Corrosion and stress-corrosion cracking of exploited storage tank steel
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CORROSION AND STRESS-CORROSION CRACKING OF EXPLOITED STORAGE TANK STEEL A. Zagórski, H. Matysiak, O. Tsyrulnyk, O. Zvirko, H. Nykyforchyn, and K. Kurzydłowski
UDC 539.375: 620.178
We study the corrosion resistance of St3S steel under loading and its susceptibility to corrosion and hydrogen-induced cracking in bottom water. Sections of a tank are distinguished according to the character of the media interacting with the metal of the inner surface in the process of operation. It is shown that bottom water is characterized by high levels of corrosion activity and that the degrees of in-service degradation of different sections of the tank are different. The worst corrosion and stress-corrosion resistance are exhibited by steel operating in contact with bottom water. Significant levels of plastic strains intensify the process of corrosion in steel and make the rates of corrosion in different sections of the tank closer to each other. The in-service degradation of steel can not only intensify the process of corrosion of the inner surface of the tank but also promote the brittle fracture of the material characterized by the elevated susceptibility to hydrogen-induced cracking.
From the viewpoint of storage and transportation of oil, the key objects are trunk pipelines and storage tanks. Recently, we have observed a significant increase in demands concerning their safe operation period, which, in most cases, was extended to 30 years [1–5]. This long operation period adds to the appearance of substantial corrosion damages of the inner surfaces of pipelines and elements of tanks. In both cases, residual water is the corrosion agent. Moreover, for the long period of operation, we observe the appearance of oil-derivative sediments containing, among others, hydrogen sulfide. It adds to the local acidification of the environment and corrosion with hydrogen depolarization, which, consequently, leads to the embrittlement of the structural material. Long-lasting operation can, therefore, cause a significant decrease in the corrosion-resistance and resistance properties (especially in the resistance to brittle fracture) of the pipe and tank materials. This is why the works aimed at the calculation of the corrosion rate and the sensitivity of materials to stress-corrosion cracking under conditions similar to the operating conditions gain significance. The research results published in [2–5] show that the degradation of pipe and tank materials strongly depends on geometric characteristics of the object. The material taken from the upper and bottom (operating in contact with residual water) areas of a pipe should be considered separately. The same is in the case of tanks. Material degradation occurs with different intensities in different fragments of the tank. The areas of the tank operating in constant contact with residual water or water originating from steam condensation are the places especially susceptible to corrosion attack. The state of the material surface and the impact of mechanical loads (connected with periodic filli
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