Investigation of the Influence of the Steel 20 Microstructure on the Carbon Dioxide Corrosion Rate
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OSION UDC 620.193.1
INVESTIGATION OF THE INFLUENCE OF THE STEEL 20 MICROSTRUCTURE ON THE CARBON DIOXIDE CORROSION RATE P. E. Yudin,1 T. M. Pugacheva,1 L. A. Kondratyeva,1 M. V. Bogatov1 Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 61 – 65, June, 2020. The influence of various heat treatment modes (annealing, quenching, tempering) of steel 20 on the carbon dioxide corrosion rate in a 3% NaCl solution in combination with carbon dioxide (CO2 ) barbotaging was studied. It is established that the quenching followed by high tempering is the most effective heat treatment of steel 20 to achieve a combination of increased mechanical properties and corrosion resistance in a carbon dioxide environment.
Key words: heat treatment, steel 20, carbon dioxide corrosion, carbon dioxide, microstructure, corrosion rate, quenching followed by high tempering. – reduction of bicarbonate to carbonate ion ÍÑΖ3 ®
INTRODUCTION
H+ + ÑÎ2– 3 ;
Carbon dioxide corrosion is one of the main destructive factors in the operation of steel pipe products. The mechanism of carbon dioxide corrosion [1 – 3] consists of several sequential stages: – a layer of carbonates is formed on the surface of the metal, in which discontinuities (pores) are formed later; – the formation and growth of pittings under a layer of iron carbonate is observed, which leads to a decrease in carbonate adhesion, the appearance of an electrochemical galvanic pair and accelerated growth of pittings; – removal of carbonate crystals, stripping of the pipe metal; – the process of the three stages above is repeated in other areas. Schematically, the process of carbon dioxide corrosion is presented in Fig. 1. In the presence of water, carbon dioxide (CO2 ) dissociates forming carbonic acid (H2CO3): CO2 + H2O ® H2CO3. Typical carbon dioxide corrosion consists of three cathodic and one anodic reactions [10]. Cathodic reactions: – reduction of carbonic acid into bicarbonate ions H2CO3 ® H+ + ÍÑΖ3 ; 1
– reduction of hydrogen ions 2H+ + 2e– ® H2. The anodic reaction is the iron dissolving: Fe ® Fe2+ + 2e–. 2+ ions exceeds When the concentration of ÑÎ2– 3 and Fe the solubility of iron carbonate, formation of a solid precipitate is observed: Fe2+ + ÑÎ2– 3 ® FeCO3.
Pipe metal FeCO3
Pitting
Samara State Technical University, Samara, Russia (e-mail: [email protected]).
Transported fluid
Pore
Fig. 1. Schematic representation of the carbonate corrosion process.
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part of the pipe is associated with structural transformations during upsetting, as well as the absence of subsequent recrystallization heat treatment, which led to a catastrophic corrosion destruction of the pipe. This example shows how a different microstructure can multiply change the resistance of the same steel to carbon dioxide corrosion. Studies on the dependence of the corrosion rate on the microstructure are known. For example, in [7] it was established that the ferrite-pearlite structure
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