PSI - Issue 13

2186 M.S. Khoma et al. / Procedia Structural Integrity 13 (2018) 2184–2189 3 M.S. Khoma, V.R. Ivashkiv, M.R. Chuchman, Ch.B. Vasyliv, N.B. Ratska, B.M. Datsko / Structural Integrity Procedia 00 (2018) 000–000 of its accumulation in defects of microstructure increases from pearlite to troostite [Hagi (1994)]. The part of diffusion-mobile hydrogen in the volume of absorbed gas is more than 2/3, only in sorbite ~ 50%. Diffusion-mobile hydrogen redistributes under the influence of gradients concentrations and tensions of various physical fields, and interacts with the matrix and structural imperfections, initiates processes of microdeformation and destruction in local volumes. Less mobile residual hydrogen remains in the high-energy traps of the crystal lattice after desorption of diffusion-mobile hydrogen. Corrosion rate growth when non-equilibrium of the steel structure increases. It can be associated with growth of the surface energy. Films of sulfides were fixed on the surface of steels after exposure in corrosive solution. Heterogeneity and defectness of sulfides increases when the non-equilibrium of the steel structure grows (Fig. 1). The most homogeneous film with low defectness is formed on the surface of sorbite (Fig. 1b). On troostite and martensite films consist of large chaotically layered crystals (Fig. 1).

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d Figure 1. Microstructure of sulfide films on the surface of steel У8 with structures of pearlite (a); sorbite (b); troostite (c); martensite (d) after exposure in a NACE solution during 96 hours. The surface of steel under the sulfide films contains numerous ulcers. The depth of ulcers is the smallest on the sorbite and increases from pearlite to troostite and martensite (Fig. 2). The bottom of the ulcers is rounded on sorbite and troostite; and is sharpened on pearlite and martensite that promotes the emergence and development of cracks. The maximum depth of local damages is observed on martensite and reaches ~ 260 μm. The obtained data correlate with resistance of the steel to corrosion cracking in the solution of NACE (Table 2). Steel with a structure of sorbite is more resistant than martensite and pearlite.

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Figure 2. The corrosion damage on the surface of steel У8 with structures of pearlite (a); sorbite (b); troostite (c); martensite (d) after exposure in the NACE solution for 96 hours.

The number of corrosion ulcers is the smallest on the sorbite, and the radius of their bottom is greater, which slows down the corrosion cracking of steel. Hydrogenation of steel in the solution of NACE increases under the influence of static loads 300 MPa. The concentration of hydrogen in troostite increases in ~ 1.8 times, in pearlite and sorbite - in ~ 1.3 ... 1.4 times (Table 2). Such results are not obtained for martensite, because the samples were destroyed faster even at lower loads. In the hydrogen sulfide environment, local corrosion cells appear on the surface, which is accompanied by the growth of microelectrochemical heterogeneity. After 96 hours of corrosion electrode potentials of martensite and troostite grows by an order of magnitude, pearlite ~ 5 times, sorbite ~ 2 times (Table 3). This indicates that a non- equilibrium structures like troostite and martensite have greater tendency to corrosion than more stable sorbite and pearlite.