| As a main desalination technique, reverse osmosis (RO) technology is widely used in the high water-consuming enterprises such as coastal power plants in China. However, RO product water has a strong corrosiveness to carbon steel pipe of water distribution network, which endangers the safe operation of the water distribution pipes. Therefore, the works of exploring the corrosion mechanism of carbon steel in RO product water and seeking an economic and efficient anti-corrosion method is urgent to be carried out, which is of great research value.Firstly, in view of the corrosion differences of carbon steel in seawater and RO product water, this paper researches on the relation between the steel corrosion and rust layer characteristics using the weight-loss method and the surface analysis techniques; explores the electrochemical characteristics of iron rust in RO product water using electrochemical measurement techniques; and then proposes the corrosion mechanism of carbon steel in RO product water and establishes the corrosion model. Secondly, according to the corrosion mechanism of carbon steel, this paper studies the inhibition effect of inhibitors on carbon steel in RO product water and puts forward an economical, efficient and environmentally friendly inhibitor formulation, and then research on the inhibition mechanism of inhibitors. Finally, the field feasibility of this inhibition method is evaluated and a comprehensive anti-corrosion program is proposed by taking Yueqing power plants as an example. In addition, this paper also proves the accuracy of some inferences, which makes the research results have much broader applicability.The main conclusions are as follows:(1) The corrosion mechanism of carbon steel in RO product water is proposed. Due to the slightly acidic of RO product water,γ-FeOOH is easy to be reduced and Fe3O4can be converted from γ-FeOOH quickly; so the rust layer of carbon steel consists of a thin outer layer (γ-FeOOH layer) and a thick inner layer (Fe3O4layer). The γ-FeOOH layer tends to promote the corrosion of carbon steel via reduction of γ-FeOOH. The Fe3O4layer works as a large cathode area and oxygen is reduced on its surface. As a result, the corrosion rate of carbon steel is accelerated greatly and determined by the limiting diffusion rate of oxygen.(2) The broad applicability of this corrosion model is proved. The corrosion behaviors of carbon steel in dilute NaCl solutions are same as that in RO product water, which means that this corrosion model is suitable for the RO product water with different salinity. The reduction potential of γ-FeOOH is related to the electrolyte pH which is directly related with the alkalinity of solution. Meanwhile, the corrosion model of carbon steel in3%NaCl solution shows that the rust layer has a promotional effect on metal corrosion and the salinity of solution will not change the performance of rust layer. Therefore, it is probable that the rust layer generated in all slightly acidic water with low alkalinity can promote the corrosion of carbon steel via reduction of γ-FeOOH. This theory can provide guidance for the anticorrosion of carbon steel in this type of solution such as seawater distillation water.(3) A compound inhibitor with high inhibition efficiency and good economy is selected, and its inhibition mechanism is clarified. This inhibitor formulation consists of sodium hexametaphosphate and zinc sulfate and the optimal ratio of two chemicals is1:1. As for bare iron, the dosage of inhibitor is6mg/L and more than95%inhibition efficiency is reached; under this condition, water treatment cost is no more than0.034yuan per ton. As for rusted iron, the dosage of inhibitor is chosen as12mg/L and about70%inhibition efficiency can be reached. The compound inhibitor can form protective film on the surface of rust layer, which makes the rust layer much compacter and the diffusion process of oxygen is hindered. |
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