A comparative H2S corrosion study of 304L and 316L stainless steels in acidic media

H₂S corrosion of 304L and 316L in oxygen-free Na₂SO₄ + Na₂S solution at pH 3 and temperature of 60 °C were investigated by EIS, potentiodynamic polarisation, multi-component Pourbaix diagrams and microstructure characterization. At similar conditions, lower corrosion rate was observed on 316L, attributed to its denser (1.5 times) and smoother (6%) surface layer and confirmed by SEM micrograph. During polarisation, H₂S increases significantly the critical current density on 304L and passivation current density, i_p, on 316L. Higher i_p on 316L was associated to simultaneous FeS₂–MoS₂ preservation, confirmed by XRD examination. H₂S could have an inhibiting effect on 304L in passivity region.     

真空低压渗碳对304与316L奥氏体不锈钢组织和性能的影响

采用真空低压渗碳技术对304和316L奥氏体不锈钢进行表面强化,利用光学显微镜、扫描电镜、Thermo-Calc热力学软件、X射线衍射仪和显微硬度计等对渗碳层显微组织、相组成及硬度分布进行分析表征,计算了奥氏体不锈钢渗碳层中不同衍射峰的偏移量及渗碳前后晶格常数的变化量。结合钼对奥氏体不锈钢渗碳过程的影响,对比研究了304和316L奥氏体不锈钢渗碳后,在渗碳层深度、表面硬度及碳化物的析出规律等方面的差异。结果表明,经750 ℃真空渗碳2.6 h后,304和316L奥氏体不锈钢晶格常数分别增加了1.33%和1.14%,形成了由膨胀奥氏体和Cr₂₃C₆组成的渗碳层,Cr₂₃C₆在渗碳层中主要以条状沿膨胀奥氏体晶界析出,表面硬度较基体硬度均提升了两倍以上。     

316L奥氏体不锈钢应力腐蚀裂纹的形成机制

利用EBSD分析了316L奥氏体不锈钢应力腐蚀穿晶裂纹的形成机制。结果表明,裂纹扩展受晶体滑移性能影响,Schmid因子小的晶粒抵制应力腐蚀裂纹扩展,Schmid因子大的晶粒有利于裂纹扩展。裂纹形核和扩展涉及晶体滑移、钝化膜破裂和阳极溶解,这些过程均在{111}滑移面上进行。在高温低应力和含氯离子的腐蚀环境的交互作用下,应力腐蚀的裂纹扩展过程符合滑移溶解机制。     

In-service stress corrosion cracking of AISI 316L stainless steel in an H2S environment

Since publication of ANSI/NACE MR0175/ISO 15156 in 2003 there has been much debate on the performance of austenitic stainless steels in oil and gas production environments, with researchers recommending relaxation of the ISO 15156 restrictions for this material. This paper describes a recent austenitic stainless steel stress corrosion cracking failure and discusses its implications for the current restrictions of the ISO standard and recently proposed relaxations of these restrictions.     

Super304H奥氏体不锈钢的高温抗氧化性能

对Super304H奥氏体不锈钢在550～800℃进行高温氧化试验,结合氧化动力学规律去研究Super304H奥氏体不锈钢的氧化机理。结果表明,Super304H奥氏体不锈钢在550～800℃氧化质量增加曲线遵循抛物线规律,在750～800℃时60 h以内氧化质量增加趋势最明显,100 h后质量增加高达0.005 mg·mm^-2。在550～750℃逐渐生成致密的氧化膜,主要由Cr₂O₃和Fe_3-xCr_xNiO₄混合氧化物和少量CuCrMnO₄构成。升高温度会促进Cr的选择性氧化,使得Cr₂O₃保护膜开裂,800℃时暴露出的Fe基体与氧原子反应生成瘤状Fe₃O₄,氧化膜厚重并伴有剥落现象。应变速率为3.2×10^-4 s^-1时,不锈钢的抗拉强度随氧化温度升高而降低,600℃的抗拉强度最大,达350 MPa; ...     

Stress corrosion cracking of cold-worked austenitic stainless steels

Stress corrosion cracking (SCC) of AISI 304L and AISI 316L stainless steels, cold-worked under various conditions (i.e. at different degrees of deformation obtained by drawing and rolling at room temperature and at liquid nitrogen temperature) has been carried out in H₂O containing 1000 ppm Cl^? at 250°C and in a boiling MgCl₂ solution. The effect of heat treatments at 400 and 900°C on the SCC of previously cold-worked steels has also been studied. Particular attention was directed towards heat treatment at 400°C. In steels deformed at room temperature, it increases the SCC resistance. By contrast, for steels deformed at liquid nitrogen temperature, heat treatment at 400°C reduces the SCC resistance if carried out for short periods of time (1–6 h). Hardness measurements, structural analyses via X-rays, scanning and transmission electron microscopy (SEM and TEM), as well as modified Strauss tests, seem to prove that reduced stress corrosion resistance is not to be related to the chromium-rich carbides precipitation which could have been accelerated by the presence of α′-martensite. Instead, they tend to suggest that perhaps this phenomenon is connected to an increase in the level of internal micro-stresses which are generated by a reciprocal re-ordering of the α′ and γ structural phases.     

Electrolyte and temperature effects on pitting corrosion of type 316LN stainless steels

Abstract

The influence of electrolyte composition and temperature on the pitting corrosion resistance of nitrogen bearing (0·015, 0·198, and 0·56%N) type 316L stainless steels has been investi gated. Anodic polarisation curves were determined in neutral chloride solution at room temperature and at elevated temperatures of 308, 318, and 333 K. Similar polarisation studies were also conducted at room temperature in 1N H₂ SO₄ and in an acidic chloride solution containing 1N H₂ SO₄ and 0·5M NaCl. The results show that the critical pitting potential E_pp in neutral chloride and acidic chloride media increases as the nitrogen content of the alloy increases, indicating that resistance to pitting increases with the addition of nitrogen. However, in 1N H₂ SO₄ solution, the transpassive potential was almost independent of the nitrogen content. As the temperature of the neutral chloride medium was increased, the E_pp values decreased, irrespective of nitrogen content. The decrease in E_pp is attributed not only to the temperature induced modification of the passive oxide film but also to chloride induced activity at the passive film/solution interface. Nevertheless, the alloy con taining 0·56% nitrogen showed better resistance to pitting at 333 K than did the alloy with 0·015% nitrogen at room temperature. SEM examination of the pitted specimens showed clear evidence of pitting for the alloy with 0·015% nitrogen, but insignificant pitting attack for the alloy with 0·56%N. However, the alloy with 0·56%N displayed some pitting attack when the experiments were conducted at 333 K.     

Fatigue behaviour of low temperature carburised AISI 316L austenitic stainless steel

Low temperature carburising (LTC) was applied to AISI316L austenitic stainless steel and its effect on microstructure and fatigue behaviour was investigated. LTC treatment enhances surface hardness and wear resistance of the steel without reducing its corrosion resistance. Surface hardness up to 1150 Vickers was achieved in the carburised layer, thanks to the formation of the so-called “S-phase”, a carbon-supersaturated austenite phase. The XRD evaluation of treated material verified expanded austenite with no evidence of carbide precipitation. Rotating bending fatigue tests showed that the low temperature carburising treatment enhances the fatigue strength of the 316L steel by 40% with respect to the untreated material due to the high residual stresses present in the treated layer. A major temperature increase was found testing the LTC specimens, with a peak value at the end of the test up to 600 °C. By air cooling the LTC specimens during the tests, a further increase of fatigue strength up to 70% was achieved with respect to the untreated material. Fatigue cracks in the surface-treated specimens always nucleated near the boundary between the carburised case and the core.     

Characterization and perspective of stress corrosion cracking of austenitic stainless steels (type 304 and type 316) in acid solutions using constant load method

The stress corrosion cracking (SCC) of the commercial austenitic stainless steels, type 304 and type 316 has been extensively investigated as functions of applied stress, sensitizing temperature, sensitizing time and the environmental factors such as pH, anion concentration, anion species (chloride ions and sulfate ions), test temperature, applied potential and inhibitor concentrations of chromate and molybdate by using a constant load method. We have found that the steady state elongation rate obtained from corrosion elongation curve becomes a relevant parameter for predicting time to failure and also for criterion on assessment of whether SCC takes place or not. The value of t_ss/t_f is also found to become an indicator for assessment of whether SCC takes place or not. Furthermore, from the results obtained, it is deduced that a unified SCC mechanism is qualitatively proposed to explain both of transgranular SCC (TGSCC) and intergranular SCC (IGSCC), where the unified SCC mechanism is basically based on a film rupture- formation event at crack tips.     

Role of thiosulfate in susceptibility of AISI 316L austenitic stainless steels to pitting corrosion in 3.5% sodium chloride solutions

The susceptibility of AISI (American iron and steel institute) 316L austenitic stainless steel alloy to pitting corrosion was assessed in 3.5% chloride solutions containing various concentration of thiosulfate ions, a main sulfide oxidant product, spanning across values of 0.001, 0.005, 0.01, 0.05 and 0.1 M, at temperatures of 23, 50 and 80°C. The potentiodynamic scan results indicated that low thiosulfate concentrations promote the chloride attack and the aggressiveness of thiosulfate species depends on the chloride to thiosulfate ratio and the test temperature. Increasing temperature apparently promotes the ionic activity of Cl^– and S₂O₃^2– The thiosulfate to chloride ratio plays an essential role in pitting the intensity of the AISI 316L stainless steel alloy and was found to be dependent on the test temperature.     

High-nitrogen austenitic stainless steels with high strength and corrosion resistance

An alloy design model has been used to develop an austenitic stainless steel containing 25–28 wt.% Cr, 22 wt.% Ni,6 wt.% Mn, 4–8wt.%Mo, and O.6–O.9 wt.% N. The steel is produced by rapid-solidification powder metallurgy with subsequent consolidation by hot isostatic pressing. The steel exhibits high yield strength and corrosion resistance, both of which increase with increasing nitrogen content, yet high ductility and impact strength are maintained when properly annealed.     

SCC evaluation of type 304 and 316 austenitic stainless steels in acidic chloride solutions using the slow strain rate technique

The stress corrosion cracking (SCC) of the commercial austenitic stainless steels type 304 and 316 was investigated as functions of strain rate, test temperature and pH in acidic chloride solutions using a slow strain rate technique. From the comparison between the parameters such as time to failure (t_f), maximum stress (σ_m) and strain at t_f (ε_f) obtained in inert and corrosive environments, the relationships between t_f^Cl(corrosive)/t_f^W(inert), σ_m^Cl/σ_m^W and ε_f^Cl/ε_f^W versus strain rate were divided into three regions for type 304 and 316 specimens: the strain rate-dominated, SCC-dominated and corrosion-dominated regions, respectively. Furthermore, it was found that the relationship between t_f^Cl/t_f^W and σ_m^Cl/σ_m^W in the SCC-dominated region became a good straight line for the type 304 and 316 specimens irrespective of strain rate, test temperature and pH. This suggests that σ_m^Cl/σ_m^W became the parameter for predicting t_f. The results obtained were compared with those obtained from the constant load method and are discussed in terms of the formation of slip steps, dissolution current and film formation rate.     

Low temperature nitriding,nitrocarburising and carburising of AISI 316L austenitic stainless steel

Abstract

AISI 316L grade ASTM F138 austenitic stainless steel specimens were low temperature plasma nitrided (LTPN), nitrocarburised (LTPNC) and carburised (LTPC) using different gas mixtures. Different expanded austenite layers formed after each thermochemical treatment. LTPN and LTPCN led to formation of nitrogen supersaturated expanded austenite (γ_N). After LTPN, a second carbon expanded austenite (γ_C) layer was formed beneath the nitrogen expanded austenite layer (γ_N). LTPC led to formation of a carbon supersaturated expanded austenite (γ_C). Scanning electron microscopy, XRD and microhardness were used to characterise the expanded austenite layers formed on the surface of the specimens. Different mechanisms of formation and growth of the layers are pointed out. XRD results show that the lattice parameter of nitrogen expanded austenite (γ_N) is higher than that calculated for carbon expanded austenite γ_C. As a consequence, the lattice expansion Δa/a for the nitrogen rich (γ_N) phase is higher than the one observed for the (γ_C) phase and the nitrogen rich expanded austenite layer displays higher hardness than the carbon rich expanded austenite layer. The LTPNC bilayer displays a less steep hardness gradient, indicating that the carbon rich expanded austenite layer can grant mechanical support to the harder nitrogen rich layer.     

304和304L奥氏体不锈钢的热加工性能研究

利用Gleeble-3500热力模拟试验机分别对304与304L奥氏体不锈钢进行单道次热压缩实验,研究了材料在变形温度900～1100℃,应变速率0.01～5 s~(-1)条件下的热变形行为及组织演变规律,并基于动态材料模型(Dynamic Materials Mod-el,DMM)对比分析304与304L奥氏体不锈钢的热加工性能。结果表明:304L变形抗力随变形温度的升高和应变速率的降低而降低,其流变曲线可分为加工硬化、动态回复及动态再结晶3种类型。通过构建304与304L奥氏体不锈钢的热加工图,发现当应变量为0.6时,304热加工窗口为970～1050℃、0.01～0.1 s~(-1)和1050～1100℃、0.03～1 s~(-1),并在高温低应变速率区域出现严重的流变失稳现象; 304L热加工窗口为950～1100℃、0.01～0.03 s~(-1)。对比可知304和304 L的可加工温度区间相似,但应变速率区间差异明显,合金元素含量的变化导致可加工应变速率区间向低应变速率方向移动。     

Corrosion behavior of austenitic 304L and 316LN stainless steel clad reinforcing bars in cracked concrete

A study has been conducted on the chloride-induced corrosion behavior of 304L and 316LN stainless steel clad reinforcing bars (rebar) in concrete and in synthetic concrete pore solution. Metallographic examination of the as-received clad bars confirmed a strong metallurgical bond at the core/clad interface and some grain growth interdiffusion of species at the interface. Both bars showed a wide variation in coating thickness around the rebar circumference, from a minimum of 0.32 and 0.60 mm to a maximum of 1.4 and 2.8 mm in the 304L clad and 316LN clad, respectively. The electrochemical results and visual examination after autopsy showed that active corrosion was yet initiated on either the solid and clad stainless steel or carbon steel rebar in the sound noncracked concrete specimens. In contrast, corrosion had initiated in the bars embedded in cracked concrete at the base of the crack and extended along or around the bars. In the concrete and synthetic pore solution tests, the current densities of both solid and clad stainless steel rebar exposed to ∼21% chloride brine solution for days between 400 and 1,500 were similar. This was also the case for current densities of the straight and bent stainless steel bars tested in the synthetic pore solution test.     

Role of nitrogen on the corrosion behavior of austenitic stainless steels

The role of nitrogen in the mechanisms of localized corrosion resistance and repassivation were electrochemically investigated using a nitrogen-bearing austenitic stainless (SUS316L) steel in 0.1 and 0.5 M Na₂SO₄ solutions and a 3.5% NaCl solution. Almost 100% of the nitrogen compounds dissolved into the bulk solution after crevice corrosion were transformed into NH₃. That is, the mole amount of ammonia in the solution was approximately equivalent to the mole amount of nitrogen dissolved in the steel. This suggests that NH₄⁺ consuming H⁺ in the pit controlled the local decrease of pH and promoted the repassivation. NO₃-N and NO₂-N were not detected by chemical analyses in the high potential and thermodynamically stable zone as NO₃⁻ in the potential-pH diagram. The repassivation in nitrogen-bearing SUS316L steel in a 0.1 M Na₂SO₄ solution was studied using the scratching electrode technique, which measured the partially destroyed passivation films on the steel. This technique showed that nitrogen dissolved in the steel has a strong repassivation capacity.     

Studies of the tensile and corrosion fatigue behaviour of austenitic stainless steels

Corrosion fatigue behaviour of four types of austenitic stainless steels were investigated in boiling 45% magnesium chloride solution at a stress ratio of 0.25 and a frequency of 0.1 Hz. Type 316LN stainless steel possessed the best resistance and type 304 stainless steel had the lowest resistance to corrosion fatigue. XPS studies on the fracture surface indicated that the presence of nitrogen as ion in the surface film of type 316LN stainless steel gave it the highest resistance to corrosion fatigue. Fractographic examination showed wholly transgranular cracking in all cases.     

A hydrogen embrittlement mechanism for sensitized types 304, 316 and 310 austenitic stainless steels in boiling saturated magnesium chloride solutions

We have already proposed a mechanism for intergranular hydrogen embrittlement (IG-HE) for solution annealed austenitic stainless steels (types 304, 316 and 310) in HCl solutions and in boiling saturated magnesium chloride solutions. The proposed IG-HE mechanism was based on martensite transformation, hydrogen-enhanced local plasticity (HELP), grain boundary sliding (GBS). Recently, it was reported that the fracture susceptibility and fracture mode for sensitized steels in boiling saturated magnesium chloride solution under an open-circuit condition were significantly different from those observed for solution annealed steels. In the present paper, the hydrogen embrittlement behavior of sensitized types 304, 316 and 310 in boiling saturated magnesium chloride solutions was explained in more details in terms of an inhibiting effect of chloride ions, martensite transformation, Cr depletion, HELP, the degree of corrosiveness through the comparison with those for the solution annealed steels. Furthermore, a transgranular HE (TG-HE) cracking mode that was not observed for the solution annealed steels was discussed as well as IG-HE. Then a TG-HE mechanism for sensitized austenitic stainless steels was proposed, while the IG-HE mechanism for solution annealed austenitic stainless steels which was discussed in details was applied to IG-HE of sensitized austenitic stainless steels. It was also pointed out that the occurrence of both TG-HE and IG-HE was explained with an identical concept.