The Corrosion Behavior Of P110 Steel In S/CO₂ Environments

Due to the increasing energy demand in the world,the high CO2/H2S content petroleum and natural gas fields in harsh environment have been mined.However,corrosion damage is a serious threat to the personal and equipment safety in this mining pipeline.It is of great significance to study and understand the corrosion process in petroleum and natural gas field.In this thesis,P110 steel is used in the experiment,since it is widely adopted in the construction site.The corrosive medium used in the experiment is the component of the simulated petroleum extract,which was provided by the CNPC(China National Petroleum Corporation)Tubular Goods Reach Institute.The experiment studied the change of corrosion rate,composition of production film,the microstructure of the corrosion product film and electrochemical characteristics under various corrosion temperatures,corrosion times,and different concentrations of S,revealing the corrosion behavior of P110 steel in different corrosion environments.The results show that the corrosion of P110 steel in CO2 environment was uniform corrosion.With the increase of treatment time,FeCO3 and FeC3 appeared in the film,which can reduce the corrosion rate because of their protection effect.The corrosion of the P110 steel matrix in S-containing environment has two forms:1)the sulfur reacts directly with the metal at the point where the corrosion product film falls off.2)the hydrolyzate produced by the S-H2O solution can penetrate the corrosion film,and act with the substrate,causing the metal surface detached in lamella.In addition,local corrosion in the S-containing environment requires a longer incubation period.That is,the corrosion occurs when the film breakdown or fall off.Compared with the temperature,the effect of sulfur concentration on accelerating the corrosion process is larger.In the CO2-containing environment,the protective FeCO3 film is formed in the initial stage,and then the corrosion products are continuously deposited onto the surface of the P110 steel.Finally,due to the severe corrosion,the outer membrane shows a sponge-like structure.In the S-containing environment,there is an uniform corrosion product on the surface of the P110 steel.With poor adhesion,part of the corrosion film drops.Over time,pitting will form at these shedding positions.In the CO2/S coexisting environment,the main components of the corrosion product film are FeCO3 and FeC3,covering the surface of the substrate as large particles in the initial stage.As the etching time is prolonged,the corrosive film particles become smaller,the gaps between particles become narrower,the gelatinous Gel-like boundary are formed among the particles,and the compactness of the corrosion product film is improved.With the increase of corrosion time,the surface of the product film chapped.With the existence of CO2,the Nyquist pattern shows a single resistive arc.As the corrosion time increases,the corrosion potential gradually decreases.When the current density is high,the Warburg impedance is founded in the low frequency region.In the S-containing environment,with the increase of corrosion time,self-corrosion potential and the solution resistance Rs both show an increase followed by decrease pattern.Besides that,Warburg impedance forms during the whole corrosion process.When the corrosion product film continuously deposited onto the metal surface,the anode reaction is affected by the diffusion process.When the two factors coexisted,the corrosion rate will be decreased from 9.76×10-3 to 0.0253 at the beginning of the corrosion reaction.This result indicates the film is not dense with a large porosity.The ions easily pass through the corrosive film to react with the matrix.The EIS shows,at 72h of corrosion,the electrode process is controlled by activation,and pitting is happened on the surface of the P110 steel.