Comparative Study Of Corrosion Behavior Of Three Types Of Offshore Steels In The Marine Environment

The ocean contains a variety of important material resources needed for human survival and development,the key to the development of marine resources and the maintenance of rights and interests is the safe service of offshore equipment,high-performance steel materials as the main structural materials for offshore equipment,its environmental suitability is also gradually gaining attention.S420,S460 and FH550 are the three low alloy and high strength steels,which have excellent toughness and fatigue resistance.The complex and harsh seawater environment has put forward more stringent requirements for marine engineering steels.In order to overcome the marine environment,marine engineering steel should have comprehensive performance such as high strength,high toughness and corrosion resistance,which is also the future development trend of high strength marine steel.Research on the corrosion behavior of offshore wind power steel structures in the marine environment is still relatively little,and it is of great theoretical significance to study the effect of corrosion behavior of high-strength steels in the seawater environment to provide theoretical guidance for the development of corrosion-resistant offshore steels.Therefore,three different strength levels of offshore steel,S420,S460 and FH550,were used as the research elephant in this project.Two influencing factors,temperature and immersion period,were chosen to study the differences in corrosion behavior of three steels compared in artificial seawater.Through outdoor exposure experiments to study the corrosion behavior of S420 steel in different zones,so as to enrich the corrosion theory system of S420 offshore steel,and the relevant indoor laboratory research results further corroborate the outdoor corrosion casting results.The main conclusions are as follows:（1）The effects of temperature and immersion period on the corrosion behavior of S420,S460 and FH550 offshore steels in artificial seawater solutions were systematically investigated.Electrochemical experiments revealed that the three steel anodes exhibited pseudo-passivation properties and the cathodic reaction was controlled by both oxygen reduction and water reduction.S420,S460,FH550 three kinds of offshore steel in300m V_{Ag/Ag Cl} potential of the saturation corrosion current density are lower than 6 m A/cm²,it shows that the three steels have high corrosion stability performance.In low-temperature seawater,the corrosion product layer formed on the steel surface is loose and porous,which cannot prevent the corrosion of steel by corrosive media.Under room temperature conditions,S460 steel shows a more excellent corrosion resistance,while FH550 steel in the three steel corrosion resistance is relatively poor.The corrosion products generated by the three steels in low-temperature and room-temperature artificial seawater mainly consist ofγ-Fe₂O₃ and Fe₃O₄ andα/γ-Fe OOH.The three strength marine steels show pitting corrosion in low-temperature seawater and accelerate the corrosion process.Therefore,it is necessary to improve the corrosion resistance of S420,S460 and FH550 steels in very cold seawater.（2）The corrosion behavior of S420 steel was compared after one year of exposure to four different zones:atmospheric zone,splash zone,tidal zone and total immersion zone.S420 steel has the highest corrosion rate in the tidal zone and the lowest corrosion rate under atmospheric conditions.The difference in corrosion rates indicates the importance of the drying period and water retention capacity of the rust layer in the tidal zone.The corrosion products consisted mainly ofγ-Fe₂O₃ and Fe₃O₄ andα/γ-Fe OOH.The persistence of the electrolyte layer favors the formation ofγ-Fe OOH,making it dominant in the tidal zone.Adequate oxygen and dry-wet cycles promote the production of Fe₃O₄,leading to its dominance in the splash zone.S420 steel exhibits a high number density of etch pits and maximum etch pit depth in the splash zone,while it exhibits a limited number of etch pits and a large etch pit volume in the total immersion zone.