Gas pipelines are facing increasingly serious CO₂ corrosion problem. This paper introduced the specific corrosive environment within the gas pipeline, then summarized up the CO₂ corrosion research progress in reaction mechanism, influencing factors and current corrosion control measures. At last, the research direction of CO₂ corrosion of gas pipeline was proposed.

The influences of dissolved oxygen, temperature, pH, carbonate and pressure in deep seawater on the cathodic protection of steel materials and the formation of calcareous deposits were reviewed. Meanwhile, overseas and domestic research status on the cathodic protection for steel materials in deep seawater was summarized. Furthermore, the parameters of the deep seawater cathodic protection design were introduced. It was proposed that it was an important measure to increase the initial current density for deep seawater cathodic protection and also with the coating. Finally, the inadequacies of the research currently on the deep seawater cathodic protection and the development direction were discussed.

Abstract：The corrosion behaviors of microarc oxidized 5052 alumina immersed in 70 ℃/7% NaCl solutions with different time were studied through the open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements. The results reveal that high temperature solution could permeate through microarc oxidation films and induce the corrosion of aluminum alloy substrate in the initial stage. After that, the corrosion products depositing in film defects could block the permeation of solution, which make microarc oxidation films exhibiting a self-sealing ability. And the film resistance keeps mostly unchangeable after initial rapidly decrease. Sealing treatment of microarc oxidation film could improve the self-sealing ability effectively.

TiO₂ films were formed on metallic titanium substrate by anodic oxidation method, and its corrosion resistance was studied in simulated deep sea hydrothermal region by hydrothermal synthesis reactor. The crystal form, chemical composition, microstructure and hydrophobic properties of oxide film and products of corrosion sample were characterized by X-ray diffraction (XRD), scanning electron microscopy(SEM) and water contact angle tester, respectively. Their electrochemical properties were investigated through polarization curves. The results showed that in the simulative deep sea hydrothermal system, the non-compact TiO₂ film which was produced on the surface of Ti and anodic oxidation sample could not protect the substrates well and the non-compact TiO₂ film was different from the anodic oxidation TiO₂ film. The corrosion potential of Ti electrode shifted 0.45 V towards negative direction and TiH₂ formed on the surface of Ti after corrosion, while the corrosion potential of Ti sample processed by anodic oxidation method changed less, and no TiH₂ was produced on its surface. The above results indicated that the TiO₂ films formed on metallic titanium substrates by anodic oxidation method had a good corrosion resistance.

In order to improve the corrosion resistance of coating and prolong its life in the marine environment. Zn-Ni composite coatings including different nickel content were prepared by oxyacetylene flame spraying process, with Zn-Ni composite powders which were prepared by spray-dried process. The protection performance and corrosion mechanism of the Zn-Ni composite coatings in seawater were tested by means of potentiodynamic polarization and electrochemical impedance, and with the analysis of SEM, EDS and XRD. Results shown that the stable corrosive potential of the coatings finally ranges from -0.98 V to -0.95 V. The presence of nickel can play a role that restrain the dense Zn(OH)₂ to be converted into the loose ZnO. At the same time, the coatings resistance R_c and charge transfer resistance R_t are both being increased and causing the coatings corrosion current being decreased gradually with the accumulation of corrosion products. Corrosion resistances of these coatings with different nickel content are obviously different, and the anti-corrosion properties of the coating with 20 mass% Ni is best.

The effects of PO₄^3- on nucleation and growth of metastable and stable pitting were investigated by cycling potentiodynamic polarization. Both metastable pitting potential E_m and stable pitting potential E_b increase with the increasing of PO₄^3- concentration. Higher PO₄^3- concentration results in lower average growth rate and peak current of metastable pitting, hence inhibits the nucleation of stable pitting. However, as PO₄^3- concentration increases, the repassivation potential of stable pitting E_p decreases, indicating that the repassivation of the pit is retarded. The reason is explained by the deposition of phosphate film around the pit mouth, which may promote the growth stability of the pit.

The high-temperature exhaust gas oxidation-acidic condensates immersion cyclic test was used to simulate diesel exhaust after-treatment environments. After oxidation at 400 ℃ in simulated exhaust gas environments with or without urea, the corrosion behavior of 304 and 439 stainless steels was investigated in the condensate solutions. The electrochemical test results indicate that 304 stainless steels oxidized in the presence or absence of urea show passive corrosion characteristics in the condensate solutions, whereas 439 stainless steels oxidized under both conditions tend to active corrosion. Corrosion product films on the specimen surfaces may be damaged during the oxidation and immersion cycles, which give rise to the formation of some pits on the specimens. As urea is added into the exhaust gas, it can promote the oxidation processes of both 304 and 439 stainless steel. As a result, the general corrosion processes of both stainless steels are accelerated by the urea addition, but the localized corrosion processes are inhibited to a certain extent.

A high-speed impingement corrosion apparatus was manufactured and its jetting velocity reached maximal 150 m/s. This apparatus could be connected with electrochemical workstation for electrochemical test. Based on the hydrodynamic theory, an impingement hydrodynamic model was built and the relationships of jetting velocity, impingement force, surface shear stress with jetting pressure were separately studied and verified by experimental methods. The impingement corrosion of stainless steel was studied by mass loss and electrochemical methods. It was found that the impingement force induced by jetting flow had great influence on impingement corrosion and corrosion morphology, and that the combination of impingement force and surface shear stress changed the anodic polarization process and accelerated electrochemical corrosion. The impingement force parameter was more reasonable for describing impingement corrosion compared with surface shear stress.

By surface scanning technology and linear scanning technology with SECM, which have a micrometer pixel, corrosion behavior of HR-2 stainless steel in 0.5 mol/L H₂SO₄+0.1 mol/L NaCl solution was studied. The results show that: open circuit potential, activation potential, and passivation potential are -0.4 V, -0.23 V, 0.0 V ~ 0.8 V relatively. The maximum current of probe is 9.5 nA when substrate current is open circuit potential 35.2nA, when substrate current is activation potential. When passive film formed on the substrate, the current of probe largely decreased. H₂ was obtained when current of probe is -0.2 V, Fe₂₊ was obtained when current of probe is 0.6 V. It is also observed that the proper distance between probe and substrate is 20 μm, and best concentration of the corrosion inhibition (sulfocarbamide) is 0.75 mmol/L.

The behavior of hydrogen evolution was determined by gas volumetric measurement over a period of 72 h at 45 ℃. The inhibiting effectiveness of ten kinds of corrosion inhibitors on Al-Zn-Si alloy powders in a solution of water with 10 mass% C₆H₁₄O₂ was compared. The investigations of Al-Zn-Si samples were carried out by use of IR, SEM and XRD. It has been shown that the hydrogen evolution of flake Al-Zn-Si powder can be inhibited by the addition of TBAB and HEDP with an optimal concentration of TBAB/Al-Zn-Si=50(mass%) and HEDP/Al-Zn-Si=100(mass%) in 2-butoxyothanol aqueous solutions the inhibition efficiency can reach 78.99% and 62.77%, respectively. At last, Pourbaix diagram was used to analyse the microgalvanic corrosion of Al-Zn-Si alloy powder in aqueous media, and the inhibition mechanism of the inhibitors TBAB was discussed briefly.

Intergranular corrosion susceptibility of diffusion bonded joints of 316L stainless steel was investigated by using microstructure inspection, electrochemical testing and analysis of grain boundary characteristic. Effect of grain size and microstructure texture on sensitization of the diffusion bonded joint was discussed. The result showed that no chromium carbide precipitation was observed in the diffusion bonded joint after 100 h treatment at 650 ℃, while chromium carbide precipitations could be seen clearly after 8 h treatment in the base material. The susceptibility of the diffusion bonded joint is much less than that of the base material. The intergranular corrosion susceptibility of 316LSS decreases with increasing grain size. Grain coarsening and increase in the amount of the twin boundaries lead to the improvement of the intergranular corrosion resistance, but the grain coarsening is the main contribution.

Different researchers make different conclusions on the reinforcement corrosion parameters after electrochemical repair. In order to give a reasonable explanation of the phenomenon, anodic polarization curve (APC) is employed to study the effects of electrochemical repair methods on polarization parameters of reinforced in simulated pore solution. The results show that the effects of different treatment duration and current density on the anodic polarization current and potential of the reinforced after various treatments. After the treatment, the anodic polarization current and reinforcement potential change a lot: the greater the power, the more the increase of anodic polarization current, the more the reduction of reinforcement potential. Over time, this change gradually weakens until smooth, which is a depolarization process, and usually takes 42 days. After depolarization, the polarization current of the reinforced is still relatively large compared with that before the treatment, usually 2~5 times, that shows the treatment can increase the polarization area of the reinforced, also 2~5 times; however, reinforcement potential changes little. By analysis of surface morphology of the reinforced, it is found that the surface of the reinforced becomes rougher after treatment, surface area increasing will make the polarization area increase, which is a reasonable interpretation of polarization current increasing after treatment.