The Study Of The Corrosion Characteristics Of X80 Pipeline Steel Under Cathodic Protection

Distribution,shape,size and composition and other parameters of the various non-metallic inclusions were statistically analyzed by means of optical microscope,scanning electron microscopy(SEM)and energy dispersive spectrometer(EDS)in the X80 pipeline steel.It was found that: non-metallic inclusions contained class A,class B,class C,D and DS five types in X80 pipeline steel according to GB/T10561-2005;non-metallic inclusions of X80 pipeline steel were mainly divided into oxides and sulfides,these inclusions were existed with complex forms.Polarization characteristics of the X80 pipeline steel were respectively examined by telectrochemical measurements technique in 3.5% Na Cl and NS4 solution.The results showed that all specimens for X80 pipeline steel exhibited active anodic dissolution behaviour,such that the anodic current density increased continuously with increasing potential.This behaviour typically indicated that a passive film is not formed on the specimen surface;Compared to fast scan rates,X80 pipeline steel at slow scan rate had a lower corrosion current density,higher polarization resistance and lower corrosion rate in the same solution.Pitting corrosion was observed to occur on X80 pipeline steels under cathodic protection.In this paper,the digital microscope,scanning electron microscopy(SEM)and cathodic polarization technique were combined to investigate the initiation and features of pitting corrosion on cathodically polarized X80 pipeline steel in 3.5% Na Cl solution and in NS4 solution at different potentials.The results showed that pitting corrosion was more likely to nucleate in non-metallic inclusion edge and expanded along the matrix under the condition of cathodic protection in 3.5% Na Cl solution.When the cathodic protection potentials were-850 m V(vs.CSE)and-900 m V,it was found that pitting corrosion occurred in the matrix and surrounding non-metallic inclusions;when the protection potential was-1000 m V,X80 pipeline steel matrix was effectively protected but corrosion pits still could generate around the non-metallic inclusions;When the protection potential reached-1200 m V,corrosion no longer appeared on the X80 pipeline steel.Non-metallic inclusions had a more negative electrochemical potential compared to X80 pipeline steel matrix,which was dissolved as the galvanic anode under the condition of cathodic protection.Pitting corrosion could occur on a cathodically protectived X80 pipeline steel when the steel experienced a current density fluctuation,and an instant,reversed potential field was generated locally at surface defects to result in the electrochemical state conversion in NS4 solution.