PSI - Issue 26

118 V.N. Kytopoulos et al. / Procedia Structural Integrity 26 (2020) 113–119 V. N. Kytopoulos / Structural Integrity Procedia 00 (2020) 000 – 000 corrosion as shown the series of Figs.(2-6), where an evident reduction in the ductility of the material can be deduced. It is mentioned that similar shifts of mechanical curves, due to hydrogen embrittlement, have been obtained elsewhere (Zhao et al. 2011). With respect to this it should be pointed out that a related convincing evidence of the occurrence of hydrogen- assisted embrittlement in the present material was recently demonstrated by microscopic indicators of a detailed SEM- microfractographic analysis (Ioakeimidis et al. 2013). Nevertheless, at this place it should be noted that there still exist problems with the full acceptance of hydrogen as universal embrittling agent of metals in corrosive salt-water environments in the sense that the chloride agents-induced aggressiveness in this corrosive environment may play a certain important role (Megn & Bayles 1987). Because of this, we have used the SEM-aided Electron Probe-X-ray Microanalysis of wavelength disperses type technique for the fractured surfaces to detect chloride-type agents. Within the sensitivity limits of this technique, 100 ppm, such agents could not be detected. Furthermore, it is known that chloride agents-promoted corrosion phenomena are attributed to long-time (months or years) related pitting formations on the materials surface. In our case, perhaps due to short corrosion times (maximum 1000 hrs), pitting micro features could not be observed. By virtue of these findings one can reasonably exclude any important role of such agents for the present investigated embrittlement processes. Moreover, at the same time, an overall shift of the J-magnetic curves to lower magnetic values with increasing corrosion time can be observed. This behavior may present another convincing indicator of magnetic hardening tendency of the steel with hydrogen accumulation. This argument may be supported by the effective reduction of the ME- energy signal due to the magnetic hardening of material observed elsewhere (Sulliran et al. 2004). In addition, one can further observe in Fig.2, the relatively small discrepancy between the maximum of the mechanical and magnetic curves. However, by close examination of the strain location points of Figs.(3-6) one can estimate that, due to shift to lower strains with time of corrosion, an increasing discrepancy occurs between these points. Thereafter, in this way, one can estimate that the rate of shift to lower strain is greater for the magnetic J-curves. This is indicative of a time processing advance in the occurrence of magnetic hardening compared the embrittlement process. In other words, this would mean in general that magnetic processes may be stronger influenced by hydrogen and as such, the magnetic properties of the material would respond with an increased susceptibility to hydrogen effects compared to mechanical ones. Certain physical properties of a low-carbon steel, determined by its mutual tensile stress and micromagnetic emission response, may considerably be influenced by cumulative atomic hydrogen produced under corrosive NaCl-water solution environment. The influencing factors can better be revealed, described and analyzed by introducing a relevant parameter of specific micromagnetic emission response and employing certain intrinsic processes of ferromagnetic and mechanical behavior of the material. In this manner one can show that an increase in the hydrogen accumulation with corrosion time leads to an associated increase of the embrittlement of steel, expressed by appreciable loss in its ductility as well as to a parallel increase in the magnetic hardening tendency expressed by a decrease in the specific micromagnetic emission response. In this way, one can reasonably estimate that the magnetic processes of the material would be more susceptible to hydrogen effects compared to mechanical processes. It is mentioned at this point, that the present study, is part of an ongoing project of the corresponding author’s team, that aims to connect electric and magnetic features with the respective stress and damage fields (Kytopoulos et al. 2019). Satisfactory results have been already obtained for metal matrix composites (Andrianopoulos et al. 1997; Kourkoulis & Andrianopoulos 2000; Kourkoulis 2001; 2002; 2003). The results of this ongoing project are to be correlated with the respective ones of projects based on detection of similar features, as, for example, various NDT techniques (Kourkoulis et al. 2006), Acoustic Emissions (Triantis & Kourkoulis 2018; 2019) and Pressure Stimulated Currents (Stavrakas et al. 2019) (although the latter is mainly applied to a completely different class of materials, i.e., brittle building geomaterials (Kourkoulis et al. 1999; 2010; Exadaktylos et al. 2001a; 2001b)). 4. Conclusions

Acknowledgements

The authors would like to express their gratitude to Prof. E. Christoforou for permitting use of the Barkhausen apparatus in the laboratory of Physical Metallurgy at the School of Metallurgical Engineering of NTUA.