KINETICS OF CORROSION OF STEEL St3 IN PO-6СT FOAMING AGENT FOR FIRE EXTINGUISHING. ELECTROCHEMICAL INVESTIGATION

Abstract

Foaming agents for extinguishing fires are to be stored in a concentrated form. Storage of concentrate in St3 steel containers is allowed. However, this metal can be corroded, which damages the storage tanks, and corrosion products of steel reduce the quality of the foaming agent. The kinetics of metal corrosion depends on the type of foaming agent and the temperature storage conditions. In this study, the corrosion behavior of St3 steel in commercial concentrate of the PO-6CT foam for extinguishing fires was studied by electrochemical methods (potentiometry, voltammetry, electrochemical impedance spectroscopy) at 27±1°С. It was found that during the testing period, which lasted for 6 weeks, the corrosion potential of steel began to undergo sharp fluctuations, but its value was generally negative than the initial value. The greatest changes in the impedance of the steel sample during the corrosion process occur in the first two weeks of contact with the corrosive medium. An equivalent circuit, which takes into account the two time constants was used to interpret the impedance data of the corrosion process which was occurred under the porous layer of corrosion products. It is shown, the charge transfer resistance (Rct) between the corrosive metal and corrosion products, the capacity of the double electrical layer (Cdl) and the electrolyte resistance in the pores of the corrosion products layer (Rpo) highly increase during this period. Subsequently, the Rct is decreased and stabilized at a level 3 times lower than the original value. The properties of the metal/electrolyte interface were stabilized, but the Cdl value and the Rpo values are increased by 5 times and by 10 times, respectively. This indicates a high degree of the surface rust and a stable state of the corrosion products layer. Polarization studies have shown that the anodic and cathodic polarization curves have a Tafel slope equals to 0.147 V and 0.187 V, respectively. This is due to the presence of an oxide-hydroxide film on the electrode surface. The calculated value of the mass index of corrosion rate is 2.46×10-8 kg×m-2×s-1. Thus, the experiment showed that the St3 corrosion proceeds with activation control of the process rate. The corrosive behavior of St3 in the PO-6CT foaming agent for fire extinguishing is a consequence of the physicochemical features of this system. Primary corrosion products of steel St3 (Fe+2 ions) can be readily oxidized to Fe+3 by dissolved oxygen of air, and Fe+3 ions undergo hydrolysis, which leads to an increase in the acidity of the foaming agent and, correspondingly, an increase in its corrosive activity.Forcitation:Grishina E.P., Kudryakova N.O., Predein A.N., Belyaev S.V. Kinetics of corrosion of steel St3 in PO-6СT foaming agent for fire extinguishing. electrochemical investigation. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2018. V. 61. N1. P. 30-36