Study Of The Mechanism & Synergistic Effect For Molybdate As An Inhibitor On A20 Carbon Steel

How to solve metal corrosion is one of the most focused questions in the commercial processes. Molybdate has become more and more interesting because it’s no poisonous and damage to environment, strong thermo stability, and synergistic effect. This paper aims at studying inhibiting efficiency, mechanism of inhibiting corrosion, and synergism effect on A20 carbon steel for molybdate in the prepared water, in order to decrease the material corrosion rate and prolong operating period and using life of device. Changing of conductivity, oxygen content, the values of pH, concentration of MoO42- and turbidity with time were determined in solutions with different concentration of MoO42-. The inhibiting performance of MoO42- is investigated by means of weight loss measurements, polarized curve method, SEM and EDS surface analysis technique. As a result, optimized synergistic way is obtained.The research work obtains results as follow:⑴Molybdate is an anodic corrosion inhibitor. The filmming process is divided into three phases of inducing, which takes 24h, growing, which 48h, and keeping. The rate of corrosion is the most rapid in inducing phase, thus the efficiency can be enhanced effectively by reducing the time. The higher of MoO42- concentration is; the denser and thicker of membrane is. The concentration should be maintained at 100 mg?L-1 in keeping phase.⑵Langelier saturation index(L.S.I) is an important factor for inhibiting efficiency. The water is scaled one when it is bigger than zero. It is easy to produce deposition membrane which consists of two layers. In the blank solution, composition in lower layer is mainly compounds containing iron oxide, in upper layer is CaCO3 mainly. The film is thin. The lower layer is FeMoO4 which is even, dense without crack, and the upper layer is CaMoO4 and CaCO3 when MoO42- is added. At the moment, the inhibiting efficiency is only about 6%. The water is corrosive one when L.S.I. is lower than zero. The inhibiting efficiency may reach up nearly 20% as the concentration of MoO42- is bigger than 100 mg?L-1.⑶When Zn2+ is added, polarizations of two poles arise at one time, the ionic rates of metal and O2 become weak, inducing phase is reduced, the film is compact, and the inhibiting efficiency is enhanced to 36.5%.⑷The inhibiting efficiency can reach 86.6% when C6H11O7Na and Zn2+ are added. When Zn2+, Co2+ and Ni2+ are mingled with MoO42-, the synergism effect is more obvious. The inhibiting efficiency increases much higher. The inhibiting efficiency enhances from 53.8% to 92.4% the concentration of MoO42- increasing from 25 mg·L-1 to 100 mg·L-1.