An investigation of corrosion-induced stress during SCC

TEM (Transmission Electron Microscope) observations show that corrosion process during stress corrosion cracking (SCC) enhances dislocation emission and motion; and microcrack of SCC initiates when the corrosion-enhanced dislocation emission and motion reaches a certain condition. The passive film or dealloyed layer formed during corrosion or SCC can induce a large tensile stress, which can assist the applied stress to enhance dislocation emission and motion, and then SCC occurs. Experiments show that the variation of SCC susceptibility of brass,α-Ti and stainless steel with the applied potential and pH value of the solution is consistent with that of the corrosion-induced additive stress. Molecular dynamics simulations show that a dealloyed layer can generate a tensile stress; and the corrosion (dealloyed layer)-induced tensile stress can assist the applied stress to enhance dislocation emission and crack propagation.     

Correspondence between Susceptibility to SCC of 7050 Aluminum Alloy and Passive Film-induced Stress at Various pH Values

The passive film-induced stress and the susceptibility to SCC of 7050 aluminum alloy in 3.5%sodium chloride solution at various pH values were investigated by slow strain rate testing(SSRT) and flowing stress differential method.The results showed that the passive film-induced stress and the susceptibility to SCC decreased with increasing pH values when pH≤7,while they increased with increasing pH values when pH7.However,the corrosion type was interpreted as exfoliation corrosion when pH=l and 14,and there was no film formed on the surface of the specimens.The whole variation plots of film-induced stress and the SCC susceptibility with pH values were both presented as a valley shape.The symbol and amount of the film-induced stress were related to the compositions of the passive film,which were analyzed using X-ray photoelectron spectroscopy(XPS).     

SCC Susceptibility of Steel 16Mn in Nitrate Solution and Its Mechanism

The SCC(stress corrosion cracking )susceptibility of steel 16Mn in nitrate solution was studied.The results showed that applied potential polarization would accelerate (anodic polarization)or retard(cathodic polarization)the SCC process.The study on phase electrochemistry revealed that there was significant difference in electrochemical performacnce between ferrite and pearlite of steel 16Mn.Pearlite preferentially corroded under the action of galvanic cell.The observation on time and in situ showed that corrosion started first at the phase boundary between ferrite and pearlite,and the pearklite gradually corroded until disappared.and then corrosion corssed the phase boundary extending into the ferrite phase.According to this,an anodic dissolution mechanism of SCC was proposed ,on which pre-existing active path and phase electrochemistry(PEAP-PEC) jointly came into action (SCC mechanism of PEAP-PEC).     

Film-induced stress enhancing stress corrosion cracking of austenitic stainless steel

A constant deflection device designed for use within a transmission electron microscopy (TEM) was used to investigate the change in dislocation configuration ahead of a crack tip during stress corrosion cracking (SCO of type 310 austenitic stainless steel in a boiling MgCI2 solution, and the initiation process of stress corrosion microcrack. Results showed that corrosion process during SCC enhanced dislocation emission, multiplication and motion. Microcracks of SCC were initiated when the corrosion-enhanced dislocation emission and motion reached critical state.A passive film formed during corrosion of austenitic stainless steel in the boiling MgCI2 solution generated a tensile stress. During SCC, the additive tensile stress generated at the metal/passive film interface helps enhance dislocation emission and motion.     

Pitting corrosion of 316L stainless steel under low stress below yield strength

Pitting corrosion of 316L stainless steel(316L SS)under various stress was studied by potentiodynamic polarization,electrochemical impedance spectroscopy(EIS)and Mott-Schottky(M-S)analysis in 3.5% NaCl solution.The results of polarization curves show that,with the increase of the stress,the pitting potentials and the passive current density markedly decrease firstly(180 MPa),and then increase greatly(200 MPa).The corresponding surface morphologies of the samples after the polarization test well correspond to the results.Mott-Schottky analysis proved the least Cl-adsorbed to the surface of passive film with more positive flat potential,indicating that a moderate stress could increase the pitting corrosion resistance of 316L SS in 3.5% NaCl solution.     

Corrosion-enhanced dislocation emission and motion resulting in initiation of stress corrosion cracking

A special constant deflection device for TEM has been designed, and then change of dislocation configuration ahead of a crack tip during stress corrosion cracking (SCC) of brass in water and of Ti-24Al-11Nb alloy in methanol and initiation of SCC can he observed in TEM In situ tensile test in TEM for brass was carried out for comparison The results show that anodic dissolution during SCC can facilitate dislocation emission, multiplication and motion, and a dislocation free zone (DFZ) is formed The stress at a particular site in the DFZ, which is an elastic zone and is thinned gradually through corrosion-enhanced dislocation emission and motion, is possibly up to the cohesive strength, resulting in initiating of a nanocrack of SCC in the DFZ or sometimes at the crack tip. Because of the action of the corrosion solution the nanocrack of SCC propagates into a cleavage or intergranular microcrack rather than blunts into a void like in situ tension in TEM     

Localized electrochemical impedance spectroscopy study on the corrosion behavior of Fe-Cr alloy in the solution with Cl− and SO4 2−

The corrosion behaviors of Fe-Cr alloy under three different pH values solutions with Cl- and SO42- were investigated by localized electrochemical impedance spectroscopy (LEIS) measurements and the corrosion products were analyzed by laser Raman spectrometry. The results show that the high corrosion resistance of Fe-Cr Alloy is attributed to a passive film which is formed more easily when the alloy contains a large quantity of Cr element. However, its corrosion resistance varies in the solutions with different pH values, especially in the initial corrosion. The average impedance values in neutral and alkaline solution are much higher than that in acidic solution because the passive film is more likely to dissolve in the acidic condition. Moreover, the destructive effect of Cl- and SO42+ ions on the passive film is also demonstrated in corrosion process through the change of the impedance value with the steeping time.     

Effects of Strain Rate on Stress Corrosion of S355 Steel in 3.5% NaCl Solutions

The stress corrosion of S355 steel in 3.5% NaCl solution under the different strain rates was analyzed with the slow strain rate test(SSRT), the stress corrosion cracking(SCC) behaviors of S355 steel under the different strain rates in the solution were investigated, and the fracture morphologies and compositions of corrosion products under the different strain rates were analyzed with scanning electron microscopy(SEM) and energy dispersive spectrometerry(EDS), respectively. The experimental results show that the SCC sensitivity index is the highest when the strain rate is 2×10-6, and the medium corrosion is the main reason resulting in the highest SCC sensitivity index. The SCC sensitivity index is the least when the strain rate is 5×10-6, and the stress is the main reason resulting in the stress corrosion. The SCC sensitivity index is the middle when the strain rate is 9×10-6, the interaction of stress and medium is the stress corrosion fracture mechanism.     

Rock fracture under anti-plane shear (Mode Ⅲ) loading

Anti-plane punch-through shear test and anti-planefour-point bending test are used to study the crack initiation and propagation under anti-plane shear (Mode Ⅲ) loading. The tensile and shear stresses at the crack tip are calculated by finite element method. The results show that under Mode Ⅲ loading the maximum principal stress σ1 at crack tip is smaller or a little larger than the maximum shear stress τmax. Since the tensile strength of brittle rock is much lower than its shear strength, σ1 is easy to reach its critical value before τmax reaches its critical value and thus results in Mode I fracture. The fracture trajectory is helicoid and the normal direction of tangential plane with the fractured helicoid is along the predicted direction of the maximum principal stress at the notch tip. It is further proved that Mode Ⅰ instead of Mode Ⅲ fracture occurs in brittle rock under Mode Ⅲ loading. The fracture mode depending on the fracture mechanism must be distinguished from the loading form.     

Investigation of stress corrosion cracking initiation of 7A52 aluminum alloy

The stress corrosion cracking(SCC) behaviour of 7A52 aluminum alloy in air and in 3.5% NaCl solution was researched by slow strain rate test(SSRT) and SEM-EDS. The SCC susceptibility was estimated with the loss of the reduction in area. The experimental results indicate that the SCC susceptibility of 7A52 aluminum alloy in 3.5% chloride solution is the highest at strain rate of 1×10-6 s-1. The lowest one is under the condition of 1×10-5 s-1. Stress concentration and anode dissolving around Al-Fe-Mn intermetallics initiate micropores which will result in microcracks. The existence of intermetallics in the microstructure may play an important role in understanding the SCC initiation mechanisms of 7A52 aluminum alloy.     

Behavior of stress corrosion cracking in a magnesium alloy

Slow strain rate testing (SSRT) was employed to study the stress corrosion cracking (SCC) behavior of ZE41 magnesium alloy in 0.01 M NaCl solution. Smooth tensile specimens with different thicknesses were strained dynamically in both longitudinal and transverse direction under permanent immersions at a strain rate of 10⁻⁶ s⁻¹. It is found that ZE41 magnesium alloy is susceptible to SCC in 0.01 M NaCl solution. The SCC susceptibility of the thinner specimen is lower than that of the thicker specimen. Also, the longitudinal specimens are slightly more susceptible to SCC than the transverse specimens. The SCC mechanism of magnesium alloy is attributed to the combination of anodic dissolution with hydrogen embrittlement. Funded by the National Natural Science Foundation of China (No. 50771093)     

Effect of hydrogen on the stress corrosion cracking behavior of X80 pipeline steel in Ku'erle soil simulated solution

Hydrogen was a key factor resulting in stress corrosion cracking (SCC) of X80 pipeline steel in Ku'erle soil simulated solution. In this article, the effect of hydrogen on the SCC susceptibility of X80 steel was investigated further by slow strain rate tensile test, the surface fractures were observed using scanning electron microscopy (SEM), and the fracture mechanism of SCC was discussed. The results indicate that hydrogen increases the SCC susceptibility. The SEM micrographs of hydrogen precharged sample...     

Effect of deteriorated microstructures on stress corrosion cracking of X70 pipeline steel in acidic soil environment

In order to investigate stress corrosion cracking (SCC) of X70 pipeline steel and its weld joint area in acidic soil environ-ment in China, two simulating methods were used: one was to obtain bad microstructures in heat affected zone by annealing at 1300°C for 10 min and then, quenching in water; the other was to get different simulating solutions of acidic soil in Yingtan in south-east China. The SCC susceptibilities of X70 pipeline steel before and after quenching in the simulating solutions were analyzed...     

X70钢在鹰潭酸性土壤中的应力腐蚀行为

在不同的阴极保护电位下,采用慢应变速率拉伸试验(SSRT)和电化学方法研究X70钢在水饱和鹰潭土壤中的应力腐蚀开裂行为,为酸性土壤地区的X70钢管线的腐蚀防护提供基础参考数据.结果表明,X70钢在实验所用的酸性土壤环境中能够发生穿晶应力腐蚀裂纹(TGSCC);SCC萌生与外加保护电位有关,外加电位较高、X70钢完全受阳极过程控制时的SCC敏感性较低;外加电位较低、X70钢受阴阳极混合电极过程控制或完全受阴极过程控制时均能发生SCC,且其敏感性随外加电位的降低而增加,且完全受阴极过程控制时的SCC敏感性大大高于其它情况.