Numerical analysis of the kinetics of propagation of hydrogen blisters in the oil-and-gas equipment
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NUMERICAL ANALYSIS OF THE KINETICS OF PROPAGATION OF HYDROGEN BLISTERS IN THE OIL-AND-GAS EQUIPMENT Yu. V. Banakhevych,1 O. V. Hembara,2, 3 and O. E. Andreikiv2
UDC 539.3: 537.22: 669.788
We consider a problem of cracking of metals (formation of blisters) in the oil-and-gas equipment. The causes of initiation of the nuclei of hydrogen blisters are investigated and, on this basis, a mathematical model of their growth is developed with regard for the electrolytic hydrogenation of the metal surface, hydrogen diffusion into the metal, and its accumulation in the internal microdefects. We also present an example of analysis of the kinetics of growth of circular blisters in low-carbon and low-alloy steels under the action of hydrogen penetrating into the metal in the process of corrosion in wet hydrogen-sulfide media. Keywords: blisters, hydrogen, hydrogen-sulfide media, pressure of hydrogen, kinetics of growth of the blisters.
Causes of Formation of the Nuclei (Microcracks) of Hydrogen Blisters At present, the problem of guaranteeing reliable operation of the oil-and-gas equipment and pipelines becomes more and more urgent. This is connected with the intensification of the processes of oil and gas extraction and their processing and some changes in the base of raw materials. Moreover, the acceleration of technological processes leads to an increase in the working pressure and more pronounced temperature oscillations. High levels of corrosion activity of the technological media and changes in the chemical compositions of processed products deteriorate the operating conditions of the oil-and-gas equipment and pipelines and lead to a significant decrease in the level of their safety. This problem becomes even more critical in the presence of corrosive components (in particular, hydrogen sulfide) in the extracted substances. As the principal feature of hydrogen-sulfide media, one can mention their ability to induce hydrogen-sulfide stress-corrosion cracking, blistering, and hydrogen-induced cracking (Fig. 1) of steels susceptible to these types of fracture. Hydrogen sulfide substantially intensifies the process of penetration of hydrogen in steel [1–6]. Mechanisms of Influence of Hydrogen Sulfide on the Hydrogenation of Steel. Hydrogen sulfide is a fairly aggressive substance responsible for the corrosion damage to the equipment as a result of electrochemical corrosion and hydrogen embrittlement [1]. Being dissolved in water, it dissociates according to the following reactions: –
+
H2 S ↔ HS + H ↔ S
2–
–
+ 2H .
The equilibrium of these reactions depends on the pH value of the medium: it contains HS– ions for pH > 7 or molecular H2 S for pH < 7. At the same time, S2 – ions appear for pH >> 7. 1 Management of Gas Mains and “Ukrtranshaz” HRSDK, Kyiv, Ukraine. 2 Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. 3 Corresponding author; e-mail: [email protected].
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 45, No. 5, pp. 17–25, September–October, 2009. Origina
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