The Study On The Effect Of Stress On Corrosion Behavior Of Pipeline Steel Under Simulated Sea Water Environment

Nowadays,the corrosion is very harmful to the marine oil pipeline,which has caused great economic loss and brought many technical problems to the corrosion researchers.On-site inspection of service pipelines revealed that most of the oil pipelines were external corrosion,and the remaining life of the pipeline reduced with the pressure increased.The detection of the other four setspipeline showed that pipe corrosion at the bend is serious than the straight.These phenomena are estimated to be stress-related.And the influence of stress on corrosion is not enough,so it is necessary to study the effect of stress on corrosion.In this paper,electrochemical methodsincluding electrochemical noise,electrochemical impedance spectroscopy and potentiodynamic polarization curves were used.In the simulated seawater solution the electrochemical corrosion behavior of X65 pipeline steel under different tensile stress conditions,X65 pipeline steel with different initial pH values under constant stress,X65 steel of ultrasonic shot peening and without ultrasonic shot peening were studied.The main conclusions are as follows:The results of electrochemical noise PSD and EDP tests under different stress conditions in this experiment indicate that the X65 pipeline is uniformly eroded in the initial 3.5%NaCl solution and the EDP is mainly distributed in the lower stage,and the corrosion morphology observation also proves this viewpoint.The electrochemical impedance spectroscopy（EIS）shows that the impedance characteristics are the same under different stress.With the increase of the stress,the curvature radius t and the charge transfer resistance Rt decrease with the increase of the stress,and the polarization in the curve The current density I_corr also increases with the increase of stress,which indicates that the corrosion rate of X65 pipeline steel increased with the increase of applied stress,and the corrosion rate is increased and the corrosion weight loss experiment also confirms this point.The electrochemical impedance spectroscopy（EIS）of 3.5%NaCl solution under constant stress shows that the charge transfer resistance of the impedance spectrum decreases with the decrease of pH value,and the corrosion current density of the polarization curve gradually increases with the decrease of pH value,which indicates that the corrosion rate of X65 pipeline steel increases.The hardness of X65 pipeline steel samples after shot peening and unshotted treatment shows that the hardness of X65 pipeline steel was improved obviously after shot peening,and the anti-fatigue ability of X65 pipeline steel was improved.The electrochemical test results of shot blasted and unshotted X65 pipeline steels,whether0YS（yield strength）or 100%YS,show that the arc radius and charge transfer resistance Rt of the X65 pipeline steel after shot peening become large.The I_corr value of the polarization curve is smaller,which shows that the anti-corrosion ability of the sample after shot peening is enhanced.Ultrasonic shot peening to produce a layer of metal stress layer,so that the metal surface residual stress from tensile stress into stress,which can slow down the rate of corrosion.