Here HI is anodic polarisation current for hydrogen charged specimen and ref I is anodic polarisation current for specimen without hydrogen (reference curve). Calculation of hydrogen concentration was done according to formula: zFv Q C abs H H . (3) Where z is the number of electrons take in reaction; F is the Faraday constant; v is the effective volume of specimen: 3 C mol cm H ; Q A s abs H ; 1 z ; C mol F 4 9,65 10 . After assigned exposition under hydrogenating conditions, all specimens were tested to failure under increasing static loading or fatigue according to 3-point bending scheme. The “load – displacement” diagram and acoustic emission (AE) signals were simultaneously registered by PC during the tests. The start of fracture process has been defined by acoustic emission method. 3. RESULTS AND DISCUSSION Process of hydrogen charging of pipeline steels at the given conditions of cathodic polarisation was characterised by following parameters: averaged meaning of cathodic current density ci ; total quantity of evaluated ev Q and absorbed abs Q hydrogen; coefficient of efficiency of hydrogen permeation in metal ev abs k Q Q and hydrogen concentration in metal HC . It can be seen that for assigned testing conditions, i.e. in deoxygenated, near-neutral pH NS4 solution and under loading by tensile stress, which simulates of operating conditions, the resistance to hydrogen absorption decreases with decreasing of steel strength. Based on received experimental results, the hydrogen concentration in given steels versus time of exposition of specimens in the hydrogenating conditions can be described by power relation: m HC A 6 10 . 3 mol cm . (4) Where Aand mare constants (see Table 4). Table 4 Meanings of constants in formula (4) Steel A m API X52 0.30 0.57 API X70 0.40 0.42 3.1Local strength The study of local strength of notched specimens in presence of hydrogen was resulted in the dependencies of total work iU for initiation of the local fracture emanating from the notch. The scheme of determination of parameter iU is presented in Fig. 3 U P d i 0 i . (5) Here the hydrogen concentration HC in metal was calculated by using of the analytical relation (4). The main observation based on these results is the existence of some critical time of exposition and as a consequence – some critical hydrogen concentration HC , when the essential
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