| Severe localized corrosion often occurred in the oil production and transportation in Chinese oilfields. The behavior of mild steel in NaHCO3-NaCl solution was chosen to be the objective of the present work. The effects of aggressive and inhibitive anions on pitting corrosion of mil steel were studied. The accelerating effect of corrosion products on pitting was also investigated. The cathodic and anodic process of mild steel was studied in the simulating solution within pits. On the basis of corrosion mechanism, the factors, which affected corrosion in gas wells and injecting water, were analyzed. A model was constructed to predict corrosion rate in gas wells.1. The anodic polarization curves of mild steel were measured in sodium bicarbonate solution of varying concentration, and the surface products were analyzed by XPS. The results showed that two anodic peaks and two passive zones were observed in all the curves. At bicarbonate ion concentration below 0.lmol/L, the two peaks were so close that the first passive zone was difficult to observe. At concentration above 0.lmol/L, the first passive zone was clear, and the second peak changed to be smaller. The results by XPS indicated that the higher potential film was Fe2O3, at low potentials, FeOOH was formed along with FeCO3, which subsequently oxidized to be Fe3O4 at the second peak.2. A transition of bicarbonate ion from aggressive to inhibitive behavior was observed. At concentrations below O.lmol/L, bicarbonate ion stimulated the reactions of film dissolution and breakdown, as the concentration increased above O.lmol/L, the ion would make the mild steel passivated. However the passivation was relatively weak, the addition of chloride and sulfate ions would make the film breakdown.3. The effects of six anions on pitting corrosion were investigated in sodium bicarbonate solution containing chloride ion by polarization measurement. The pitting potentials were shifted to more positive potentials by the addition of these anions, indicating suppression of pit nucleation. The inhibition decreased in the order Cr2O72> PO43> NO2"> MoO42">B4O72"~WO42', which could be explained by the competitive adsorption of anions on the active site of surface with chloride ions.4. The effects and mechanisms of three anions on the pit growth were studied. Theresults showed that PO and MoO42" could retard the pit propagation, however Cr2O72" ion stimulated pit growth. The different effects of the anions on pit propagation were explained by the opposite effects on pH value within pits. The pH value in molybdate-containing solution increased as a result of polymerization of MoO42', while in dichromate-containing solution, pH value decreased due to hydrolysis of Fe3+and Cr3".5. Severe pitting corrosion was observed in the solutions containing high concentration of bicarbonate and chloride ion, and low concentration of calcium ion. This was attributed to the formation of galvanic cell with a large area of cathode and a small anode. The area covered with corrosion product behaved as the cathode of the cell, and the uncovered area behaved as the anode. An expression for anode dissolution rate was derived on the basis of corrosion electrochemical principal:The larger the area ratio and the potential difference of cathode to anode were, the bigger the anode dissolution current was.6. The corrosion process of mild steel in pit simulating solution was controlled by the diffusion of hydrogen ion; the anodic dissolution took place by Bockris mechanism. A critical pH value or salt concentration was not observed for mild steel. The logarithm of corrosion rate was inversely proportional to the pH value: log Vc=0.968-0.444pH. A small change in pH value within pits would lead a marked change in corrosion rate.7. The dissolved oxygen and pH value had significant effects on corrosion rate of mild steel in produced water of oilfield. Increasing pH value and lowering the content of oxygen decreased the corrosion rate. However, pH above 8.5 would cause severe crevice corrosion. Therefore pH val... |
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