Evolution Of Local Corrosion Of X80 Riser Induced By Dynamic Phase Boundary

The riser system of the offshore oil and gas development drilling platform faces corrosion in multiple zones of the marine environment such as the marine atmosphere,the splash zone,the tidal range,and the seawater full immersion zone.Among them,the corrosion rate is the fastest in the splash zone,and the local corrosion is the most prominent in the tidal range.However,the mechanism of the electrode process of micro-corrosion at the interface has not been clarified.This article focuses on the impact of dynamically changing corrosion environment on the metal electrode process.The tidal range appears as a dynamic waterline phase boundary;the spray splash area appears as a dynamic dispersed liquid film.This paper mainly uses local electrochemical testing methods such as concentric three-electrode array and close-packed array electrodes to study the corrosion electrochemical characteristics of X80 steel in static waterline,dynamic waterline,static dispersion liquid film and dynamic dispersion liquid film system,respectively.In order to find out the evolution of local corrosion induced by the riser in the dynamic phase boundary of the tidal range,and the mechanism of the dynamic dispersion liquid film in the splash zone of the spray to accelerate the corrosion.Aiming at the static waterline corrosion system,the electrochemical characteristics of local corrosion of X80 steel were studied by using the concentric three-electrode array test method.In the thickness range of 20μm-500μm,9 gradients were set to test the corrosion potential and coupling current distribution of the lower electrode surface covered by uniform seawater liquid films of different thicknesses.The results show that the liquid film thickness below 100μm has a higher driving force for local corrosion and has a greater tendency to local corrosion;corrosion rate is relatively stable at 100μm-300μm;when the thickness is above300μm,the corrosion further develops.Tilt the electrode surface by 2°,add the solution to the third row of electrodes,simulate the three-phase line interface area of the waterline,test the electrochemical impedance spectra of the electrodes under the waterline in the uncoupled and coupled states,and find that the electrodes are all coupled after coupling.The anodic polarization has occurred and the corrosion has increased.Aiming at the dynamic waterline corrosion system in the simulated tidal range,the corrosion of the electrode surface was tested with a three-electrode array of concentric circles when the moving cycle time was T=1h,T=3h,T=5h,T=8h,and T=12h.Potential,coupling current distribution,and electrochemical impedance spectroscopy of local electrode units.The results show that the dynamic waterline movement period will affect the electrochemical stability of the system.As the period becomes longer,the waterline movement speed gradually decreases,and a stable electrochemical corrosion field begins to form on the electrode surface,and the corrosion kinetics is relatively stable.And when the dynamic waterline movement period T=3h,the local corrosion ability of the system is the strongest,and the local corrosion rate is the highest.With a three-electrode test system of concentric circles,the corrosion potential of the electrode surface,the distribution of coupling current and the electrochemical impedance spectra of the local electrode unit under different cycle times of the dynamic waterline movement period T=3h were tested.The results show that in the dynamic waterline seawater corrosion system,local corrosion can respond quickly,and the number of cycles will also affect the corrosion stability of the electrode surface.When n=4,the electrode surface begins to form stable anode and cathode differentiation.With the further increase of the number of cycles,a stable electrochemical corrosion field is formed on the electrode surface,and the corrosion kinetics is relatively stable;The cathode and anode are further differentiated,the anode position is relatively fixed and tends to be concentrated,the anode current tends to be stable,and a stable local corrosion development condition is formed.The self-made close-packed electrodes were used to test the coupling current distribution on the electrode surface under different liquid film dispersion degree and different coupling time,and extract the relevant electrochemical parameters.It is found that with the increase of the dispersion degree of the liquid film,the increasing of the three-phase line interface area at the edge of the droplet accelerates the local corrosion process by increasing the area of the cathode.However,this acceleration is not infinite on the surface of a limited electrode,and a stable electrochemical corrosion field will gradually form on the surface of the electrode.The coupling effect polarizes the electrodes to each other,reduces the potential difference between the anode and the cathode,and reduces the corrosion driving force.After a period of polarization,the potential difference tends to be stable,and the corrosion kinetics of the system is relatively stable.The self-made close-packed electrodes were used to test the evolution law of local corrosion of X80 steel induced by the dynamic dispersion liquid film in the splash zone.The initial state of the test is the distribution characteristics of the interface coupling current under the action of the dynamic dispersion liquid film under the state of uniformly dispersed droplets and uniformly dispersed seawater crystals.The results show that the initial distribution state of the electrode surface is different,and the dynamic dispersion liquid film process significantly accelerates the corrosion kinetics compared with the static process.This is mainly due to the imbalance in the response of the fast and slow steps of the interfacial corrosion process to the changes of the external corrosion environment,which leads to the continuous high-speed transient electrochemical process of the electrode process.