| 254SMo super austenitic stainless steel is a special stainless steel that was developed by the 1970 s with a high content of Cr, Ni, Mo and a moderate amount of N and Cu alloy elements, with good mechanical properties and corrosion resistance, it is widely used desalination, pulp bleaching, flue gas desulfurization, and other areas of the harsh corrosion environment. But due to its higher nickel content, nickel prices rising and the shortage of nickel resources, a low nickel content, excellent corrosion resistance and good mechanical properties of super duplex stainless steel 2507 has been developed in the 1980 s, it has gradually developed to be partly replace 254 SMo super austenitic stainless steel in many areas. Since the 21 st century, with the rapid growth of world population and the rapid development of social economy, fresh water resources are gradually scarce, so the seawater desalination processing has been become a important way to solve the shortage of fresh water resources. At present, the most widely used is multi stage flash desalination technology, according to this method constructed desalination processing equipment accounted for more than 70% of the global total. However, the equipment used in the past is prone to occur pitting and crevice corrosion in the long-term service environment, the super austenitic stainless steel and super duplex stainless steel become the best alternative materials with their more excellent corrosion resistance. Therefore, studying corrosion resistance law and corrosion mechanism of two super stainless steels used as multi stage flash desalination plant material in different sea conditions, it has important practical significance of the multi stage flash desalination device selection and application.This paper based on the research status at home and abroad as well as multi stage flash desalination device’s working conditions, using electrochemical workstation and Factsage thermodynamic calculation software to study corrosion resistance of two super stainless steels under seawater conditions of different pH value, temperature and the concentration of Cl~-, and to explore and analyse on its corrosion mechanism. At the same time, corrosion morphology of samples were observed after electrochemical tests by using scanning electron microscope. The main conclusions are as follows:(1) With the decreasing of seawater pH value, pitting resistance of two super stainless steels weakened after the first increase; under the same condition of pH value, the corrosion tendency of two super stainless steels is equivalent, and pitting resistance of 2507 super duplex stainless steel is stronger than 254 SMo super austenitic stainless steel;(2) With the increasing of seawater Cl~- concentration, pitting resistance of two super stainless steels gradually increased; under the same condition of Clconcentration, the corrosion tendency of two super stainless steels is equivalent, and pitting resistance of 2507 super duplex stainless steel is stronger than 254 SMo super austenitic stainless steel;(3) With the increasing of seawater temperature, pitting resistance of two super stainless steels weakened after the first increase; under the same condition of temperature, pitting resistance of 2507 super duplex stainless steel is stronger than 254 SMo super austenitic stainless steel, and the corrosion tendency of two super stainless steels is equivalent below 60 ℃, the corrosion tendency of 254 SMo super austenitic stainless steel is weaker than 2507 super duplex stainless steel above 60 ℃;(4) In strong acid, high temperature(T > 60 ℃) and containing Cl~- harsh corrosive environment, two super stainless steels both occur metastable pit.(5) Pourbaix diagram shows that throughout the process of corrosion, Cr was transformed into Cr2O3 and Cr(OH)2+ which could be oxidized to the passive film was adhered to the surface to protect the substrate; N mainly combined with H+ around the corrosion holes form NH4+ to inhibit self-catalytic process of pitting; Mo into MoO42-adsorbed on the substrate surface enhanced passive stability of materials on Cl~-. |
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