Experimental Research On Stress Corrosion Cracking Of X70 Pipeline Steel In Nitrate Solution

API X-70 pipeline steel is a special grade designated for the West-East Natural Gas Transportation Project in China,because of very long distance of pipelines and complicated and variable environment along the pipelines across numerous regions,the safety of them becomes very important.The soil where oil transportation pipelines pass through is partially enriched with nitrate due to application of chemical fertilizer and industrial pollution.In nitrate environment, X70 pipeline steel may have potential hazard of SCC when it is put into service,but there are no reports yet about the researches on its SCC behavior in nitrate environment.Therefore,it is of important practical significance to research stress corrosion of X70 steel in nitrate environment.As a sub-project of the National Science & Technology Support Program in the Eleventh Five-year Plan Period "Research and engineering application of safety assessment of pressure-bearing equipment in corrosive and high-temperature environment" (2006BAK02B02-02),SSRT and WOL test methods are adopted to conduct experimental research on stress corrosion sensitivity and crack propagation rate of X70 submerged arc welded pipeline steel with spiral welds made by Bao Steel under complex medium condition of NO₃^--H₂S-Cl^-in this research.The SSRT results show below:(1) Both the stress corrosion sensitivity index I_SCC(δ) and I_SCC(S) of X70 steel in a single saturated sodium nitrate medium(800g/L) are very small without remarkable stress corrosion tendency.(2) Both I_SCC(δ) and I_SCC(S) in NO₃^--H₂S-Cl^- complex medium containing sodium nitrate are small,while the case is contrary when the medium does not contain sodium nitrate.(3) The elongation(δ) of X70 steel specimen is relatively low in the corrosive medium environment of 1000ppm H₂S+5%Cl^-,I_SCC(δ),I_SCC(ψ) and I_SCC(S) are all very big,indicating that stress corrosion tendency is remarkable.(4) In the same corrosion system,as H₂S content increases,stress corrosion sensitivity of X70 steel increases.The WOL test results show below:(1) The crack propagation rate of X70 WOL specimen in the near-neutral saturated sodium nitrate environment(800g/L) is very low,the da/dt of base mental is in the range of 1.431×10^-9 mm/s～2.861×10^-9mm/s averagely,and that of weld zone is 3.471×10^-9～3.987×10^-9mm/s,at the same K_I,da/dt of weld zone is higher than that of base metal.(2) In the saturated sodium nitrate environment containing NaNO₃(800g/L) and H₂S(50, 500,1000ppm) and NaCl(1%,3%,5%),the average crack propagation rate da/dt of base metal is relatively low,which is 4.973×10^-11～2.531×10^-8mm/s.In the experimental environment,the order of magnitudes of crack propagation rate da/dt of X70 steel at approximately equal K_I: heat-affected -zone＞weld zone＞base mental.(3) In the complex medium environment not containing sodium nitrate,the crack propagation rates of base metal,heat affected zone and weld are slightly higher than those in the environment containing saturated sodium nitrate.