Influence of Soil Corrosion and Transported Products on the Service Life of Welded Joints of Oil and Gas Pipelines
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INFLUENCE OF SOIL CORROSION AND TRANSPORTED PRODUCTS ON THE SERVICE LIFE OF WELDED JOINTS OF OIL AND GAS PIPELINES О. E. Andreikiv1 and О. V. Hembara2
UDC 620.197.5: 669.788
We develop methods for the evaluation of the service life of welded joints of the pipes of oil and gas pipelines taking into account the action of the main operating factors on the corrosion rate. We quantitatively analyze the influence of hydrogen released from the transported products on the rate of soil corrosion and residual life of different zones of welded joints of underground pipes. Keywords: soil corros1ion, welded joints, hydrogenation of material, corrosion rate, static corrosion crack growth, residual life.
It is known that the corrosion rate of welded joints (WJ) is higher than for the base metal (BM) and, in many case, the service life of the entire structure depends on their strength. The complex processes running in the zones of WJ and their influence on corrosion were investigated in [1–5]. The electrochemical inhomogeneity of the surface in the “BM–heat-affected-zone (HAZ)–weld” zone was observed [3]. It was established [4, 5] that the weld metal (WM) of heat-resistant steels degrades more intensely than the metal of the other zones of WJ as a result of which its hardness, strength, and plasticity eventually become lower than the corresponding characteristics of the BM and the HAZ. Clearly, this heterogeneity is responsible for different corrosion rates in the individual regions of WJ. Hence, the evaluation of the service lives of metal structures and their WJ should be based on the data of testing samples and the results of simulation of the kinetics of corrosion processes with regard for the hydrogenation of various zones of the WJ. On the basis of the well-known theoretical concepts and some experimentally justified hypotheses, a physicochemical model was constructed [6–8] and mathematical relations were proposed for the description of the corrosion-mechanical fracture of materials of pipes of underground oil and gas pipelines with regard for the intensification of this process by diffusion-mobile hydrogen. It is shown that hydrogen absorbed by the metal increases the corrosion rate more than twice and, therefore, makes its residual life almost twice shorter. In the present work, we apply this approach to the quantitative evaluation of the influence of hydrogenation on the rate of soil corrosion and the residual life of welded joints. Statement of the Problem Consider a part of the pipe of a gas pipeline containing a weld (Fig. 1а). The pipe lies in clay and sandy soils, i.e., under the conditions of nonuniform aeration favorable for the formation of galvanic corrosion zones: 1 2
Franko Lviv National University, Lviv, Ukraine.
Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine; e-mail: [email protected] (corresponding author).
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 49, No. 2, pp. 52–58, March–April, 2013. Original article submitted November 20, 2012. 1068-8
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