Effects of alternating current on corrosion of a coated pipeline steel in a chloride-containing carbonate/bicarbonate solution

In this work, the alternating current (AC)-induced corrosion of a coated pipeline steel was studied in a chloride-containing, concentrated carbonate/bicarbonate solution, which simulated the trapped high pH electrolyte under coating, by potentiodynamic polarization measurements, immersion tests and surface characterization technique. It was found that an application of AC resulted in a negative shift of corrosion potential of the steel, caused an oscillation of anodic current density, and degraded the steel passivity developed in the solution. With the increase of AC current density, the corrosion rate of the steel increased. At a low AC current density, a uniform corrosion occurred, while at a high AC current density, pitting corrosion occurred extensively on the steel electrode surface. At individual applied AC, there was a higher electrochemical dissolution activity of the coated steel electrode containing a 1 mm defect than that of the electrode containing a 10 mm defect.     

Synergistic Effects of Hydrogen and Stress on Corrosion of X100 Pipeline Steel in a Near-Neutral pH Solution

In this work, scanning vibrating electrode technique and local electrochemical impedance spectroscopy measurements were used to investigate the effects of stress and hydrogen on electrochemical corrosion behavior of a X100 pipeline steel in a near-neutral pH solution. The stress distribution on the test specimen was calculated using the finite element method. Results demonstrated that the hydrogen-charging enhances the local anodic dissolution of the steel, contributing to the formation of a layer of corrosion product. However, there is little difference of the charge-transfer resistance between the regions with and without hydrogen-charging due to rapid diffusion of hydrogen atoms throughout the specimen with time. When the local stress concentration is not significant enough to approach the yielding strength of the steel, the steel is still in a relatively stable state, and there is a uniform distribution of dissolution rate over the whole surface of the steel specimen. Although the stress-enhanced activation is not sufficient to result in an apparent difference of current density of the steel, the activation of the steel would activate dislocations, which serve as effective traps to the charged hydrogen atoms. With the increase of hydrogen concentration, the hydrogen-enhanced anodic dissolution becomes dominant.     

Effects of tritiated hydrogen peroxide in tritiated water containing Cl and CO3 on behavior of welded type S32550 duplex stainless steel

We report an investigation of the corrosion susceptibility of welded S32550 Duplex stainless steel in the presence of tritiated hydrogen peroxide, chloride and carbonate, which are found in radioactive aqueous solutions. It is well known that a structure transformation occurs during welding, this could lead to localized corrosion of the welded zone. The electrochemical behavior of welded S32550 steel was studied using cyclic voltammetry and electrochemical impedance spectroscopy to provide an indication of mechanisms and oxide layer modifications. Increasing hydrogen peroxide concentration produces several effects. Although the corrosion potential does not change, the prepassive current is higher and, depending on passive potentials and hydrogen peroxide concentration, the passive oxide layer and its characteristics change showing the importance of the radiolytic species in passivity. Also, the breakdown potential shifts towards more positive values on increasing ³H₂O₂ concentration. Thicker passive oxide layers should limit localized corrosion. Examination of the impedance spectra indicates ionic diffusion in the outer oxide passive layer and a diffusion barrier effect for the inner oxide. As the hydrogen peroxide concentration is further increased, these effects appear more pronounced. Carbonate ions should keep the alkaline buffer pH constant giving protection from localized corrosion, and ³H₂O₂ should enhance the characteristics of the inner passive oxide layer. Due to the effects of these two parameters: alkaline pH kept constant at the electrode surface and enhancement of the characteristics of the inner oxide layer by ³H₂O₂, no pitting is observed in presence of chloride ions. Also, an equation, giving the pitting potential limit is derived. The ability of the nucleation sites to propagate as metastable pits is limited by the presence of the ³H₂O₂ and CO²⁻₃ buffers.     

Das Korrosionsverhalten von Eisen,Zink, Aluminium und Chromnickelstahl im System Methanol-Wasser-Chlorwasserstoff

Corrosion behaviour of iron, zinc, aluminium and chromium nickel steel in the system methanol/water/hydrogen chloride The influence of water on the corrosion behaviour of pure iron, zinc, aluminium and normal 18 9 CrNi steel in hydrogen chloride containing methanol has been studied. The immersion tests and the tracing of potential dynamic current density potential curves were carried out at 20°C, oxygen being excluded. Even small quantities of water may give rise to a considerable decrease of corrosion rates. This effect is attributed in the case of iron, zinc and chromium nickel steel to increased hydrogen overvoltage, which exhibits a sudden jump between 0 and 15 Vol.-% water. Heterogenous cathodic limiting reaction currents are observed on iron. In the case of aluminium the change in corrosion rates toward lower values is caused by the inhibited anodic partial reaction. Addition of 5 Vol.-% water gives rise to a displacement of the pitting potential by approx. 400 mV in the positive direction, accompanied by a decreasing corrosion rate by about three orders. Anodic polarization of chromium nickel steel reveals that this steel can be passivated only beyond a critical water content of approx. 50 Vol.-%. A further increase of the water content gives rise to a sudden decrease of the critical passivation current density and the passivity current density. Simultaneously, the passivation potential moves toward more negative values.     

氢对310不锈钢钝化膜的影响

采用电化学和ＳＴＭ方法研究了氢对３１０不锈钢阳极极化过程,钝化膜形成过程和表面形貌特征以及膜耐点蚀性能的影响,并进行了相应的理论分析。研究表明：氢会降低不锈钢的自腐蚀电位,缩短钝化区域并使其向低电位方向偏移;降低反应电阻,增大钝化电流工,降低膜在纳米尺度上的结晶度和耐点蚀性能。     

Interpretation of electrochemically and chemically investigated corrosion behaviors of 316L SS in acid chloride environments

The electrochemical behavior of 316L stainless steel was investigated in acid chloride environments, and pitting potentials were determined electrochemically and chemically. An increase in the anodic maximum current density was observed upon decreasing the cathodic potential from which the scan was initiated to determine the polarization curve. To determine the critical pitting potential through the chemical method, the potential was increased by increasing the concentration of ferric ions in ferric chloride while holding the chloride ion concentration constant with sodium chloride. When 316L stainless steel was immersed in 15 g/1 of FeCl₃6H₂O containing the same chloride ion concentration as 5% NaCl with pH=2 at 57°C, the corrosion potential increased to 0.47 V (SHE) within two minutes due to initial passivation. Immediately after reaching 0.47 V (SHE), which was just above the pitting potential of 0.45 V (SHE) determined electrochemically in 5% NaCl (pH=2, 57°C), the corrosion potential continuously decreased, indicating the onset and propagation of pitting corrosion. A correlation between the electrochemical and chemical methods can be verified if the proper measurements are made and the observations are properly interpreted.     

Micro-electrochemical characterization of corrosion of welded X70 pipeline steel in near-neutral pH solution

The local corrosion behavior of welded X70 pipeline steel in near-neutral pH solution was studied by micro-electrochemical measurements, including scanning vibrating electrode and local electrochemical impedance spectroscopy. The microstructure of the welded steel was observed by optical microscopy and scanning electron microscopy. It is demonstrated that the microstructure of weld metal consists of acicular ferrite and grain boundary ferrite, while that of heat-affected zone is a mixture of acicular ferrite, bainitic ferrite and a few martensite/austenite microconstituents. The microstructure of base steel is typically ferrite and pearlite. Electrochemical corrosion mechanism of welded X70 steel does not experience change upon hydrogen-charging, or stressing, or both. Hydrogen-charging is capable of enhancing the local anodic dissolution of the steel. The resistance of corrosion product layer decreases with hydrogen-charging, and heat-affected zone has the largest dissolution current upon hydrogen-charging. The increase of applied stress enhanced the anodic dissolution of welded X70 steel, especially the heat-affected zone, in near-neutral pH solution. Maximum current is observed in heat-affected zone, and increases with the increase of applied stresses. The total synergistic effect of hydrogen-charging (10 mA/cm²) and applied stress (550 MPa) on anodic dissolution of welded X70 steel in near-neutral pH solution is determined to be within the range of 5.7 and 6.5, with a maximum value encountering in heat-affected zone.     

Corrosion of 304 stainless steel exposed to nitric acid-chloride environments

In an effort to examine the combined effect of HNO₃, NaCl, and temperature on the general corrosion behavior of 304 stainless steel (SS), electrochemical studies were performed. The corrosion response of 304 SS was bifurcated: materials were either continuously passive following immersion or spontaneously passivated following a period of active dissolution. Active dissolution was autocatalytic, with the corrosion rate increasing exponentially with time and potential. The period of active corrosion terminated following spontaneous passivation, resulting in a corrosion rate decrease of up to five orders of magnitude. The length of the active corrosion period was strongly dependent on the solution volume-to-surface area ratio. This finding, coupled with other results, suggested that spontaneous passivation arises solely from solution chemistry as opposed to changes in surface oxide composition. Increasing NaCl concentrations promoted pitting, active dissolution upon initial immersion, a smaller potential range for passivity, longer active corrosion periods, larger active anodic charge densities preceding spontaneous passivation, and larger corrosion current and peak current densities. In contrast, intermediate HNO₃ concentrations promoted active dissolution, with continuous passivity noted at HNO₃ concentration extremes. During active corrosion, increased HNO₃ concentrations increased the anodic charge density, corrosion current density, and peak current density. The time required for spontaneous passivation was greatest at intermediate HNO₃ concentrations. Susceptibility to pitting was also greatest at intermediate HNO₃ concentrations: the pit initiation and repassivation potentials decreased with increasing HNO₃ concentration until the HNO₃ concentration exceeded a critical concentration beyond which susceptibility to pitting was entirely eliminated. Increasing solution temperature increased the susceptibility to both pitting and active dissolution.     

Effect of halide ions on passivation and pitting Corrosion of copper in alkaline solutions

The passivity of copper in NaOH and borate buffer solutions containing chloride, bromide and fluoride ions was studied by using cyclic voltammetry and potentiostatic current transient measurements. At scan rates ≥ 20 mVs^?1, the addition of halide ions does not nearly affect the cyclic voltammograms in the absence of pitting. But they differ considerably in presence of pitting. The pitting potential was found to depend on the solution pH and decreased linearly with increase of logarithm of halide ion concentration. The current transients in the passivity as well as in the pitting potential regions were analyzed. Before the pitting, i-time curves were rather similar to those obtained in the plain solutions. At times > 20 seconds, the current varies linearly with the reciprocal of the square root of time indicating diffusional characteristics of the metal corrosion through the passive layer. In presence of pitting, the pitting current versus time relations fit the Engell-Stolica equation.     

Effects of corrosion product deposit on the subsequent cathodic and anodic reactions of X-70 steel in near-neutral pH solution

The effects of corrosion product deposit on the subsequent anodic and cathodic reactions of X-70 steel in a near-neutral pH solution were investigated by localized electrochemical impedance spectroscopy (LEIS), scanning vibrating micro-electrode (SVME) and macroscopic EIS measurements as well as surface analysis technique. It is found that the deposit layer formed on the steel surface is porous, non-compact in nature. The presence of a corrosion product layer would enhance adsorption, but significantly inhibit absorption and permeation of hydrogen atoms into steel. It is due to the porous structure of the deposit that generates a spatial separation of cathodic and anodic reaction sites, resulting in an increased effective surface area for hydrogen adsorption and, simultaneously, a “blocking” effect on hydrogen absorption and permeation. The deposit enhances greatly anodic dissolution of the steel, which is attributed to the adsorption of the intermediate species and the resultant “self-catalytic” mechanism for corrosion of the steel in near-neutral pH solution. In the presence of corrosion product deposit on the pipeline steel surface, pipeline corrosion, especially pitting corrosion, is expected to be enhanced. Stress corrosion cracks could initiate from the corrosion pits that form under deposit. However, deposit does not contribute to hydrogen permeation, although the hydrogen evolution is enhanced.     

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.     

Quantitative characterization by micro-electrochemical measurements of the synergism of hydrogen, stress and dissolution on near-neutral pH stress corrosion cracking of pipelines

Occurrence of stress corrosion cracking of pipelines under a near-neutral pH condition depends on the synergism of stress, hydrogen and anodic dissolution at the crack tip of the steel. In this work, micro-electrochemical techniques, including localized electrochemical impedance spectroscopy and scanning vibrating electrode technique, were used to characterize quantitatively the synergistic effects of hydrogen and stress on local dissolution at crack-tip of a X70 pipeline steel in a near-neutral pH solution. Results demonstrate that, upon hydrogen-charging, the anodic dissolution of the steel is enhanced. The resistance of the deposited corrosion product layer depends on the charging current density. There is a non-uniform dissolution rate on the cracked steel specimen, with a highest dissolution current density measured at crack-tip. For a smooth steel specimen, the synergistic effect factor of hydrogen and stress is equal to 5.4, and the total effect of hydrogen and stress on anodic dissolution of the steel is 7.7. In the presence of a crack, the hydrogen effect factor, stress effect factor and the synergistic effect factor are approximately 4.3, 1.3 and 4.0, respectively. The total effect factor is up to 22.4, which is very close to the 20 times of difference of crack growth rate in pipelines in the presence and absence of the hydrogen involvement recorded in the field.     

Untersuchungen über die Lochfraßkorrosion des passiven Eisens in chlorionenhaltiger Schwefelsäure

Investigation into pitting corrosion of passive iron in sulphuric acid containing chloride ions Pitting corrosion of metallic materials is generally connected with presence of a surface layer giving rise to a local differentiation of the electrochemical behaviour of the metal surface. The pitting corrosion by halogen ions on passive metals is investigated using passive iron in chloride ion-containing sulphuric acid as the model system. Quantitative data are presented concerning the mechanism and kinetics of the individual processes giving rise to pitting corrosion in a chloride ion concentration range covering three powers of ten, and in the whole potential range of iron passivity, from the Flade potential to the transpassive breakthrough potential. Pit formation normally follows a linear kinetic law, the rate depending in particular from the chloride ion concentration and from the thickness of the passive layer. The growth of pit diameters follows a linear kinetic law, too; the dissolution current density in the pits depends from the chloride ion concentration. Comparative investigations carried out on active iron, and potential distribution as measured in the pits show that the metal is active in the pits, too. The heterogeneous mixed electrode condition — active pit/passive metal surface — is stabilised by resistance polarisation. The investigations so far do not permit any statement concerning the specific effect of the chloride ions.     

硫脲对非调质钢在HCl酸洗溶液中的缓蚀作用

目的 研究硫脲(TU)对非调质钢在HCl溶液中的缓蚀作用.方法 通过动电位极化曲线、电化学阻抗谱研究硫脲对非调质钢在HCl溶液中的缓蚀效应,采用KH-7700型三维视频显微镜观察非调质钢的腐蚀形貌.结果 非调质钢在不合缓蚀剂的HCl溶液中的自腐蚀电位(Ecorr)为-0.566V,自腐蚀电流密度(Jcorr)为12.57 mA/cm2.随着HCl溶液中硫脲浓度的增加,非调质钢的自腐蚀电流密度(Jcorr)逐渐减小,反应电阻(Rct)逐渐增大,界面双电层电容Cdl降低,缓蚀效率逐渐增加,阴极极化曲线几乎重合,而阳极极化曲线逐渐正移.当HCl溶液中加入5g/L硫脲时,缓蚀效率达91.17％,效果好于市售酸洗缓蚀剂AS-30,能够有效消除非调质钢在HCl酸洗液中的腐蚀麻点.结论 硫脲对非调质钢在HCl溶液中具有明显的缓蚀效应,能够有效消除非调质钢酸洗过程中的腐蚀麻点.     

Electrochemical corrosion behavior of 300M ultra high strength steel in chloride containing environment

The electrochemical corrosion behavior of 300M ultra high strength steel in chloride containing environment was investigated by potentiodynamic polarization technique, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The results show that uniform corrosion occurs on 300M steel during the electrochemical measurements because no anodic passivation phenomenon is observed on polarization curves within the measurement range. The tests also show that 300M steel is highly susceptible to chloride containing solution, which is characterized by corrosion current density increasing with the addition of chlorides, and corrosion potential shifting towards positive direction and corrosion resistance decreasing, positively suggesting that chloride ions speed up the corrosion rate of 300M steel. Meanwhile corrosion products on the 300M steel surface formed during the salt spray test are too loose and porous to effectively slow down the corrosion rate. Additionally, a schematic structure of uniform corrosion mechanism can explain that 300M steel has better property of stress corrosion cracking (SCC) resistance than stainless steels.     

Environmental factors affecting pitting corrosion of type 304 stainless steel investigated by electrochemical noise measurements under potentiostatic control

Electrochemical noise measurements on anodically polarised type 304 stainless steel surfaces in contact with buffer solutions of neutral pH were performed to study the effect of chloride ions in the nucleation of pitting corrosion. Passive layer stability and susceptibility to pitting corrosion after pickling and passivation at different environmental conditions were also investigated by means of electrochemical current noise measurements under cathodic and anodic polarisation. According to the obtained experimental results pits nucleate independently on the presence of chloride ions. It has been also shown that protectiveness of stainless steel surfaces after pickling strongly depends on the relative humidity of the environment in which the surface is subsequently passivated.     

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.     

Use of Electrochemical Noise (EN) Technique to Study the Effect of sulfate and Chloride Ions on Passivation and Pitting Corrosion Behavior of 316 Stainless Steel

In the present paper, studies were conducted on AISI Type 316 stainless steel (SS) in deaerated solutions of sodium sulfate as well as sodium chloride to establish the effect of sulfate and chloride ions on the electrochemical corrosion behavior of the material. The experiments were conducted in deaerated solutions of 0.5 M sodium sulfate as well as 0.5 M sodium chloride using electrochemical noise (EN) technique at open circuit potential (OCP) to collect the correlated current and potential signals. Visual records of the current and potential, analysis of data to arrive at the statistical parameters, spectral density estimation using the maximum entropy method (MEM) showed that sulfate ions were incorporated in the passive film to strengthen the same. However, the adsorption of chloride ions resulted in pitting corrosion thereby adversely affecting noise resistance (R _N). Distinct current and potential signals were observed for metastable pitting, stable pitting and passive film build-up. Distinct changes in the values of the statistical parameters like R _N and the spectral noise resistance at zero frequency (R°_SN) revealed adsorption and incorporation of sulfate and chloride ions on the passive film/solution interface.     

Elektrochemisches Verhalten von Aluminium und Möglichkeiten des Korrosionsschutzes in der Praxis

Electrochemical behaviour of aluminium and possibilities of practical corrosion protection The characteristic passivity of aluminium is shown by potentialcurrent density diagrams of materials AlMg2Mn0.8 and AlMgSi0.5 in seawater and in special hard water. These diagrams indicate the value of the passivity area, which is limited in anodic direction by the pitting potential. In the corrosion-system the pitting potential is influenced by the alloying elements in aluminium and also by the inhibitors in the corrosive medium. Zinc in aluminium diminishes the passivity area and sodium chromate in the special hard water enlarges the passivity area. Comparison of the corrosion behaviour of AlMg2Mn0.8 and high purity aluminium with respect to only the pitting potential does not permit conclusions to be drawn about the corrosion probability since pitting corrosion can only occur when a critical value is exceeded. The corrosion-sensitive aluminium-recycling material can be cathodically protected by reducing the potential below this value with help of galvanic anodes.     

Electrochemical corrosion behavior of X80 pipeline steel in a near‐neutral pH solution

In this work, the electrochemical corrosion behavior of X80 pipeline steel was investigated in a near‐neutral pH solution using electrochemical impedance spectroscopy (EIC) and photo‐electrochemical (PEC) measurements as well as X‐ray photo‐electron spectroscopy (XPS) technique. The effects of hydrogen‐charging and stress were considered. The results show that the steel is in an active dissolution state, and a layer of corrosion product is formed and deposited on the electrode surface, which is subjected to further oxidation to form ferric oxide and hydroxide. Photo‐illumination enhances anodic dissolution of the steel when it is under anodic polarization due to destroying of the corrosion product film. When the steel is under cathodic polarization, the cathodic current density decreases upon laser illumination due to the photo‐oxidation of hydrogen atoms generated during cathodic reactions, which behaves as an anodic reaction to offset the cathodic current density. Hydrogen‐charging and stress decrease the corrosion resistance of the steel and enhance the dissolution rate of the steel.