Synergistic Effect Of Cu And Cr On The Localized Corrosion Of Low Alloy Steels

It is one of the effective methods to improve the corrosion resistance of steel by adding alloy elements.Alloy elements Cu and Cr can significantly improve the uniform corrosion resistance of steel,but their influence on localized corrosion is still controversial,which is mainly related to the distribution of alloy elements in the matrix.At present,there are few reports about the synergy between Cu and Cr in steel,and there is also a lack of research on the effect of this synergy on localized corrosion.Due to the complex service environment and variable component structure,the actual engineering components are often in service in acid environment,such as the urban air environment with serious industrial pollution and the crevice between components.In this paper,we designed and fabricated three kinds of low alloy steels containing Cu or/and Cr elements.We adopt immersion test combined with various analysis and test methods to investigate the mechanism of localized corrosion induced by inclusions in steel.The synergetic effect of Cu and Cr in steel was studied by the first principles calculation for understanding of localized corrosion mechanism.By comparing the corrosion behavior of three kinds of steel in the acid NaCl solution,it is found that the localized corrosion of the steel containing both Cu and Cr is the most serious,followed by the steel containing only Cr,but almost no localized corrosion in the steel containing only Cu.The localized corrosion originates from inclusions in the three steels.All of them showed the preferential dissolution of sulfides in the complex inclusions,but no dissolution of oxides.After immersion,Cu particles deposited on the area where sulfide dissolved of the steel containing Cu,which can inhibit the further corrosion of the matrix;the hydrolysis of Cr3+ in the steel containing Cr can reduce the pH value of the local environment and accelerate the corrosion of the matrix.A comprehensive and systematic statistical analysis of inclusions in the three steels was carried out.It was found that the types of inclusions in the three steels are basically the same,mainly composed of(Si,Al)oxide surrounded by(Mn,Cu,Cr)sulfides.Thermodynamic calculations showed that the precipitation order of inclusions in steel is(Si,Al)oxides,followed by(Mn,Cu,Cr)sulfides.The proportion of SiO2 precipitation in oxides is higher than Al2O3,and the amount of MnS in sulfides is higher than Cu2S and CrS,which is consistent with the experimental results.There is no obvious difference in the number,size and distribution of inclusions in the three steels.The enrichment of Cu and Cr on MnS region of the complex inclusion was observed.Compared with steel containing only Cu or Cr,the enrichment degree of both elements increased when they coexisted,and the increase of enrichment degree of Cr element is higher than that of Cu element.In order to clarify the enrichment behavior and mechanism of Cu and Cr on MnS inclusions,first principles was used to study the segregation of Cu and Cr in?-Fe matrix and the adsorption behavior on MnS surface.It is found that Cu and Cr are more inclined to segregate on the surface of ?-Fe than dissolving in the matrix,and the driving force of segregation of Cu is stronger than that of Cr.Both Cu and Cr can be adsorbed stably on MnS surface,and the adsorption of Cu is more stable than that of Cr.According to the analysis of structure,energy and electronic structure,pre adsorbed Cr on MnS surface can promote the co-adsorption of Cu,and pre adsorbed Cu can also promote the co-adsorption of Cr.The promotion effect of Cu on Cr is stronger than that of Cr on Cu,which is in agreement with the experimental results.Furtherly,we selected three low index surfaces of MnS and ?-Fe to build nine interfaces.The calculated results of energy and electronic properties showed that the Fe(110)//MnS(110)interface is the most stable one among the nine interfaces.MnS tends to precipitate along the close-packed(110)surface of the matrix Fe with(110)surface.Cu and Cr also tend to segregate from the Fe matrix to the Fe(110)/MnS(110)interface.These results are confirmed by TEM experiments.