PSI - Issue 13

M.S. Khoma et al. / Procedia Structural Integrity 13 (2018) 2184–2189 2185 2 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

(FeS), and kansite (Fe 9 S 8 ) films on iron and steel surface depends on the hydrogen sulfide concentration in solution. Surface films demonstrate a different effect on the corrosion processes, hydrogenation and stress corrosion cracking resistance, what depends on the presence of defects in the films, the structure and chemical composition of the metal [Ćwiek (2009), Shoesmith et al. (1980), Ma et al (2000)]. Damages appear as a result of corrosion and hydrogen embrittlement, and corrosion cracking occurs when the load is applied. The influence of the steels structure on its stress corrosion cracking under the loads in hydrogen sulfide environment is insufficiently studied. The aim of the study is to determine the influence of the steels structure on its corrosion, hydrogenation and corrosion cracking in the NACE hydrogen sulfide solution. 2. Materials and methods. Investigated materials were steels 45 and У8. Steels were thermally treated in such regimes: annealing, quenching, high, medium and low tempering. The equilibrium ferrite-pearlite (steel 45) and pearlite (У8) structures were obtained after annealing of steels at 800ºС, for 0.5 hours, and cooling in the furnace. Steels were quenched after annealing at 800ºС for 0.5 h and cooling in oil. High, medium and low tempering were carried out at 650; 450 and 200ºС (steel 45) and at 600, 400 and 200°С (steel У8), followed by cooling in the furnace. Structures of sorbite, troostite and martensite were obtained. A scanning electron microscope EVO-40XVP with INCA Energy 350 microanalysis system was used for metallographic investigations. The corrosion rate was investigated by gravimetric method in solution 5%NaCl + 0.5% CH 3 COOH, saturated by Н 2 S, for 96 h. Hydrogen sulfide was synthesized by the reaction Al 2 S 3 + 6H 2 O → 3H 2 S + 2Al (OH) 3 in stainless steel chamber. During the experiment, hydrogen sulfide was bubbled through a solution at a rate ~0.2 ml/min. The concentration of absorbed hydrogen was determined by vacuum extraction at 200 (C H200 ) and 800ºC (C H800 ). The total hydrogen concentration of С НΣ was determined as the sum of the C H200 and CH 800 concentrations. The predisposition to hydrogen sulfide corrosion cracking was determined by slow stretching of cylindrical specimens with a diameter of the working part d = 6.4 mm. Stretching speed was 10 -5 m/s. The cell was filled with a solution of NACE, which was vigorously saturated with hydrogen sulfide (100 ml / min) for an hour. During the experiment, hydrogen sulfide was bubbled through a solution at a rate ~0.2 ml/min. Resistance to corrosion cracking was evaluated by the ratio  a /  e , where  a is the time to sample destruction at air and  e – at at the NACE solution 3. Results and discussion 3.1. Corrosion and hydrogenating of steel У 8 in the NACE solution Corrosion of steel У8 in the solution of NACE has been investigated. A corrosion rate of pearlite is 1.2...2.1 g/(m 2 /h). The concentration of absorbed hydrogen is 5.7...7.5 ppm, where the proportion of diffusion-mobile hydrogen reaches ~ 75% (Table 1). The corrosion rate of sorbite is ~ 20% higher than that of pearlite (Table 1). The concentration of absorbed hydrogen is greater than in pearlite in 1,5 ... 2 times. The part of diffusion-mobile hydrogen is 50 ... 54% (Table 1). Corrosion rate of troostite is 1.5-1.9 times as large as pearlite (Table 1). The concentration of absorbed hydrogen is almost the same as in sorbite, but the part of diffusion-mobile hydrogen in it increases to 64 ... 78% (Table 1). The corrosion rate of martensite is the highest and is 2.0 ... 5.1 g/(m 2 /h). Also the highest concentration of absorbed hydrogen is fixed: 8.5 ... 22.4 ppm. The part of diffusion-mobile hydrogen is ~ 67% (Table 1).

Table 1. Corrosion rate and hydrogenating of steel У8 in the solution NACE (t = 720 h) Structure Perlite Sorbite

Sorbite 2.3…3.5 5.4…11.9 8.4…15.2

Martensite 2.0…5.1 2.5…16.1 8.5…22.4

Corrosion rate, g/(m 2 /h)

1.2…2.1 4.2…4.9 5.7…7.5

1.4…2.6 4.9…6.2 9.0…14.8

C H200

Hydrogenating, ppm

С Н 

When the structure of the steel У8 changes in the sequence: pearlite - sorbite – troostite, its dispersion growth and the surface area of the grain boundaries increases. Since the high-defect grain boundaries of ferrite- cementite in the pearlite structure have low activation energy of hydrogen absorption (18.4 kJ/mole), the probability