The direct cause for the explosion accident occurred by Dong Huang pipeline is that the corrosion of pipeline was enhanced by coastal soil, which in turn resulted in thinning and then cracking of the pipe. Because localized corrosion cracking in coastal soils is a serious threat to the integrity and security for the pipeline, the effect of the coastal soil environment on localized corrosion for oil and gas pipelines was analyzed systematically. Typical corrosion environmental characteristics were acquired and then analyzed firstly for the specified coastal soil, which in general is rather different from the terrestrial soil and seabed soil distinctly. The coastal soil is characterized as a mixture of gas/liquid/solid multi-phases with high salt- and water-content and good air aeration, while it experiences dry-wet cycles periodically. The effect of the coastal soil environment on the initiation and propagation of localized corrosion for oil and gas pipelines is discussed. Finally, the trends and difficulties of the study in oil and gas pipeline localized corrosion are analyzed, and in the end, a scheme of micro-e electrochemical study is peculiarly prospected.

The corrosion performance of Ni free and Mn alloyed high nitrogen austenitic stainless steels (HNSSs), including uniform corrosion, intergranular corrosion, crevice corrosion, pitting corrosion and repassivation,was investigated by means of immersion test and electrochemical test. The effect of cold work and sensitization treatment on the microstructure, characteristic of passive film and corrosion resistance of the HNSSs was examined. It was found that the solution-annealed (SA) HNSSs had much weaker resistance to uniform- and intergranular-corrosion compared to the SA 316LSS. The sensitization-treatment (ST) hardly affected the uniform corrosion resistance of all the steels, but resulted in rapid degradation of the resistance to intergranular-corrosion, especially for the Mo free HNSSs. The SA HNSSs had a better resistance to crevice- and pitting-corrosion than the SA 316LSS, in particular the Mo bearing HNSSs. The ST degraded the resistance to crevice- and pitting-corrosion of the HNSSs. The cold work degraded the pitting-corrosion resistance of the HNSSs in chloride-containing solutions through thinning the passive film and decreasing the stable oxides in the passive film, but it improved the repassivation ability of the HNSSs. The ST-induced precipitation of χ phase degraded the corrosion resistance and repssivation ability of the HNSSs, and such degradation became aggravated with increasing cold work degree. Furthermore, the relevant corrosion mechanism is also discussed.

316L stainless steel is widely used for enhancing the pitting resistance of pipelines in gas field. The corrosion environment is complex and diversified in different working districts of gas field. Therefore, it is necessary to develop a model for predicting the pitting resistance of pipelines serving in different corrosive environments. Critical pitting temperature (CPT) is considered as a criterion for evaluating the pitting resistance of stainless steel. Based on a survey on the operation situations of gas field, the relevant data of CPT for 316L stainless steel is acquired by potentiodynamic polarization method in solutions with various Cl^- concentrations and pH values, which are selected to correspond with the real environments in operation. Then, an artificial neutral network (ANN) model is developed to predict the CPT, and therewith to compare with the measured data. The results show that the CPT decreases with the increase of Cl^- concentration, but on which pH value has little influence. The developed ANN model has good ability to predict the CPT of 316L stainless steel, and can be used for the prediction of CPT in complex environments in gas field. It is also revealed that there is no interactive effect between Cl^- concentration and pH value, and Cl^- concentration was the main influencing factor on the CPT. Therefore, Cl^- concentration will be peculiarly concerned with for the implementation of a corrosion control project in gas field.

The corrosion behavior of X70 pipeline steel in a near-neutral pH solution NS4 with and without sulfate-reducing bacteria (SRB) respectively was studied by means of electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and microscopic observation. The results showed that the corrosion rate of X70 steel increased with time in NS4 solution without SRB; The growth process of the SRB in the solution NS4 can be divided into three phases: logarithmic phase (1~3 d), stable growth phase (4~7 d) and death phase (7~14 d). The influence of SRB on the corrosion rate of X70 pipeline steel in the solution NS4 with SRB was related to the growth process of SRB. A compact biological film could form on the steel surface in the stage of logarithmic phase and stable growth phase, which was conducive to enhancing the protectiveness of corrosion products on the steel and therewith the corrosion rate of X70 steel in the NS4 with SRB was lower than that without SRB. While the corrosion of X70 steel in the NS4 with SRB in the death phase was more serious than that without SRB, while the scale of corrosion products became thicker gradually, then the biological film broken and therewith the corrosion rate increased.

Effect of hydrostatic pressure on the electrochemical corrosion of X100 pipeline steel in 0.5 mol/L NaHCO₃+0.03 mol/L NaCl solution was investigated by means of methodspotentindynamic polarization, electrochemical impedance spectroscopy and Mott-Schottky. The results show that with the increasing hydrostatic pressure the number and the size of corrosion pits increase,while the activity of ions in the solution also increases, which in turn promotes the adsorpting of Cl^-in the passive film. As a result, the corrosion rate and the corrosion current density are increased. The component of the passive film transforms from oxide or hydroxide formed by low pressures to carbonate by high pressures, which may result in the decrease of corrosion-resistance and increase of probability of pitting corrosion nucleation. The passivation film formed on the X100 steel exhibits characteristics of n-type semiconductor. The increase amount of Cl^- in the passivation film promotes the increase of lattice imperfections and facilitates the cracking of the oxide film.

The corrosion behavior of X70 mild steel was investigated in the case of water deposited in CO₂ pipeline by means of weight loss method and scanning electron microscopy (SEM). Results showed that general corrosion rate of X70 steel decreased with the increase of CO₂ pressure from 4 to 12 MPa at 35 ℃. FeCO₃ film and severe general corrosion was observed at 4 MPa, while thin and discontinuous corrosion product and serious pitting corrosion attack were found under other CO₂ pressure conditions. It is of great corrosion risk to transport CO₂ in gaseous or supercritical status for X70 mild steel if free water is formed in the pipeline.

The electrochemical corrosion behavior of X70 steel in CO₂ saturated solution with various concentrations of acetic acid was studied by means of potentiodynamic polarization measurement and EIS. Whilst the influence of acetic acid on the top localized corrosion (TLC) of X70 steel pipelines was examined in a set of high temperature and high pressure autoclave. The results showed that in the CO₂ saturated solution, the X70 steel was inactive state. While the free corrosion potential of X70 steel shifted positively and its corrosion current density increased with the increasing concentration of acetic acid, which may be related to the rising H⁺ concentration, so that the cathodic reaction was promoted. In addition, because of the presence of acetic acid, the corrosion scale become loose and easy to detach from the substrate. Finally a pitting corrosion model was proposed to describe the initiation and evolution of the pitting process.

The effect of strain on the pitting behavior of Q235 carbon steel in solutions of NaHCO₃+NaCl was studied by means of potentiodynamic polarization measurement, EIS and XPS. The results show that in a solution of 0.2 molL^-1 NaHCO₃+0.01 molL^-1 NaCl, a strain of 8% leads to an obvious increase of the pitting potential E_b of Q235 steel, but as the Cl^- concentration increased, the difference between the E_b values of strain and strain free samples decreased. When the Cl^- concentration increased to 0.1 molL^-1, the difference of E_b values disappeared. Besides, strain caused lower impedance, smaller charge transfer resistance R_ct and decreased the ratio Fe³⁺/Fe²⁺ in the passive film, thereby reduced the stability of passive film. The phenomenon that strain caused the increase of E_b was attributed to that the strain promoted the anodic dissolution of Fe which in turn promoted the preferential adsorption of HCO₃^- on the surface, as the result the harmful effect of Cl^- on the passive film was inhibited. As the ratio HCO₃^-/Cl^- in the solution decreased, the effect of HCO₃^- decreased and finally disappeared.

The corrosion inhibition of octyl isoquinolinium bromide on Q235 carbon steel in HCl solution was studied by means of mass loss test, potentiodynamic polarization curves and electrochemical impedance spectroscopy. The results show that octyl isoquinolinium bromide is a mix-type inhibitor which can inhibit the anodic- and cathodic-reactions. The inhibition efficiency increases with the concentration of inhibitor and decreases with temperature. The adsorption of octyl isoquinolinium bromide on Q235 carbon steel surface accords with Laugmuir isothermal adsorption. The adsorption equilibrium constant K_ads is big, which can explain the strong adsorption ability of inhibitor for the steel.

The corrosion behavior of low alloy steels, such as 27SiMn, 30CrMnSi, 30CrMnTi, 40Cr and Q550 was studied comparatively in high-mineralized coal mine water, tap water and NaCl solution by means of immersion test, salt spray test and electrochemical methods as well as XRD and SEM with EDS. The results showed that elements Cr and Si can significantly affect the corrosion resistance of low alloy steels: in chloride containing media, Si exhibits stronger effect than Cr on the corrosion; whereas in the presence of oxygen (water and gas phase), Cr exhibits stronger effect than Si. Therefore, 30CrMnTi and 40Cr have better corrosion resistance in the coal mine water. The corrosion behavior of the low alloy steels in the coal mine water depend not only on the role of alloying elements but also on the compactness of the formed limescale induced by mineral ions. The limescale, on one hand, can fill the porous rust layer, which slows down the corrosion rate of the steel, however, on the other hand, due to the low concentration of oxygen at the zone below the limescale, where the corrosion of the steel can be accelerated. Finally the corrosivity of three corrosive media can be ranked as the order from severe to mild: 3.5%NaCl solution, high mineralized mine water and tap water.

The microstructure, corrosion behavior and the influence of corrosion on mechanical performance of 5A90 Al-Li alloy and 2D12 alloy were studied comparatively by means of X-ray diffraction, metallurgical analysis, scanning electron microscopy and marine atmospheric exposure test, while the relevant corrosion mechanism was also discussed. The results showed that the strengthening phases δ ′-Al₃Li and T-Al₂MgLi of 5A90 Al-Li alloy are fine and dispersed quite uniformly. 5A90 Al-Li alloy showed corrosion characteristics in micro-scale apparently different from 2D12 alloy, i.e. the corrosion of the former occurred simultaneously at grain boundaries and in grains. The corrosion process of 5A90 Al-Li alloy did not follow the power function in terms of corrosion mass loss. The mechanical performance degradation of 5A90 Al-Li alloy due to corrosion is apparently faster than that of 2D12 alloy. The fracture manner of 5A90 Al-Li alloy is mainly dimple fracture, while the environmental corrosion promotes the transition from ductile fracture to intergranular brittle fracture.

The microstructure and corrosion morphology of extruded alloys Mg-xZn-yGd (x=1~3, y=0~3) were observed by optical microscopy (OM), scanning electron microscopy (SEM). Results show that after extrusion the microstructure of the alloys is obviously refined and further refined by Gd addition, i.e. the average grain size decreases from (30±3) μm for Mg-3Zn to (10±2) μm for Mg-3Zn-3Gd. The dynamic recrystallization occurs during the extrusion, while the second phase particles tend to distribute as belts along the extrusion direction. Mg-xZn-yGd alloy is sensitive to pitting corrosion in the Hank's solution. Among others the corrosion of Mg-3Zn-2Gd is the fastest,while that of Mg-3Zn is the slowest. After solid solution treatment, the corrosion rate of Mg-3Zn-2Gd and Mg-3Zn-3Gd decreases from (0.605±0.025) and (0.352±0.021) mg/(cm²h) to (0.085±0.010) and (0.167±0.020) mg/(cm²h) respectively. With increasing Zn content the corrosion current density decreases and the high-frequency capacitance increases gradually; with the increase of Gd content, the corrosion current density and high-frequency capacitance of the alloys rise firstly and then reduce gradually. Mg-3Zn alloy exhibits aminimum corrosion current density (8.65×10^-3 mA/cm²)and a maximum Faraday resistance R_t (3312 Ω).

Nano-ZnS was prepared by hydrothermal method and polyaniline was prepared by chemical oxidation respectively, which then were blended to prepare a series composite materials of polyaniline modified by nano-ZnS. Finally the relevant composite coatings were applied on Q235 carbon steel. The acquired composite materials and coatings were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The electrochemical corrosion performance of the composite coatings in 3.5% (mass fraction) NaCl solution was investigated by potentiodynamic polarization and electrochemical impedance. The results showed that the nano-ZnS was uniformly dispersed in the polyaniline matrix and therewith the anticorrosive performance of the coatings was improved significantly. Among others the coating with 50% (mass fraction) ZnS exhibited the best anticorrosive performance with a protection efficiency 99.9% after immersion in 3.5%NaCl solution for 7 d. Besides, after immersion for 30 d, the coating surface morphology changed obseriously, but the coating is still dense and could provide good protectiveness for the substrate material even up to 30 d.

A rust converter coating, using vinyl chloride-acrylic copolymer as film-forming substance and phosphoric acid as rust converter, was prepared on a rust Q235 carbon steel. Then the influence of the mass fraction of phosphoric acid on the coating performance was studied by means of salt spray tests and electrochemical impedance spectroscopy (EIS). The reaction of phosphoric acid with the rust was examined through microscopic analysis of the surface morphology of the coating. The results showed that phosphoric acid can react with the lepidocrocite in the rust, forming a dense conversion layer on the steel. When the mass fraction of phosphoric acid in the coating is 10%, the property of the conversion layer showed better corrosion resistant performance than other coating samples with lower mass fraction of phosphoric acid.