Research On Corrosion Resistance Of Marine Hose Steel In Seawater And Carbon Dioxide Corrosion Environment

In recent years, oil leaks and other serious accidents occur frequently due to the corrosion of marine pipelines. Although pipelines for oil exploitation and transportation include a variety of materials and structures, the marine hose has become a great alternative to rigid pipelines because of its excellent performance, and it has broad prospects for development. The middle layer material is critical to the whole performance of marine hose, and the steel in middle layer needs both high strength and excellent corrosion resistance. Up to now, the market and technology of marine hose are almost monopolized by foreign companies, and relative research in our country is still in its infancy, so it is urgent to develop high-strength and corrosion resistant steel used in marine hose to break the foreign monopoly. The research object is static marine hose steel with high-strength and excellent corrosion resistance, and this paper studies its component, microstructure, mechanical properties, corrosion resistance and corrosion mechanism. The main research work and results are as follows:(1) To meet the requirement of corrosion resistance, the component method of experiment steel is using low C, Mn, P and S contents, and add in Cr, Mo and Ti elements. The austenite cooling process and microstructure evolution law of marine hose steel are studied through the methods of thermal expansion and microstructure observation, and the static CCT curves are established. Results show that when the cooling rate is less than 5?/s, the microstructure in room temperature is ferrite, pearlite and bainite; when the cooling rate is more than 10?/s, the microstructure is granular bainite and lath bainite. The larger the cooling rate, the smaller the distance between the bainite laths and the more densely and orderly arranged the laths.(2) The optimum parameters for rolling and heat treatment are determined through laboratory experiments combined with the actual production process. The heating temperature of hot rolling is 1150?1200?; the finish rolling temperature is 950?980?; the temperature after water cooling is 820?850? and after air cooling is 300?350?; the air cooling rate is 2?5?/s. The heating temperature of quenching is 900 "C and holding time is 15min; tempering temperature is 350? and holding time is 30min. The marine hose steel after this heat treatment process has good mechanical properties and the microstructure is fine ferrite and tempered martensite.(3) After the experiment of full immersion corrosion in seawater, the average annual corrosion rate of marine hose steel is calculated using weight-loss method. The surface profile and phase composition of the corrosion product are observed and analyzed to explore the corrosion mechanism. Results show that the corrosion of the marine hose steel with this designed alloy composition and rolling and heat treatment processes is uniform and the steel surface does not appear obvious corrosion pits. The average annual corrosion rate of the steel is 0.11?0.12mm/a. The corrosion products include lepidocrocite, magnetite, iron oxide and Goethite.(4) After carbon dioxide corrosion experiment, the average annual corrosion rate of marine hose steel is calculated using weight-loss method. To determine the applicability of the designed steel and explore its corrosion mechanism, the surface profile and phase composition of the corrosion product are observed and analyzed. Results show that when the corrosion environment is 75? and 1.2MPa the corrosion of the steel is uniform and the average annual corrosion rate of the marine hose steel is 1.23mm/a in liquid phase and is 0.03mm/a in gas phase. The corrosion products include lepidocrocite, iron oxide and sidenite.(5) Through electrochemical corrosion experiment, the polarization curve of the experimental steel is tested using dynamic potential method. The polarization behaviors of the steel in different corrosion solutions are compared. Results show that the experiment steel does not appear anodic passivation in the corrosion process, and the steel's corrosion tendency increases with the increase of chloride ions. The smaller the pH values of the corrosion solution, the weaker the corrosion resistant ability of the experiment steel. The destruction of chloride ion in acidic solution on experimental steel is more serious than in neutral solution.