pitting potentials. A crevice-free cell has been used to perform the electrochemical tests.An exponential equation is obtained in the regression analysis between the pitting potential and chemical composition which allows an estimate of the pitting resistance of these new duplex stainless steels.     

点缺陷模型在2205双相不锈钢中的应用

采用电化学极化曲线和电化学阻抗技术对2205双相不锈钢在0.1%、1.0%及3.5%（质量分数,%）三种不同浓度的NaCl溶液中的腐蚀性能进行测试,采用点缺陷模型(PDM)对测试结果进行建模与分析。研究结果表明,2205双相不锈钢随着溶液浓度的升高抗点蚀能力下降,这是由于在钝化膜的生长过程中,氧离子缺陷产生于金属/膜界面,消耗于膜/溶液界面,而金属离子缺陷产生于膜/溶液界面,消耗于金属/膜界面;氧离子缺陷的迁移导致钝化膜的生长,而金属离子缺陷的迁移使得钝化膜发生溶解。同时,根据PDM模型理论并从金属相角度出发对2205不锈钢建立钝化膜溶解模型,可知2205双相不锈钢奥氏体相γ上的钝化膜可能比铁素体相α优先发生溶解。     

Influence of pH on the electrochemical behaviour of a duplex stainless steel in highly concentrated LiBr solutions

The objective is to study the influence of pH on the corrosion and passive behaviour of duplex stainless steels (DSS) using potentiodynamic measurements, potentiostatic tests and electrochemical impedance spectroscopy (EIS).DSS are spontaneously passive in heavy brine LiBr solutions. Under potentiostatic conditions at applied anodic potentials within the passive domain an equivalent circuit with two time constants is the most suitable model to describe the corrosion mechanism in the interface electrolyte/passive film/metal. pH modifies the electrochemical properties of the passivity of the alloy in a 992 g/L LiBr solution reducing its resistance with the applied potential.     

氮元素对25-6Mo3型双相不锈钢耐孔蚀及缝隙腐蚀性能影响的研究

从孔蚀与缝隙腐蚀两方面考察氮对25-6Mo3型双相不锈钢耐蚀性能的影响。结果表明:随含氮量增加,材料的孔蚀阻力增大,缝隙腐蚀速率下降,且存在关系式:CCT(℃)=17.3+78.4N％(wt)。含氮量越高,氮在钝化膜中的富集程度越大,对于含0.28％N的钢,其最高富集系数可达14;随含氮量增高,孔蚀位置从Y相转移到α相,含0.096％N是孔蚀位置发生转移的分界点。模拟闭塞区试验表明,含氮量越高,闭塞溶液的最终pH值越大,用Nessler特性试剂检验,发现0.28％N钢的闭塞液中存在着NH,从而证实了双相钢中同样存在着氮元素的缓蚀效应。可见氮元素的作用是通过膜内富集,改善γ相耐蚀性能以及与H~+结合形成NH_4~+从而抑制闭塞液酸度变化达到缓蚀效应而得以实现的。     

Effect of W on stress corrosion cracking susceptibility of newly developed Ni-saving duplex stainless steels

Effect of W on stress corrosion cracking behavior (SCC) of Ni-saving duplex stainless steels (Fe18Cr6Mn3Mo0.4N (2.13, 5.27)W, in wt%) was investigated in 4 M NaCl solution using a slow strain rate test method. The change in the W content from 2.13 to 5.27 wt% marginally affected the tensile properties of the investigated DSSs. Alloying W clearly improved the pitting initiation resistance and repassivation tendency of the investigated alloys, but the SCC susceptibility was not remarkably decreased by addition of W. The slight enhancement in the SCC resistance of the alloy containing 5.27 wt% W was revealed to be correlated with the accelerated galvanic corrosion between the ferrite and austenite phases as a result of the W partitioning preferentially into the ferrite phase which could encourage the propagation of pitting.     

Laser Raman microscopic studies of passive films formed on type 316LN stainless steels during pitting in chloride solution

The surface films formed on type 316LN stainless steels (SS) with different nitrogen contents, during potentiodynamic polarization in acidified 1 M NaCl solution, were characterized by Laser Raman Spectroscopy (LRS). LRS confirmed the presence of oxides and oxychlorides of iron and chromium, hydrated chlorides and nitrates in the film. Raman mapping showed increasing nitrate content in the film with increasing nitrogen content. The film on the uncorroded material showed the presence of chromium and molybdenum oxides. The improvement in pitting corrosion resistance of type 316LN SS with increasing nitrogen content was attributed to increased amount of nitrates in the passive film.     

Austenitic-ferritic stainless steels: A state-of-the-art review

Austenitic-ferritic stainless steels, more commonly known as duplex stainless steels, or DSS for short, consist of two basic phases. One is austenite, A, and the other is ferrite, F, present in about equal amounts (but not less than 30% each). The two phases owe their corrosion resistance to the high chromium content. Compared to austenitic stainless steels, ASS, they are stronger (without sacrificing ductility), resist corrosion better, and cost less due to their relatively low nickel content. DSS can be used in an environment where standard ASS are not durable enough, such as chloride solutions (ships, petrochemical plant, etc.). Due to their low nickel content and the presence of nickel, DSS have good weldability. However, they have a limited service temperature range (from −40 to 300°) because heating may cause them to give up objectionable excess phases and lower the threshold of cold brittleness in the heat-affected zone of welded joints. State-of-the art DSS are alloyed with nitrogen to stabilize their austenite, and in this respect the nitrogen does the job of nickel. Also, nitrogen enhances the strength and resistance to pitting and improves the structure of welds. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 20–29, October, 1997.     

Depassivation-repassivation behavior of type-312L stainless steel in NaCl solution investigated by the micro-indentation

Repassivation behavior of type-312L stainless steel containing 6% of molybdenum was examined in NaCl solution using in situ micro-indentation technique, together with type-304 and 316L stainless steels. High stability of the passive film formed on the type-312L stainless steel was also examined by depth profiling analysis of passive films using glow discharge optical emission spectroscopy (GDOES). In 0.9 mol dm⁻³ NaCl solution at 296 K the type-304 and 316L stainless steels are passive only up to 0.3 V (SHE), above which pitting corrosion occurs. In contrast, no pitting corrosion occurs on type-312L stainless steel. Despite the significant difference of the pitting corrosion resistance, the repassivation kinetics of the three stainless steels, examined by micro-indentation at 0.3 V (SHE), is similar. The presence of molybdenum in the stainless steel does not influence the repassivation kinetics. The charge required to repassivate the ruptured type-312L stainless steel surface increases approximately linearly with the potential, even though the passivity-maintaining current increased markedly at potentials close to the transpassive region. Repassivation occurs without accompanying significant dissolution of steel, regardless of the stability of passive state. Depth profiling analyses of the passive films on the type-312L stainless steels formed at several potentials revealed that molybdenum species enrich in the outer layer of the passive film, below which chromium-enriched layer is present. The permeation of chloride ions may be impeded by the outer layer containing molybdate, enhancing the resistance against the localized corrosion of the type-312L stainless steel.     

固溶处理对低镍双相不锈钢耐点蚀及强韧性能的影响

采用“C/N+Mn代Ni”的合金设计方法,制备出了一种具有良好热加工性能的低镍双相不锈钢21Cr-DSS,并研究了固溶处理工艺对新钢种微观组织、强韧性及耐点蚀性能的影响。结果表明:21Cr-DSS在热轧过程中铁素体比奥氏体更容易发生动态软化。固溶温度比保温时间对21Cr-DSS中两相比例的影响更明显,且奥氏体晶粒尺寸稳定性优于铁素体。随固溶温度升高,21Cr-DSS的强韧性得到改善,但当温度超过1100℃后,强韧性下降。在1050℃固溶处理30 min时,21Cr-DSS具有最佳的综合性能,此时强塑积为58.9 GPa%,-40℃低温冲击功为84 J,在3.5%氯化钠溶液中的点蚀电位为0.43V。21Cr-DSS比常规LDX2101具有更优异的热加工性能和强韧性,且耐点蚀性能基本相当。与AISI 304奥氏体不锈钢相比,21Cr-DSS的强度和耐点蚀性能明显更优。     

Microstructure and stress corrosion cracking in simulated heat-affected zones of duplex stainless steels

The effects of nitrogen content and the cooling rate on the reformation of austenite in the Gleeble simulated heat-affected zone (HAZ) of 2205 duplex stainless steels (DSSs) were investigated. The variation of stress corrosion cracking (SCC) behavior in the HAZ of 40 wt% CaCl₂ solution at 100 °C was also studied. Grain boundary austenite (GBA), Widmanstatten austenite (WA), intergranular austenite (IGA) and partially transformed austenite (PTA) were present in the HAZ. The types and amounts of these reformed austenites varied with the cooling rate and nitrogen content in the DSS. U-bend tests revealed that pitting corrosion and selective dissolution might assist the crack initiation, while the types and amounts of reformed austenite in the HAZ affected the mode of crack propagation. The presence of GBA was found to promote the occurrence of intergranular stress corrosion cracking. WA, IGA and PTA were found to exhibit a beneficial effect on SCC resistance by deviating the crack propagation path.     

00Cr24Ni6Mo2N双相不锈钢耐腐蚀性能分析

00Cr24Ni6Mo2N不锈钢是双相不锈钢，在正常情况的组织中有α铁素体相和γ奥氏体相，各占约50％。经分析它的击破电位(Eb)、保护电位(Ep)及室内加速腐蚀试验和海港挂片试验，都证实它具有优良的耐点蚀性能。经测试它同时具有良好的耐应力腐蚀和耐冲刷腐蚀能力。