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INFLUENCE OF THE PRODUCTS OF ELECTROCHEMICAL DESTRUCTION OF PROTECTIVE COATINGS ON THE PROPERTIES OF PIPE STEEL IN LOW-ALKALINE MEDIA S. О. Osadchuk,1 L. І. Nyrkova,1, 2 А. О. Rybakov,1 S. L. Mel’nychuk,1 N. О. Hapula,1 and S. М. Ostapyuk3

UDC 621.791:621.643

We investigate the influence of the products of electrochemical destruction of polymeric tape protective coatings formed in low-alkaline media under the conditions of cathodic polarization at 1.5 V (s.c.e.) and without cathodic polarization for different times on the corrosion, electrochemical, and stress-corrosion properties of pipe steel. It is shown that, for 2.5 months, the pH value shifts toward the alkaline region and the electric conductivity of the medium increases. We reveal an increase in the corrosion rate of pipe steel and its susceptibility to stress-corrosion cracking in the medium of model soil electrolyte in which the destruction of the coating occurs for 2.5 months both under the conditions of cathodic polarization and without it. It is demonstrated that the chemical composition of the polymeric film remains almost constant in the process of destruction. Keywords: electrochemical destruction of protective coatings, low-alkaline media, cathodic protection, corrosion rate of pipe steel, products of electrochemical destruction of protective coatings.

It is known [1] that almost all insulating coatings are permeable for water and oxygen and, as a result, electrochemical reactions occur under coatings and, in the presence of a defect, these reactions are intensified in the region of the defect and in the zones adjacent to it. This is why insulating coatings exfoliate from the metal of the pipe. Under the conditions of polarization, these processes are accelerated. Thus, in 1977, the researchers gave the first place to the adhesion of insulating coatings and the second place to the resistance to cathodic exfoliation. However, in 1985, the resistance to cathodic exfoliation was shifted to the first place [1]. Note that the information about the influence of the products of electrochemical destruction of polymeric protective coatings on the corrosion, electrochemical, and stress-corrosion properties of pipe steels available from the literature is insufficient. In what follows, we present the experimental results of investigations of the influence of the products of electrochemical destruction of polymeric tape coatings formed in low-alkaline media under the induced potential of cathodic protection and without this potential on the corrosion, electrochemical, and stress-corrosion properties of pipe steels. The present paper is a continuation of the investigations originated in [2]. Methodical Aspects of Experimental Investigations A model soil electrolyte (MSE) is chosen as a working medium because it is most close to the actual conditions. The chemical composition of the MSE is as follows: 0.037 mg/liter potassium chloride (KСl), 1 2 3

Paton Institute of Electric Welding, Ukrainian National Academy of Sciences, Kyiv, Ukraine. Corresponding author; e