PSI - Issue 22

Olha Zvirko et al. / Procedia Structural Integrity 22 (2019) 299–304 Olha Zvirko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

301

3

rate of 1.0 mV  s -1 under room temperature. The steels were tested in NS4 test solution simulated soil environment. Chemical composition of NS4 test solution is presented in Table 1. The solution containing (mg/L) 14440 Na + ; 25400 Cl – ; 129 K + ; 5.0 SO 4 2 – ; 2.5 Li + ; 600 Ca 2+ ; 522 HCO 3 – ; 518 Mg 2+ ; 140 Ba 2+ ; 5.0 NO 3 – ; 389 Sr 2+ ; 3.6 F – ; 0.25 Fe 2+ ; 100 Br – ; 0.5 Mn 2+ ; 21 I – ; 1.0 Al 3+ ; 5.0 PO 4 3 – ; 52 NH 4 + ; 18 SiO 2 2 (alkalinity – 455), simulated aqueous condensate in gas transit pipelines, was also used as the test medium. The test solutions were prepared from analytical grade reagents. The solutions were treated neither during the preparation stage nor during the test itself. The basic electrochemical characteristics of steels (corrosion potential Е corr , corrosion current density i corr , the Tafel constants b c and b а of the cathode and anode reactions respectively) were determined by the graph-analytic method. The polarisation resistance R p was calculated using the Stern-Geary equation:

 Е /  і = R p = K/i corr ,

(1)

where K = b a ·b c / [2.3· (b a + b c )] is a constant.

Table 1. Chemical composition of NS4 test solution. Components KCl

NaHCO 3 CaCl 2  2H 2 O MgSO 4  7H 2 O

Concentration in g/L 0.122 0.483

0.181

0.131

3. Test results and discussion 3.1. Deterioration of brittle fracture resistance of pipeline steels under operation

The mechanical properties experimentally determined for the investigated pipeline steels in as-received state and after operation are presented in Table 2. Long-term service of transit pipelines caused, as a rule, some decrease in strength properties of steels, as it was demonstrated by Gabetta et al (2008). However, characteristics of brittle fracture resistance of the steels, namely the impact toughness KCV and fracture toughness J 0.2 (the value of J-integral corresponding to 0.2 mm crack growth), were sharply decreased as a result of long-term operation. Accordingly, these characteristics can be considered as mechanical parameters of in-service material degradation.

Table 2. Mechanical properties experimentally observed for the studied pipeline steels. Pipeline steel Steel state

Impact toughness KCV (J/cm 2 ) KСV deg /KСV in J

0.2 , MPa∙√m

X52

As-received

196

-

412

X52-10 X52-10 X52-12 X52-12 17H1S 17H1S 17H1S 17H1S 17H1S 17H1S

Operated 30 years, top Operated 30 years, bottom Operated 30 years, top Operated 30 years, bottom

57 60 72 77

0.29 0.31 0.37 0.39 0.81 0.67 0.56 0.61 0.24 -

-

79

-

127 322

As-received

200 162 134 112 122

Operated 28 years Operated 29 years Operated 31 years Operated 40 years Operated 53 years

-

175 201

- -

48

3.2. Susceptibility of pipeline steels to corrosion degradation under operation

The potentiodynamic polarisation curves for the pipeline steels with different strength (the 17H1S, X60 and X70 steels) in the as-received state and after long-term operation, determined in NS4 solution simulated groundwater, are presented in Fig. 1. The obtained potentiodynamic curves showed no active-passive transitions in the investigated