Corrosion Of X80 Steel Weld Joint In Acidic Red Soil Extract Solution

Pipeline steel is a long-distance,safe,and economical transportation tool for oil and natural gas.Weld technology is widely used in the connection of oil and gas pipelines.And the weld joints are often affected by environmental media during the service process,which is a weak link in the pipeline structure.Corrosion failure damage often occurs here.Currently,pipeline steel has been laid in large quantities in acidic red soil with high water content,low electrical conductivity,high CO₂ content,high salt content,and strong corrosion.However,the corrosion behavior of X80 pipeline steel welded joints in acid red soil has not been reported.In this thesis,using the methods of immersion and disbonded coating experiments,the corrosion behavior of weld joints in acidic red soil was systematically studied by electrochemical tests,metallographic microscope,X-ray diffractometer,scanning electron microscope and 3D ultra-depth microscopy.The main results of the research are as follows:1. The corrosion rates of the base metal?BM?,heat affected area?HAZ?and weld zone?WZ?of the weld joint were compared in the acid red soil leaching solution.After35 days immersion in the soil solution,the corrosion products on the weld zone possess the densest structure and the best mechanical properties.However,the corrosion product on the heat affected zone tends to get cracking and form the wide micro-cracks.2. The corrosion reaction for WZ,HAZ and BM specimens are dominated by electro-chemical polarization during the separated immersion in the acid soil solution.The corrosion rate of three specimens slightly decreased at the initial stage,then rapidly increased,and eventually remained stable until the end of the immersion.The corrosion rate of HAZ specimen is always higher than those of WZ and BM specimens throughout the separated immersion in the acidic soil solution.3. Under the disbonded coating,the corrosion morphology of BM,HAZ and WZspecimens was mainly pitting corrosion at holiday and bottom,and uniform corrosion was mainly in the middle of crevice.At the same position of the disbonded coating,the corrosion resistance of the base metal of the X80 steel weld joint is the best,followed by the weld zone,and the worst in the heat affected area.4. In the acidic soil solution,the corrosion rates of BM,HAZ and WZ specimens were the highest at the opening,followed by the bottom,and the lowest in the middleof the crevice under the disbonded coating.The gradient distribution of aggressive ions in rectangular gaps may be one of the reasons for this phenomenon.5. Under the disboded coating,the corrosion rates of BM,HAZ and WZ specimens first decreased and then increased with the increase of the immersion time.The corrosion sensitivities of each zone of the weld joint under the disbonded coating were far greater than those in the immersion solution.The micro-environment of peeling caused by the failure of the coating had a great promoting effect on the corrosion rate of each zone of the X80 pipeline steel weld joint.