Corrosion Resistance Mechanism Of Cr-Mo-Sn Microalloyed Low-alloy Steel In Tropical Marine Atmosphere

Based on the urgent need of low-alloy steel for infrastructure construction in the development of China's major marine strategy,this paper used microstructure characterization,electrochemical performance,wet/dry accelerated test,corrosion big data monitoring,phase analysis and other methods to study the influence of Cr,Mo and Sn element content and microstructure differences on the corrosion laws and mechanism of steels.Finally,a Cr-Mo-Sn corrosion-resistant low-alloy steel suitable for the harsh service environment of the tropical marine atmosphere was developed.Using a combination of machine learning and outdoor exposure tests,corrosion resistance evaluation model of low-alloy steel in marine atmospheric environment was constructed and verified.The results showed:The addition of Cr in the low-alloy steel could improve the corrosion resistance of the steel subtrate and reduce the corrosion rate of the steel.Cr was enriched in the inner rust layer.In addition to forming its own corrosion-resistant and stable Cr2O3,Cr could also be doped in ?-FeOOH and Fe3O4 to form thermodynamically stable ?-(Fe,Cr)OOH and FeCr2O4.These products destroyed the original Fe(O,OH)6 network structure of the rust layer,provided more nucleation sites for rust particles,and promoted the refinement of the rust layer.Among them,3 Cr steel achieves the best corrosion resistance.The addition of Mo in 3Cr steel improved the corrosion resistance of the steel subtrate and the compactness of the rust layer.The addition of 0.1 wt.%Mo effectively inhibited the cathodic reaction of the steel and increased the corrosion potential of the steel subtrate.Mo was evenly distributed in the rust layer.In addition to increasing the value of the protective index ?/?*in the rust layer,it could also form its own insoluble oxide MoO2,reducing pores and cracks in the rust layer,and improving the compactness of the rust layer.The addition of Sn in 3Cr steel could further improved the compactness of the rust layer.Sn exhibited an inhibitory effect on the anodic dissolution of steel of steel.Sn was evenly distributed in the rust layer in the form of insoluble product SnO2.It promoted the conversion of ?-FeOOH to ?-FeOOH.The combined effect of the two effects improved the compactness of the rust layer.In addition,Sn could reduce the active dissolution sites on the steel surface and consume H+in the environment,thereby inhibiting the tendency of corrosion pits to deepen under rust.The composite addition of Cr-Mo-Sn suppressed the cathodic and anode reaction and improved the corrosion resistance of the steel matrix.The synergistic effect of Mo-Sn made Cr,Ni,Cu,Mo,Sn further transformed to stable form,Cr2O3,NiFe2O4,CuO,MoO2 and SnO2.Under the synergistic effect of each element,the protective index ?/?*of the rust layer was increased,the defects of the rust layer were reduced,and the corrosion resistance of steel was further improved.In addition,the Mo-Sn compound addition further inhibited the development of corrosion pits depth.The microstructure difference had a certain influence on the corrosion resistance of 3Cr-Mo-0.1%Sn steel.The fine-grained microstructure effectively inhibited the cathodic reaction process of steel,the difference in the internal composition and microstructure of the grains was reduced,and the corrosion resistance of the substrate was improved.The existence of a large number of grain boundaries in the fine-grained microstructure promoted the rapid densification of the rust layer.The grain boundaries served as a rapid diffusion channel for alloying elements,which was beneficial to the formation of Cr2O3,NiFe2O4,SnO2 and MoO2,improving the protection of the rust layer.There are a large number of dislocations in the grains of the coarse-grained microstructure,and the local strain is very high.The electron transmission channel was increased,and the unevenness of the microstructure was higher,resulting in the worst corrosion resistance of the coarsegrained microstructure.The random forest algorithm was used to construct a comprehensive corrosion evaluation model of low-alloy steel in tropical marine atmosphere,and the data from outdoor exposure tests proved that both models have good generalization ability.According to the importance analysis,Cr played a major role in reducing the corrosion rate,while Sn played a major role in inhibiting the longitudinal development of corrosion pits,and Mo had a certain influence in both aspects.The optimal additive content of Cr,Mo and Sn of low-alloy steel were:2.5 wt.%,0.25 wt.%and 0.22 wt.%,respectively.Comparing the exposure data with the results of the indoor accelerated test,the corrosion mechanism of steel under the two conditions was consistent,which proved that the indoor accelerated test method wass reasonable and feasible.