Corrosion Resistance Of 316L Stainless Steel Composite Pipelines And Its Welding Joints

316L stainless steel composition pipe is consisted of inner pipeline of 316L stainless steel and outer pipeline of 20G steel.The design idea of 316L stainless steel composition pipe is based on that the inner pipeline has good corrosion resistance and outer pipeline has high strength.Therefore,316L stainless steel composition pipes have been widely used in oil,natural gas transportation fields for the good mechanical property and good corrosion resistance.The investigation on the corrosion reistance of 316L stainless steel and its welding joints in the oil&gas field environment is very necessary.The corrosion resistance of 316L stainless steel and its welding joints was investigated by SEM,EDS and electrochemical measurments in the pitting environments with different Cl~-concentration and pH values.The results implied that Cl~-concentration has little effect on the open circuit potential and pitting potential in low Cl~-concentrations(150mg/L).In this case,the passive film of stainless steel has strong corrosion resistance and can not dissolve until the applied potential unless exceed 1200 mV,which indicated that pitting does not occur and no CPT is observed.With the increase of Cl~-concentration(>3700mg/L),although there are some fluctuations in the open circuit potential,a downward trend is still visable.The break potential of 316L stainless steel decreased with the increase of Cl~-concentration significantly,which indicates that the Cl~-concentration has great influence on the corrosion resistance of stainless steel.The CPT decreased from 14?to 6?with the concentration of Cl~-content,while CPT almost keep as a constant with pH value,which indicated that Cl~-concentration has great influence on the CPT of 316L stainless steel,but pH vale has a little influence on the CPT of 316L stainless steel.Moreover,the results also indicated that there was no synergestic effect on the CPT of 316L stainless steel.for the transversal welding,the weld heat affected zone shows the coarsr microstructure and lower Cr content(only 14.65%),meanwhile,there are some segregation phase with higher Cr content was observed in the longitudinal weld,but the diameter and density of weld oxide particles are very small.Small heat-affected zone,there is no Cr depleted zone,316L transverse weld corrosion occurs mainly in the heat-affected zone,followed by welding area.Due to higher Cr content,F316L root weld area and 316L base metal have the higher break potential and highe corrosion resistance.Because the segregation phase with higher Cr content and the small weld oxide particles in the longitudinal weld,the break potential of longitudinal weld is 100mV higher than that of transversal welding,suggesting the higher corrosion resistance than transversal welding.