Influence of the Corrosiveness of a Medium on the Ratio of the Cathodic Protection Current to the Ultimate Diffusion Cur
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INFLUENCE OF THE CORROSIVENESS OF A MEDIUM ON THE RATIO OF THE CATHODIC PROTECTION CURRENT TO THE ULTIMATE DIFFUSION CURRENT FOR KH70 PIPE STEEL L. І. Nyrkova,1,2 S. О. Osadchuk,1 А. V. Klymenko,1 А. О. Rybakov,1 S. L. Mel’nychuk,1 and S. М. Prokopchuk1
UDC 629.197.5:544.652
We study the dependence of the ratio of the cathodic protection current to the ultimate diffusion current for Kh70 steel within the range of protective polarization potentials (– 0.75)–(– 1.05) V (relative to the silver-chloride reference electrode) standardized by the DSTU 4219 in media with different degrees of corrosiveness. It is shown that, within the рН range 5.6–9.6, as the potential shifts to more negative values, the ratio jc. p. / jO 2 increases from 0.5 to 138. It was discovered that, for the identical protective polarization potentials in a section of the gas main, the corrosion rate in a defect of the protective coating of steel may not only decrease but also activate the process of hydrogen reduction. It is reasonable to determine a value of the ratio jc. p. / jO 2 above which the process of decomposition of soil electrolytes is accelerated and the hydrogenation of the pipe wall in a defect of the coating becomes possible for every medium and take into account this value both in the design of the cathodic protection and in the process of monitoring in the course of operation to prevent stress-corrosion cracking.
Keywords: Kh70 pipe steel, protective polarization potential, cathodic protection, residual corrosion rate, ratio of the cathodic protection current to the ultimate diffusion current.
All main pipelines, independently of the operating conditions, are subject to corrosion protection by coatings and to electrochemical protection (except overhead pipelines). Their protective state is periodically monitored by measuring a series of parameters specified in the normative documents [1, 2–6] among which we can mention the minimum and maximum protective potentials and the rate of residual corrosion of the pipe metal. The requirements to protective potentials established in different countries are different. Thus, in particular, in the normative documents of Ukraine and the countries of the former Soviet Union, it is customary to regulate the minimum and maximum values of protective potentials, including the values in the absence of the Ohmic component [1–3, 7]. Moreover, the EN 12954 [8] and NACE SP0169 [4] standards also indicate the necessity of taking into account the properties of soils. Noted that, in a given individual section of a main pipeline, the recommended values of protective potentials may turn out to be insufficient to guarantee satisfactory protection or excessive. The applied protection is regarded as efficient if the residual corrosion rate does not exceed 0.01 mm/yr [1, 9]. However, as follows from the world experience, even if the above-indicated parameters lie within the ranges normalized by the standards, the development of corrosion processes, including stress corrosion cracking (SCC), remains possible. In
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