PSI - Issue 13

2188 M.S. Khoma et al. / Procedia Structural Integrity 13 (2018) 2184–2189 5 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

The high concentration of absorbed hydrogen inevitably worsens the propensity of ferrite-pearlite to corrosion cracking. It may be due to the imperfection of its structure, in which a large amount of hydrogen accumulates between the plates of ferrite and cementite, without creating significant stresses in the lattice. Influence of the static load (  = 300 MPa) on the hydrogenation of steel 45 in the solution NACE is less then that of the steel У8 (Table 5). The concentration of hydrogen in the pearlite-ferrite and sorbite under load increases in ~ 1,1 times, in troostite - in ~ 1,2 ... 1,3, and in martensite - in ~ 1,4 ... 16 times. a b c d

Figure 4. Corrosion damage on the surface of steel 45 of ferrite perlite (a); sorbite (b); troostite (c); martensite (d) structures after exposure in NACE solution for 96 h.

Table 5. Susceptibility of steel 45 to stress corrosion cracking in NACE solution. Structure Time to destruction in air/NACE, min  a /  e *С � 0 Н  /С Н  Ferrite-pearlite 179/57 3.1 1.1 Sorbite 101/72 1.4 1.1 Troostite 125/37 3.4 1.3 Martensite 115/28/ 4.1 1.4 С   - hydrogenation under load 300 MPa; С 0  - hydrogenation without load.

Н

Н

The initial microelectrochemical heterogeneity of ferrite-perlite is 20 ... 55 mV, sorbite - 15 ... 30 mV, troostite - 18 ... 35 mV, and martensite - 15 ... 30 mV (Table 6). Non-continuous sulfide films and local defects are formed on the surface of steel 45 in NACE solution. As a result, microelectrochemical heterogeneity of the troostite increases by ~7 times, sorbite and martensite in 3-5 times, and ferrite-pearlite - twice (Table 6). Microelectrochemical properties of steel 45 with different structures correlate with corrosion damage. Table 6. Microelectrochemical properties of steel 45 in the initial state (  Е) and after corrosion (  Е  ) in the NACE solution. Structure  Е, mV  Е  , mV Ferrite-pearlite 20…55 30…100 Sorbite 15…30 50…140 Troostite 18…35 100…250 Martensite 10…25 20…170 The corrosion of steels У8 and 45 in the NACE solution is localized. The average size of the ulcers on the steel У8 is 50 ... 80 μm and the deepest of them are on martensite (~ 260 μm). The average size of the ulcers on the steel 45 is 45 ... 65 μm, and they are the deepest on troostite (~ 210 μm). Corrosion products are more compact on the steel 45. The corrosion rate, hydrogenation and the tendency to corrosion cracking of steel У8 in the solution depend on its structure and increase in a row: pearlite, sorbite, troostite and martensite. The corrosion rate and hydrogenation of steel 45 for pearlite-ferrite are mach larger then for sorbite and troostite.