Corrosion Behavior Of NiCrMoV Steel Welded Joints In Simulated Deep Ocean Environment

NiCrMoV steel,as a new generation of deep-sea engineering steel,is used for the construction of pressure-resistant shells of deep-sea equipment.The corrosion problem caused by the welding joints of pressure-resistant shells exposed to seawater directly affects the service safety of deep-sea equipment.Therefore,studying the corrosion behavior of welded joints in the deep sea environment is very important to ensure the safe operation of equipment.In this paper,OM,SEM,EDS,TEM and other material analysis and characterization techniques and corrosion electrochemical test methods are used to study the corrosion behavior of welded joints in deep and shallow sea environments.The research results showed that the corrosion rate and corrosion morphology of welded joints showed huge difference in the deep and shallow environment.The deep sea environment reduces the overall corrosion rate of the material,but the corrosion morphology is changed from the general corrosion which is mainly happened in the shallow sea environment to the pitting localized corrosion.The corrosion of NiCrMoV welded joints is mainly controlled by diffusion.Corrosion rate in typical region is affected by microstructure differences.The corrosion rate in the heat affected region is the fastest,the base metal is the second,and corrosion rate of the weld seam is the slowest.Among the deep sea environmental factors,temperature has the greatest influence on material corrosion,followed by dissolved oxygen content and hydrostatic pressure.The coupling effect of environmental factors in the deep sea leads to the corrosion of the material of the welded joint along the grain boundary in the depth direction.The welded components of pressure-resistant shells of deep-sea equipment are subjected to external loads during service,which easily cause stress concentration.Therefore,the corrosion sensitivity test of welded joints under different stress loading conditions was carried out by using the corrosion electrochemical test method.The results showed that the corrosion rate of welding joints increased with increasing stress loading level.But it did not cause stress corrosion cracking of welded joints.Stress loading promoted the formation of pitting nucleation and metastable pitting on the surface of the material,and increased the pitting sensitivity of welded joint materials.