Studies of the Inhibiting Effects of Sodium Molybdate for Carbon Steel in Aqueous Solutions Containing Sulfur Compounds

• Main Authors: How, Way-Shin, 郝維新
• Other Authors: Chung-Jen Hsu
• Format: Others
• Language: zh-TW
• Published: 1996
• Online Access: http://ndltd.ncl.edu.tw/handle/65920861389673002458

碩士 === 逢甲大學 === 材料科學研究所 === 84 === This study was done by potentiodynamically scanning method to investigate the inhibitive effect of sodium molybdate on carbon steels in sodium sulfite or disulfite solutions. The polarization data obtained were then transferred to the REDIF program to calculate the corrosion current densities, Tafel slpoes and polarization resistances. The effect of sulfite or disulfite concentrations,molybdate contents, pH values and solution conductivities on the corrosion rates were examined. In addition, weight loss experiments were executed to check the reliability of the electrochemical corrosion data. The adsorption model, possible corrosion reactions, and inhibitive mechanisms concerned are also presented. The experimental results and interpretations are summarized as follows: 1.Sodium sulfite has dual behaviors, i.e., it can stimulate as well as inhibit the corrosion rate. At pH 7 and sodium sulfite concentration above 0.005M, it behaves as a corrosion inhibitor. At pH 9 or 11, OH- ions are dominant in the solution. In the acidic solutions (pH=3 to 5), sodium sulfite acts as a corrosion stimulator similar to the self- catalyzed reaction of chloride ion as proposed by earlier investigators. 2.The corrosion properties and reactions of sodium disulfite are analogous to those of sodium sulfite. Sodium disulfite also has dual behaviors as sodium sulfite. 3.In the strong alkaline solutions test ed, the icorr obtained is very low (in the order of uA/cm^2). 4.Sodium molybdate in general has an inhibitive effect on carbon steel corrosion. However, at pH=5, in the solutions containing 0.00002M molybdate, their icorr is higher than those didn't contain sodium molybdate. This phenomenon can be explained as the formation of the fragmented passive film at low molybdate concentration, which will increase the icorr. 5.Superfluous sodium molybdate (above 0.001M in the sulfite solutions) will decrease its inhibition efficiency, probably due to its hindering the formationof the ferrous-ferric-molybdate complex. 6.The weight loss and electrochemical experiments provide similar trends in regarding to the variations of icorr as functions of pH, sodium molybdate as well as sodium sulfite and disulfite.