Detection and characterisation of intergranular stress‐corrosion cracking on austenitic stainless steel

Intergranular stress‐corrosion cracking (IGSCC) on a sensitised type AISI 304 stainless steel specimen was monitored simultaneously by acoustic emission, electrochemical noise, elongation measurements and a digital imaging system. The specimen was exposed to an aqueous sodium thiosulphate solution in combination with a constant load. It was established that before the final fracture two large cracks and numerous smaller cracks had developed. Detection and characterisation of the stress‐corrosion processes which generated these cracks are discussed. The results confirm and generalise previously established correlations between various parameters obtained by the implemented characterisation methods and IGSCC processes. Additionally, a clear differentiation between crack related and crack non‐related AE signals was made based on an analysis of the AE signals. The relationship between the crack lengths calculated by means of digital image correlation analysis and the electrochemical current noise was also established.     

Stress corrosion behavior of 6082 aluminum alloy

Stress corrosion tests of 6082 aluminum alloy were carried out by using a three-point bending fixture while holding at 50% of yield strength state through different immersion times in 1.5% NaCl electrolyte solution. The electrochemical impedance spectra and dynamic electric potential polarization curves were measured to indicate the stress corrosion behavior of the alloy. Optical microscopy, scanning electron microscopy, and X-ray energy spectrum analysis were applied for microstructural investigations. The results show that all of the Nyquist electrochemical impedance spectra consisted of high- and low-frequency double capacitive arcs. However, an increase in immersion time while holding at 50% of yield stress resulted in a corresponding increase in the corrosion current density, leading to gradual corrosion depth growth, and a decrease in the corrosion resistance of the alloy. 6082 Aluminum alloy included AlMnFeSi, Mg₂Si, and Si secondary phases. The different secondary phases presented different stress corrosion behaviors. Stress corrosion cracks were generated at the boundaries of AlMnFeSi and matrix or within the AlMnFeSi phase. Crack direction is always perpendicular to the tensile stress applied. Mg₂Si secondary phase was self-corroded as its corrosion potential is lower than that of the matrix. As the electric potential of Si is higher than that of the matrix, corrosion occurred at the matrix side of the boundary between Si and matrix.     

奥氏体304不锈钢在活化态下的应力腐蚀与马氏体相变

用ＳＳＲＴ和电化学方法研究了３０４不锈钢在酸性氯化物溶液中的应力腐蚀与形变诱发马氏体的关系．结果表明形变诱发马氏体相不仅有利于应力腐蚀裂纹的形核而且它的选择性溶解构成了裂纹扩展的活性通道．文中讨论了裂纹沿马氏体相扩展的机制．     

Electrochemical noise during non‐stationary corrosion processes

In the present work, electrochemical noise (EN) was measured in three different types of experimental set‐ups in order to obtain and compare various types of corrosion. Simultaneously with EN measurements, a parallel technique was used, regarding the type of experimental set‐up: a computer visualization system combined with optical microscopy, or measurements of mechanical changes. In order to distinguish between these corrosion types and to assess corrosion rate, the measured EN signals were analysed by two different techniques: spectral and statistical. On the basis of estimated significant spectral parameters (power spectral densities of current and voltage noise, spectral noise resistance) and statistical parameters (standard deviation, localisation index and noise resistance), the passive state, localized corrosion types and uniform corrosion can be recognized. However, by these parameters it is not possible to distinguish between certain localized corrosion processes: metastable pitting, initiation and growth of stable pits, stress‐corrosion cracking. It was ascertained that these corrosion processes exhibit unstable nature, and consequently generated EN signals are usually non‐stationary. Since stationarity is required for spectral and statistical analysis, the unstable nature of these processes was recognized as the main reason for unreliable results. It is believed that these types of signal analysis give poor information about corrosion processes which can be easily identified from EN signal itself: transient corrosion processes and transitions between different corrosion processes.     

The influence of the cathodic process on the interpretation of electrochemical noise signals arising from pitting corrosion of stainless steels

The use of electrochemical noise (EN) measurements for the investigation and monitoring of corrosion has allowed many interesting advances in the corrosion science in recent years. A special advantage of EN measurements includes the possibility to detect and study the early stages of localized corrosion. Nevertheless, the understanding of the electrochemical information included in the EN signal is actually very limited. The role of the cathodic process on the EN signals remains uncertain and has not been sufficiently investigated to date. Thus, an accurate understanding of the influence of the cathodic process on the EN signal is still lacking. On the basis of different kinetics of the oxygen reduction it was established that the anodic amplitude of transients arising from pitting corrosion on stainless steel can be decreased by the corresponding electron consumption of the cathodic process. Therefore, the stronger the electron consumption, the weaker the anodic amplitude of the EN signal becomes. EN signals arising from pitting corrosion on stainless steel can be measured because the cathodic process is inhibited by the passive layer. This was confirmed by means of EN measurements under cathodic polarisation. Since the cathodic process plays a decisive role on the form of transients arising from pitting corrosion, its influence must be considered in the evaluation and interpretation of the EN signals.     

应力应变下TC2钛合金在模拟海水环境中的腐蚀行为

目的研究TC2钛合金在模拟海水中的表面电化学及腐蚀行为,以及不同形变对其的影响。方法制备U形试样,进行模拟海水浸泡试验,采用电位测试、交流阻抗及极化曲线测试、扫描电镜(SEM)、X射线衍射(XRD)等方法进行分析。结果在240 d模拟海水中浸泡试验期间,无形变TC2钛合金表面钝化膜阻抗值随时间延长,先迅速增加,后缓慢增加,腐蚀电位持续升高,因此其表面电化学反应下降,腐蚀速率较低。45°形变TC2钛合金试样的表面钝化膜阻抗值先略微上升,后下降,最终低于初始阻抗值。90°形变试样的表面钝化膜阻抗值持续下降。经过240d浸泡后,无形变的试样表面出现微小的点蚀,45°形变试样表面点蚀密度增加,90°形变试样表面垂直压应力方向出现裂纹。XRD结果显示,形变处Ca~(2+)、Mg~(2+)等离子的吸附增加,这可能与表面粗糙度增大,TC2钛合金表面活性增加有关。结论在模拟海水环境中,无应变试样耐腐蚀性较强,应变导致TC2钛合金表面钝化膜破裂,点蚀增加,甚至出现裂纹,增加了TC2钛合金的应力腐蚀敏感性。     

Das elektrochemische verhalten von Schweißverbindungen

The electrochemical behaviour of weld seams On the basis of reproducible electrochemical tests in macro and micro ranges a correlation is derived between electrochemical data and the corrosion behaviour of weld seams of steels of different compositions and aluminium welded with dissimilar fillers. In austenitic welds differences of the rest potential may give rise to the formation of local elements, and in passive welds there may exist active zones showing pitting and intercrystalline corrosion. The area and location of the active and passive region can be defined by current density potential curves. The susceptibility to selective corrosion is established by measurements on micro regions.     

Spannungsrißkorrosion von Stählen in flüssigem Ammoniak

Stress corrosion cracking of steels in liquid ammonia The apparatus developped for the investigation of stress corrosion cracking of steel in liquid ammonia under controlled electrochemical conditions is described. The parameters of the experiments were set by a computer which also stored and correlated the experimental data. Cylindrical samples of the welding steel W. Nr. 1.0143 and of the steel STE 355 (W. Nr. 1.0562) in liquid ammonia containing ammonium chloride or lithium perchlorate as the electrolyte developped cracks only at negative electrode potentials in the regions of active dissolution and hydrogen deposition. Other parameters including contamination of the solution by air were unimportant. The results are explained by hydrogen induced stress corrosion cracking. No embrittlement was observed with passive samples. However, ultimate tensile strengths in liquid ammonia were clearly lower than at air also for samples breaking without formation of cracks. Experiments with notched sheets resulted in sharp, essentially transcrystalline cracks. Passivation of these samples was difficult in the region of the notch indicating the danger of anodic stress corrosion cracking.     

X80管线钢在近中性pH溶液中腐蚀行为研究

目的研究X80管线钢在近中性p H溶液中的腐蚀与应力腐蚀裂纹萌生行为。方法采用电化学实验和浸泡实验研究X80管线钢在近中性p H溶液中的腐蚀行为,采用慢应变速率拉伸实验研究X80管线钢在近中性p H溶液中,在自腐蚀电位和外加电位下的应力腐蚀裂纹萌生行为。结果 X80管线钢在近中性p H溶液中的极化曲线只有活化区,没有钝化区,其自腐蚀电位约为-750 m V,浸泡195天后,试样表面没有氧化膜出现,但是观察到点蚀坑。在自腐蚀电位下,X80管线钢试验表面有大量的应力腐蚀裂纹;在-500 m V阳极外加电位下,X80管线钢试验表面几乎没有观察到应力腐蚀裂纹;在-850 m V阴极外加电位下,X80管线钢试验表面的应力腐蚀裂纹很少,但是随着外加阴极电位负移到-1300 m V时,X80管线钢试验表面的应力腐蚀裂纹增多。结论 X80管线钢在近中性p H溶液中发生均匀腐蚀,但是夹杂物剥落能在X80管线钢表面形成点蚀坑。在近中性p H溶液中,在自腐蚀电位下,X80管线钢应力腐蚀裂纹萌生敏感性最强;外加阴极电位抑制应力腐蚀裂纹萌生,但是随着外加阴极电位的负移,应力腐蚀裂纹萌生敏感性增强;外加阳极电位下,由于均匀腐蚀的作用,应力腐蚀裂纹萌生敏感性较弱。     

Elektrochemische Untersuchung der Rißbildung bei Schwingungsrißkorrosion im stabil-passiven Werkstoffzustand

Electrochemical investigation of crack initiation during corrosion fatigue of stainless steels in the passive state The corrosion fatigue behaviour of three stainless steels - ferritic (12% Cr), austenitic (type 316 Ti) and austenitic-ferritic (type 31803; Duplex stainless steel) - was studied under rotating bending moments in aqueous sulphuric acid of 30°C. An instrumental set-up for recording the transient currents of specimens during potentiostatically controlled corrosion fatigue is described. Based on this transient current signal technique, three stages on the corrosion fatigue process can be discerned. In the incubation period, small stochastic current transients are caused by the response of the passive layer to alternating stresses and environmental conditions. The appearance of sinusoidal current signals indicates crack initiation whereas the phase angle between a fixed marker - i.e. a light barrier signal -, and the anodic amplitude represents the site of initiating cracks. Finally, the crack growth period is characterized by an increasing cell current and steadily growing sinusoidal current signals caused by the interplay of microplastic and repassivation processes at the crack tip.     

De-alloying and dissolution induced cracking of the Zinc-iron δ phase

The preferential anodic dissolution of zinc from the brittle zinciron δ phase has been investigated in acidified sulphate solutions. From electrochemical measurements, light optical and scanning electron microscopy, electron microprobe analysis and X-ray diffraction experiments it is deduced that this process results in the formation of considerable concentration gradients within the alloy phase that set up a local state of stress and thus induce the formation of cracks. By the interplay of de-alloying and crack propagation the corrosion degradation of the δ phase is significantly enhanced. Diffusion driven phase transformations to form the iron-rich intermediate phases, Γ₁ and Γ, were not observed. The ultimate corrosion morphology that results from the de-alloying of the δ phase is rather a sponge structure which appears to consist of porous ferrite with a zinc concentration that considerably exceeds its room temperature solubility limit.     

载荷对砂浆中碳钢腐蚀的影响

给砂浆试样施加三点载荷,测试砂浆内碳钢的腐蚀电位、腐蚀电流密度和阻抗等随着时间的变化;结合砂浆试样表面裂纹的分布,研究了载荷大小对砂浆内钢筋腐蚀的影响。结果表明,载荷对砂浆中碳钢腐蚀的影响可分为两个阶段,浸泡初期,载荷主要影响溶液在砂浆中的渗透过程;后一阶段载荷主要通过改变钢筋的应变状态而加速钢的腐蚀。随着时间的延长,溶液渗透到钢筋表面且引起钢筋腐蚀后,钢筋的腐蚀电位和腐蚀电流密度都趋于稳定并与载荷有关,载荷较高时钢筋的腐蚀电位较负、腐蚀电流密度较高。     

The effect of electrolyte stirring and laminar flow on measured electrochemical noise

In the present paper the influence of various types of electrolyte movement on the characteristics of measured electrochemical noise (EN) has been investigated. For this reason the measurements were performed during different phases: in a still solution, during stirring, as well as in laminar flow. In order to relate the measured EN to the development of corrosion processes, digitized images of the electrodes were recorded continuously during these measurements. After the tests were finished, the corroded electrode surfaces were examined also by the scanning electron microscope (SEM). It was established that the characteristics of EN, in general, change significantly with the movement of the solution. On the other hand, no clear difference was observed between the EN signals measured during stirring and those obtained during laminar flow. This observation, combined with the results obtained in a passive environment and in a very aggressive still solution, confirmed that the main source for the change of EN characteristics during electrolyte movement is the transformation of corrosion processes. The direct effect of the electrolyte movement on the measured EN (modulation of signals due to spatio‐temporal flow disturbances) was found to be insignificant compared to the influence of the corrosion processes themselves.     

Effect of W and Ce additions on the electrochemical corrosion behaviour of 444-type ferritic stainless steel

In order to achieve excellent heat resistance and corrosion resistance, high melting-point and rare earth elements were added to ferritic stainless steel. The effect of W and Ce addition in ferritic stainless steel on the corrosion resistance in 0.1?M NaOH solution was studied. Potentiodynamic polarisation curves indicated that W- and Ce-containing samples had lower corrosion rate and lower passive current density. The electrochemical impedance spectroscopy measurements revealed that W–Ce-containing samples had higher polarisation resistance values and thicker passive films. Capacitance analysis indicated that the semiconducting behaviour and the properties of passive film remain unchanged. The addition of W and Ce led to a decrease in donor and acceptor density, which improved the passive film stability.     

Micro-electrochemical characterization of corrosion of pre-cracked X70 pipeline steel in a concentrated carbonate/bicarbonate solution

The electrochemical corrosion behavior of a stressed, pre-cracked X70 pipeline steel was studied in a bicarbonate/carbonate solution by electrochemical and micro-electrochemical measurements, numerical calculation and surface analysis technique. The effects of stress and potential on passivity, corrosion and electrochemical behavior of the steel at crack-tip were mechanistically determined. It was found that the passive film formed at crack-tip was less stable than that formed in the region ahead of the crack. Moreover, the crack-tip is more susceptible to pitting corrosion than other region of the specimen. The applied stress enhances the anodic dissolution of the steel. In particular, the stress concentration at crack-tip further increases the local anodic dissolution rate. The enhancement of the anodic dissolution of the steel at crack-tip is also resulted from the formation of a galvanic couple, i.e., the crack-tip as an anode and the surrounding region as a cathode.     

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.     

Discussion on the equivalent conductivity and resistance of stress corrosion cracks in eddy current simulations

This study evaluates the numerical modeling of stress corrosion cracks from the viewpoint of eddy current simulations. Five stress corrosion cracks are artificially introduced into austenitic stainless steel plates measuring 25 mm in thickness, and then eddy current inspections are conducted to gather eddy current signals and destructive tests performed to confirm the true profiles of the cracks. The cracks are carefully introduced so as not to cluster, and the data obtained enables discussion of the modeling of a single crack quantitatively. Subsequent numerical simulations model each crack as a rectangular region with a fixed width and uniform conductivity, and evaluate the equivalent width and conductivity suitable for modeling the crack. The evaluation reveals that it is not reasonable to correlate the size of a crack and only its width or conductivity, and larger cracks tend to have greater resistance, which is defined as the width divided by conductivity. Furthermore, the values of width and conductivity depend also on the exciting frequency and probe used; however, the resistance remains almost unchanged by the experimental condition.     

Discussion on modeling of thermal fatigue cracks in numerical simulation based on eddy current signals

This study evaluates modeling of thermal fatigue cracks by the finite element method from the view point of eddy current testing. Five artificial thermal fatigue cracks introduced into type 304 stainless steel plates were prepared for the research. Eddy current signals were gathered by a differential type plus point probe and subsequent destructive tests were performed to confirm the true profiles of the cracks. Numerical simulation based on the results of eddy current testing and destructive tests were carried out to show how the thermal fatigue cracks should be modeled in numerical simulations. The results of the numerical simulations revealed that thermal fatigue cracks tend to be much less conductive than stress corrosion cracks if they are assumed to have uniform conductivity inside. The results also imply that taking consideration of magnetization induced by the thermal fatigue process enables eddy current signals to be analyzed more quantitatively.     

Study on the effect of PbO on the passive behavior of alloy 690 in sodium hydroxide aqueous solution

The corrosion behaviors of alloy 690 in 1 wt pct sodium hydroxide aqueous solutions with and without PbO were characterized by electrochemical measurements, including potentiodynamic polarization, electrochemical impedance spectroscopy and Mott-Schottky measurement. The results reveal that there is a great difference between the corrosion behaviors of alloy 690 in the two tested solutions. The corrosion current density in PbO-containing solution is higher than that in the solution without PbO, which may be attributed to the anodic dissolution of metallic lead deposited at the early stage of polarization. The electrochemical impedance of the specimen in PbO-free solution is higher than that in the solution containing PbO. The slope of the Mott-Schottky curve in Pb-containing solution decreases notably compared with that in the Pb-free solution, which can be interpreted by the chromium depletion in the duplex structure of passive films.     

Effect of Strain Rate on Cathodic Reaction During Stress Corrosion Cracking of X70 Pipeline Steel in a Near-Neutral pH Solution

The effect of strain rate on cathodic reactions of X70 pipeline steel during stress corrosion cracking in a near-neutral pH solution was investigated by electrochemical impedance spectroscope and potentiodynamic polarization curve measurements as well as slow strain rate tests. A local additional potential model was used to understand mechanistically the role of strain rate in electrochemical cathodic reaction. It was found that an application of elastic stress would not affect the electrochemical stable state of the steel specimen at a macroscopic scale. Under a weak cathodic polarization, the interfacial charge-transfer process occurring on steel contains both cathodic and anodic reactions. Since the anodic reaction process is still significant, localized dissolution could occur even at such a cathodic potential, resulting in generation of corrosion pits. These pits could be the start sites to initiate stress corrosion cracks. Strain rate affects the corrosion reaction, which is associated with the generation of dislocation emergence points and slip steps on the specimen surface, resulting in a negative local additional potential to enhance the cathodic reaction locally.