Effect of surface polishing on SCC susceptibility of sensitised type 304 stainless steel

Abstract

The effect of surface polishing on the stress corrosion cracking (SCC) susceptibility of sensitised SUS 304 stainless steel was studied by using the well controlled surface polishing method which has been developed for the surface preparation of the semiconductor material. The SCC susceptibility of the polished specimen was examined by using the slow strain rate testing method, at which the specimen was kept at a constant potential in 0·1M Na₂SO₄ solution kept at 368 K. The most effective suppression effect on the SCC susceptibility was found in the case of surface polishing using alumina slurry, followed by water soluble fullerene and diamond slurry, compared with the mechanical polishing with #0/6 paper. Surface roughness and pitting potential of polished surface were compared with the SCC susceptibility of polished sensitised SUS 304 stainless steel. It was found that the SCC susceptibility was decided mainly by the roughness factor rather than pitting resistance. Experimental evidences suggest that the SCC susceptibility was decided by the crack initiation being controlled by surface roughness.     

Study of the correlation between corrosion resistance and semi‐conducting properties of the passive film of AISI 316L stainless steel in physiological solution

In this work the corrosion resistance of AISI 316L biomedical stainless steel was assessed through electrochemical impedance spectroscopy (EIS) measurements in Hanks' solution at 37 °C. Specimens were immersed in the electrolyte during 21 days. Semi‐conducting properties of the passive film naturally formed on the surface of the metallic material during the test were evaluated through the Mott–Schottky approach. The aim was to investigate the correlation between corrosion resistance and semi‐conducting properties in the physiological solution. The corrosion resistance was found to decrease with the immersion time. The density of defects in the passive film increased accordingly as indicated by the Mott–Schottky plots. The passive film presented a semi‐conducting behavior with a duplex character. Above the flat band potential the behavior was typical of an n‐type semiconductor whilst below such value it was typical of a p‐type semiconductor. The results from EIS measurements and Mott–Schottky were in good agreement, suggesting that the corrosion resistance of biomedical stainless steels may be associated with the semi‐conducting properties of the passive films formed during immersion in physiological medium.     

Effect of chloride concentration on passive film properties on copper

The semiconducting properties of passive films formed on copper, in anaerobic alkaline sodium chloride solution, were studied using Mott–Schottky analysis and electrochemical impedance spectroscopy, based on the point defect model. Results showed that the corrosion resistance increased with increasing potential, which was attributed to a well crystallised, refined grain structure, and a thicker passive film at higher potential. P-type semiconducting characteristics were obtained with or without chloride. The density of copper vacancies was approximately 10²⁰?cm^?3, and increased with the increasing chloride concentration, which was attributed to faster film-formation in a higher chloride environment. The diffusion coefficient of the defects, a key dynamic parameter for passive film breakdown, was in the range of 10^?16–10^?15?cm²?s^?1, and increased with increasing chloride concentration, thus leading to a greater probability of pitting.     

The inhibitive action of bound water in the passive film of stainless steel against chloride corrosion

The stability of passivated 304 stainless steel specimens in the presence of chloride ions was investigated in relation to the amount of bound water in the passive film. As a measure of the stability the induction period for pit initiation and number of the anodic fluctuation prior to pit initiation were used. The amount of bound water was controlled by varying of passivation potentials, passivation times and passivation temperatures. Two kinds of bound water with different inhibitive action against corrosion by chloride ions were found in the passive film.     

The inhibitive action of bound water in the passive film of stainless steel against chloride corrosion

The stability of passivated 304 stainless steel specimens in the presence of chloride ions was investigated in relation to the amount of bound water in the passive film. As a measure of the stability the induction period for pit initiation and number of the anodic fluctuation prior to pit initiation were used. The amount of bound water was controlled by varying of passivation potentials, passivation times and passivation temperatures. Two kinds of bound water with different inhibitive action against corrosion by chloride ions were found in the passive film.     

Stress corrosion cracking of 304L stainless steel in molten salts media

A tensile apparatus with constant strain rate, suitable for corrosion tests in molten salts has been used to determine the mechanical characteristics of 304L stainless steel in molten NaClCaCl₂ at 570°C fromα= f [(Δl)/(l₀) %]curves for different strain rates. Potentiometric measurements, executed simultaneously, have determined the electrochemical conditions during mechanical testing. Results show a strain rate field in which the stress corrosion cracking occurs.     

Effect of cutting process on the stress corrosion susceptibility of AISI 304L stainless steel

During manufacturing of a component, cutting, turning, grinding, and milling operations are inevitable and these operations induce surface residual stresses. In this study, it is shown that, depending on the process employed for cutting, residual stresses generated at the cut surfaces can vary widely and they can, in turn, make the cut surfaces of austenitic stainless steel (SS) prone to stress corrosion cracking (SCC). An austenitic SS 304L plate was cut using three different procesess: bandsaw cutting, cutting using the cut-off wheel, and shearing. Surface residual stress measurement using the X-ray diffraction (XRD) technique is carried out close to the cutting edges and on the cross-section. SCC susceptibility studies were carried out as per ASTM G36 in 45% boiling magnesium chloride solution. Optical microscopic examination showed the presence of cracks, and confocal microscopy was used to measure the depth of cracks. The study confirmed that high tensile residual stresses present in the cut surfaces produced by cut-off wheel and shear cutting make the surfaces susceptible to SCC while the surfaces produced by bandsaw cutting are resistant to SCC. Hence, it is shown that there is a definite risk of SCC for product forms of austenitic SS with cut surfaces produced using cutting processes that generate high tensile residual stresses stored for a long period of time in a susceptible environment.     

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。     

The effects of pH on the repassivation of 304L stainless steel

The rate of anodic film growth on freshly generated 304L stainless steel surfaces is presented as a function of electrode potential and pH. For all pH values between 0 and 14 the kinetics are controlled by ion migration under high electric field, with two distinct sequential kinetic regions. The high field parameters are presented, and show that in the early stages the film grows with a lower activation energy than in the later stages.     

The effect of cold rolling regime on microstructure and mechanical properties of AISI 304L stainless steel

In this paper, the effect of different thickness reductions by cold rolling on the microstructure and mechanical properties of AISI 304L austenitic stainless steel were investigated. The hot rolled steel strips were subjected to cold rolling at 0 °C from 10 to 90% thickness reduction. Microstructures, strain-induced martensitic transformation and mechanical properties of the cold-rolled specimens were characterized by X-ray diffraction, Feritscope measurements, optical metallography, hardness and tensile tests. The resulting transformation curve showed a sigmoidal shape with the saturation value of strain-induced martensite of approximately 100%. A good agreement was found between the experimental results and the Olsen–Cohen model. The results indicated that formation of strain-induced martensite clearly resulted in a significant strengthening of the steel.     

堆内构件304L焊接件在除氧和氯离子环境中的应力腐蚀开裂研究

采用高温高压慢应变速率拉伸试验方法(SSRT),研究了在除氧环境下不同浓度氯离子对304L焊接件在模拟一回路高温高压硼锂水介质中氯致应力腐蚀开裂的影响。结果表明:当氯离子浓度为1 mg/L时,其各项力学性能指标与惰性氮气空白样一致,表明在低浓度氯离子除氧条件下,304L焊接件没有应力腐蚀敏感性。随着氯离子含量(1~50mg/L)的增加,304L焊接件应力腐蚀敏感指数变化呈增加趋势。断口侧面没有观察到明显的裂纹,氯离子浓度越低,断口缩颈现象越明显,表明主要是机械断裂。氯离子浓度较高时,棱角部分出现微裂纹源,可能与棱角部分应力更为集中相关。所有样品的断裂位置都在焊缝和热影响区,在焊缝和热影响区发生了巨大的形变,离断口越近,变形越严重。焊接部位是304L堆内构件中薄弱环节,应该成为应力腐蚀开裂和其他性能检测的重点部位。     

XPS studies on passive film formed on stainless steel in a high-temperature and high-pressure methanol solution containing chloride ions

The composition and structure of passive film formed on 316L SS immersed in an anhydrous methanol solution (water content < 0.05 wt%) containing 0.42 wt% LiCl at 323-473 K were investigated by X-ray photoelectron spectroscopy (XPS), and compared with those of film formed in an aqueous solution. The passive film formed in the methanol solution was mainly composed of Fe and Cr oxides, and it possessed a double-layered structure consisting of an Fe oxide-rich outer layer and a Cr oxide-rich inner layer. Dissolution of the Fe-rich layer and densification of the Cr-rich inner layer were observed, especially at high temperatures. However, these were suppressed in an aerated methanol solution at 423 K or below, probably due to the barrier effect of adsorbed oxygen. No Ni compound contributed to composing the passive film, even at higher temperatures. The ratio of OH⁻ to O²⁻ was small and decreased with an increase in temperature (the presence of oxygen suppressed the decrease, especially at 423 K or below). The chloride ions were concentrated in the Fe-rich outer layer, and they penetrated more deeply than that in the aqueous solution into the passive film formed in the methanol solution.     

Comparison of corrosion behaviour and passive film properties of 316L austenitic stainless steel in CO2 and N2 environments

This study investigated the susceptibility to pitting corrosion of 316L in CO₂ and N₂ environments at temperatures from 30 to 80°C in 3?wt-% NaCl at pH 4. Results from cyclic polarisation technique confirm greater pitting susceptibility of 316L in the CO₂ environment. Electronic properties and composition of the passive film were identified by electrochemical impedance spectroscopy, Mott–Schottky, and X-ray photoelectron spectroscopy. Increasing temperature negatively affects the passive film stability, and its influences are amplified in the presence of CO₂ as compared to N₂. In the CO₂ environment, the passive film becomes porous with the increasing temperature leading to higher defects (donor/acceptor densities).     

Hydrogen effects on the passive film formation and pitting susceptibility of nitrogen containing type 316L stainless steels

The effects of hydrogen on the passivity and pitting susceptibility of type 316L stainless steels have been investigated with alloys containing different nitrogen contents (0.015, 0.198 and 0.556 wt.% N). The study revealed that electrochemically pre-charged hydrogen significantly reduced the pitting resistance of alloys conatining 0.015 and 0.198 wt.% nitrogen contents. In alloy with highest nitrogen content (0.556 wt.% N), an increase in the passive film current density with hydrogen was observed without affecting breakdown potential. Auger electron spectroscopy (AES) analysis of the passive film indicated the presence of nitrogen in the passive film. On other hand, for hydrogen charged samples, nitrogen was found to be significantly less in the passive film. In Electrochemical impedance spectroscopy (EIS) measurement, the decrease in semi-circle radius of Nyquist plot, and the polarization resistance, R_P associated with the resistance of the passive film was observed with hydrogen, indicating that hydrogen decreased the stability of the passive film. The present investigation indicated that precharged hydrogen deteriorated the passive film stability and pitting corrosion resistance in these alloys, and the increase in nitrogen content of the alloy offsets the deleterious effect of precharged hydrogen.     

Grain size effects on the SCC susceptibility of a nitrogen steel in hot NaCl solutions

The effect of grain size on the exhibited stress corrosion cracking (SCC) resistance of an austenitic stainless steel containing 0.97 wt.% nitrogen was investigated in this work. In order to vary the grain size, the as-received steel (AR) was heat treated (HT) at 550 and at 1100 °C for 1 h. Cert specimens of the AR and HT N-containing steels were then machined and tested in a Cortest machine using slow strain rates ranging from 7.95 × 10⁻⁶ s⁻¹ to 7.06 × 10⁻⁷ s⁻¹. The corrosion environment was hot water, as well as a 30% NaCl solution at 90 °C. In all the steel conditions, SCC was directly related to active pitting development. In all the cases, pits were found to preferentially form at Mn-Al inclusions, but were not able to grow too deep probably as a result of active repassivation reactions. It was found that the drop in the exhibited mechanical properties of the steels such as yield strength, UTS and % elongation was particularly enhanced for the coarse grained steel HT at 1100 °C. A SCC susceptibility index confirmed that this was indeed the case. Apparently, a drop in the yield strength tends to reduce the mechanical stability of the passive layer promoting its breakdown. Finally, quasi-cleavage and the lack of appreciable plastic deformation in some of the fracture surfaces indicated that the steel underwent some degree of embrittlement when exposed to these aqueous environments.     

Comparative study of structural and semiconducting properties of passive films and thermally grown oxides on AISI 304 stainless steel

A comparative study including structural characterization and semiconducting properties of passive and thermally grown oxides on AISI 304 stainless steel is performed by near field microscopy, Auger electron spectroscopy, capacitance measurements and photoelectrochemistry. This comparative investigation is performed on thermally grown oxides at different temperatures and passive films formed at different potentials. The results obtained by characterization techniques show that the thickness of both oxides increases with increasing formation temperature and potential and allow discussing grain size and surface roughness vs. formation temperature. Capacitance measurements reveal semiconducting behavior of both iron and chromium oxides constituting whole passive and thermal oxides. These results allow extracting and discussing space charge layer thickness and doping densities of iron and chromium oxides in relation with formation conditions. The photocurrent results show a variation of the quantum efficiency with formation temperature and potential and a constant band gap value whatever the nature of the considered film. The variation of the quantum efficiency with applied potential in accordance with Pool–Frenkel model allow extracting and comparing donor densities with those obtained by capacitance measurements.     

Effects of temperature on the oxide film properties of 304 stainless steel in high temperature lithium borate buffer solution

The paper mainly investigated the effects of temperature on the oxide film properties of 304SS in lithium borate buffer solution by electrochemical measurements and XPS analysis. As temperature increased, the protective property of the film degraded and structure varied from a single layer to a double-layer. Whatever the temperature, the oxide film exhibited an n-type and p-type semiconductor in the potential range above and below the flat band potential, respectively. The electronic properties were assigned to a Fe–Cr spinel inner layer and a defective Fe–Cr oxide outer layer. The related growth mechanisms of the oxide film were also discussed.     

Corrosion behaviour of AISI type 304L stainless steel in nitric acid media containing oxidizing species

The corrosion behaviour of AISI type 304L stainless steel (SS) in different concentration of 0.01 M, 1 M and 5 M HNO₃ in presence of oxidizing ions at different temperatures has been evaluated. The main objective of this study is to assess the corrosion resistance of type 304L SS in non-radioactive conditions encountered during storage of liquid nuclear waste. Electrochemical impedance spectroscopy (EIS) and laser Raman spectroscopy (LRS) has clearly brought out the deleterious effect of oxidizing species on the passive film leading to increased corrosion along with increase in HNO₃ concentration and higher temperature.     

The effect of cysteine on the corrosion of 304L stainless steel in sulphuric acid

The effect of cysteine on the corrosion of 304L stainless steel in 1 mol l⁻¹ H₂SO₄ was studied using open-circuit potential measurements, anodic polarization curves, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). All the electrochemical measurements obtained in the presence of low cysteine concentration (10⁻⁶-10⁻⁵ mol l⁻¹) presented the same behaviour as those obtained in the absence of cysteine, a passivated steel surface. However, for higher cysteine concentrations (10⁻⁴-10⁻² mol l⁻¹), a different behaviour was observed: the corrosion potential stabilized at a more negative value; an active region was observed in the anodic polarization curves and the electrochemical impedance diagrams showed an inductive loop at lower frequencies and a much lower polarization resistance. These results show that the presence of cysteine at high concentration turns the surface of 304L stainless steel electrochemically active, probably dissolving the passivation layer and promoting the stainless steel anodic dissolution. SEM experiments performed after immersion experiments at corrosion potential were in good agreement with the electrochemical results.