The Studies Of The Corrosion Mechanism Of X70 Pipeline Steel In The Neutral Solutions And Its Surface Protection Methods

The 21st century is a peak period of the building of the gas transmission pipelines in China. In the project of "Transport gas from west to east", the pipeline from Lunnan to Shanghai is the jumping-off point of the large-diameter, high-pressure gas transmission pipelines used in China, and the steel used for this pipeline is a high-intensity, high-toughness pipeline steel in grade X70. In the long-distance transportation of gas and oil, the safety of the pipelines has an important significance. Now the studies of the X70 steel are mostly done with the mechanical methods, for the mechanical behavior showed under press, and the magnetic flux leakage (MFL) was also used for corrosion monitoring of X70 steel in some literature, but how the steel is corroded in corrosive environment and what the kinetic processes are have not been studied. However, the property to be passive easily makes steels local corroded easily, for example ,the pitting corrosion. In case of severe corrosion, the damage will be unthinkable. So, to understand the corrosive mechanism of X70 steel and to find out the methods of controlling and effective monitoring its corrosion are very important.In this thesis, we attempted on one hand to study the corrosion electrochemical behaviors of the X70 pipeline steel in the neutral solutions and give explanations to its corrosion mechanism, on the other hand to investigate the surface protection methods about the X70 pipeline steel and try to find better methods for protection.In the third chapter of this thesis, the corrosion electrochemical behaviors of X70 pipeline steel in the neutral solutions were studied by electrochemical measurements such as electrochemical noise(EN), polarization curve and electrochemical impedance spectroscopy(EIS), and surface techniques such as SEM and EDX. The corrosion mechanism was investigated as well. The data of EIS measured were fitted by the software of Zview, the corresponding equivalents were applied to simulate the corrosion process and mechanism of the X70 pipeline steel in the neutral NaCl solutionand the surface of the X70 steel sample after polarization in the neutral solution was detected by SEM and EDX. The results showed that in neutral NaCl solution , the oxide film formed on the X70 steel was not in its integrity. Pitting corrosion was prior to occur on the discrete area and it made the steel corrode seriously; in the neutral Na2SO4 solution, the obvious passivation region rose. However the passivated current density was fluctuated, which illuminated that the film formed on steel was not stable and broken easily. The surface tests showed that the content of oxygen in the surface of the X70 steel sample polarized in the neutral Na2SC>4 solution was much higher than normal, which illuminated that the oxid of Fe maybe formed on the surface of the steel and to some extent, it covered the surface to protect the base metal.In the fourth chapter of this thesis, we attempted to study the inhibition effect of the rare-earth inhibitor—La(NO3)3 to X70 pipeline steel. Neutral NaCl solution was adapted for these experiments. We utilized the polarization curve and EIS as well as SEM and EDX to study the inhibition effect. The results showed that: La(NC>3)3 belonged to the film-formed mixed inhibitor, whose restraint effect to the anodic and cathodic reaction of the steel was affected by the concentration of itself. La(NC>3)3 as inhibitor was high efficiency and non-toxicity. It may react with the OH"1 ions generated in the oxygen reduction reaction on the cathodic sites and form lanthanum hydroxide (such as La(OH)3) and La2O3. The insoluble La(OH)3 (the solubility products, Ksp of LaiOH)^ =5.2xlO~20) and L^Os may cover compactly over the whole electrode surface, block the reaction active sites and inhibit the diffusion of oxygen to the electrode surface, which results in the decrease of corrosion rate of X70 pipeline steelIn the fifth chapter of th-s thesis, we attempted to study the protective impact of anodic polarization as a surface protection method to the X70 steel. The 0.5mol/L NaHCC>3 solution was used as the electrolyte for the anodic polarization. According to the polarization curve tested in the 0.5mol/L NaHCC>3 solution, we adapted the 0.4V(vs.SCE) as the polarization potential. The X70 steel samples polarized during different time or with the addition of Na2MoO4 in the electrolyte were respectively tested in neutral NaCl solution by electrochemical measurements. The results showedthat: it was bad that the protective effect by polarization only in 0.5mol/L NaHCCh solution to the steel. However, the X70 steel sample, which polarized in the 0.5mol/L NaHCCh solution with Na2MoC>4 added, showed better effect. More detailed measurements need to be done in the future.