Stress Corrosion Behavior Of X70 Pipeline Steel In Simulated Seawater Environment

With the rapid development of China’s economy,the development of offshore oil and gas resources is imminent due to the sharp increase in energy demand and the gradual depletion of onshore oil and gas resources.The complex marine environment poses a significant threat to the safe operation of pipeline steel,mainly including p H value,Cl^-concentration,temperature,microorganisms,and salt content.This paper systematically studies the stress corrosion behavior of X70 pipeline steel in simulated seawater solution through slow strain rate tensile testing（SSRT）at different strain rates,p H values,Cl^-concentrations and applied potentials,combined with microscopic observation methods such as scanning electron microscopy and Zeiss microscope.The main conclusions are as follows:1)Comparative experiment results of slow strain tensile properties of X70 pipeline steel at different strain rates in simulated seawater solution and air show that X70 pipeline steel has the highest stress corrosion sensitivity when the strain rate is 2×10^-6/s,which has the largest plastic loss.Its reduction of area loss is 32.76%,which is much greater than those（25%）when the strain rates are 1×10^-6/s and 4×10^-6/s.When the strain rate is slow,electrochemistry dominates due to the long immersion time of the sample in the solution.When the strain rate is fast,mechanical action dominates.When the strain rate is 2×10^-6/s,the electrochemical and mechanical synergistic effects reach a balance.Thus,the stress corrosion trend of the pipeline steel is the highest at this strain rate,and its stress corrosion sensitivity is the highest.2)The slow strain tensile tests results in simulated seawater solutions with different p H values show that the decreases of tensile strength and yield strength are the most significant only in simulated seawater solutions with p H=3 while the effect are not significant in simulated seawater solutions with p H=7.7 and p H=8.1.This is due to the increase in hydrogen ions in the simulated seawater solution at p H=3,resulting in enhanced anodic dissolution.The introduction of hydrogen ions also increases the trend of hydrogen induced cracking in pipeline steel,reducing the strength of X70 pipeline steel.From the perspective of the plastic loss index-area shrinkage loss rate,X70 pipeline steel is the most sensitive to stress corrosion in simulated seawater solutions with p H=8.1,followed by p H=3,and 7.7 as the least sensitive.When p H=8.1,the simulated seawater solution belongs to high p H,and the anodic dissolution of pipeline steel is strengthened,especially in areas with large defects such as crack tips,where the corrosion rate is faster and the stress sensitivity is strongest.3)The order of stress corrosion sensitivity of X70 pipeline steel in simulated seawater solution under different chloride ion concentrations is:3.5%>5%>2%.The stress corrosion sensitivity is the greatest when the chloride ion concentration is 3.5%,which is due to the small radius and strong adsorption of Cl^-in simulated seawater.Cl^-gradually adsorbs on the surface of the corrosion layer,resulting in the breakage of the product film so that corrosion was accelerated and its stress sensitivity is strongest.When the concentration of Cl^-is low（2%）,it will not produce sufficient concentration of adsorption.When the concentration of Cl^-is high（5%）,the corrosion will further increase and the product film on the surface of X70 pipeline steel becomes thicker,which hinders the contact of Cl^-and oxygen with the substrate and further reduces the corrosion rate as the results of the reduced stress corrosion sensitivity.4)The stress corrosion sensitivity of X70 pipeline steel under applied potential is ranked in order of magnitude:-950 m V>-1100 m V>-800 m V.When the applied potential is-950 m V,the cathodic reaction generates a small amount of hydrogen that diffuses into the metal surface.As the reaction proceeds,a certain amount of hydrogen atom diffuses from the substrate surface into the metal crystal,causing adverse effects on the microstructure and properties of the material.A crack source forms and expands.Superposing the anodic reaction,the X70 steel sample will fracture with no obvious necking,accompanied by certain quasi-cleavage brittle fracture characteristics.When the applied cathode potential is-800 m V,the cathode current effectively inhibits the anodic dissolution reaction and the stress corrosion sensitivity is minimal.Therefore the applied potential（-800 m V）achieves the best inhibition effect on the stress corrosion of the X70pipeline steel.