Electrochemical Behavior Research Of Pipeline Steel In Simulation Soil Solution

For pipeline steel used in oil transportation, soil corrosion is one of the mostserious corrosion. As a long distance pipeline material, pipeline steel is difficult to betested in its servicing environment. Existing electrochemical method is not suitable tomonitor of the corrosion behavior of pipeline steel for long time and long distance. Theemerging electrochemical noise method, which has advantage of continuous monitoringat long distance, has had great progresses on the testing and analysis technologies.In this paper, by chosen NaHCO₃-Na₂CO₃simulated soil system, applyingpolarization curve, electrochemical noise and galvanic corrosion methods, theconcentration of Cl^-and Na₂CO₃and the influence of surface roughness wereinvestigated. The following conclusions were made:The electrochemical noise research of X70and X80pipeline steel inNaHCO₃-Na₂CO₃solution system shows that the degree of corrosion increases with theincrease of concentration of Na₂CO₃. The noise curve is shown that potential curvereduces and current curve is away from zero. The increase of electrochemical potentialin noise curve and the curve of current noise is close to zero indicates that passivationoccurred; the reduce of potential noise curve and the curve of current noise being farfrom zero, indicate that material is under corrosion. The noise peaks in the curve is theresult of alternation of pitting corrosion and passivation, both of which are in dynamicequilibrium.In the Cl^-containing NaHCO₃-Na₂CO₃solution, pitting corrosion behavior of X70pipeline steel has three clear stages, namely instable phase, metastable pitting nucleationphase and pitting phase. Cl^-affects the nucleation time of metastable pitting andsignificantly shortens the metastable pitting nucleation time. Adding of Cl^-leads to the falling of potential values in X70and X80pipeline steelpotential noise curve, the curve of current noise current value being away from zero, theincreasing of fluctuation amplitude and shortening the pitting induction period. It alsoreduces self-corrosion potential of polarization curve and increases the passive current.The smoother surface of X70and X80pipeline steel, the better the corrosion resistancein the containing Cl^-solution.X70and X80pipeline steel galvanic corrosion study showed that when theconcentrations of Na₂CO₃is small, X80pipeline steel is easy to become anodecorrosion. When the concentration of Na₂CO₃is increased, Cr in X70steel is easy to bedamaged, reducing its corrosion resistance and making it become anode corrosion.