Stress corrosion cracking initiation under the disbonded coating of pipeline steel in near-neutral pH environment

A novel test setup has been used in this study to simulate stress corrosion cracking initiation under a disbonded coating on an X-65 pipeline steel. In this setup, the synergistic effects of cyclic loading, cathodic protection and soil solution environment under disbonded coatings have been considered. When the X-65 pipeline steel was exposed to the test environment, there existed a wide range of corrosion products on the steel surface in the gradient of cathodic protection. Increasing the test time and the maximum stress increased the possibility of stress corrosion cracking initiation in regions with a high susceptibility to pitting corrosion.     

Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure

An investigation has been undertaken to establish the effect of surface preparation method on the susceptibility of a 304 stainless steel to stress corrosion cracking under simulated atmospheric corrosion conditions. MgCl₂ was deposited onto four-point bend specimens, which were then placed in a chamber with a relative humidity of 45% and temperature of 60 °C. These test conditions were designed to reflect external exposure of stainless steel components in industrial plant, including nuclear reactor components, situated in a coastal region, but with the severity of the exposure conditions enhanced to allow discrimination of the effect of surface preparation in a short timescale (up to 1500 h). Four surface preparation methods were evaluated: transverse grinding, longitudinal grinding, transverse dressing using an abrasive flap wheel, and transverse milling. For each case, surface topography, surface defect mapping, near-surface microhardness mapping, residual stress and electron back-scattered diffraction measurements were undertaken. Stress corrosion cracks were observed for the ground and milled specimens but not for the dressed specimens, with cracks apparently originating at corrosion pits. The density of cracks increased in the order: transverse ground, milled and longitudinal ground, with the cracks notably much smaller in length for the transverse ground condition. The propensity for cracking could be linked to the high residual stress and apparent nanocrystalline microstructure at the surface. There was a greater propensity for pitting to initiate at local defect sites on the surface (laps, deeper grooves). However, the tendency was not overwhelming, suggesting that other factors such as more general roughness or the distribution of MnS inclusions had an influence, perhaps reflecting the severity of the environment.     

Effect of inclusions on initiation of stress corrosion cracks in X70 pipeline steel in an acidic soil environment

The effect of inclusions on the initiation of stress corrosion cracking (SCC) X70 pipeline steel was investigated in an acidic soil solution using slow strain rate test, scanning electron microscopy and energy-dispersive X-ray techniques. The results demonstrated that stress corrosion cracks are not initiated in X70 steel when it is under anodic polarization. At cathodic polarization, hydrogen evolution is enhanced, and hydrogen is actively involved in SCC processes. Two types of inclusions exist in the steel and play different role in crack initiation. The inclusions enriching in Al are brittle and incoherent to the metal matrix. Microcracks and interstices are quite easily to be resulted in at the boundary between inclusions and metal. There is no crack initiating at inclusions containing mainly Si.     

Einfluß der Oberflächenbeschaffenheit auf die Beständigkeit nichtrostender,austenitischer Chrom-Nickel-Stähle gegen transkristalline Spannungsrißkorrosion

Influence of surface conditions on the resistance of stainless austenitic bromium nickel steels to transcrystalline stress corrosion Grinding weld seams may result in an increased susceptibility to stress corrosion cracking of the areas treated in that way. This susceptibility may be eliminated by an ultimate pickling step; the thickness of the layer to be removed is 0.15 to 100 μm, depending on surface roughness. The susceptibility to stress corrosion cracking of the ground zones can be determined, however, only in the boiling 42% MgCl₂-solution; no stress corrosion cracking could be produced in solutions containing from 10 to 3O% MgCl₂ or 10 to 2O% and even 40% CaCl₂. Another possibility to eliminate stress corrosion cracking susceptibility is an ultimate sand blasting which produces compressive residual stresses in the ground surface.     

Electrochemical polarization behavior of X70 steel in thin carbonate/bicarbonate solution layers trapped under a disbonded coating and its implication on pipeline SCC

Potentiodynamic polarization measurements were performed on X70 pipeline steel in thin carbonate/bicarbonate solution layers trapped under a disbonded coating. A conceptual model was developed to illustrate the effects of the thickness and concentration of the trapped solution layer, cathodic protection (CP) potential and stress on stress corrosion crack initiation and propagation in pipelines. It was found that the passivity of the steel depended on the solution layer thickness, and the passive current density decreased with the thinning of the solution layer. With an increasing solution concentration, the role of the solution layer thickness in the steel passivity became unapparent, which was attributed to a strong passivating ability of bicarbonate and carbonate ions. Furthermore, with the decrease of the solution layer thickness, the pitting potential was shifted negatively. However, an increase of the solution concentration enhanced the resistance of the steel to pitting. A pre-application of CP would degrade the passivity of the steel due to the hydrogen-enhanced activity of the steel. Moreover, an applied stress shifted the pitting potential negatively, and decreased the passive potential range.     

Corrosion behavior of API 5L X‐60 pipeline steel exposed to near‐neutral pH soil simulating solution

Steel gas pipelines are exposed externally to damage by surface corrosion and cracking phenomena. They are the main deterioration mechanism under coating failure and cathodic protection (CP) that can reduce the structural integrity of buried gas transmission pipelines where the soil aggressiveness and bacterial activity appear. Corrosion phenomenon is accentuated by the soil parameters influence such resistivity, pH, temperature, moisture content and chemical composition of electrolytes contained in the soil. Soil parameters influence on pipeline steel corrosion behaviour exposed in near‐neutral pH soil simulating solution has been investigated by potentiodynamic polarisation and EIS method. Results showed that the steel corrosion increases, corrosion current density increases with temperature in the range from 20 to 60 °C. The associated activation energy has been determined. Impedance curves showed that the charge transfer resistance (R_t) increases with increasing immersion duration. Parameters such as corrosion current density (I_corr), polarization resistance (R_p), and soil resistivity (ρ) can serve as the parameters for evaluation of soil corrosivity.     

Investigating the mechanism of stress corrosion cracking in near-neutral and high pH environments for API 5L X52 steel

Stress corrosion cracking behaviour of API-5L-X52 steel under cathodic protection in near-neutral and high pH conditions was studied using slow strain rate test method and electrochemical measurements. The slow strain rate test showed ductile and brittle fracture feature at low and high applied potentials, respectively. In order to identify the mechanism contributes in stress corrosion cracking; the electrochemical potentiodynamic polarisation test was done at fast and slow sweep rate. The results revealed that at near-neutral pH condition the anodic dissolution at crack tip was the dominant mechanism. While at high pH medium, the hydrogen based mechanism was dominant.     

Cracking of martensitic alloy EP-823 under controlled potential

The susceptibility of martensitic Alloy EP-823 to stress corrosion cracking was evaluated with and without an applied cathodic potential using the slow-strain-rate (SSR) testing technique. The magnitude of the applied potential was based on the corrosion potential determined by cyclic polarization. The cracking susceptibility in an acidic environment at different temperatures was expressed in terms of the true failure stress (σ_f), time to failure (TTF), and ductility parameters, including percent elongation (%El) and percent reduction in area (%RA). The data indicate that the magnitudes of σ_f, TTF, %El, and %RA were reduced due to cathodic charging. The scanning electron microscopic evaluations of the primary fracture surface of the specimens used in SSR testing revealed a combination of ductile and brittle failures. Further, the secondary cracks at the gauge section of these specimens were characterized by branching.     

Intergranular stress corrosion cracking of copper in nitrite solutions

Semi-hard tubes of deoxidized high phosphorous copper with different levels of tangential residual stresses have been exposed to nitrite solutions in a laboratory heating circuit. After characterization of investigated materials influence of temperature, location of heating, concentration of solution, electrochemical potential, and atmosphere on stress corrosion cracking susceptibility of those copper tubes has been investigated. Threshold stress for crack initiation has been determined. Maximum duration of experiments was 1 month. Breakthrough time of tubes has been measured as criterion for susceptibility to SCC.To vary stress level in a wider range constant load tests on tubes with different degrees of cold working (annealed, semi-hard, hard) were done by mounting them in a steel frame.Stress corrosion cracks were always intergranular. A tenorite layer covered surface of cracked copper tubes. A reaction scheme for ammonia formation is presented. Necessary conditions for formation of stress corrosion are shown and critically discussed.Results show that intergranular cracking takes place at much lower stress levels below yield strength when compared to literature data on transgranular cracking above yield strength. For transgranular cracking cross slipping and cleavage formation as cracking mechanism is confirmed while for intergranular cracking chemical dissolution of grain boundaries plays a more important role.     

The effect of pH,copper content and ammonia concentration on the stress corrosion cracking susceptibility of an aged Cu−4%Ti alloy

Stress corrosion cracking tests of an aged Cu?4% Ti alloy in various copper-ammonia solutions revealed that the alloy was highly susceptible to stress corrosion cracking in highly alkaline solutions with preconcentrated copper, but almost immune in Mattsson solutions (pH 3.9–10.0). The surface film (Cu₂O) may be not an important factor in stress corrosion cracking in this alloy system. Increase in preconcentrated copper content and ammonia concentration leads to a decrease in the time to failure. Preconcentrated copper enhances the stress corrosion susceptibility by promoting preferential dissolution of copper through acceleration of the cathodic reaction. In the highly susceptible solutions, the corrosion process seems to be mainly controlled by diffusion of cupric complex ions and the preferential corrosion of copper under concentration polarization.     

Stress corrosion cracking mechanism of prestressing steels in bicarbonate solutions

Prestressing steels occasionally fail by a process named “stress corrosion cracking”. This process has not been fully elucidated and several theories exists in order to explain the cases in which real structures have collapsed. This paper briefly mentions the different theories and identifies the progress in understanding whether it is necessary to use a testing method, which is able to separate the different steps and mechanisms contributing to the failures.This paper presents the methodology used for inducing controlled localized attack to study the susceptibility of the high strength steels resistance to stress corrosion cracking (SCC). The method is designed to study the growth of cracks initiated from a mechanical notch; the crack is not produced by fatigue.It consists of several stages: coating of the bar with epoxy resin, generation of a small notch, constant load and controlled potential test in the media, mechanical test in air and fractographic study. It allows us to calculate the crack propagation rate and the fracture toughness in the same test.Finally, it has been possible to apply the surface mobility mechanism (SMM) in order to identify the SCC mechanism that operates.     

The improved corrosion resistance of steel in water after abrasive blasting with alumina

Polarization resistance studies and cathodic delamination studies provide evidence that steel abrasively blasted with alumina exhibits a lower initial rate of corrosion in distilled water than similar steels blasted with steel grit or abraded with silicon carbide paper. Auger spectroscopy and electron microprobe analysis revealed the presence of significant aluminum on the surface, largely in the form of embedded particles but possibly also as a constituent of the oxide film. Non-metallic abrasive blasting may have application in the chemical modification of metal surfaces.     

Peculiarities of the Corrosion Protection of Steel Underground Pipelines

The topics of enhancing the corrosion protection efficiency of pipelines are examined. Improuving the quality of the protective coatings enables one to preclude corrosion under the disbonded insulation, as well as the stress corrosion cracking, but causes the increase in the stray-current corrosion and other undesirable effects that are easily removed with the aid of electrochemical protection. Checking the insulation of pipelines upon completed construction by cathodic polarization ensures the reproducibility of the design parameters of the cathodic protection. The topics of the influence of insulating components, as well as protective and anodic groundings, on the effectiveness of corrosion protection are considered.     

New insight into the pit-to-crack transition from finite element analysis of the stress and strain distribution around a corrosion pit

A finite element (FE) analysis has been undertaken to evaluate the stress and strain distribution associated with a single corrosion pit in a cylindrical steel specimen stressed remotely in tension. A key observation was the localisation of plastic strain to the pit walls (just below the surface of the specimen). Simulation of a growing pit in a static stress field indicated corresponding plastic strain rates that were commensurate with values associated with stress corrosion cracking. This observation introduces a wholly new concept in understanding of the evolution of stress corrosion cracks from pits and correlates with recent X-ray tomography measurements.     

Influence of local stress on initiation behavior of stress corrosion cracking for sensitized 304 stainless steel

To investigate the influence of local stress on initiation behavior of stress corrosion cracking (SCC) for sensitized Type 304 stainless steel, cracking process during constant load SCC test was monitored and recorded with an in situ crack observation system. The changes in number of cracks, sum of crack length and cracked area on the specimen surface with test time were identified from the cracking images analyzed by image processing. In the SCC tests, many cracks were initiated and coalesced on the surface, and the coalescence of cracks played an important role to primary crack growth. The influence of applied stress on crack initiation was different from that on crack growth. In addition, there was a difference between influences of stress on incubation period to crack initiation and crack initiation rate. Due to these differences, a stress of 0.8S_y was thought to cause relatively many cracks compared with 0.5S_y and 1.3S_y (S_y = 200 MPa). Through quantitative estimation of distribution in local stress around a crack by finite element analysis method, it was deduced that the crack initiation is influenced not only by bulk stress applied at the end of the body, but also by local stress formed around pre-existing cracks. According to pre-existing cracks, stress enhancement accelerates the crack growth, while the stress relaxation causes the suppression of new crack initiation. Based on the experiment and analysis results, three types of growth process were suggested, which are caused by propagation itself, by new crack initiation at vicinity of the crack tip, and by coalescence of approaching cracks. Then, it was concluded that, in order to predict/simulate the cracking behavior of this SCC system, the influence of local stress on the crack initiation should be taken into account.     

Alumina grit blasting parameters for surface preparation in the plasma spraying operation

This paper examines how the grit blasting process influences the surface roughness of different sub-strates, the grit residue, and the grit erosion. The influence of grit blasting conditions on induced sub-strate residual stresses is also discussed. Aluminum alloy, cast iron, and hard steel were blasted with white alumina grits of 0.5,1, and 1.4 mm mean diameters. Grit blasting was performed using either a suction-type or a pressure-type machine equipped with straight nozzles made of B4C. The influence of the follow-ing parameters was studied: grit blasting distance (56 to 200 mm), blasting time (3 to 30 s), angle between nozzle and blasted surface (30°, 60°, 90°), and blasting pressure (0.2 to 0.7 MPa). The roughness of the substrate was characterized either by using a perthometer or by image analysis. The grit residue remain-ing at the blasted surface was evaluated after cleaning by image analysis. The residual stresses induced by grit blasting were determined by using the incremental hole drilling method and by measuring the de-flection of grit-blasted beams. Grit size was determined to be the most important influence on roughness. The average values of R_a and Rt and the percentage of grit residue increased with grit size as well as the depth of the plastic zone under the substrate. An increase of the pressure slightly increased the values of Ä_a and Rt but also promoted grit breakdown and grit residue. A blasting time of 3 to 6 s was sufficient to obtain the highest roughness and limit the grit breakdown. The residual stresses generated under the blasted surface were compressive, and the depth of the affected zone depended on the grit diameter, the blasting pressure, and the Young’s modulus of the substrate. More-over, the maximum residual stress was reached at the limit of the plastic zone (i.e., several tenths of a mil-limeter below the substrate surface).     

Effects of surface preparation on pitting resistance, residual stress, and stress corrosion cracking in austenitic stainless steels

Surface finishing treatments such as shot blasting and wire brushing can be beneficial in improving the integrity of machined surfaces of austenitic stainless steels. These operations optimize in-service properties such as resistance to pitting corrosion and stress corrosion cracking (SCC). In this study, ground steel surfaces were subjected to a series of sand blasting and wire brushing treatments. The surfaces were then characterized by their hardness, surface residual stress state, and resistance to stress corrosion and pitting corrosion. Some samples were selected for depth profiling of residual stress. It is found that surface hardening and the generation of near-surface compressive residual stress are the benefits that can be introduced by sand blasting and brushing operations.     

TC2钛合金弹性片裂纹原因分析

针对航空发动机尾喷管弹性片多次出现裂纹故障,利用视频显微镜、扫描电镜等设备,通过对弹性片进行外观检查、断口分析、能谱分析、金相检查以及流场和应力分析等方法,确定弹性片裂纹性质和产生原因。结果表明:故障弹性片裂纹性质为多源疲劳,起源于弹性片薄板间的焊缝位置;飞机垂直尾翼处流场异常,造成气动载荷较大,是弹性片产生裂纹的主要原因;同时,弹性片选用0.5 mm厚的TC2薄板强度储备不足,焊缝位置存在未焊合缺陷,形成较大的应力集中,降低了弹性片的抗疲劳性能,促进了疲劳裂纹的萌生。提出相应的改进建议,避免类似故障的发生。     

The stress corrosion cracking behavior of austenitic stainless steels in boiling magnesium chloride solutions

The change in the mechanism of stress corrosion cracking with test temperature for Type 304, 310 and 316 austenitic stainless steels was investigated in boiling saturated magnesium chloride solutions using a constant load method. Three parameters (time to failure; t_f, steady-state elongation rate; l_ss and transition time at which a linear increase in elongation starts to deviate; t_ss) obtained from the corrosion elongation curve showed clearly three regions; stress-dominated, stress corrosion cracking-dominated and corrosion-dominated regions. In the stress corrosion cracking-dominated region the fracture mode of type 304 and 316 steels was transgranular at higher temperatures of 416 and 428 K, respectively, but was intergranular at a lower temperature of 408 K. Type 310 steel showed no intergranular fracture but only transgranular fracture. The relationship between log l_ss and log t_f for three steels became good straight lines irrespective of applied stress. The slope depended upon fracture mode; −2 for transgranular mode and −1 for intergranular mode. On the basis of the results obtained, it was estimated that intergranular cracking was resulted from hydrogen embrittlement due to strain-induced formation of martensite along the grain boundaries, while transgranular cracking took place by propagating cracks nucleated at slip steps by dissolution.     

Stress corrosion cracking of Ni2MnGa magnetic shape memory alloy in water and moist atmosphere

We prepared two kinds of cracks (surface cracks and transfixion cracks) to in situ study stress corrosion cracking of seven-layered (which is denoted as 7 M or 14 M) off-stoichiometric Ni₂MnGa magnetic shape memory alloy in deionized water and moist atmosphere under constant deflection at room temperature. The experimental results showed that the cracks could nucleate and propagate continuously or discontinuously for a distance after some incubation time, and then arrest in water or moist atmosphere under constant deflection. The delayed cracking of Ni₂MnGa alloy is regarded as reduction of the surface energy induced by adsorption of water molecule.