Research On The Corrosion Resistance Of The Weld Of L245NB/316L Bimetal Composite Pipe

During the process of oil and gas transportation,the corrosion of pipeline steel is widespread.Especially in the acidic gas environment,ordinary carbon steel can not meet the demand on corrosion resistance.Considering the economic and security factors,the bimetal composite pipes were chose to solve this problem.In this paper,the outer carbon steel of the bimetal composite pipes ensures its rigid condition,meanwhile the inner layer ensures the corrosion resistance.The composite pipes are usually connected by welding.Due to the various factors in the welding process,the weld seam becomes the weakness area.This paper’mainly discusses the corrosion resistance of the interior and exterior of the composite pipe welded joint.For the exterior of the welded joint,the effect of temperature and CO2 on corrosion resistance in the standard NS4 neutral solution were investigated.The result showed that the corrosion rate raised with the increase of the temperature and the addition of CO2,and the impact of which was obvious for L245NB.Based on the analysis of experimental date,it was found that the corrosion rate of L245NB was far greater than the weld metal.At temperature of 20 ℃ and 40 ℃,there is hardly any corrosion product.With the increase of temperature,the corrosion products appeared on the surface of the sample.At the temperature of 80 ℃,the corrosion products of Fe2O3 and FeCO3 covered on the surface of L245NB,which weakened the corrosion as well as protected the based metal.Due to the good corrosion resistance,the deposition could be found on the surface of the weld metal only at the higher temperature.Electrochemical analysis results indicated that the corrosion potential of the weld metal was more positive than L245NB,and its corrosion current density was smaller than L245NB,and the charge transfer resistance of weld metal was greater than L245NB.Therefore,the electrochemical reaction rate is smaller than L245NB.In brief,the raise of temperature and the addition of CO>has a promoting effect on the electrochemical reaction of the weld metal.For the interior corrosion resistance of the welded joint,electrochemical method was considered to study its corrosion resistance of the substrate of 316L stainless steel.The result showed that with the increase of temperature and the concentration of S2-,and the addition of CO2,the electrochemical reaction rate increased.When the concentration of S2-is low,the CO2 gas and S2-had a synergistic effect,and the electrochemical reaction rate was further increased.When the concentration of S2-increased to 0.5 mol/L,the two factors inhibited each other,and the electrochemical reaction rate decreased.Under the same condition,the electrochemical test results showed that the electrochemical reaction rate of 316L was superior than the weld metal.