Slow Strain Rate Test Studies Of Stress Corrosion Cracking Of Pipeline Steels

Stress corrosion cracking (SCC) of pipeline steels has several kinds of influenceing factors, including applied potential, CO2 content, temperature and mechanical condition etc.These factors should be studied widely,which decide the complexity of SCC problem. In addition,the study on cracking characteristic,mechanism and condition influence still need lots of work.The slow strain rate test method (SSRT) is used in our test. In near-neutral pH and high pH soil-simulation liquor, many tests simulating the SCC behavior of X70 pipeline steel are operated under different potential. Lots of analysis to the condition factors of X-70 SCC steel in near neutral pH liquor is done.And The results are as follows:1. In near-neutral pH solution, the cracking mode is transgranular with the feature of quasicleavage.The function of SCC factors are as follows:(1)The additional potential takes an important role in the SCC behavior of X70 pipeline steel: the susceptibility to SCC increased with decreasing cathodal potential,HIC is the dominant reason,the crackles on the surface are almost vertical to stress;On the country, at anodic potential,there is a critical potential,below which the dominant reason is anode-dissolving supported by hydrogen,and the crackles on the surface are not vertical to stress.(2)The temperature variety doesn't influent SCC sensitivity's variety with the variety trend of electric potential, but the temperature influent the SCC sensitivity:Same electric potential, there's little difference in SCC sensitivity between 30℃and 50℃.While below 30℃, SCC sensitivity increase.The thesis finds the critical potential below different temperature.(3) Strain Rate plays an important part in SCC,and a critical Strain Rate exist:When the local Strain Rate is higher than the upper limit, along with the increment, corrosion fatigue(CF) or even mechanism crack will happen;when the local Strain Rate is lower than the descend limit,SCC will be repressed. The variety of electric potential decides variety of Strain Rate bound.And the bound will enlarge with the negative increasing of electric potential.2.In high pH solution, the cracking mode is intergranular with the mechanism of anode dissolve. the influence of electric potential resembles it in near-neutral pH solution.3. The increment of CO2 leads the increase of H+.So the action of HIC is more evident,and the SCC sensitivity increases.