Research Of Corrosion Mechanism On Welded Joint Of 5Cr Steel By Fusion Welding

Low Cr steel have extremely application prospect in traditional pipeline steel because of its have a certain corrosion resistance.5Cr steel is a new development of the domestic steel grade.Compared to its material and processing properties of extensive research,the corrosion resistance of welded joint are rarely reported.Therefore,based on this new steel grade of 5Cr steel.Research on temperature and CO₂ partial pressure effect on corrosion rate,polarization curves and corrosion product film morphology of 5Cr steel in the formation of the welding joint under different filler material（Stainless steel wire,Flux-cored wire,Ordinary wire）by weight-loss method,electrochemical method and surface analysis technology of combining the way.Analysis the mechanism of corrosion resistance.Find that match the wire material and ensure the corrosion resistance of weld.Combined with weight-loss method and electrochemical method to research and analysis the corrosion rate that 5Cr steel welded joint under different temperature and CO₂ partial pressure.The results show that under the condition of the temperature of 40℃ and CO₂ partial pressure of 1 MPa,the average corrosion rate of size of the stainless steel wire welding joint: V_W<V_HAZ<V_WM.The average corrosion rate of size of the flux-cored wire welding joint: V_W<V_HAZ<V_WM.And the average corrosion rate of size of the ordinary wire welding joint: V_W>V_HAZ>V_WM.Comparing the average corrosion rate of the weld found that:cored welding is minimum,and ordinary weld is the largest,is about 67 times that of the cored welding seam,stainless steel weld fall in between,but also is only 1/13 of the common weld.With the increasing of temperature,the corrosion rate of stainless steel weld showed a trend of increase with the decrease of the first,attain to the minimum in60℃,about 0.0664mm/a.The corrosion rate of cored welding is increases with the increasing of temperature.Whereas increase first,then decrease trend in ordinary weld,reached the maximum in 60℃,about 1.5112mm/a.Three kinds of weld corrosion rate with the increase of the CO₂ partial pressure.In view of the different weld corrosion degree,based on the corrosion model in the case of electrode potential,to separate the sample in each region of welded joint,respectively under different temperature,CO₂ partial pressure extraction of polarization curve,combined with the dynamics and thermodynamics theory to analyze it.The results show that under the condition of the temperature of 40℃ and CO₂ partial pressure of 1 MPa,compared with the heat affected zone and the parent metal,stainless steel weld and cored from the corrosion potential of weld seam werethe most positive,corrosion current density are the smallest,namely the corrosion tendency is small,low corrosion rate.And ordinary weld from the corrosion potential of the youngest,corrosion current density is the largest.With the increase of temperature,weld from the corrosion potential of stainless steel are moving to the negative first,corrosion current density increase with the decrease of the first,corrosion tend to be the smallest when 60℃,the corrosion rate is lowest.And cored welding seam from corrosion potential continued to negative,corrosion current density increased,as well as an increase with the temperature,the corrosion tendency increases,corrosion rate to rise.Ordinary weld from the corrosion potential appeared move to the negative first,the corrosion current density increased after the first decreases,and at 60℃ the corrosion tendency is the largest,the corrosion rate is highest.With the increase of CO₂ partial pressure,three kinds of the corrosion potential of weld seam were negative,corrosion current density showed a trend of increase,This means that the corrosion tendency increases with the increase of CO₂ partial pressure,corrosion rate to rise.Analysis of the corrosion product film by SEM、EDS and XRD.The results show that under the condition of the temperature of 40℃ and CO₂ partial pressure of1 MPa,stainless steel weld and cored welding seam are not or only the product of a layer of thin film formation,there are scratches and Cr content is higher than that of heat affected zone and the base in the surface,namely the weld corrosion resistance is superior to the heat affected zone and the parent metal.Ordinary weld formation of corrosion product film crack shaped and appear Cr enrichment phenomenon,but the Cr content still below the heat affected zone and parent metal,so the poor corrosion resistance of weld.The stainless steel weld and cored welding surface had no obvious change in the different temperatures,there are slight uniform corrosion and somewhat corrosion occurs,and pitting corrosion intensifies with increasing of the temperature.The stainless steel weld with granular crystal deposition in the surface of 60℃,presumably for Fe carbonic acid double salt compounds.Ordinary weld product membrane was stratification obviously,inner lining crack with amorphous characteristics,energy spectrum analysis may for Cr compounds,the outer membrane shows characteristics of granular crystal,may for the Fe（CO3）.With temperature rise,the outer membrane of improving density and coverage of substrate protective enhancement.With the increase of CO₂ partial pressure,the corrosion product film density increased,but there are still a part of the bare metal,corrosion is increasing.At the same time,the welding joint under different filling materials on line scan found,mainly containing elements of Fe,Cr,O.Stainless steel weld and cored welding seam area were higher than in other areas of Cr content.Whereas ordinary weld zone of Cr content is much lower than other areas,on the contrary O content is the highest.It is shows that ordinary weld corrosion is most serious compared with thestainless steel weld and cored welding seam.This article research results will provide theoretical basis for the 5Cr steel structured application,for 5Cr steel welding technology provides a new way of thinking.