Electrochemical Properties of Steels in a Model Hydrogen Galvanic Couple
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ELECTROCHEMICAL PROPERTIES OF STEELS IN A MODEL HYDROGEN GALVANIC COUPLE V. H. Zakharchuk, O. T. Tsyrul’nyk, and H. M. Nykyforchyn
UDC 621.325:669.539.43
We study the effect of preliminary hydrogenation of carbon and low-alloy steels on their electrochemical behavior in a hydrogen galvanic couple “hydrogenated–nonhydrogenated steel.” The hydrogenation conditions being equal, the current of a hydrogen galvanic couple is determined not by its electromotive force but by the degree of polarization of the electrodes, which depends on the ratio between their areas. The polarization relations show that the electrochemical processes in a hydrogen galvanic couple obey the laws of electrochemical kinetics and have an activation nature. The current of a hydrogen galvanic couple is proportional to the concentration of absorbed hydrogen in steels, and, correspondingly, this dependence can serve as a basis of a new method for the nondestructive testing of hydrogen content in steel. As the hydrogen content in steels increases, their electrochemical properties approach the properties of a juvenile surface: the electrolytic potential becomes more negative, the polarization decreases, and the rate of anodic reactions grows sharply.
It is customary to explain the effect of hydrogen absorbed by steel on its electrochemical characteristics by the hydrogen cold working of surface layers, which shifts the electrolytic potential to the positive side, increases the capacitance of the electric double layer, and stimulates the anodic dissolution of carbon and low-alloy steels in acid solutions [1 – 6]. However, despite the mentioned substantial influence of absorbed hydrogen on the anodic behavior of steels (see, e.g., [1, 2, 5, 6]), including stainless [3, 4], quantitative assessments of the intensification of their corrosion have not been obtained. In neutral and alkaline media, the surface of steels is covered with barrier oxide films, and, therefore, the mechanism of the influence of absorbed hydrogen is different. In the course of its desorption, hydrogen reduces the oxide films, imparting the juvenile properties to the surface: it shifts the electrolytic potential to the negative side and intensifies the electrode processes. The laws of changes in the electrolytic potential depending on the intensity of hydrogen flow, which had been studied thoroughly, served as a basis of the well-known electrochemical method of evaluating the hydrogen permeability through metal membranes [7]. However, the effect of absorbed hydrogen on the corrosion behavior of steels in neutral media has been studied only under conditions of the action of a galvanic couple “hydrogenated–nonhydrogenated steel,” i.e., a hydrogen galvanic couple (HGC) [8 – 10]. It has been shown that hydrogenated steel in a HGC plays the role of an anode, and its current can be hundreds of times (and, for a plastically deformed anode as at the tip of a crack, more than a thousand times) as high as the corrosion current of nonhydrogenated steel. The formation of such micro-HGC explai
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