Study On Galvanic Corrosion Of Stainless Steel / Carbon Steel Gathering Pipeline Connection In Gas Field Of CO 2

Dabei area gas field contained high salinity and CO2, the design of pipeline construction adopts316L/L360QCS,304/L245NCS pipeline steel, Galvanic corrosion would happen owing to the difference of physicochemical property between stainless steel SS(316L、304) and carbon steel CS(L360QCS、L245NCS) while once SS were connected to CS, serious threats will come to pipelines. In this study, the corrosion behavior of316L/L360QCS、304/L245NCS couples were stuied by Electochemical method、Weightloss method、Scanning electron microscope(SEM)、Energy spectrum analysis technology(EDS) and X-ray diffraction(XRD), the effects of potential differences between CS and SS、temperature、corrosion time、ratio of cathode and anode aeras、flowage of aggressive medium on couples and galvanic effects were discussed and galvanic corrosion regular pattern was gained. Conclusions of study are as follows:L360QCS、L245NCS acted as anodes in316L/L360QCS、304/L245NCS couples and had a rapider corrosion rates than themselves alone. Potential differences between316L and L360QCS were higher than304and L245NCS, both potential difference of. carbon steel between stainless steel and driving force of galvanic corrosion decresed The corrosion rates of L360QCS and L245NCS increased with the temperature rising, temperature changes influenced the formation rate of corrosion products and topography dramatically. Corrosion product film on carbon surface built more quickly and density of corrosion product film increased as temperature went up. EDS and XRD analysed that the main component of corrosion product of carbon steel were FeCO3. There were linear relation between corrosion rate with time. When the area of carbon steel were the same to stainless steel, bigger galvanic current density and more negative galvanic potential were gained at higher temperature and Evans diagrams could calculate galvanic current density and galvanic potential correctly. Galvanic effects would get down slightly as corrosion time extended at different temperature, Among the temperatures, the biggest reduction of galvanic effects were at80℃,60℃came second and last was30℃. Difference of potential between316L-L360QCS were200mv bigger than304-L245NCS couple at any temperature, then the galvanic effect of latter couple was smaller than the former. Galvanic current density and galvanic effects increased with the ratio of cathode and anode aeras amplifing, in addition, galvanic potential shifted positively and corrosion rate of anode steel increased, Logarithmic of galvanic effects approximated linear relationship between ratio of cathode and anode aeras and when cathode areas were not the same to anode, Evans diagrams could not be used to calculate galvanic current density and galvanic potential. Dynamic corrosion experiments indicated that fluid corrosive medium could raise the corrosion rates of CS and galvanic couples, but it had a little influnce on galvanic effects. Galvanic effects of304/L245NCS were smaller than316L/L360QCS and resiatance of local corrosion by CO2on L245NCS was stronger than L360QCS.316L/L360QCS couple could be replaced by304/L245NCS couple for decreasing galvanic effects in actual working condition owing the aspects of material corrosion performance.Corrosion rates of carbon steel exceeded0.076mm/a ruled by SY/T Standards, and it is necessary to take efficacious design of corrosion control on pipelines construction, corrosion inhibitor added or cathodic protection measures usually are effective and feasible to welded dissimilar metal pipes; when pipelines were connected by flanges, electrical insulating measures could prevent each metal on connecting directly to avoid galvanic corrosion.