Corrosion Behavior of Nitrogen Containing Hot Rolled 304LN in Nitric Acid Medium

The corrosion behaviour of 304LN stainless steels containing three different nitrogen content (0.132, 0.193, 0.406 wt% N) was investigated by potentiodynamic anodic polarization technique, in 1, 4, 6 M nitric acid and simulated high level waste (HLW) medium. The results showed that all three alloys exhibited good corrosion resistance in nitric acid and simulated HLW and the corrosion properties were found to be similar. Owing to the spontaneous formation of the protective chromium oxide passive film in nitric acid and simulated HLW, increasing the nitrogen content of the alloy did not indicate any discernable effect on the corrosion resistance in both media. It was also found that the oxidizing ions present in simulated HLW did not deteriorate the passive film stability of the nitrogen containing alloys. In chloride medium, the highest nitrogen content 304LN stainless steel showed a profound increase in pitting corrosion resistance when compared to the lower nitrogen content alloy. Optical and scanning electron microscopy was carried out to obtain information about the microstructure. The results of the investigation are discussed in the paper.     

Corrosion Behavior of Sensitized 304 SS in Nitric Acid Medium Containing Oxidizing Ions

The corrosion behavior of 304 stainless steel (SS) in the as-received and sensitized conditions has been investigated at room temperature and at 60 °C in the presence of oxidizing ions in 6 and 8 M nitric acid media. Corrosion of 304 SS in nitric acid was far more severe in sensitized condition than in as-received condition. Further, decrease in corrosion resistance was observed with increase in the temperature of nitric acid. The corrosion potential shifted to noble direction with increase in the concentration of nitric acid and the potential shifted further to transpassive potential at 60 °C. Increase in corrosion potential towards transpassive domain led to severe intergranualar attack. Impedance spectra showed two time constant which could be attributed to the redox reaction on the surface of oxide film. However, the effect of oxidizing metal ions was not prominent.     

Analysis of Electrochemical Current Noise From Metastable Pitting of SS304L in NaCl Solutions

Electrochemical noise emanating from a corrosion situation gives indication about the nature and form of corrosion. An attempt has been made to analyse electrochemical current noise signal generated under potentiostatic condition (for metastable pitting) for SS304L–NaCl system. To begin with polarisation plots of SS304L were obtained in various test solutions to precisely know pitting potential of SS304L. It is found that E_pit and i_pass increase on increasing chloride content. The electrochemical current noise was measured at potentials 20–30 mV below the pitting potentials. The current time record shows two types of current transients; (i) slow rise and rapid decay and (ii) rapid rise and slow decay. Power spectral density analysis of current noise shows that the power (A²/Hz) of the signal measured at metastable pitting range increases with increasing chloride. Sampling frequency has to be properly selected otherwise some of the spikes are not recorded and as a result, size and shape of few current transients is altered.     

Corrosion Compatibility Studies of Stainless Steel 304L in Flowing Liquid Lead Bismuth Eutectic

Stainless steel 304L is being considered as a structural material for some components in the lead–bismuth eutectic (LBE) target facility operating at a temperature of about 250°C. Data available on the corrosion of SS 304L in LBE for long duration exposure at these temperatures are scanty. This paper gives the corrosion behaviour of SS 304L after the exposure of 6500 h in LBE in a non-isothermal loop with oxygen concentration of ~4 × 10^?10 wt% at temperatures of 250 and 350°C and with flow velocity of 16 cm s^?1. The composition of the surface film was analysed by energy dispersive X-ray (EDAX) and X-ray photoelectron spectroscopy techniques. To record the changes in the mechanical properties of SS 304L upon exposure tensile tests of the tensile specimens exposed to LBE were performed in air at room temperature and the fractured surfaces were examined by scanning electron microscope (SEM). Changes in microstructure and elemental composition of the exposed surface of SS 304L were studied using SEM and EDAX. While no changes were observed in the mechanical properties and microstructure due to prolonged exposure to LBE at either temperature, the specimens exposed to 350°C after 6500 h showed a corrosion rate of 1.2 × 10^?3 μm h^?1. While no penetration of LBE into the grain boundaries was observed at either temperature after an exposure of 6500 h, a minor depletion of nickel was noted in specimens exposed at 350°C.     

Electrochemical Behaviour of Surface Modified AISI 304 Grade Stainless Steel in Ringer’s Solution

Stainless steels are widely used as orthopedic implant materials. The main problems with these implant materials are fatigue fracture, fretting fatigue, wear and corrosion. Surface mechanical attrition treatment (SMAT) is a newly developed method to improve the materials properties and performance. In the present study, AISI 304 SS was subjected to SMAT using 2, 5 and 8 mm ? 316L SS balls for 15, 30, 45 and 60 min at 50 Hz under vacuum. SMAT of 304 SS increased the surface roughness, induced the formation of mechanical twins, strain induced martensite phase, and increased the defect density, which is a function of the size of the balls and treatment time. SMAT using 5 and 8 mm ? balls significantly decreased the corrosion resistance whereas treatment using 2 mm ? balls enabled an anodic shift in E_corr and a marginal decrease in i_corr. However, increase in surface roughness leads to the formation of a defective passive film and a decrease in breakdown potential.     

304L不锈钢焊缝在混凝土模拟孔隙液中的点蚀行为

采用动电位极化曲线、电化学阻抗谱、Mott-Schottky曲线等电化学方法研究了以308 L为焊丝的304 L不锈钢焊接接头在不同氯离子含量的混凝土模拟孔隙液中腐蚀行为和电化学规律.随Cl-增加,304 L不锈钢焊接接头的三个区域(母材、焊缝和热影响区)在混凝土模拟孔隙液中的自腐蚀电位、点蚀电位及电荷转移电阻降低,钝化膜中载流子密度和焊接接头的点蚀坑数量增加.在同浓度的腐蚀溶液中,308 L的焊缝区域耐蚀性最佳,热影响区次之,304 L基体表现出低的电荷转移电阻和高的掺杂浓度使得母材的耐蚀性最差.      

Position Dependence of SS304 and Iron Phases in Welded Specimen by Neutron Diffraction and Bragg-Edge Transmission Method

We investigated the nondestructive examination method by neutron scattering. A welded plate of SS304 (fcc) and iron (bcc) was investigated using neutron diffraction and the Bragg-edge transmission method. We tried to clear the positions of the welded area, SS304 phase, and iron phase and to determine the lattice spacing. The determined value of d ₁₁₁ for SS304 is 2.07471(8) Å using neutron diffraction. With Bragg-edge transmission, it is possible to collect the information of a specimen as two-dimensional (2-D) images using a 2-D position sensitive detector at a pulse neutron source. The determined d ₁₁₁ of SS304 phase indicates between 2.0745(8) and 2.0752(9) Å depending on the measurement points using Bragg-edge transmission. The same tendency was also seen in iron phase. The determined d ₁₁₀ of the iron phase is 2.02802(1) Å using neutron diffraction. The determined d ₁₁₀ of iron phase indicates between 2.0266(7) and 2.0321(13) Å using Bragg-edge transmission. The determined d using diffraction exists within that of Bragg-edge transmission. In order to clarify the position dependency of the phase and lattice spacing, the combined diffraction and Bragg-edge transmission is effective.     

Pitting Corrosion Behavior of Stainless Steel 304 in Carbon Dioxide Environments

The pitting corrosion behavior of stainless steel (SS) 304 in aqueous CO2-H2S-CI-environment was investigated by potentiodynamic cyclic anodic polarization and electron probemicroanalysis (EPMA). The experimental results show that the pitting corrosion susceptivityof SS 304 increases with the increase of temperature. Chlorine ion is the prerequisite for pittingcorrosion of SS 304 in H2S-CO2 environments. There is a linear relatiotxship between the pittingcorrosion potential (Eb-100) and chlorine ion concentration, and Eb-00 becomes noble with in-creasing pH value of the solution with or without H2S. pH value has little effect on the protec-tion potential with the presence of Hz S. H2S increases strongly the pitting corrosion susceptivi-ty and deteriorates the pitting corrosion resistance of SS 304 in CO2 environments. The obser-vations by EPMA show that SS 304 in CO2-saturated NaCI solution (3%) with H2S sufferspitting corrosion accompanied with intergranular corrosion.     

Fracture and the formation of sigma phase,M23C6, and austenite from delta-ferrite in an AlSl 304L stainless steel

The decomposition of delta-ferrite and its effects on tensile properties and fracture of a hot-rolled AISI 304L stainless steel plate were studied. Magnetic response measurements of annealed specimens showed that the transformation rate of delta-ferrite was highest at 720 °C. Transformation behavior was characterized by light microscopy, transmission electron microscopy, scanning electron microscopy, and energy-dispersive spectroscopy on thin foils. The initial transformation of delta-ferrite (δ) to austenite (γ) and a chromium-rich carbide (M₂₃C₆) occurred by a lamellar eutectoid reaction, δ⇄M₂₃C₆ +γ. The extent of the reaction was limited by the low carbon content of the 304L plate, and the numerous, fine M₂₃C₆ particles of the eutectoid structure provide microvoid nucleation sites in tensile specimens annealed at 720 °C for short times. Sigma phase(σ) formed as a result of a second eutectoid reaction,δσ +γ. Brittle fracture associated with the plate-shaped sigma phase of the second eutectoid structure resulted in a significant decrease in reduction of area (RA) in the transverse tensile specimens. The RA for longitudinal specimens was not affected by the formation of sigma phase. Tensile strengths were little affected by delta-ferrite decomposition products in either longitudinal or transverse orientations. Y. Shen, formerly with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, is deceased.     

Influence of thermal aging on the intergranular corrosion resistance of types 304LN and 316LN stainless steels

Intergranular corrosion (IGC) resistance of types 304LN and 316LN stainless steels (SS) thermally aged at 823, 873, and 923 K for various durations was assessed by ASTM A262 practice A test (electrolytic etch test) and electrochemical potentiodynamic reactivation (EPR) test. The results indicated that the type 316LN SS has significantly improved IGC resistance compared to 304LN SS. Based on the results of these tests, time-temperature-sensitization (TTS) diagrams were developed for both alloys. The secondary precipitates formed during thermal aging treatments were electrochemically extracted and analyzed by X-ray diffraction (XRD) to determine the types of precipitates formed during the aging treatments. The results indicated that the precipitates were mostly of M₂₃C₆ carbides.     

Effect of Ball Burnishing Treatment on the Fatigue Behavior of 316L Stainless Steel Operating Under Anodic and Cathodic Polarization Potentials

Corrosion fatigue (CF) behavior of AISI 316L was investigated in a 3 pct NaCl aqueous solution at an R = ? 1 stress ratio and a frequency of 60?Hz at room temperature. The test scale specimen was 7 cm². The passive (0 mV_Ref), pitting (120?mV_Ref), and cathodic (? 1400?mV_Ref) polarization potentials were statically applied and recorded during CF tests until the samples were broken. The shaft material surface was treated with a ball burnishing (BB) process. By the results, the fatigue behavior of AISI 316L was affected by polarization potential and surface treatment. Under 0?mV potential charged tests for 5?×?10⁵ cycles, BB treatment raised the CF strength of the shaft material from about 448 to 702?MPa with a percent 57 increase. Fractographic observations revealed that corrosion pits occurred during the experiments where anodic potential was applied and that transgranular surface fractures occurred in all cases.     

含铝304和316L奥氏体不锈钢热轧板材焊接性能

 对加Al质量分数为4%的304、2%的316L不锈钢热轧板材的焊接性能进行了研究。采用手工氩弧焊（TIG）的焊接方法,利用光学显微镜对焊缝的显微组织进行分析,利用电子探针（EMPA）分析焊接母材的元素分布,并对焊接接头进行力学性能测试。组织和力学性能的研究结果表明：含铝304和含铝316L合金热轧板分别选用ER308L,ER316L作为焊接材料,经TIG焊接后,焊缝无裂纹、气孔等缺陷,接头具有良好的强度和塑性,焊接接头力学性能接近于其母材;热影响区组织与母材组织基本一致,焊缝与母材熔合良好,组织良好,加铝304和316L不锈钢具有良好的焊接性能。     

Correlation of Electrochemical Noise Parameters with Degree of Sensitization in AISI Type 316LN Stainless Steel

Degree of sensitization in 316LN stainless steel (SS) specimens sensitized at 898, 923 and 948 K for 500 h was obtained using double loop electrochemical potentiokinetic reactivation (DLEPR) technique as 7.4, 14.5 and 9.3% respectively. The sensitized specimens were pulsed polarized so that only Cr-depleted regions of the sensitized grain boundaries contributed to the electrochemical noise (EN) study. The DOS values evaluated from DLEPR technique were correlated with the EN parameters viz. standard deviation of current, σ_I and characteristic charge, q and characteristic frequency, f _n , obtained from shot noise analysis in order to assess the extent of intergranular corrosion (IGC) attack in 316LN SS. The plot of σ_I versus time showed highest σ_I values for the specimen sensitized at 923 K for 500 h, indicating high grain boundary dissolution and hence, severe IGC attack, whereas the specimen sensitized at 898 K for 500 h showed the least σ_I values indicating lower dissolution and least IGC attack. 316LN SS specimen heat-treated at 948 K for 500 h showed intermediate grain boundary dissolution rate. The charge q, determined from σ_I versus time plot showed a good correlation (>99%) with the DOS values obtained from DLEPR experiments. The power spectral density values of the current signal in the frequency independent region were found to have excellent correlation with these observations. The above findings were further supported by scanning electron microscopic examination which showed an increase in grain boundary width in the sensitized specimens when the heat-treatment temperature was raised from 898 to 923 K and on further increasing the temperature to 948 K, a marginal decrease in the grain boundary width was observed.     

Influence of Sigma Phase on Pitting Resistance Depending on Solidification Mode in AISI316L Weld Metal

The pitting corrosion resistances were investigated in the AISI 316L stainless steel weld metals with respect to the sigma phase precipitation in a 0.1 M NaCl solution. The modified flux-cored arc welding filler wires were fabricated at various chromium and nickel equivalent ratios using the flux-cored arc welding process. As the Cr/Ni equivalent ratio increased, the precipitated σ phase content increased in the temperature range of 923 K to 1123 K (650 °C to 850 °C), and the specimen that was aged at 1123 K (850 °C) precipitated the σ phase rapidly. The hardness increased with increasing sigma contents. During the potentiodynamic anodic polarization test, the specimens that were aged at 923 K to 1123 K (650 °C to 850 °C) exhibited lower pitting potentials than the as-weld metal. Additionally, the specimens that were aged for longer times exhibited lower pitting potentials. The pits occurred preferentially in the ductility dip cracking in specimen 1, whereas intergranular pits occurred in the sigma phase regions along the vermicular ferrite and acicular ferrite grain boundaries in specimens 2 and 3.     

Corrosion Behaviour of Chrome–Manganese Austenitic Stainless Steels and AISI 304 Stainless Steel in Chloride Environment

Electrochemical behaviour of chrome–manganese austenitic stainless steels (Cr–Mn ASS) and AISI 304 stainless steel (SS) is evaluated in various chloride (Cl^?) concentrations (Cl^? free to 20,000 ppm) to simulate rural, industrial and marine environment. Potentiodynamic polarization and electrochemical impedance spectroscopy has clearly shown that with increase in Cl^? concentration, the corrosion rate of both Cr–Mn ASS and AISI 304 SS increases and polarization resistance decreases. Comparatively, Cr–Mn ASS is more affected by Cl^? concentration than AISI 304 SS. This is attributed to relatively low Cr content and lack of Ni. The findings have been explained with the help of point defect model. However, in less aggressive environment of up to 100 ppm Cl^? concentration, Cr–Mn ASS may be a candidate material as a cheaper substitute of AISI 304 SS. Ways of improving corrosion resistance of Cr–Mn ASS by alloying with various elements have also been discussed. It is argued that a dedicated effort is needed to improve corrosion resistance of Ni-free or low-Ni Cr–Mn ASS.     

Intergranular Corrosion Behavior of 304LN Stainless Steel Heat Treated at 623 K (350 °C)

Low temperature sensitization of 304LN stainless steel from the two pipes, differing slightly in chemical composition, has been investigated; specimens were aged at 623 K (350 °C) for 20,000 hours and evaluated for intergranular corrosion and degree of sensitization. The base and heat-affected zone (HAZ) of the 304LN-1 appear resistant to sensitization, while 304LN-2 revealed a “dual” type microstructure at the transverse section and HAZ. The microstructure at 5.0-mm distance from the fusion line indicates qualitatively less sensitization as compared to that at 2.0 mm. The 304LN-2 base alloy shows overall lower degree of sensitization values as compared to the 304LN-1. A similar trend of degree of sensitization was observed in the HAZ where it was higher in the 304LN-1 as compared to the 304LN-2. The weld zone of both the stainless steels suffered from cracking during ASTM A262 practice E, while the parent metals and HAZs did not show such fissures. A mottled image within the ferrite lamella showed spinodal decomposition. The practice E test and transmission electron microscopy results indicate that the interdendritic regions may suffer from failure due to carbide precipitation and due to the evolution of brittle phase from spinodal decomposition.     

浓硫酸中904L不锈钢焊接接头的耐蚀性能

采用动电位极化曲线和电化学阻抗谱研究了904L不锈钢基体和焊接区在浓硫酸溶液中的腐蚀行为,并简要分析其腐蚀机理.实验结果表明:焊接区和基材的阴极极化曲线均为氢还原反应过程,而阳极极化曲线则有较大的差异.焊接区相对于基材,其自腐蚀电位降低且阳极电流增大,焊接对阳极反应起促进作用.904L不锈钢基体和焊接区的腐蚀主要由电化学反应步骤控制,两者的Nyquist图特征相似,均由单一的容抗弧构成,有一个时间常数.基体的耐蚀性能优于焊接区.      

A Simple Approach to the Determination of Threshold Stress Intensity for Stress Corrosion Cracking (K ISCC) and Crack Growth of Sensitized Austenitic Stainless Steel

This article discusses the intricacies associated with the determination of threshold stress intensity for stress corrosion cracking (K _ISCC) of narrow regions such as the sensitized microstructure of austenitic stainless steel and presents a simple approach to the accurate determination of K _ISCC of a sensitized stainless steel. K _ISCC and crack growth rates of solution-annealed and sensitized AISI 304 stainless steel in the 42 wt pct MgCl₂ environment at 427 K (154 °C) were determined using the circumferential notch tensile (CNT) technique. The results presented here validate the ability of the CNT technique to overcome some of the fundamental difficulties in determination of the K _ISCC of narrow regions, using the traditional techniques. This article also discusses the mechanistic aspects of the difference in fractographic features of the sensitized and solution-annealed stainless steels.     

Deformation-induced microstructure and martensite effects on transgranular carbide precipitation in type 304 stainless steels

Plastic deformation of 304 stainless steel (SS) induces transgranular (TG) carbide precipitation, which is critically dependent on deformation-induced microstructural changes occurring during thermal treatment of the SS. Uniaxial deformation of the 304 SS to 40% strain produces a high density of intersecting micro-shear bands composed of heterogeneous bundles of twin-faults and about 12–17% strain-induced α′-martensite at the intersections of the twin-faults. Thermal treatment of 670°C for 0.1–10 h, however, results in a rapid annihilation/transformation of the strain-induced martensite and the concurrent formation of zones containing mixed thermal martensite laths and fine-grained austenite, though the thermal martensite also decreases with increasing heat treatment time. Simultaneous with these thermomechanically-induced microstructural changes, TG chromium-rich carbides form at intersections of twin-faults and on fine-austenite or thermal martensite boundaries in the SS; however, no correlation between strain-induced α′-martensite and carbides was observed in this work. The mechanisms of deformation-induced microstructure and (strain-induced and thermal) martensite effects on TG carbide precipitation in 304 SS are discussed.