Hydrogen concentration near the tip of a corrosion crack
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HYDROGEN CONCENTRATION NEAR THE TIP OF A CORROSION CRACK O. V. Gembara, Z. O. Terlets’ka, and O. Ya. Chepil’
UDC 620.191.33:620.193
In evaluating the long-term damage of a metal as a result of electrochemical corrosion, it is important to determine the kinetics of inflow of hydrogen ions to the cathodic domains of the metal surface (the third problem). However, the diffusion of ions of the far and near fields of an electrolyte, especially in such thin volumes as a crack, has been studied insufficiently [1 – 4], and the corresponding mathematical models have not been constructed. In the present work, we make an attempt to do this. Computational Model As is well known [2], an electrochemical reaction takes place in the course of contact between an electrolyte and the surfaces of a crack, which has domains with different potentials. This reaction includes several processes, which proceed in different ways on different domains of the surface. In evaluating stress corrosion fracture, the key role belongs to competing anodic and cathodic processes on the clean surface near the crack tip. The cathodic processes are concentrated on the islets of the passivating film, which is formed and destroyed due to cyclic loading of the metal [5]. It is customary to think that the electrolyte solution is acid, and, hence, on the cathodic domains, hydrogen will be reduced with the formation of gas bubbles. Therefore, the pressure of gaseous hydrogen in these bubbles will serve as a driving force, which induces its dissolution on the metal surface with diffusion into the prefracture zone. To describe this phenomenon, we propose the following computational model: The metal surface under study is covered with islets of the passivating film. As a result of the cathodic reaction, hydrogen reduction and molization take place on each islet, and then bubbles are formed (Fig. 1). Most probably, the center of each bubble will be located near the point of zero charge, where, as is well known [3], wetting of the surface is minimum. The size of such bubbles is fairly small since they arise on the cathode. Hence, we may assume that the thickness of the surface layer of the electrolyte covering a bubble will be commensurable with its size. Further, since the force of hydrogen pressure pH2 in a small volume has to get balanced with the surface tension in the surface layer, this pressure can reach great values. Therefore, we assume that there exists a linear relation between the bubble volume Vm and hydrogen pressure in it pH2 : V m (t ) = pH2 (t )Vm(1) ,
(1)
Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, Lviv, Ukraine. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 1, pp. 109 – 111, January – February, 2008. Original article submitted April 7, 2006. 1068–820X/08/4401–0133
© 2008
Springer Science+Business Media, Inc.
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O. V. GEMBARA , Z. O. T ERLETS ’KA,
AND
O. YA. CHEPIL’
Fig. 1. A scheme of the formation of a hydrogen bubble in an electrolyte on a metal surface. where
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