Study On Carbon Steel Sulfide Stress Corrosion Cracking Suspectibility In Refinery

Stress corrosion crack(SCC) is one of the most dangerous damage in petroleum industry. As the S content of the crude oil increases rapidly,the corrosion problems of the equipment and pipeline in petrochemical factory become more serious in our country.In recent years,we have experienced over 30 corrosion accidents from a certain refinery,based on which we carry out the experiment of the SCC behaviors of 20# carbon steel pipeline in acidic water by means of stressing C-rings.First,we investigated the influence of microstructure on the sulfide stress corrosion crack(SSCC) behaviors of weld and body parts.The effects of the environmental factors,such as temperature,pH and Cl~- concentration,on the SSCC behaviors of weld samples were investigated secondly.At last,we discussed SSCC behaviors of weld samples under the protection by the heat treatment process,Ni-P plating coating and cathodic protection method.Whatever results were obtained in laboratory or factory,it Showed that the metal weld was the most sensitive to SSCC,which was attributed to its ribbon pearlite structure and high hardness.When temperature is 20℃-50℃,especially between 20℃and 40℃,the pipes crack easily in a short time.Meanwhile,the samples are easier to crack at pH of 3-4,but insensitive to SSCC when the pH is above 5.However,the Cl~- ion has little effect on the SSCC of 20# carbon steel pipes when its concentration is less than 2.5%,but its effect on corrosion acceleration is obvious when the concentration is more than 2.5%.We obtained the homogeneous ferrite structure through annealing treatment and uniform ferrite and pearlite structure through normalizing,whose hardness decrease after the heat treatment.The heat treatment method weakens the sensitivity of pipes SSCC.Although no cracks,the surface of sample with Ni-P coating is damaged slightly after the immersion.To protect the pipes form SSCC,it is important to improve the quality of the Ni-P coating.The cathodic protection prefers to protect the pipes from SSCC and uniform corrosion.