Predicting the External Corrosion Rate of X60 Pipeline Steel: A Mathematical Model

Abstract

The need for predicting pipeline service life and improving risk assessment relating to corrosion hazards requires establishing a correlation between the corrosion rate (CR) of pipeline steel and its coating condition, cathodic protection (CP) levels and surrounding soil conditions. This paper presents a systematic study of the CR of bare and coated—with and without a dent or holiday defect—X60 pipeline steel in simulated field environments. Three CP scenarios, i.e., no, optimized, and over-protection, were studied to cover a wide range of possible CP conditions that pipeline steel may encounter in the field. Two types of salt solutions (sodium chloride or sodium sulfate) with a variation of temperatures (10 °C, 40 °C, 65 °C) and pH values (2, 7, 12) were investigated to simulate different levels of soil corrosivity. A mathematical model was developed to reveal the impact of various parameters and their interactions on the CR of X60 steel. The coating condition was the most important factor. The individual effects of other factors including temperature, pH, salt composition and CP were not shown to be significant. Instead, the interactions between temperature and salt composition, and particularly the interaction between pH and CP appeared more important in determining the overall CR.