On the fundamentals of electrochemical corrosion of X65 steel in CO2-containing formation water in the presence of acetic acid in petroleum production

Electrochemical corrosion behavior of X65 steel in CO₂-containing oilfield formation water in the presence of acetic acid (HAc) was investigated by various electrochemical measurements and analyses as well as thermodynamic calculations of ionic concentrations, reaction rate constants and equilibrium electrode potentials. A conceptual model was developed to illustrate corrosion processes of steel in oilfield formation water system. The anodic reactions of the steel contain a direct dissolution of Fe, Fe → Fe²⁺ + 2e, and the formation of corrosion scale, FeCO₃, by Fe + → FeCO₃ + H⁺ + 2e. The cathodic processes contain the reduction of H⁺, , H₂O and HAc, where reduction of HAc has the least negative equilibrium potential and thus dominates the cathodic process. With addition of HAc in the solution, both cathodic and anodic reaction rates increase remarkably. It is attributed to the fact that HAc inhibits or degrades the formation of protective scales due to the decrease of solution pH. Upon electrode rotation, the measured impedance decreases with the increase in HAc concentration. The FeCO₃ scale will not form on electrode surface. When HAc concentration is less than 1000 ppm, the adsorbed intermediate product is not significant, resulting in generation of a low-frequency inductive loop in EIS plots. When HAc concentration is more than 3000 ppm, the adsorption of intermediate product is significant, generating overlapped capacitive semicircles in EIS measurements.     

The effects of sulfate reducing bacteria on corrosion of carbon steel Q235 under simulated disbonded coating by using electrochemical impedance spectroscopy

The effect of sulfate reducing bacteria (SRB) on the corrosion of the carbon steel Q235 has been investigated in the crevice under the simulated disbonded coating in the soil-extract solutions (SES). The results of electrochemical impedance spectroscopy (EIS) show that the corrosion rate is inhibited in the SES with SRB during the stationary phase of SRB, but enhanced during the death phase. The comparison of the polarization (R_p) and the charge transfer resistances (R_t) has indicated that the biofilm seriously influences the reactive procedure of metal/solution interface. SRB is found in the pits on the surface of the steel.     

Characterization of steel corrosion in mortar by various electrochemical and physical techniques

In the study characterization of steel corrosion in concrete at the macro- and micro-level was performed. Physical (electrical-resistance probes) and electrochemical techniques (coupled multi-electrode arrays) were implemented in order to upgrade the general information that conventional electrochemical techniques can provide. Measurements were performed in mortar exposed to periodic wetting and drying. Steel corrosion damage was assessed by micro X-ray computer tomography (CT) and SEM. The results were compared and interpreted. By combined use of micro-CT and electrochemical methods, new insights into the corrosion mechanisms of steel in concrete were obtained.     

Electrochemical corrosion and impedance study of SAE1045 steel under gel-like environment

To present simulative study on corrosion of metal by sewage sludge, three kinds of gel-like systems based on SiO₂ gel and polyacrylamide gel are proposed. Comparative results of electrochemical polarization and impedance studies together with coupon test of SAE1045 steel under these gel-like surroundings and deionized water are investigated. Obvious pitting corrosion characteristic of the steel can be seen in gel-like systems with decreasing I_corr and more negative value of E_corr. It is considered to be caused by electric field concentration effect at defected sites in covering layer on the surface of the steel formed by gel particles.     

Experimental and theoretical investigation of 3-amino-1,2,4-triazole-5-thiol as a corrosion inhibitor for carbon steel in HCl medium

The inhibition effect of 3-amino-1,2,4-triazole-5-thiol (3ATA5T) was investigated in 0.5 M HCl on carbon steel (CS) by electrochemical impedance spectroscopy and potentiodynamic measurements at various concentrations and temperatures. Results showed that the correlation between experimental (inhibition efficiencies, ΔG_ads, E_a) and quantum calculation parameters (dipole moment, E_HOMO, E_LUMO). The high inhibition efficiency was declined in terms of strongly adsorption of protonated inhibitor molecules on the metal surface and forming a protective film.     

Electrochemical corrosion of X65 pipe steel in oil/water emulsion

The electrochemical corrosion behavior of X65 pipeline steel in the simulated oil/water emulsion was investigated under controlled hydrodynamic and electrochemical conditions by rotating disk electrode technique. Results demonstrated that mass-transfer of oxygen plays a significant role in the cathodic process of steel in both oil-free and oil-containing solutions. Electrode rotation accelerates the oxygen diffusion and thus the cathodic reduction. The higher limiting diffusive current density measured in oil-containing solution is due to the elevated solubility of oxygen in oil/water emulsion. The anodic current density decreases with the increase of electrode rotating speed, which is attributed to the accelerated oxygen diffusion and reduction, enhancing the steel oxidation. Addition of oil decreases the anodic dissolution of steel due to the formation of a layer of oily phase on steel surface, increasing the reaction activation energy. The steel electrode becomes more active at the elevated temperature, indicating that the enhanced formation of oxide scale is not sufficiently enough to offset the effect resulting from the enhanced anodic dissolution reaction kinetics. The corrosion reaction mechanism is changed upon oil addition, and the interfacial reaction is activation-controlled, rather than mass-transfer controlled. When sand particles are added in oil/water emulsion, there is a significant increase of corrosion of the steel. The presence of sands in the flowing slurry would impact and damage the oxide film and oily film formed on the steel surface, exposing the bare steel to the corrosive solution.     

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.     

The inhibition activity of ascorbic acid towards corrosion of steel in alkaline media containing chloride ions

The activity of ascorbic acid towards steel corrosion in saturated Ca(OH)₂ solution containing chloride ions was investigated in this study. Concentration and time dependence of the protective properties of the passive film were acquired by electrochemical impedance spectroscopy. The best inhibitive performance, i.e. the longest pitting initiation time was obtained in the presence of 10⁻³ M ascorbic acid, while both lower and higher concentrations showed shortening of the pitting-free period. The overall behaviour of ascorbic acid was attributed to its ability to form chelates of various solubility having various metal/ligand ratios and oxidation states of the chelated iron. The assumption of ascorbic acid assisted reductive dissolution of the passive layer at higher inhibitor concentrations was confirmed by cyclic voltammetry and ATR FTIR spectroscopy. It is proposed that the overall inhibitive effect at lower concentrations is due to the formation of insoluble surface chelates and the effective blocking of the Cl⁻ adsorption at the surface of passive film. A pronounced inhibitive effect observed after the pitting had initiated was ascribed to the formation of a resistive film at the pitted area.     

The role of some thiosemicarbazide derivatives as corrosion inhibitors for C-steel in acidic media

Corrosion inhibition of C-steel in 2 M HCl was investigated in the absence and presence of different concentrations of some thiosemicarbazide derivatives namely, 1-ethyl-4(2,4-dinitrophenyl) thiosemicarbazide (I), 1,4-diphenylthiosemicarbazide (II), 1-ethyl-4-phenylthiosemicarbazide (III). Weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy techniques were employed. Impedance measurements showed that the double-layer capacitance decreased and charge-transfer resistance increased with increase in the inhibitors concentration and hence increasing in inhibition efficiency. Potentiodynamic polarization study showed that all the inhibitors act as mixed-type inhibitors. Results obtained reveal that the inhibition efficiency (%IE) follows the sequence: I > II > III. The adsorption of these compounds on C-steel surface obeys Temkin’s adsorption isotherm and has a chemisorptions mechanism.     

Initial stages of corrosion pits on AISI 1040 steel in sulfide solution analyzed by temporal series micrographs coupled with electrochemical techniques

This work presents a study of the initial instants in the pitting corrosion of AISI 1040 steel, analyzed by temporal series micrographs coupled to an open circuit potential (E_oc) and polarization curves. During the E_oc measurement, the pit induction time and the initial pit growth in MnS inclusions was detected in alkaline sulfide solution. The pit area behavior has two distinct rate of area changes in specific regions directly associated to current slope changes. Finally, it was possible to create a three-dimensional model of the pit depth evolution on the metal, using Faraday’s law and the bullet-shaped geometry.     

In situ characterization by localized electrochemical impedance spectroscopy of the electrochemical activity of microscopic inclusions in an X100 steel

A localized electrochemical impedance spectroscopy (LEIS) technique was used to characterize in situ the micro-electrochemical activity of inclusions contained in an American Petroleum Institute (API) X100 steel in a near-neutral pH solution. It is found that there exists an electrochemical heterogeneity between inclusions and the adjacent steel matrix. Consequently, a galvanic couple is formed to result in the locally preferential dissolution. The local electrochemical activity of the inclusion depends on its composition. A Si-enriched inclusion is associated with a high electrochemical activity, and the preferential dissolution of the inclusion generates a local microvoid, whose further dissolution initiates a corrosion pit. An aluminum oxide-enriched inclusion is more stable than the adjacent steel matrix. The preferential dissolution would occur on the steel, causing the “drop-off” of the inclusion and generating a corrosion pit.     

Improved corrosion resistance in simulated concrete pore solution of surface nanocrystallized rebar fabricated by wire-brushing

Continuous surface nanocrystallization (SNC) of rebar was achieved through wire-brushing process. A uniform NC layer with thickness of 25 μm and average grain size of 50 nm was formed on the rebar surface. Due to the enhanced passivation performance of the NC layer, corrosion resistance of the SNC rebar was significantly improved in Cl⁻-containing saturated Ca(OH)₂ solution. High-energetic crystal defects of the nano-grains leads to the faster passivation and enhanced stability of the passive film of the SNC rebar.     

Understanding the effects of electrode inhomogeneity and electrochemical heterogeneity on pitting corrosion initiation on bare electrode surfaces

The effects of electrode inhomogeneity (EI) and electrochemical heterogeneity (EH) on pitting corrosion initiation have been analysed by revisiting research findings reported in the literature and experimental evidences obtained in our laboratories using the wire beam electrode (WBE) method. Two mechanisms of pitting corrosion initiation have been identified on bare metal surfaces exposed directly to electrolytes. For WBE surface under free corrosion or low anodic polarisation conditions the initiation of pitting corrosion was found to be due to the disappearance of minor anodes, leading to accelerated dissolution of a few remaining major anodes. The nucleation stage of pitting corrosion appeared to be controlled by EI, while the propagation stage appeared to be determined by EH. For WBE surface under large anodic polarisation the initiation of pitting corrosion was found to be due to the formation of active new anodic sites, which is in agreement with the conventional mechanism of pitting nucleation.     

Experimental and theoretical studies of thiazoles as corrosion inhibitors for mild steel in sulphuric acid solution

The inhibition effects of 2-amino-5-mercapto-1,3,4-thiadiazole (2A5MT) and 2-mercaptothiazoline (2MT) on mild steel corrosion in 1.0 M H₂SO₄ were studied with potentiodynamic polarization, linear polarization resistance and electrochemical impedance spectroscopy techniques. It was shown that both 2A5MT and 2MT act as good corrosion inhibitors for mild steel protection. The high inhibition efficiencies were attributed to the simple blocking effect by adsorption of inhibitor molecules on the steel surface. The effects of the presence of extra NH₂ group and N atom in 2A5MT on the ability to act as corrosion inhibitors were investigated by theoretical calculations.     

Long-term monitoring of atmospheric corrosion at weathering steel bridges by an electrochemical impedance method

Weathering steel corrosion was monitored for one to two years under natural atmosphere by an electrochemical impedance technique. Two identical comb-shape weathering steel sheets embedded in epoxy resin were used as monitoring probe electrodes at two different bridges in Japan. Impedances at 10 kHz (Z_10kHz) and 10 mHz (Z_10mHz) were automatically measured every hour. Coupons (50 × 50 × 2 mm³) prepared from the same steel sheets were exposed together to measure the corrosion mass loss. The average (Z_10mHz)⁻¹ value for half to one year exposure correlated well with the average corrosion rate determined from the corrosion mass loss.     

The importance of live biofilms in corrosion protection

As observed before, Al 2024 was passive in artificial seawater (AS) in the presence of a protective biofilm of Bacillus subtilis WB600. When antibiotics were added to the AS to kill the bacteria in the biofilm, pitting occurred within a few hours as indicated by characteristic changes in the impedance spectra. The corrosion potential E_corr decreased at the same time to values observed in sterile AS. Addition of the antibiotics to sterile AS had no effect on corrosion behavior.     

N-Phenylcinnamimide and some of its derivatives as inhibitors for corrosion of lead in HCl solutions

The anodic and cathodic polarization behavior of lead electrode was studied galvanostatically in HCl solutions of various concentrations. Increasing the acid concentration enhances the rate of the anodic dissolution of the metal and the rate of hydrogen evolution reaction, with anodic and cathodic Tafel slopes equal to 30 and 115 ± 5 mV decade⁻¹, respectively. The effect of addition of N-phenylcinnamimide and some of its derivatives, N-(p-nitrophenyl)cinnamimide, N-(p-methylphenyl)cinnamimide, and N-(p-methoxyphenyl)cinnamimide, as inhibitors on the kinetic of the anodic and cathodic reactions of lead in 0.1 M HCl solutions was also studied. These compounds inhibit both the rate of anodic dissolution and the rate of hydrogen evolution reactions without affecting the Tafel slopes. This result indicates that the used compounds were of mixed-type. The inhibition efficiency of these additives increases in the order: N-(p-nitrophenyl)cinnamimide < N-phenylcinnamimide < N-(p-methylphenyl)cinnamimide < N-(p-methoxyphenyl)cinnamimide. The equilibrium constant and the free energy of adsorption process have been calculated and discussed.     

Modeling pitting corrosion by means of a 3D discrete stochastic model

Pitting corrosion is difficult to detect, predict and design against. Modeling and simulation can help to increase the knowledge on this phenomenon as well as to make predictions on the initiation and progression of it. A cellular automaton based model describing pitting corrosion is developed based on the main mechanisms behind this phenomenon. Further, a sensitivity analysis is performed in order to get a better insight in the model, after which the information gained from this analysis is employed to estimate the model parameters by means of experimental time series for a metal electrode in contact with different chloride concentrations.     

Probabilistic analysis of pipelines with corrosion defects by using FITNET FFS procedure

A probabilistic method based on the corrosion module of the FITNET FFS procedure is presented to perform the structural integrity analysis for pipelines. The comparison of the failure pressure determined by both deterministic and probabilistic methods shows that even for a high safety factor, the deterministic method may predict nonconservative results. FITNET FFS procedure predicts a similar probability to that given by the Shell-92 model. The sensitivity analysis of both the uncertainty of the variables and the underlying model shows that the corrosion depth has the most significant contribution to the pipeline failure at an inspection interval of 8 years.     

Localized corrosion behaviour of reinforcement steel in simulated concrete pore solution

The correlation of localized corrosion behavior and microstructure of reinforcement steel in simulated concrete pore solutions was investigated. The SEM/EDS analysis showed that most of ferrite, minor amount of pearlite and some MnS inclusions existed on the steel surface. The SKPFM results indicated a higher corrosion tendency at the ferrite grain boundaries, pearlite grains and MnS inclusions. The EIS and electrochemical polarization measurements demonstrated the influence of pH and chloride concentration on the corrosion behavior. In situ optical observations and AFM images revealed a detail of the localized corrosion behavior, which was in good agreement with the results from the other measurements.