Modelling and evaluation of corrosion rate in X 65 steel pipes used in natural gas lines by examining factors originating from the ground and affecting the structure from outside

Steel pipelines that used for transportation of natural gas and petroleum usually pass from different places having different soil characteristics. Due to different soil structures and other corrosive factors, pipelines are tend to corrode at different rates. In this study, the effects of moisture content, salinity content, pH, AC potential and cathodic protection levels on the corrosion rate were investigated as the factors affect corrosion in steel structures originating from the ground and external sources. The rate of the corrosion caused by these factors on the X 65 steel structure was mesaured by corrosion coupons in experimental set up with application of the factors at different levels The number of experiments to be carried out was reached as a result of experimental design with the Taguchi statistical method. L16 orthogonal array was used for 4 different levels of 5 corrosion rate affecting factors and 16 different experimental studies were conducted to obtain corrosion rate values for each experiment. Based on the statistical analysis results, effect of the changes in moisture content of the soil on corrosion rate was higher compare to other factors. Considering the underground pipelines usually pass inside the neutral environments (pH levels between 6-8) effect of changes in pH value on corrosion rate is the less than the other factors. In addition, with the regression analysis performed in the Minitab program, linear and non- linear regression equations were created to calculate the corrosion rates. Corrosion rate results obtained with the regression equations were compared with the test results performed in different test environments. By calculating the corrosion rate depending on different factors; selection or changing of pipeline route, selection of material wall thickness and planning of pipeline control and maintenance periods can be made in the most ideal way. Also comparison of corrosion rates with literature findings shows that corrosion rate levels change between outstanding and good classification ranges according to corrosion resistance classification of steel materials. KEY WORDS: Corrosion rate, steel pipeline, soil characteristic, statistical analysis, experimental