Corrosion Study of API 5L X60 Gas Pipelines Steels in NS(4) Simulated Soil

Abstract : External corrosion and cracking are the major threats and the principal mechanisms of buried pipelines deterioration. Indeed they reduce the structural integrity of the transmission system of gas pipelines. Investigations carried out on GZ1 site line, in Algeria, after about 30 years of exploitation under high pressure revealed that under certain conditions the not protected surfaces in contact with the ground developed many failures by corrosion and cracking: coating failure, reductions in thickness, pitting, etc. In this study the methodology was defined initially, by an expertise of the characteristic parameters of working GZ1 line which broke due to external corrosion, and then a simulation, in laboratory, of the conditions and the corrosion damage mechanisms. We were particularly interested in the corrosive electrolytic medium choice because it can simulate NS4 soil solution and the protection system by using corrosion inhibitors containing polyphosphates. Results showed that the corrosion potential is slightly moved towards the anodic values, when the pH of soil solution tends to a light acidity. This shift goes with a clear reduction of anodic and cathodic current densities and the polarization resistance Rp decreases. In the explored temperature range and simulating soil, corrosion current density increases with temperature increase. The corrosion activation energy increases with temperature according to the soil environment. Nyquist diagrams present the same general form for corrosion according to the slightly neutral pH for various immersion times in the steel/test solution interface.