PSI - Issue 13

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 13 (2018) 1215–1220 Hryhoriy Nykyforchyn / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 1. Time dependencies of potential E pol of the polished surface of carbon steels in 0.3% NaCl solution depending on content of carbon C in steel: 1 – steel 20 (0.20% C); 2 – steel 35 (0.35% C); 3 – steel U8 (0.80% C); 4 – steel U12 (1.20% C).

Fig. 2. Dependence between potential E pol of the polished surface of carbon steels in 0.3% NaCl solution and carbon content C C in steel, as well as potential Е fr of the fracture surface of specimens made of the X52 pipeline steel in the as- received condition (Х52) and after 30 years of operation (Х52 - 10 and Х52 -12) fractured after impact toughness testing. Line: data of regression analysis.

Table 1. Influence of long-term operation of the ferrite-pearlite X52 pipeline steel on electrochemical properties in 0.3% NaCl solution and on its impact toughness. Pipeline steel Steel state Potential E pol of polished steel surface (V) Potential E fr of fracture surface (V) Impact toughness KCV (J/cm 2 ) X52 As-received -0.486 -0.494 196.0 X52-10 Operated 30 years -0.492 -0.580 83.0 X52-12 Operated 30 years -0.490 -0.557 90.5

The significant difference in time dependences of potential Е fr of fracture surfaces of specimens for the X52 pipeline steel in the as-received state and after long-term operation was revealed. In particular, the difference between curve 1 (X52 steel in the as-received state) and, on the other hand, curves 2 and 3 (X52 st eel in the post operated state) increases with an increase in exposure time of specimens in the electrolyte (Fig. 3). The difference between electrode potential Е fr of the fracture surface of specimens made of the X52 pipeline steel in as-received and post-operated states reached almost 100 mV after exposure time of 500 sec. Such significant difference can be explained by the fact that the fracture surface is the result of crack propagation through local sites characterized by minimal resistance of material to fracture. Therefore, changes in the metal that were initiated during its long-term operation and caused its mechanical properties degradation are the most distinguishable on the specimen fracture surface. This phenomenon also effect s on electrochemical behaviour of the specimen fracture surface. It is resulted in increasing sensitivity of electrochemical properties measured on