Research On Corrosion Behavior And Mechanism Of 3Ni Weathering Steel In Harsh Marine And Atmospheric Environment

With the development of the strategy of "Marine Power" and the emphasis of the country on the status of the ocean,it is urgent to solve the corrosion resistant steel and its protection technology for high-performance building structures,and more stringent requirements are put forward for the corrosion resistant ability of steel materials in the marine atmosphere.However,the corrosion resistance and protection technology of existing steel materials cannot fully meet the service requirements of marine building structures.Especially in the harsh environment of the South China Sea,such as high salt fog,high humidity and heat,and strong radiation,the corrosion problem of steel materials is more prominent,which becomes the bottleneck of building a maritime power.3Ni weathering steel,as a new generation of weathering steel for marine atmospheric environment,has been successfully applied in the low-salinity marine atmospheric environment and achieved good demonstration results,but it can not cope with the marine atmospheric service environment of high salt and high humidity in the South China Sea.Therefore,3Ni weathering steel with higher quality is urgently needed to be developed in order to provide material support for the smooth implementation of our marine strategy of continuously expanding and extending to the far-reaching sea.In this paper,the effects of La.Cr and Mo elements on the surface electronic work function,the physicochemical properties of the rust layer and the evolution behavior of the rust interface of 3Ni weathering steel were systematically studied by using the first-principles calculation combined with the simulation corrosion experiment,electrochemical test,morphology analysis and phase analysis,and the mechanism of the influence of Cl-concentration on the corrosion behavior of 3Ni weathering steel was explored.The main findings show that:La element transformed the original elongated MnS and rhomboid Al2O3 into circular(La,S,P)Ox,and the inclusions were refined and the number of inclusions increased,and the Austenitic core location was increased and the grains were refined.The potential difference between the modified inclusions and the surrounding matrix is reduced and the corrosion resistance of the matrix is improved at the initial stage of corrosion.In addition,La covers the cathode site in the rust layer in the form of La2O3,resulting in negative charge accumulation,cathode polarization and inhibition of cathode reaction,thus improving the charge transfer resistance and corrosion resistance of 3Ni weathering steel.Cr is beneficial to enhance the corrosion resistance of 3Ni weathering steel,but the effect is controlled by Cl-concentration.When NaCl concentration is less than 0.5wt.%,Cr is doped into α-Feooh to form α-(Fe,Cr)OOH,which breaks the double-chain structure of α-FeOOH and forms ultrafine α-FeOOH,resulting in the presence of cation-selective compact packed aggregates.The refining effect of Cr on α-FeOOH and the cation selectivity of inner rust layer are enhanced with the increase of Cr content in steel matrix.In addition,the cation selectivity of α(Fe,Cr)OOH and Cr(OH)3 can enrich the inner rust layer with Ni and Cu,improve the surface electronic work function of the rust layer,reduce the potential difference between the rust layer and the substrate,and effectively improve the stability and protection ability of the rust layer.However,when NaCl concentration is greater than 0.5wt.%,although the beneficial effect of Cr still exists,the acidification caused by the hydrolysis of Cr3+ and the interaction of β-FeOOH will destroy the inner rust layer,resulting in Cl-and O2 reaching the matrix through cracks.Under the action of autocatalysis,the local corrosion will be intensified and the protective effect of the rust layer will be reduced.The rust layer of 3Ni-Mo weathering steel can resist Cl-invasion and has excellent corrosion resistance to Marine atmosphere.The steady-state rust layer of 3Ni-Mo steel has a three-layer structure,and there are differences in morphology,microhardness and phase composition between different rust layers:the microhardness of the outermost layer is 15-30 HV,mainly composed of γ-FeOOH,which does not have corrosion resistance.The microhardness of the interlayer is about 80-120 HV,which is mainly composed of Fe2O3/Fe3O4.The presence of a certain amount of NiFe2O4 makes the layer electronegative and able to repel Cl-.The microhardness of the innermost layer is 25-40 HV,which increases with the increase of Mo content and is mainly composed of α-FeOOH and β-FeOOH.The innermost layer is the main component of the stable rust layer,and its junction with the substrate is an alkaline interface with a large number of Na+.Due to the enrichment of NiFe2O4,CuFe2O4 and insoluble oxides of Cu and Mo,the interface is much more electronegative than the intermediate layer,which prevents the interface acidification and local corrosion caused by Cl-.The increase of Mo content can increase the relative content of α-FeOOH in the rust layer,shorten the formation time of alkaline steel rust interface,and enhance the compactness of the innermost layer.The steady-state rust layer with alkaline steel rust interface can be formed in 384 h by adding lwt.%Mo element in 3Ni weathering steel.The effect of Mo element on improving the corrosion resistance of 3Ni weathering steel is mainly reflected in three aspects.First,MoO2,MoO3,molybdate and CuO are deposited together on the surface of the matrix and in the innermost layer,which can repair the cracks in the corrosion pit and rust layer,and reduce the negative and anode adsorption sites on the matrix.Secondly,Mo promotes the formation of NiFe2O4 and CuFe2O4,strengthens the electronegativity of the interlayer and the steel rust interface,and constructs the alkaline steel rust interface by double Cl-discharge mechanism.Thirdly,Mo increases the nucleation position of Fe oxides and hydroxyl oxides in the process of forming oxides,resulting in nanoscale corrosion products.The stable rust layer further prevents Cl-from entering the innermost layer,inhibits the formation of β-FeOOH and promotes the formation of α-FeOOH in the innermost layer.The necessary conditions for the formation of stable rust layer in severe Marine atmospheric environment were studied,and the corrosion evolution process and mechanism of 3Ni-La,3Ni-Cr and 3Ni-Mo weathering steels were elucidated.It provides technical support and theoretical guidance for the application of 3Ni weathering steel in severe Marine atmospheric environment.