PSI - Issue 22

Olha Zvirko et al. / Procedia Structural Integrity 22 (2019) 299–304

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Olha Zvirko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

potential range, only active dissolution. The similarity of the polarisation curves (Fig. 1) for the 17H1S and X60 steels was observed for both studied steel states – as-received and operated. Electrochemical behaviour of the X70 steel differed from others. It can be seen in Figure 1 that the intensity of anode reactions on the X70 steel was less than that on the 17H1S and X60 steels. Both 17H1S and X60 steels in as-received states were characterized by the same corrosion resistance, and the highest corrosion resistance was typical for the X70 steel. The electrochemical characteristics of the studied pipeline steels are presented in Table 3.

a

b

Fig. 1. Potentiodynamic polarisation curves for the 17H1S, X60 and X70 pipeline steels in the as-received (a) and operated (b) states in NS4 test solution.

Table 3. Influence of long-term operation on electrochemical properties of the studied pipeline steels in NS4 solution. Pipeline steel Steel states Corrosion

Corrosion current density i corr (μА/сm 2 )

Tafel constant b c (V)

Tafel constant b a (V)

Polarisation resistance R p , (k  сm 2 )

potential Е corr (V)

17H1S 17H1S

As-received

-0.683

1.85 4.20 1.81 3.86 0.67 2.24

-0.090 -0.083 -0.090 -0.090 -0.089 -0.088

0.062 0.058 0.063 0.056 0.061 0.060

8.63 3.53 8.90 3.89

Operated 30 years -0.687

X60 X60 Х70 Х70

As-received

-0.664

Operated 25 years -0.696

As-received

-0.518

23.49

Operated 37 years -0.642

6.92

Corrosion and electrochemical characteristics of the operated pipeline steels in NS4 solution were significantly deteriorated due to long-term service in comparison with the as-received steels. Electrochemical activation of pipeline steels caused by long-term service manifested in an increase of intensity of cathode and anode processes on the degraded steels, in an increase of corrosion current density, in an decrease of polarisation resistance and in a shift of corrosion potential values towards more negative ones for the degraded steels compared with the steels in the as-received state (Table 3). It was revealed that degree of corrosion degradation caused by long-term operation of gas mains was the highest for the high strength X70 steel. Thus, corrosion current density i corr for the 17H1S and X60 steels increased in about 2.1 – 2.2 times and at the same time that for the X70 steel – in about 3.3 times. Despite corrosion degradation caused by operation, the X70 steel was characterized by higher corrosion resistance in NS4 solution than both 17H1S and X60 steels. Trends in changes in the electrochemical characteristics caused by in service degradation were similar for all investigated materials as it can be seen from the tests results. Different sensitivity of electrochemical characteristics of the studied steels to in-service degradation was revealed. Therefore, relative changes in them for the same degradation degree were compared: relative changes in corrosion potential and Tafel constant values were insignificant, however corrosion current density i corr and polarisation resistance R p were