Localized dissolution of carbon steel used for pipelines under constant cathodic polarization conditions. Initial stages of defect formation

• N.A. Gladkikh, M.A. Maleeva, L.B. Maksaeva, M.A. Petrunin, A.A. Rybkina, T.A. Yurasova, A.I. Marshakov and R.Kh. Zalavutdinov

A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky pr. 31, 119071 Moscow, Russian Federation

Abstract: This work is aimed at studying the possibility of initiation and growth of local (pit-like) defects under cathodic polarization, both in neutral and alkaline solutions, as well as the effect of hydrogen on the local dissolution of tube carbon steel and on the defect formation under permanent cathodic polarization conditions. A combination of optical and electrochemical methods of research allowed us to study the kinetics of the initial stages of localized cathodic dissolution of steel in detail. In fact, the initiation of pit-like defects under cathodic polarization of carbon steel in the presence of chloride ions in the electrolyte was observed experimentally. The incubation period of defect formation under various conditions was determined with high accuracy, and the factors affecting the incubation period of defect initiation were determined. It has been shown that the dependence of the density of appearing corrosion defects on the potential passes through a maximum, while the time until the appearance of the first defect decreases as the potential is shifted towards negative values. It has also been shown that the dependence of the density of corrosion defects on the time of the test passes through a maximum, i.e., some of the appearing defects are metastable and stop functioning after a few dozen hours. It has also been found that the increased rate of hydrogen penetration into the metal caused by addition of a metal hydrogen absorption promoter (thiourea) stimulates both the initiation and development of local defects at cathodic potentials. An increase in thiourea concentration, rate of hydrogen penetration into the metal and the amount of hydrogen absorbed by steel results in a shortening of the incubation period, on the one hand, and in an increase in the number of pit-like defects, on the other hand.

Keywords: localized corrosion, cathodic protection, pitting

Int. J. Corros. Scale Inhib., 2018, 7, no. 4, 683-696 PDF (1 014 K)
doi: 10.17675/2305-6894-2018-7-4-14

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