Effects Of Na₂MoO₄ And Tea On Pitting Corrosion Behavior Of Q235 Carbon Steel

Pitting corrosion is difficult to detect So pitting corrosion often causes severe and destructive corrosion accident and causes great losses. So the study of the control of pitting corrosion has important practical significance. Corrosion inhibitor is a kind of very effective anticorrosion method. In this thesis, slow potentiodynamic polarization curves test, EIS test, Mott-Schottky test, SEM, EDX, XPS, metallographic microscope, MCT test and SPM technology were used to study the effect of Na2MoO4 and TEA on pitting corrosion behavior of Q235 carbon steel in 0.02 mol/L NaCl+0.1 mol/L NaHCO3 solutions. The electrochemical characteristic parameters of potentiodynamic polarization curve and the inhibition mechanism of Na2MoO4 and TEA were analyzed. The effect of different content of Na2MoO4 and TEA on pitting corrosion of Q235 carbon steel in NaCl+NaHCO3 solution was studied. The sensitive position of pitting corrosion of Q235 carbon steel and where the inhibitor adsorbs preferentially on the Q235 carbon steel surface were researched. The change of surface potential and amount change of active sites on Q235 carbon steel surface were discussed. The main conclusions are as follows.(1)With Na2MoO4 added, Eb of Q235 carbon steel in NaCl+NaHCO3 solution increases.The Eb stays the same with TEA added. As Na2MoO4-TEA added, Eb increases greatly. In solutions with Na2MoO4, TEA or Na2MoO4-TEA, corrosion potential and passivation current density decrease. The resistance value and capacitive reactance bow radius of Q235 carbon steel passivation coating is larger and the Nd is lower in solutions with Na2MoO4-TEA. So the protectiveness and stability of Q235 carbon steel surface film in solution with Na2MoO4-TEA. Na2MoO4-TEA show better corrosion inhibition effect than Na2MoO4.(2)Na2MoO4 and TEA distribute on the film of Q235 carbon steel randomly in proportion. The adding of Na2MoO4 makes the adsorbing capacity of TEA increase. The main composition of corrosion inhibition film are Fe2(MoO4)3, Fe2O3 and TEA. The absorption of TEA can replenish the imperfection of corrosion inhibition film. The protective effect of film is far greater. So Na2MoO4 and TEA inhibit corrosion together.(3)When the additive amount of Na2MoO4 is relatively less, the Eb and Eb-Ecorr stay the same. The Eb and Eb-Ecorr increase with incremental amounts of Na2MoO4. In the condition of the same corrosion inhibition effect, adding TEA can reduce amount of Na2MoO4. When the adding amount of Na2MoO4 keeps changeless, the Eb and Eb-Ecorr of Q235 carbon steel increase first then decrease.(4)The inclusions of Q235 carbon steel include manganese compound, sulfide and aluminium compound. The inclusions distribute dispersedly. They are mainly rounded or just like potato shape. MCT and SEM tests show that manganese compound is the main induced source of pitting corrosion of Q235 carbon steel. Na2MoO4 deposits on the surface of Q235 carbon steel unequally and deposits in the area of manganese compound inclusion preferentially. Later, by certain reaction it produces protective film containing Mo and then inhibits pitting corrosion.(5)Potential peak intensity of Q235 carbon steel in NaCl+NaHCO3, NaCl+NaHCO3+Na2MoO4, NaCl+NaHCO3+Na2MoO4+TEA solutions are all quite low when adding low constant potential. Potential peak can’t exist stably and no steady active sites exist. In the condition of adding constant potential 0.3 V, the active patch is activated. Na2MoO4-TEA can effectively inhibit the number of the active sites and decrease the intensity of the potential peaks. Therefore, the deposition and absorption of compounded corrosion inhibitors are better. It can effectively inhibit the produce and evolution of early active sites which may transform corrosion hole, thus reducing pitting corrosion sensibility and improving pitting corrosion potential.