Cathodic Polarization Potential Research Articles

Carbon nitride interlayer-enhanced TiO2/WO3 nanorods with surface oxygen vacancies for durable photocathodic performance on 304 stainless steel in simulated seawater

To achieve a long-term photocathodic protective effect on metals in the marine environment, efficient charge mobility and electron storage are essential. Herein, we develop an oxygen vacancy (VO)-involved TiO2-wrapped WO3 nanorod composite by PPy-derived carbon nitride interlayer (TiO2/CNx/WO3) for the photoelectrochemical cathodic protection of 304SS in 3.5 % NaCl solution. The introduced CNx can serve not only as an electron channel to accelerate carrier migration but also as an efficient reductant to generate VO at the interfaces of TiO2 and WO3 during crystallization. The incorporated VO tunes the electronic structure and produces the mid-gap states, which lower the energy barrier for electron-hole separation. Meanwhile, WO3 nanorods as an electron self-storage pool make the photocathodic protection functional in darkness. Consequently, the as-prepared TiO2/CNx/WO3 photoanode yields a photocurrent density of 18.23 μA·cm−2 and a photoinduced cathodic polarization potential of −306 mV in simulated seawater. Even in the absence of light illustration, it can polarize 304SS to approximately −200 mV and continuously to 300 mV after 12h. This work will offer a universal strategy for the rational design and fabrication of highly efficient semiconductor composite materials in the field of photocathodic anticorrosion.

Electrochemical formation and characterization of Ni-Sc intermetallic compounds in LiF-CaF2 eutectic melt

The present investigation aimed to examine the cathodic behavior of Sc(III) on a reactive nickel electrode within a fluoride molten salt system. The electrochemical formation process of Ni-Sc alloy in LiF-CaF2 eutectic melt was investigated using cyclic voltammetry and square wave voltammetry electrochemical techniques. The formation of Ni-Sc intermetallic compounds induces a depolarization phenomenon, hence promoting the underpotential deposition of scandium onto the Ni electrode. The standard Gibbs free energy of formation for Ni-Sc intermetallic compounds were calculated by the use of open-circuit chronopotentiometry measurements. Furthermore, Ni-Sc alloys were fabricated via potentiostatic and galvanostatic electrolysis methods and subsequently characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The results indicated that a sequential emergence of Ni5Sc, Ni2Sc, and combined NiSc+NiSc2 intermetallic compounds escalates as the cathodic polarization potential. Multifarious Ni-Sc intermetallic compounds can be synthesized by controlling the electrolysis conditions, providing new insights for the preparation of Ni-Sc alloy and establishing a theoretical foundation for the synthesis of Ni-Sc intermediate alloy for solid-state reactive cathodes.

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.

Optimization of Pulsed Polarization Condition for Efficient Electrochemical CO2 Reduction

Introduction Massive emission of CO2 gas from fossil fuel combustion causes the severe global greenhouse effect. Electrochemical reduction of CO2 using renewable energy has been attractive due to its simple methodology, while the conversion efficiency may be lessened due to undesirable side reactions such as water decomposition. In this study, CO2 was covered to CO on Ag electrode in aqueous KHCO3 solution using potentiostatic pulse polarization, and optimal pulse condition to improve the conversion efficiency was explored. Experiment An electrochemical cell was composed of a cathode compartment containing an Ag working electrode (W.E.) and an Ag/AgCl reference electrode and an anode compartment containing a Pt counter electrode. Two compartments were separated by a Nafion membrane. An Ag-W.E. was mechanically polished and potentiostatically pulse-polarized in a CO2-saturated 0.1 M KHCO3 solution to form CO. After some polarization period, the conversion efficiency ( η=[CO]/([CO]+[H2]) ) of CO2 to CO was evaluated by using gas chromatography as a function of various parameters such as cathodic polarization potential (E c) and time (t c), rest potential (E a) and time (t a), KHNO3 concentration, etc. Result and discussion When E c was varied, the highest η was obtained around –1.5 to –1.6 V. When t c was varied at E c = –1.6 V, the highest η was obtained around t c = 3 s, as shown in Fig. 1. The t a did not affect η. These results reveal that the t c influences the conversion efficiency viacontrolling the diffusion layer thickness of reactants such as CO2 molecular. The highest η at the optimal pulse condition was ca. 0.9 and is comparable to the value obtained by the previous work using ionic liquid solution (T. Oguma et al., Electrochemistry, 88 (2020) 451–456). Fig. 1. Relationship between the cathodic polarization time t c and reduction efficiency Figure 1

ЗАКОНОМІРНОСТІ КОРОЗІЙНОГО РОЗТРІСКУВАННЯ ТРУБНОЇ СТАЛІ 10Г2ФБ У МОДЕЛЬНОМУ ГРУНТОВОМУ СЕРЕДОВИЩІ ЗА КАТОДНОЇ ПОЛЯРИЗАЦІЇ

Purpose. Investigate the regularities of corrosion cracking of 10G2FB steel under cathodic protection.Methodology. The following methods were used: slow strain rate, scanning electron microscopy, electrolytic hydrogenation, mass measurement.Results. The regularities of corrosion cracking of pipe steel 10G2FB in near neutral soil environment NS4 in the range of potentials from the corrosion potential to -1.2 V were investigated. According to the results of a complex of corrosion-mechanical, electrochemical and physical studies, it was found that with a shift in the cathodic polarization potential in the range of -0,75 V ® -0,95 V ® -1,05 V ® -1,2 V ® -0.95 V the coefficient of susceptibility of this steel to stress corrosion cracking KS increases correspondingly, 1,09 ® 1,11 ® 1,13 ® 1,26. The concentration of hydrogen which penetrating into steel at these potentials changes nonmonotonically: 0 ® 0 ® 0,057 ® 0,018 mol/dm3. The rate of residual corrosion with a potential shift in the series Еcor ® -0,75 V ® -0,95 V ® -1,05 V decreases first sharply, then slowly: 0.035 mm/year ® 0.005 mm/year ® 0.0009 mm/year ® 0.0004 mm/year, i.e. at high cathodic potentials, the applied polarization is spent on the decomposition of the aqueous electrolyte with the release of hydrogen, which penetrates into the steel and causes brittle cracking, which is confirmed by an increasing in the part of brittle fracture in the surface morphology of the specimens.Scientific originality. New results of fundamental research concerning the regularities of stress-corrosion cracking of ferrite-pearlite class steel of pipe assortment 10G2FB under conditions of cathodic protection in the range of potentials from the corrosion potential to -1.2 V have been obtained. It was revealed that a feature of the effect of cathodic polarization in the indicated range of potentials when assessing the tendency to stress corrosion cracking by the KS coefficient is an increasing in the relative narrowing and a decrease in the relative elongation, which generally indicates the embrittlement of the metal under the contact with corrosive medium and potential. Strength characteristics remain almost the same. The greatest tendency to stress-corrosion cracking is observed at a polarization potential of -1.0 V or more negative.Practical value. The developed methodology for a complex study of the regularities of stress- corrosion cracking was used for study of 10G2FB steel of the pipe assortment in a model soil environment NS4 under conditions simulating operating conditions. The new data obtained on the regularities of stress-corrosion cracking of steel will be useful for preventing the stress-corrosion cracking of main gas pipelines during operation.

Stress corrosion cracking behavior induced by Sulfate-reducing bacteria and cathodic protection on X80 pipeline steel

Oil and gas pipelines with cathodic protection (CP) are often affected by Sulfate-reducing Bacteria (SRB), so it is of great relevance to understand the effect of SRB and CP potentials on the stress corrosion cracking (SCC) behavior of pipeline steel. In this paper, the SCC mechanisms in SRB-inoculated and sterile near-neutral soil environments were investigated at different CP potentials. Electrochemical tests and slow strain rate tensile tests showed that at −850 mV the pipeline steel was in a cathodic protected state. At −1000 mV and −1200 mV, strong cathodic polarization promotes the hydrogen evolution reaction, while SRB not only promotes pitting to provide a basis for crack nucleation, but also has a poisoning effect. The results show that the strong cathodic polarization potential and SRB increase the SCC susceptibility of pipeline steel and the hydrogen embrittlement (HE) risk.

Stress-corrosion cracking of pipe steel under complex influence of factors

The stress-corrosion cracking (SCC) of X70 type steel of a controllable rolling for manufacture of pipes was studied in the conditions of complex influence of factors such as full immersion into the solution and periodic wetting, open circuit potential and cathodic polarization potentials (−0.75 Vs.c.e., −1.0 V, −2.0 V), accumulated cyclic stresses simulating a long operation of the gas pipeline, presence of stress concentrator in the form of a notch, influence of protective polymeric coatings with different transition resistance and their combinations. Different combinations of stress-corrosion factors were simulated in the laboratory, their influence on the X70 type steel of a controllable rolling for manufacture of pipes was investigated and the susceptibility of that steel to SCC under these conditions was estimated. The KS coefficient (the ratio of the relative reduction of the sample in air to its relative reduction in the solution) was proposed for estimating the sensibility to SCC. The results of investigations confirmed that mechanical stresses, corrosion-aggressive medium and protective potential are the main factors, while other ones are additional and can increase or decrease the influence of the main factors, without changing the process mechanism.

Preparation of Co3O4@ZnO core-shell nanocomposites with intrinsic p-n junction as high-performance photoelectrodes for photoelectrochemical cathodic protection under visible light

In this work, Co3O4@ZnO nanocomposite photoelectrodes with a core-shell structure were prepared, enabling enhanced photoelectrochemical activity under visible light illumination. This was demonstrated by the photoinduced cathodic polarization of 304 stainless steel in 3.5 wt% NaCl solution. The morphology, composition, crystalline structure and optical absorption properties of the photoelectrodes were characterized by various analysis techniques and UV–vis diffuse reflectance absorption spectroscopy. Results show that, compared to the ZnO nanorods, the Co3O4@ZnO nanocomposite photoelectrodes have a red shift of optical absorption and enhanced photoelectrochemical cathodic polarization on stainless steel under visible light. The enhanced photoelectrochemical performance is attributed to the narrow band gap of Co3O4 and the p-n junction formed at the Co3O4/ZnO interface. The Co3O4@ZnO nanocomposite photoelectrodes fabricated in this work achieve a photocurrent density of 10.5 μA/cm2 and a cathodic polarization potential of −720 mV (saturated calomel electrode) for 304 stainless steel in 3.5 wt% NaCl solution under visible light.

Hydrogen Embrittlement of Dissimilar Steel Welding Joints

Aiming at the problem of hydrogen embrittlement in dissimilar steel welded joints, the hydrogen permeation behavior and stress corrosion sensitivity of the material in sea water were studied by hydrogen permeation test, electrochemical test and slow strain rate tensile test (SSRT), and the fracture morphology was observed with SEM. The results show that the corrosion resistance and hydrogen permeation characteristics of dissimilar steel welded joints are obviously different. The corrosion resistance of the weld zone is between two parent materials. The corrosion resistance of A side HAZ is the worst, and hydrogen evolution reaction is easy. Under the condition of different polarization potential, the material gradually changes from ductile fracture to brittleness with the negative shift of the polarization potential. When the cathodic polarization potential is negative to -950 mV, the welding joints have obvious characteristics of hydrogen embrittlement.

Uniform fine particles of ZrO2 as reinforcement filler in the electrodeposited Cu-ZrO2 nanocomposite coating on steel substrate

Monodispersed spherical particles of ZrO2 were synthesized in an aqueous medium by homogeneous precipitation without being using any capping agent. Results revealed that size and uniformity of the synthesized particles were greatly dependent upon the experimental conditions. SEM, XRD, and FT-IR were used for characterization of selected particle systems. SEM analysis showed the spherical morphology of the as-prepared particles, which remained unchanged even after calcination at 750 °C. XRD results presented poor crystalline nature of the as-prepared solid while the calcined solid was perfectly crystalline. Furthermore, Cu-ZrO2 Nanocomposite coatings, containing variable amounts of the incorporated ZrO2 nanoparticles were deposited on a steel substrate by the electrodeposition from a dispersion of ZrO2 particles in aqueous solutions composed of copper sulfate, boric acid, and sulphuric acid. Effect of ZrO2 concentration in the plating bath applied current and stirring rate of the coating mixture on the co-deposited ZrO2 particles in the composite coatings were evaluated. It was noted that these parameters had noticeable effect on the particles content of the composite coatings. The deposited composite coatings were analyzed for their surface morphology and microhardness respectively by scanning electron microscope and microhardness tester. Similarly, wear resistance of the coated samples was determined by using Ball-On-Disc Tribometer, attached to data acquisition system for continuous monitoring of friction force at the sliding contacts. Selected batches of the coated substrates were tested for their response to corrosion tests at room temperature. It was observed that the amount of the embedded ZrO2 particles in the composite coatings affected the cathodic polarization potential of the test samples. Conclusively, amount of the dispersed ZrO2 particles in the copper matrix turned out to be the key parameter in controlling the properties of the composite coatings.

Effect of Ball Burnishing Treatment on the Fatigue Behavior of 316L Stainless Steel Operating Under Anodic and Cathodic Polarization Potentials

Corrosion fatigue (CF) behavior of AISI 316L was investigated in a 3 pct NaCl aqueous solution at an R = − 1 stress ratio and a frequency of 60 Hz at room temperature. The test scale specimen was 7 cm2. The passive (0 mVRef), pitting (120 mVRef), and cathodic (− 1400 mVRef) polarization potentials were statically applied and recorded during CF tests until the samples were broken. The shaft material surface was treated with a ball burnishing (BB) process. By the results, the fatigue behavior of AISI 316L was affected by polarization potential and surface treatment. Under 0 mV potential charged tests for 5 × 105 cycles, BB treatment raised the CF strength of the shaft material from about 448 to 702 MPa with a percent 57 increase. Fractographic observations revealed that corrosion pits occurred during the experiments where anodic potential was applied and that transgranular surface fractures occurred in all cases.

Effect of BEO in the electrodeposition process of Ni/diamond composite coatings for preparation of ultra-thin dicing blades: Experiments and theoretical calculations

Ni/diamond dicing blades is the main tool for scribe of silicon wafer at present. In order to decrease the width of dicing slot on the wafer, it is necessary to reduce the thickness of blades, and to increase the hardness, toughness and wear resistance of the Ni/diamond composite coatings in the process of electrodeposition. In this paper, 1,4-bis(2-hydroxyethoxy)-2-butyne (BEO) was used as an organic additive in the composite baths containing nickel amino-sulfonate, nickel chloride, boric acid, sodium dodecyl sulfate (SDS) and diamond particles in sizes of 3–5 µm, in order to improve the properties of Ni/diamond coatings and produce ultra-thin Ni/diamond dicing blades. The textures of Ni/diamond composite coatings were mainly Ni (200) and Diamond (111) since the addition of BEO in the baths inhibited the growth of Ni (111) and Ni (220). Quantum chemical calculations and molecular dynamics (MD) simulations showed that BEO could adsorb at the nickel surface strongly and inhibit the electrodeposition of nickel atoms. With the increase in concentration of BEO in the baths, the cathodic polarization potentials shifted to more negative direction and the thicknesses of the coatings decreased. Adding the appropriate amount of BEO (0.1–0.2 g/L) in the baths, the roughness of the coating decreased the number of individual diamond particles in the coating increased, and the hardness and the wear resistance of the coating was improved. When the thickness of Ni/diamond composite coating on aluminum alloy wheeled substrate was 15 µm, the width of its dicing slot was 22 µm. However, the addition of BEO in the bath cannot change the adhesive wear mechanism of the coating.

Cathodic polarization behavior of the structural steel wires under different prestressing conditions

Cold-drawn structural steel wires prestressed to different levels and cathodically polarized at various potentials were investigated using electrochemical techniques and slow strain rate tests. The potentiodynamic polarization revealed that prestressing enhances the active anodic dissolution of the structural steel wires. The corrosion current density and corrosion potential were observed to vary with prestressing levels applied to the structural steel wire specimens. The structural steel wire prestressed to 80% of its original tensile strength and cathodically polarized at −1500mV exhibited the highest current densities and lowest corrosion potentials after potentiodynamic polarization, which indicate that the above prestressing and cathodic polarization conditions lower the corrosion resistance of the material. Moreover, the tensile results show that the structural steel wire prestressed to higher levels and cathodically polarized at lower potentials was more susceptible to degradation of the tensile properties. The structural steel wire prestressed to 80% level and cathodically polarized at a potential of −1500mV exhibits the lowest UTS and ductility. The tensile fracture surfaces of the steel wires prestressed and cathodically polarized under above conditions exhibit mostly quasi-cleavage brittle fracture character. Furthermore, the brittle regions were observed to increase with increasing the prestressing levels and decreasing the cathodic polarization potentials applied to the structural steel wires.

CATHODIC DISBONDING OF INDUSTRIAL CHLORINATED RUBBER-BASED PRIMER USED IN RUBBER/METAL COMPOSITES: AN ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY ANALYSIS

ABSTRACT Chemlok 205 primer is a common bonding agent used together with a top coat in rubber/metal–bonded composites. Recent studies indicated that a stand-alone Chemlok 205 primer failed when its metal substrate underwent corrosion. The failure was suspected to be caused by coating degradation, followed by cathodic disbonding, induced from the corrosion reaction. Therefore, in this work, the effect of cathodic polarization on the coating delamination was further investigated. The work intends to focus on understanding the adhesion failure processes involved. Thus, electrochemical impedance spectroscopy (EIS) was employed on a stand-alone industrial coating Chemlok 205 primer on mild steel substrate subjected to cathodic polarization potential of −0.8 V versus Ag/AgCl reference electrode in 0.5 M NaCl. The coating barrier properties were interpreted through Nyquist and Bode plots of the EIS plots for a selected time interval. The data obtained were modeled against three distinguished electrical equivalent circuits representing the stages of coating degradation against time. Analysis through Nyquist and Bode plots indicated that the stand-alone Chemlok 205 primer coating experienced a significant drop in coating impedance before experiencing adhesion loss. The changes in the coating capacitance and coating resistance indicated that the coating underwent degradation first before adhesion failure.

A Study on the Susceptibility to SCC of 7050 Aluminum Alloy by DCB Specimens.

The stress corrosion cracking (SCC) of different aging states for 7050 aluminum alloy in 3.5% sodium chloride aqueous solution has been studied by means of double cantilever beam (DCB) specimens, cathodic polarization, scanning electron microscope (SEM), transmission electron microscope (TEM) and time-of-flying second ion mass spectrometer (ToF-SIMS). The results showed that the susceptibility to SCC (Iscc) of 7050 aluminum alloy decreases with increasing the aging time. When a cathodic polarization potential of −1100 mV was applied to DCB specimens, the ion current intensity of hydrogen (IH+) near the crack tip and Iscc increased obviously, thus the degree of the diffusion of hydrogen into the grain boundary become more serious. The observation of microstructure indicated that the precipitates on the grain boundary become coarse and are sparsely distributed with increasing the aging time of 7050 aluminum alloy.

Corrosion fatigue behaviors of steel wires used in coalmine

The corrosion fatigue (CF) behaviors of the mining steel wire in different solutions at different applied polarization potentials were investigated in this paper. The surfaces and fracture morphologies of the steel wire at different applied potentials were observed by scanning electron microscope (SEM). The results showed that the CF life of steel wire in acid solution is the shortest. Moreover, the strong anodic polarization potential greatly reduced the CF life of steel wire, while the strong cathode potential did not reduce the CF life. For the smooth steel wire, the hydrogen impacted mainly on the plastic deformation of the wire surface. There was obvious dimple in the fatigue source zone of the wire when coupled with anode potential, and the area of the dimple increased with the increase of the applied anode potential. Conversely, the fatigue source zone of the fracture was relatively smooth at cathode polarization potential, which indicated that the crack propagation followed the mechanism of hydrogen induced cracking.

EFFECT OF CATHODIC POLARIZATION ON HYDROGENEMBRITTLEMENT SUSCEPTIBILITY OF X80 PIPELINESTEEL IN SIMULATED DEEP SEA ENVIRONMENT

Pipeline steels utilized in deep seawater are usually protected cathodically.However,inappropriate operations of cathodic protection systems cause hydrogen embrittlement failures to these high strength steels in seawater which result from the application of excessive negative potentials,leading to massive generation of hydrogen at the protected pipelines' surface.With high strength steels increasingly widely used in the deep sea environment,the basic research to the cathodic protection and susceptibihty to hydrogen embrittlement of high strength steels under such a circumstance is still unfortunately relatively lack and urgently needed to supplement.Electrochemical measurement,hydrogen permeation current detection,slow strain rate tensile test(SSRT) and fracture morphology analysis,therefore,were employed to investigate effect of cathodic polarization level on the susceptibihty of API X80 pipeline steels to hydrogen embrittlement in simulated deep seawater in the present work.The results showed that the applied cathodic polarization potentials significantly affected hydrogen permeation and hydrogen-induced cracking behavior of X80 steels immersed in deep seawater.A linear relationship was found between the hydrogen permeation current densities and cathodic polarization potentials applied according to the findings of the potential dynamic polarization and hydrogen permeation current measurements.SSRT tests suggested that X80 pipeline steelsimmersed in simulated deep seawater didn't show susceptibility to hydrogen embrittlement at open circuit potential,and thus the optimum cathodic protection potential range was supposed to be above —900 mV(vs saturated calomel electrode).Under such a cathodic polarization potential,the hydrogen permeation current densities of X80 pipeline steel specimens were less than 0.1157μA/cm2 and their mechanical properties didn't decrease remarkably.Once the cathodic polarization potentials lower than —900 mV,however,hydrogen permeation current densities and calculated hydrogen embrittlement coefficients Ψ of X80 pipeline steels increased significantly,exhibiting higher susceptibility to hydrogen embrittlement in simulated deep seawater.Furthermore,macro-and micro-morphologies of fracture surface of X80 pipeline steels after SSRT test indicated that the fracture morphology transformed from a dimpled pattern with ductile fracture to a quasi-cleavage pattern when cathodically polarized to lower than-900 mV,i.e.showing obvious brittle failure characteristics.

Electrochromism of rutile nanowires, vertically aligned along the [001] direction, due to alkali metal ion intercalation

Rutile nanowires were grown along the [001] direction and perpendicular to the fluorine doped tin oxide coated glass substrate (R⊥) by a solvothermal process using titanium isopropoxide as the precursor in acidic solution. The edge length of the nanowires obtained in isopropyl alcohol is ten times less than that of the nanowires obtained in water. Due to the high ionic diffusion rate along the rutile [001] direction and good optical transmittance, R⊥ was used as an electrochromism electrode. When −2 V was applied, the color of the electrode became blue with a 60% change in the transmittance for 30 s due to the cathodic polarization potential in LiClO4/PC electrolyte (where PC is propylene carbonate). The lithium ion intercalated/extracted densities of R⊥, P25-coated and rutile nanowires-coated electrodes are 138.6/129.1 mC cm−2, 93.6/83.2 mC cm−2 and 31.7/15.9 mC cm−2 respectively. After 50 cycles, the R⊥electrode still performs very well. Furthermore, a large amount of sodium cation intercalation into TiO2 was observed for the first time at −1.0 V. According to these results, the R⊥electrode is a promising candidate to be a multi-function device.

Effects of SRB on cathodic protection of Q235 steel in soils

AbstractThe effects of sulfate reducing bacteria (SRB) on cathodic protection (CP) of the Q235 steel in the soils have been studied by bacterial analyses, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and energy‐dispersive X‐ray analysis (EDX). The results showed that the pH value of the soil around the steel gradually increased, the number of SRB and the corrosion rate of the steel decreased, and the CP efficiency increased with the increasing of applied cathodic potential. At the cathodic polarization potential of −1050 mV, SRB still survived in the soils. At the same potential, the CP efficiency in the soil without SRB was higher than that with SRB, and the corrosion rate of the steel in the soil with SRB was much higher than that without SRB. The cathodic current density applied for the steel in the soil with SRB was bigger than that without SRB at the same cathodic potential.

Effect of pH and Temperature on Stress Corrosion Cracking of API X60 Pipeline Steel

Stress corrosion cracking (SCC) is the result of the combined influence of tensile stress and a corrosive environment on a susceptible material. This paper analyzes the understanding mechanisms of stress corrosion cracking (SCC) of API X60 pipeline steel. The susceptibility to SCC and mechanism was investigated using slow strain rate tests (SSRT). The SSRT were performed at strain rate of 25.4 × 10−6 mm/sec in a glass autoclave containing a soil solution called NS4 with pH of 3 and 10 at room temperature and 50°C. Cathodic polarization potentials of −200 mV referred to Ecorr was applied. SCC properties were evaluated by SSRT in simulated soil solution (NS4 solution) that is typical environment for SCC under cathodic charging. The results of ratio reduction area (RRA), time to failure ratio (TFR) and elongation plastic ratio (EPR) indicate that X60 pipeline steel was more susceptible to SCC at pH 3 independently of the temperature. At high pH the SCC susceptibility was higher at 50°C. Scanning electron microscopy (SEM) observations of these specimens showed a brittle type of fracture with transgranular appearance. The failure and SCC mechanism of X60 steel into NS4 solution was hydrogen based mechanism. Hydrogen absorption by X60 pipe steel may result in local corrosion (primarily, SCC) of underground pipeline steel. SEM observations revealed the presence of internal cracks in some of these specimens.