Zvirko O. Development of methodology for diagnostics of corrosion-hydrogen degradation of long-term operated structural steels

The object of the study is diagnostics of degradation degree of long-term operated structural steels by destructive and non-destructive methods for assessment of their condition. The purpose of the research was to develop methodology of destructive and non-destructive methods of diagnostics of metal condition on the basis of investigation of corrosion and hydrogen degradation regularities of structural steels, taking into account peculiarities of influence of operating conditions and hydrogenating media on materials damaging, and to develop methodological recommendations for diagnostics of technical condition of steels of a number of critical structures. Research methods: mechanical tests with determination of hardness, strength and plasticity characteristics, impact toughness, static crack resistance, resistance to stress corrosion cracking; electrochemical and gravimetric methods for determining corrosion resistance; metallographic analysis of structure; microfractographic studies of fracture mechanisms; methods of mathematical statistics. A negative role of several operational factors in degradation of carbon and low-alloyed structural steels is established. For the first time, a stage of development of dissipated damaging for ferrite-pearlite pipeline steels is divided into two periods: formation of chaotically oriented defects and formation of lamination cracks oriented in rolling direction. Mechanism of ferrite-pearlite pipeline steels embrittlement under operation consisted in carbon compounds enrichment not only grain boundaries, but also intragranular defects boundaries, is revealed. A new method of conservative estimation of degradation of structural steels operated in corrosion-hydrogenating environments that considers sensitivity of operated metal to stress corrosion cracking is developed. Mechanical approaches to diagnostics of in-service degradation of steels by destructive methods taking into account the effect of intensive dissipated damaging on their mechanical characteristics and establishing criteria of consistency of their estimation are developed. A new scientific and methodical approach concerning usage of electrochemical analysis of fracture surfaces for diagnostics of degradation of ferrite-perlite pipeline steels is substantiated. The correlation dependence between operational changes in brittle fracture resistance of metal and in electrode potential of its fracture surface is determined. A complex of diagnostic features of hydrogen induced macrodelamination in pipe wall of main pipelines is supplemented with a new diagnostic electrochemical feature, namely, polarization resistance; it is proposed to predict risk assessment of opening macrodefect at pipe outside by its decrease. The non-destructive electrochemical method of diagnostics of brittle fracture resistance characteristics of steels is improved and its application is extended to use for class of stainless martensitic steels. Universality of correlation dependence between the operational changes in brittle fracture resistance and in polarization resistance for carbon and low-alloyed ferrite-pearlite steels of a number of critical metal structures is determined. The obtained results and developed methodologies were used on a number of enterprises of Ukraine, namely on branches Affiliate “Lvivtransgaz” and Affiliate “Prykarpattransgas” of PJSC “Ukrtransgas”, and Dobrotvirska TPP of PJSC “DTEK Zakhidenergo”.