Effect Of Marine Service Environment On Electrochemical Corrosion Behavior And Hydrogen Embrittlement Under Cathodic Protection Of X80 Steel

Pipeline has always been used as a main mode for oil and gas transportation,and its failure in service is mainly caused by the corrosion of pipeline steel.Lots of researches focused on the corrosion behavior in various soil environments outside the pipeline.However,with the consumption of land resources,more and more efforts are devoted to exploit marine resources,and the environment outside the pipeline changes.Because of the complex and corrosive marine environment,the failure of submarine pipelines is mostly caused by outside environment corrosion,and which is difficult to repair and maintain.Therefore,it is imperative to study the corrosion mechanism and effect of different changing factors in marine environment for safe service of pipeline in marine engineering.In this paper,focused on the main characteristics of the marine environment,which include temperature,dissolved oxygen,static and dynamic load,the effects on the electrochemical corrosion behavior and hydrogen embrittlement under cathodic protection of X80 pipeline steel in p H=7.7±0.2,3.5wt.%NaCl simulated seawater were studied,and the influence law and mechanism of various marine environmental factors were discussed for providing basic data and theoretical guidance to the application and protection of X80 pipeline steel in marine engineering.The main results and research progress are as follow:(1)Temperature,dissolved oxygen and pre-strain included,the influence laws and mechanisms of steady-state variables on the electrochemical behavior of X80 steel in3.5wt.%NaCl solution were investigated.The local anode reaction rate increased with the rise of temperature,but the local cathode reaction rate was almost unchanged.The promoting effect of temperature on corrosion was realized by accelerating anodic dissolution.The cathode process was closely related to the concentration of dissolved oxygen and the Evans diagram was obtained.With the decrease of the concentration of dissolved oxygen,the corrosion rate decreased significantly and the reaction changed from the oxygenation reaction controlled by mass-transfer to the hydrogen evolution reaction controlled by activation at 0.5mg·L^-1dissolved oxygen.The effect of pre-strain on the corrosion of X80 steel was not obvious.This was mainly attributed to the hindering effect of compact?-Fe OOH corrosion scale formed by anodic dissolution.The corrosion rate decreased with the increase of pre-strain.(2)In 3.5wt.%NaCl solution,the dissolved oxygen in the marine service environment could change the hydrogen embrittlement sensitivity of X80 steel under cathodic protection.In the environment with dissolved oxygen,X80 steel under OCP condition had a certain stress corrosion sensitivity,and the stress corrosion sensitivity decreased firstly and then increased with the negative shift of the cathodic protection potential.When the protection potential reached the hydrogen evolution potential,the hydrogen-induced plastic loss increased and hydrogen embrittlement appeared under-1200m V.In the environment without dissolved oxygen,the hydrogen-induced plastic loss increased with the negative shift of the cathodic protection potential,and the critical cathodic protection potential,which showed the hydrogen embrittlement sensitivity at-1100m V,increased.The cathode protection potential changed the subsurface hydrogen concentration in steel,and the apparent hydrogen concentration of c_appwas positively correlated with the hydrogen embrittlement sensitivity of the steel.(3)The influence law of cyclic stress ratio(R)and loading frequency(f)on the mechanical-electrochemical effect on matrix/solution interface of X80 steel in 3.5wt.%NaCl solution was investigated and the physical mode of corrosion progress under cyclic stress was proposed.The electrochemical corrosion mechanism of X80 steel in3.5wt.%NaCl solution was unchanged by the application of cyclic stress.In 1h early stage corrosion,the values of corrosion rate promoting factor(CRPF)under cyclic stress were positive,which showed that the corrosion process was promoted by applying cyclic stress.With the decrease of the cyclic stress ratio(R),the value of CRPF increased and the relationship between CRPF and R was CRPF=3.084×10^-4×(1-R).With the increase of the loading frequency(f),the corrosion promoting effect increased firstly and then kept steady state when f>1.0Hz,which attributed to the mass transfer controlled process.In 10h later stage corrosion,because of the formation of surface corrosion products,which inhibited the corrosion process,CRPF changed oppositely and the corrosion rate under cyclic stress condition smaller than that without stress.With the decrease of R,the inhibition of corrosion scale became more obvious.The loading frequency had little effect on the corrosion rate.Moreover,the application of cyclic stress promoted the occurrence of local corrosion.(4)An electrochemical hydrogen permeation equipment that can be used in conjunction with the fatigue testing machine was designed,and the effects of constant stress,cyclic stress ratio(R)and loading frequency(f)on the hydrogen permeation process of X80 steel under cathodic protection were studied.The absorption and diffusion process of hydrogen had changed by applying stress.With constant stress,it was found that hydrogen-absorption resistance factor HPRF(absorb)and hydrogen-diffusion resistance factor HPRF(diffuse)were both negative,which indicated that the constant stress promoted the permeation of adsorbed hydrogen atoms and accelerated the diffusion of hydrogen atoms in the matrix.The constant stress increased;the promoting effect increased.The value of HPRF(absorb)was positive and the absorption process was inhibited when under low R value cyclic stress;the value of HPRF(absorb)was negative and the absorption process was promoted when under high R value cyclic stress.The values of HPRF(diffuse)were negative,and increased with the R value increased,which indicated that the promoting effect increased.There was a transitive frequency(f_t),when the loading frequency f<f_t,HPRF(absorb)increased and HPRF(diffuse)decreased with the loading frequency raised;when the loading frequency f>f_t,HPRF(absorb)changed to negative which indicated the hydrogen absorption process was promoted,HPRF(diffuse)converted to positive which showed the hydrogen diffusion process was inhibited,and more hydrogen atoms were trapped in steel.When maximum stress?_max=200MPa and cyclic stress ratio R=0.4 were unchanged,the transitive frequency f_t=1.0Hz.