Research On Soil Corrosion Of20#Steel Buried Gas Pipelines In Chongqing

Soil corrosion is the main reason for the failure of buried metal system. With therapid development of the economy and urbanization in Chongqing, the vast buried gaspipe system builds the sprawling and complex metal network under the ground.20#steelplays a very important role in the field of buried gas pipeline. Owing to soil corrosion,however, the situation is not optimistic. Soil corrosion will bring damage to the buriedpipe, and even cause fire and explosion. It poses a great threat to the economy, energy,environment and personal security. Therefore, in order to ensure the safe and highlyefficient operation for buried gas pipeline, the corrosion and protection work shoud becarried out urgently.The main research contents and results of this paper were as follows:①Soil samples were collected from the typical area in Chongqing, and the soilcorrosivity of each sampling point was analyzed. Physical factors （Soil Bulk Density,Character, Density, Porosity, Moisture Content）, chemical factors (pH, Salinity, contentof SO₄^2- and Cl^-) and electrochemical factors （Resistance, Oxidation-ReductionPotential） for soil samples were monitored. Soil corrosivity was evaluated by singleindex method and multiple index method. The result showed that soil corrosivity inChongqing mainly focused on moderate degree and heavy degree.②In order to study the corrosion resistance of20#steel, corrosive current densityof20#steel in simulated soil was tested by polarization curve. The result indicated thatthe corrosion resistance of20#steel was at moderate level in Chongqing.③Test of buried20#steel pieces was designed by uniform experiment. Thecorrosion form of the specimen was observed, and the results suggested that the20#steel was mainly damaged through pitting corrosion. In addition, the importance of soilcorrosion factors to the corrosion of20#steel was analyzed. It was found that the orderof importance as follows: Moisture Content> Content of Cl^-> Content of SO₄^2->pH> Character. Compared with other factors, the weight of Moisture Conten was thebiggest and its value reached56.73%. ForCl^-and SO₄^2-, the weights were about20%.However，as for pH and Characrter, the corrosion impacts were negligible because ofthe values of weight were less than1%.④According to the buried test, a regression model, which was applied topredicting the corrosion rate of20#steel in soil of Chongqing, was builded through SPSS software. The prediction model was as follows:（y_i|^）=4.133-0.095x₁+0.006x₂+0.002x₃.⑤Four kinds of3PE coated sample with the defect diameter of0mm,0.8mm,2mm and5mm respectively were immersed in the simulated soil solution and measuredby electrochemical impedance spectroscopy. The results showed that with the increaseof defect size, the protection performance of the3PE coating was decreased. During thewhole immersion period, the protection performance of the intact3PE/20#steel coatingsystem was very high. For the coating system with the defect of0.8mm, the protectionperformance at the beginning of the immersion was as well as the intact one. However,the protection performance reduced to the same level as the coating system with thedefect of2mm at last. Besides, the coating system with the defect of0.8mm and2mmwere damaged moderately. It deserved to note that the protection performance of thecoating system with the defect of5mm was failed throughout the immersion period.