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

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enough sensitive to pipeline steels degradation and could be informative parameters of their state changes under long term operation. Evaluation of polarisation resistance is enough easy, therefore a usage of this parameter is enough prospective for an evaluation of degradation degree of steels. Relative changes in polarisation resistance (R p deg /R p in ) for the 17H1S, X52-10 and X52-12 pipeline steels in the as-received (R p in ) state and after different time of operation (R p deg ), measured in the test solution simulated aqueous condensate in gas transit pipelines, are presented in Fig 2. Degradation of brittle fracture resistance of the studied pipeline steels under operation (Table 2) was accompanied by a decrease in polarisation resistance.

Fig. 2. Relative changes in polarisation resistance (R p deg /R p in ) for the 17H1S, X52-10 and X52-12 pipeline steels in the as-received (R p in ) and operated (R p deg ) states measured in the solution simulated aqueous condensate in gas pipelines.

Consequently, deterioration of a number of electrochemical characteristics of the operated pipeline steels with different strength, especially corrosion current density and polarisation resistance, indicated their corrosion degradation, which was evidently caused by in-service degradation due to mutual effect of corrosion hydrogenating environments and working stresses under their long-term operation. 3.3. Correlation between changes in impact toughness and polarisation resistance of pipeline steels caused by long term operation A possibility of estimation of impact toughness of pipeline steels based on changes in polarisation resistance caused by long-term operation was analyzed. In the present work, a systematic deterioration of mechanical (impact toughness) and electrochemical (polarisation resistance) parameters of pipeline steels caused by long-term operation was observed. The correlation between changes in these parameters for the low-carbon low-alloyed ferrite-pearlite API 5L grade X52 pipeline steels is shown in Fig. 3. Using regression analysis, the dependence satisfies the following relation: KCV deg /KCV in = -0,308+ 1,309·[R p deg /R p in ], (2) where KCV in and KCV deg – impact toughness of the as-received and operated steel, respectively; R p in and R p deg – polarisation resistance of the as-received and operated steel, respectively. The dependence KCV deg /KCV in – R p deg /R p in is the basis of non-destructive electrochemical method for evaluation of in-service degradation of pipeline steels. An acceptable correlation revealed between relative changes in polarisation resistance R p deg /R p in and impact toughness KСV deg /KСV in of pipeline steels caused by their in-service degradation (Figure 3) enables an evaluation of in-bulk material properties changes, namely impact toughness, by measurements of electrochemical characteristics changes. Having initial properties of the material (for example, impact toughness of the as-received pipeline steel according to certificate or impact toughness of the steel of the reserved pipe), its actual properties can be predicted.