Stress Corrosion Process Research Articles

INNOVATIVE INSULATING MATERIAL IN THE ASPECT OF ENSURING THE SAFETY OF HYDROCARBON TRANSPORTATION

Ensuring continuous and safe operation of the main oil and gas pipelines is a critical aspect for preserving the environment and efficient use of hydrocarbon resources. More than half of accidents on trunk gas pipelines occur due to corrosion wear of the metal caused by insufficient protection of the insulation coating. The consequences of such incidents have a devastating impact on the environment and human lives: spilled oil and oil products pollute the soil, and leaking gas can lead to explosions and forest fires.Corrosion processes hiding under the coating due to poor adhesion and due to non-fulfillment of their functions by the coating lead to increased peeling of insulation and the formation of new corrosion foci, including stress-corrosion processes. Consequently, imperfect insulation coating and insufficient adhesion to metal are one of the main causes of accidents on main pipelines.The purpose of this work was to study and prove the chemical interaction of the functional groups of asmol insulation compositions with the metal surface. In the course of work, the following tasks were solved:- assessment of the ratio of initial adhesion parameters obtained in laboratory and route conditions and their values after 1.5 and 6 years of operation;- study of the IR spectrum in order to prove the presence of functional groups in the asmol material;- study of the protective capacity of the asmol coating structure in an aggressive corrosive environment at different temperatures.Methods were used to measure the quantitative adhesion of the anti-corrosion coating and the peel area during cathode polarization, in accordance with State Standard R 51164-98. The composition of the asmol material was studied by IR spectroscopy.It has been shown that functional groups, the presence of which has been confirmed by spectral analysis, of asmol material in the insulation coating make it possible to chemically adhere to steel. Indirect proof of this is the stability of the adhesion strength over time, the cohesive separation of the asmol coating in determining adhesion indicators under trace conditions and the relatively small area of coating detachment during cathode polarization.

In situ monitoring of stress corrosion at the Alloy 625|NaCl interface

Digital holography was used for in situ monitoring of the dynamic stress corrosion processes occurring at the Alloy 625|0.5 M NaCl interface. The introduction of elastic deformation to Alloy 625 through constant extension rate tensile loading increased the anodic and corrosion currents. These increases were attributed to the increased stress caused by elastic deformation, which increased the number of defects in the oxide film on the surface of Alloy 625 and promoted stress corrosion; this was verified by the fact that the accept density (NA) was nearly ten times greater with elastic deformation than without elastic deformation. Intergranular corrosion (IGC) was induced without elastic deformation. IGC induction occurred because chloride ions were readily adsorbed at the grain boundaries and were more active than the grain bodies. However, the application of elastic deformation induced cracks on the alloy surface to enhance stress corrosion, and it inhibited the development of IGC because the cracks were more susceptible to corrosion than the grain boundaries. In-line digital holography revealed that the cracks were initiated in a certain area and progressively advanced to adjacent areas with applied elastic tensile stress.

Evaluation of welded joints of austenitic stainless steels at high temperatures in the presence of SO2, SO3, and O2

This study undertakes a comprehensive examination of crack formation in welded joints of austenitic stainless steel 304, particularly in high-temperature corrosive environments prevalent in sulfuric acid production units. Initially, an investigation was conducted on a steam superheater that had been operational for about 19 years, where a crack formation was discovered. This unit, subjected to a working temperature of approximately 620 °C and exposure to gases composed of SO2, SO3, and O2, allowed for the analysis of corrosion products formed during the crack propagation. Building on the initial findings, a more controlled study was undertaken, focusing on the cracks formed in welded joints using the coated electrode E308L for joining 304H stainless steel plates under similar corrosive conditions. The specimens, constructed using materials typically utilized in sulfuric acid production units, were designed based on the double-beam principle, enabling exposure to both tensile stresses and a corrosive environment at high temperatures. Over a span of 122 days, or roughly 3000 h, microstructural analyses were conducted, revealing that the formation and propagation of cracks preferentially occurred through fragile microconstituents, notably the sigma phase. These findings mirrored the sigma phase formation observed in the initial superheater study and point to a stress corrosion process acting synergistically with sigma phase development, and consequently seen as a probable cause of failure. This comprehensive evaluation, therefore, indicates a significant role of the sigma phase and corrosive environments in influencing crack formation and propagation in welded joints of austenitic stainless steels, which require further studies for preventative strategies in industrial setups.

Effect of cathodic protection potential on stress corrosion susceptibility of X80 steel

By simulating the different electrochemical characteristics of the crack tip and crack wall in the stress corrosion process of X80 steel through the dynamic potential polarization test with different scanning rates, it is reasonable to predict that the crack tip of tensile sample might still be in anodic dissolution state even under cathodic protection, and meanwhile, the hydrogen atoms generated by the cathodic reaction would have a facilitating effect on the anodic dissolution of crack tip. Thus, with the cathodic potential being negatively shifted, the hydrogen-promoted anodic dissolution and the cathodic protection effect may compete at the crack tip of steel, simultaneously. Combined with the results of electrochemical hydrogen permeation and SSRT (Slow strain rate tensile test), the authors quantitatively analyzed the effect of cathodic polarization potential on the kinetics of anodic dissolution at the crack tip, and the results showed that the cathodic protection effect on the specimen at the critical potential (about −950 mV) for stress corrosion of X80 steel is sufficient to offset the hydrogen-promoted anodic dissolution effect caused by the enrichment of hydrogen atoms at the crack tip, compared to that of the free-corrosion condition. Therefore, the dominant factor that caused a significant increase in stress corrosion cracking (SCC) susceptibility, which was indicated by the comprehensive loss rate IΣ>20% of X80 steel and brittle fracture morphology obtained through SEM, was the hydrogen embrittlement mechanism rather than the anodic dissolution mechanism.

In-situ electrochemical analytical effect of aging treatment on stress corrosion behavior of Al–Zn–Mg–Cu aluminum alloy

The stress corrosion cracking (SCC) behavior of Al–Zn–Mg–Cu alloys in different aging treatment states was dynamically studied by slow strain rate tensile (SSRT) combining in-situ electrochemical impedance spectroscopy (EIS). By analyzing the influence of microstructure and precipitation phase on stress corrosion resistance under different aging conditions, it was found that the stress corrosion process can be divided into three stages, the corresponding charge transfer resistance (Rct) shows three evolutionary patterns, with the maximum value occurred when the alloys were under the maximum stress. The regression re-aging (RRA) treatment obtained coarse and discrete grain boundary precipitates (GBPs) with higher Cu content, which improved the SCC resistance of alloys significantly.

Stress corrosion behavior and microstructure analysis of Al-Zn-Mg-Cu alloys friction stir welded joints under different aging conditions

The friction stir welding joints of Al-Zn-Mg-Cu alloy were prepared, then their stress corrosion evolution during slow strain rate tensile testing was studied by in-situ electrochemical impedance spectroscopy. Slow stress corrosion process can be divided into three stages according to the corrosion trend, the corresponding charge-transfer resistance also shows three evolutionary patterns. Grain boundary precipitates in thermal machine affected zone (TMAZ) of the pre-weld RRA treatment are more discrete and coarser, and have a higher Cu content than those of pre-weld double-stage aging treatment TMAZ, which reducing the potential difference with the surrounding substrate and exhibiting higher stress corrosion resistance.

Numerical Simulation of the Stress Corrosion Rupture Zone for Deep Tunnels

Stress corrosion is a geological phenomenon that damages the long-term stability of underground projects. A comprehensive approach for stress corrosion prediction of underground projects combining creep experiment, particle flow code (PFC)and Finite element method (FEM) is proposed. Calibration of the numerical model was undertaken according to the data obtained from laboratory tests. Based on the numerical model, biaxial and triaxial stress corrosion compression tests were conducted to investigate the stress corrosion mechanism of Jinping marble. Through numerical tests, the stress corrosion process was visually observed. According to the development history of microcracks, the stress corrosion rupture process of surrounding rocks can be divided into 4 stages, i.e., rapid growth, slow growth, rapid growth and instant leap. The process was controlled by driving stress ratio and confining stress. On or near the tunnel wall, most stress corrosion microcracks are induced by tensile stress and the direction is parallel to the direction of the maximum principle stress. Near the yield zone of tunnel, the ratio of shear microcracks becomes larger with increasing confining stress. And the amount of microcracks is larger at the rupture stage than that on the tunnel wall, which makes the stress corrosion rupture process look like a ductile failure process. There is a linear relationship between the logarithm of stress corrosion lifetime and driving stress ratio if the confining stress keeps constant. Stress corrosion lifetime will increase when pc becomes higher or driving stress ratio becomes lower. Based on the laboratory and numerical tests, the stress corrosion rupture depth of the tunnel section was predicted using FEM, which can optimize the length of bolts.

Passivation behaviors of 6082 aluminium alloy under stress corrosion process

AbstractWe systematically studied the passivation process of 6082 aluminium alloy under the bending stress situation by combining electrochemical measurement techniques with three‐point bending stress fixture designed by our lab, and then examined the microstructures of corroded specimens to analyze electrochemical corrosion mechanism in 1.5% NaCl solution. The results show that secondary Mg2Si phase acts as the anodic electrode, leading to the self‐corrosion of Mg2Si phase. As a result, the spots of self‐corroded Mg2Si phase within grains act as initial pitting corrosion site, combined with tiny, massive precipitated Mg2Si particles at the grain boundaries and bending stress, leading to the failure of surface of the 6082 aluminium alloy. The corrosion current density increases from 3.422 × 10−7 to 13.77 × 10−7 A/cm2 when bending stress level was increased from 0% up to 100% of yield stress. Passive film formation process occurred between polarization potential area of −1.05 and −0.65 V. Sectional microstructural investigations show that the corrosion starts penetrating vertically into the material before it develops corrosion paths extending parallel to the surface, leading to massive stress‐induced corrosion cracks. The maximum corrosion depth increases from ∼24 μm on specimen without any stress applied to 85 μm when bending stress of 100% yield strength is applied.

Stress-corrosion behavior and characteristics of the friction stir welding of an AA2198-T34 alloy

To better understand the stress-corrosion behavior of friction stir welding (FSW), the effects of the microstructure on the stress-corrosion behavior of the FSW in a 2198-T34 aluminum alloy were investigated. The experimental results show that the low-angle grain boundary (LABs) of the stir zone (SZ) of FSW is significantly less than that of heated affected zone (HAZ), thermo-mechanically affected zone (TMAZ), and parent materials (PM), but the grain boundary precipitates (GBPs) T1 (Al2CuLi) were less, which has a slight effect on the stress corrosion. The dislocation density in SZ was greater than that in other regions. The residual stress in SZ was +67 MPa, which is greater than that in the TMAZ. The residual stress in the HAZ and PM is −8 MPa and −32 MPa, respectively, and both compressive stresses. The corrosion potential in SZ is obviously less than that in other regions. However, micro-cracks were formed in the SZ at low strain rate, which indicates that the grain boundary characters and GBPs have no significant effect on the crack initiation in the stress-corrosion process of the AA2198-T34. Nevertheless, the residual tensile stress has significant effect on the crack initiation during the stress-corrosion process.

Time-dependent model for sand grain deflection including contact maturing under sustained load

The hypothesis of contact maturing advocates static fatigue at contacts between sand grains as the key cause contributing to time effects in sand. The focus of the paper is on a model of an individual grain subjected to a sustained load applied through two steel plates. The grain is characterized by the roughness of its surface. The distinct element method is used to construct a model on two scales: the grain scale and the contact scale. Assemblies of bonded sub-particles are used to model both the grain and the contact region. The bond model includes the stress corrosion process, which simulates decaying strength of bonds and fracture. Two components of the time-dependent grain deflection under sustained loads are the displacement owed to sub-critical fracturing of asperities on the grain surface at the contacts and the creep of the core mineral in the grain. The model demonstrates what may be difficult capturing in physical testing. The simulated nominal contact evolves as the number of contact points increases due to sub-critical fracturing of asperities, leaving the contact firmer. The contact evolution process, as observed in simulations, is consistent with the contact maturing hypothesis.

Modeling time-dependent deformation behavior of brittle rock using grain-based stress corrosion method

This research presents a grain-based stress corrosion model to simulate time-dependent failure of brittle rock. Time-dependent deformation of rock is considered as the manifestation of stress corrosion that may occur inside grains and on grain boundaries. A damage-rate law is adopted and modified to describe continuous degradations of parallel-bond diameters and contact properties in the stress corrosion process. A detailed numerical scheme with self-adaptive determination of time step is used to simulate the time-dependent deformation behavior of rock. The results show that both the short-term mechanical properties and the fatigue test results of Lac du Bonnet granite can be reproduced by the proposed method. In addition, compared with the original parallel-bond stress corrosion method, the modified grain-based stress corrosion method predicts the failure time of rock better under low driving-stress ratios. With the increase of the driving-stress ratio, the failure time of the numerical specimen decreases, and the creep curves tend to have a stair-stepping shape. The numerical simulation results show that an increase of 15 MPa confining pressure under a driving-stress ratio of 0.6 can lengthen the failure time of the rock from 139 days to 11 years.

Theoretical Considerations of Numerical Model for Hydrogen Diffusion Behavior of High-Strength Steel Under Combined Action of Tensile Stress and H 2 S Corrosion, 인장응력과 H 2 S 부식의 복합조건 하에서 고강도 강재의 수소확산 거동 분석을 위한 Numerical 확산모델과 이론적 고찰

The hydrogen diffusion and trapping model with a numerical finite difference method (FDM) was modified and extended to accommodate H2S corrosion and scale forming processes of high-strength steel under tensile stress condition. The newly proposed diffusion model makes it possible to clearly understand combined effect of tensile stress and H2S corrosion process on hydrogen diffusion behaviors. The core concept of this theoretical approach is that overall diffusion behavior is separated into diffusion process through two respective layers: an outer sulfide scale and an inner steel matrix. Diffusion coefficient values determined by curve-fitting permeation data reported previously with the newly proposed diffusion model indicate that the application of tensile stress can contribute to continual increase in the diffusivity in the sulfide scale with a high density of defect. This suggests that the scale with a lower stability under the stress condition can be a key parameter to enhance hydrogen influx in the steel matrix. Consequently, resistance to hydrogen assisted cracking of the steel under tensile stress can be decreased significantly.

Effect of static mechanical load on intergranular stress-corrosion cracking of 7050-T7451 C-Ring specimens

ABSTRACTIn this paper, 7050-T7451 C-Ring stress-corrosion alternate immersion test in neutral 3.5% NaCl solution was conducted to investigate the mechanism of stress-corrosion cracking (SCC). The SCC sequence was proved to comprise dealloying, pitting, micro-cracks, and intergranular (IG) cracks. Besides, anodic dissolution characterised by reticular IG cracks within pitting zones and IG Dimple Rupture is suggested to be the main operative mechanism during this static stress-corrosion process. Since pitting originates from coarse constituents whose number can be reduced by optimised solution treatment, the formation of pitting can, therefore, be delayed by adopting more effective solution treatment before artificial aging to enhance the SCC resistance.

Corrosion fatigue crack growth modelling for subsea pipeline steels

Based on the recent development of research for environment-assisted cracking (EAC), a crack growth model for the corrosion fatigue (CF) of subsea pipeline steels is proposed in this paper. The model adopts a two-component structure in consistence with the physical fact that CF is a mixed process of stress corrosion (SC) and hydrogen-assisted cracking (HAC). Anodic dissolution (AD) and hydrogen embrittlement (HE) are believed to be responsible for SC and HAC and their models are integrated in the general model within a framework of fracture mechanics. The model is then applied to modelling the CF crack growth of X65 pipeline steel and the results show that the shape of the modelled crack propagation curve can be controlled well using the proposed model, and both the exacerbated cracking rate and behavior features can be captured with appropriate consideration of the effects of mechanical and environmental parameters, such as loading frequency, stress ratio, temperature, and hydrogen concentration, etc.

Cluster analysis of stress corrosion mechanisms for steel wires used in bridge cables through acoustic emission particle swarm optimization

Stress corrosion is the major failure type of bridge cable damage. The acoustic emission (AE) technique was applied to monitor the stress corrosion process of steel wires used in bridge cable structures. The damage evolution of stress corrosion in bridge cables was obtained according to the AE characteristic parameter figure. A particle swarm optimization cluster method was developed to determine the relationship between the AE signal and stress corrosion mechanisms. Results indicate that the main AE sources of stress corrosion in bridge cables included four types: passive film breakdown and detachment of the corrosion product, crack initiation, crack extension, and cable fracture. By analyzing different types of clustering data, the mean value of each damage pattern’s AE characteristic parameters was determined. Different corrosion damage source AE waveforms and the peak frequency were extracted. AE particle swarm optimization cluster analysis based on principal component analysis was also proposed. This method can completely distinguish the four types of damage sources and simplifies the determination of the evolution process of corrosion damage and broken wire signals.

Влияние почв на коррозию стальных труб. Моделирование стресс-коррозионных процессов

Влияние почв на коррозию стальных труб. Моделирование стресс-коррозионных процессов

Effect of precipitate state on the stress corrosion behavior of 7050 aluminum alloy

The effect of precipitate size and state on stress corrosion property of 7050 aluminum alloy has been investigated for the first time. The results suggest that 7050 aluminum alloy with fine precipitates in matrix and discontinuous copper-rich precipitates at grain boundary exhibits excellent stress corrosion resistance. The different precipitates in matrix lead to electrochemical heterogeneity and different corrosion susceptibility, due to the different coherence with matrix and alloy element matrix solute content. The discontinuous grain boundary precipitates can hinder corrosion cavities connecting into a continuous crack, and copper-rich grain boundary precipitates can retard hydrogen embrittlement, during the stress corrosion process. Furthermore, the in-situ electrochemical impedance spectroscopy (EIS) in real time suggests that precipitate state has obvious influence on passivation during the stress corrosion process. Fine nanoscale precipitates in matrix are beneficial to maintaining continuous passivation, and the copper-rich grain boundary precipitate is beneficial to repassivation.

OPERATIVE WAY FOR ADJUSTMENT TEMPERATURE OF GAS LEAVING OUT OF COMPRESSOR STATION

Discrete control of the temperature of gas, supplied to the gas pipeline, generates temperature impulses. Each temperature impulse introduced into the gas flow induces unstable thermodynamic processes that encourage the activity of stress-corrosion processes that destroy the pipeline. For improving the quality of adjustment temperature conditions of exploitation gas pipeline, should be used variable frequency drive of fans in gas air coolers, which also helps to reduce specific energy consumption for transportation of gas. Algorithm has been designed to realize the forward adjustment temperature of gas leaving out of compressor station. Algorithm allows predicting the heating rate of gas pipeline with the environment, during all seasons of year. It’s based on dispatcher data. The algorithm combines discrete and frequency adjustment of air coolers fans, equipped with a variable frequency drive.

Phase field modelling of stress corrosion

The evolution of surfaces exposed to an aggressive environment and mechanical load is studied. This is a process of stress corrosion that leads to pitting, crack initiation and growing cracks. In conventional fracture analyses a known or a postulated crack is required. A serious complication is that a large part of the lifetime of a crack or a surface flaw is spent during the initiation of the crack. The knowledge of the mechanisms leading from a pit, flaw, scratch, etc. to a crack is very limited. The motivation for the present study is to provide a model that will increase the understanding of the transition from stress induced surface roughening and pitting to growing cracks.The evolution of the originally flat surface involves free strain energy, chemical energy and gradient energy. A phase field model is used to capture the driving forces that the free energy causes. The flat surface is unstable and develop a waviness. Initially while the waves are shallow a spectrum of favoured spatial frequencies are found to be in accordance with the Asaro–Tiller–Grinfeld theory. Later the surface curvature becomes larger at the depressions than at the higher parts of the surface. This increases the growth rate of formed pits. The pits finally develop into cracks. Also massive branching of pits and cracks is observed.

In-Beam Stress Corrosion Tests for Welded 308 Stainless Steel in Pure Water at 473 K

Stress corrosion tests were performed for welded 308 stainless steel under proton irradiation at 473 K. The concentration of oxygen and hydrogen in the feed water was controlled to be below 5 and 10 ppb, respectively. The in-beam loading condition was 0 and 300 MPa in tension under an irradiation dose rate of 1.3 × 10−7 dpa/s. The electrochemical corrosion potential (ECP) was also measured during the tests. After the corrosion tests, the specimen surface at the fusion zone was examined by SEM for all specimens. Extensive electrochemical reactions on the specimen surface were implied by ECP measurement under the in-beam loading condition. The initiation of surface cracking followed by coalescence of numerous larger corrosion pits at the boundaries of ferrite phases in the austenitic matrix was detected for the in-beam specimens at 300 MPa. Thus, the initial process of stress corrosion cracking at weld metal would be accelerated under irradiation.