Local Dissolution And Crack Initiation Induced By Inclusions In P110S Oil Casing Steel

As a sulfur resistant oil casing steel,P110S oil casing not only has high yield strength,but also has strong H₂S fracture resistance.However,in sulfur-containing environment,its corrosion resistance is poor and can cause pitting corrosion,which limits its further use.The main failure types of P110S oil casing steel are pitting corrosion and stress corrosion cracks The inclusions in oil casing steel are the possible initiation points of pitting corrosion and SSC cracks.On the one hand,inclusions can cause pitting corrosion by local dissolution and then cause stress concentration,finally cause crack initiation.On the other hand,inclusions can also act as hydrogen traps to capture hydrogen atoms and cause hydrogen induced cracks.Therefore,this paper studies the local dissolution and crack initiation caused by inclusions in P110S oil casing steel.The corrosion behavior of P110S oil casing steel in Na₂S solution was characterized by electrochemical experiment and immersion experiment.The results show that in the slightly sulfurous environment,with the increase of Na₂S content,the corrosion process of P110S oil casing changes from cathodic control to mixed control;As the pits’numbers and sizes increase,the corrosion rate of casing increases.Inclusions and other locations are the common sources of pitting corrosion,the local corrosion caused by inclusions in P110S oil casing steel was studied by SEM,in-situ SVET and SKPFM.The results show that the main inclusions in P110S steel are Al-Ca-O-S inclusions composed of Ca-S inclusions and Al-O inclusions,with a size about 5μm;Inclusions in Na₂S solution can cause local dissolution and generate corrosion products,the corrosion products have shielding effect,which makes the local dissolution current gradually decrease.The potential differences between Ca-S inclusions,Al-O inclusions and matrix cause driving forces of galvanic corrosion.In addition to causing local dissolution,inclusions can also act as hydrogen traps to cause cracks.Therefore,the hydrogen bubbles,hydrogen induced cracks and stress corrosion cracks caused by inclusions as hydrogen traps were studied by electrochemical hydrogen charging,hydrogen thermal desorption experiment,hydrogen escape experiment and U-bend experiment.The results show that the inclusions in P110S oil casing steel acting as hydrogen traps can cause hydrogen bubbles and hydrogen induced cracks;A large number of cracks appear in the cross section of U-bend samples in Na₂S solution.And the cracks mainly originate from inclusions,which indicates inclusions can cause SSC cracks.