.35 Crmo Steel In The Saturated Solution 2 </ Sub> Co <sub> Corrosion Behavior Research

In this thesis,autoclave test and the electrochemical corrosion test,together with weight loss method,AFM,XRD and SEM,was used to study CO2 corrosion behavior of drilling tools steel 35CrMo in oil well produced carbon dioxide saturated solution at 30-120℃,CO2 partial pressure(0.5-2MPa)and its time is 10-72h. What's more,the stress corrosion cracking (SCC) behaviour of 35CrMo steel after corrosion in different CO2 partial pressure was studied using slow strain rate test (SSRT) technique.and the results were discussed through the empirical electronic theory (EET) in the valence electronic structures of the 35CrMo steel. The results follow as:(1) The corrosion rate increases with CO2 partial pressure and the temperature rised then decreased after the temperature is over 90℃, The corrosion rate is highestat 90℃and the CO2 partial pressure is 2MPa.All corrosion rates are higher than NACE standard, belong to extremely serious corrosion;(2) Using HTHP autoclave to study specialty of corrosion product film of 35CrMo steel with CO2 saturated and effect of which on corrosion by weight loss,SEM and EDS at 90-120℃,CO2 partial pressure(2MPa)and its time is 72h.The results showed that Corrosion products mainly were FeCO3 Fe3C and other oxide of Fe at 90℃,but were (Ca,Mg)CO3 besides Fe3C and FeCO3 at 120℃.So the corrosion rate at 90℃is higher than it at 120℃;Using autoclave to study specialty of corrosion rate of 35CrMo steel with CO2 saturated and effect of which on corrosion time,which showed that With extension of time, corrosion rate increases first and then decreases the highest corrosion rate is after 32h.Under high power SEM,some erosion pit gradually formed at first and then accelerated the sample corrosion,so corrosion of 35CrMo steel was initially by local corrosion;(3) Using electrochemical experiments combined with atomic force microscope(AFM) observation 35CrMo tracking in CO2 corrosion medium at 90℃,from the micro level,combining with phase change of X-ray diffraction analysis,showed that the potential of local corrosion mutations occur 503mV-518mV and the surface of 35CrMo appeared deep corrosion pit. So the initial intergranular corrosion and corrosion due to the C pushed to the diffusion at speed different,Primary battery formed between the poor Cr parts of grain boundaries and matrix of sample;(5) By the SSRT,CO2 corrosion has an impact on the mechanical properties of steel 35CrMo.In the corrosion process, hydrogen diffusion inside the material promotes the local plastic deformation and reduces the materials toughness, which make the fracture stress reduced. So rupture happens finally. When the specimens were corroded,this action of promoting fracture increases with the increase of CO2 partial pressure which stimulates the tendency to brittle fracture on steel 35CrMo;(6) By calculating the valence electron structure parameters and their change of steel 35CrMo based on Empirical Electron Theory of Solids and Molecules(EET), it can be concluded that the hydrogen bonds are formed after the hydrogen atoms into the lattice, which leads to anisotropy of crystal lattice and a change in the various properties of materials. In the alloy structures, the interaction of alloying elements is stronger than it between hydrogen and alloying elements.So alloying elements play a leading role, making the change by the entrance of hydrogen decrease.leading to the decrease of hydrogen embrittlement of steel 35CrMo and preventing the aggravation of the corrosion effectively.