The effect of sulfate reducing bacteria (SRB) on the hydrogen permeation rate through ferrite-pearlite and sorbite steels of quite similar chemical composition was studied using a specially designed facility. Tests were carried out in synthetic sea water, sterile or inoculated with bacteria, at potentials corresponding to cathodic protection (À 800 mV to À 1400 mV NCE ). Cathodic polarization within the studied potential range did not stop the metabolism of SRB. Presence of SRB was found to increase the hydrogen permeation rate, to form S 2À ions, to increase the polarization current, to modify the impedance spectrum and to change the appearance of cathodic deposits in comparison with sterile conditions. The promoting effect of SRB on the hydrogen uptake was concluded to be the result of the increase in polarization current due to the formation of the less protective layer of cathodic deposits on the steel surface, the presence of S 2À ions and the possible decrease in pH. Despite the similar tendencies, the effect of SRB on hydrogen uptake was more pronounced in the case of sorbite steel. The bacteria action can cause hydrogen deterioration of steel at potentials, recognized as safe ones at cathodic protection.

At the slow strain rate tensile tests done using the specially designed facility, the decrease in the elongation to fracture, reduction of area, fracture energy and no effect on the strength have been stated for the low alloy ferrite-pearlite and sorbite steels, polarized in synthetic sea water at potentials corresponding to the cathodic protection (À 800 to À 1400 mV SCE ). Presence of SRB promotes the plasticity loss, being especially pronounced at potentials À 1100 to À 1200 mV SCE . At higher cathodic polarization, the plasticity estimated in inoculated and in sterile water equalizes. The effects have been correlated with the contents of absorbed and of permeable hydrogen. The promotion of hydrogen charging and the plasticity loss by SRB at the low and medium applied cathodic polarization has been accounted for the observed production of S À2 ions and inhibition of deposit formation. The negligible effect of SRB at the high cathodic polarization has been suggested to be a result of the suppression the SRB growth due to the high alkalization of the near surface solution. The same amount of hydrogen produces the less detrimental effect on the sorbite than on the ferrite-pearlite steel. However, at the similar cathodic polarization, the sorbite steel absorbs the highest amount of hydrogen and reveals the most pronounced degradation. Cathodic protection of constructions subjected to the action of SRB in the sea water should provide the conditions, under which no fragment of marine construction could be polarized by potential corresponding to the maximum degradation of the plastic properties of steels (À 1100 to À 1200 mV SCE ).

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The specially designed, fully equipped, and computerized devices for the underwater long-term measurements have been used to study the hydrogen permeation rate through the structural steels under natural sea water conditions. By applying constant cathodic potentials, the polarization current and the hydrogen permeation rate were recorded. The temperature and the concentration of S À2 ions in sea water were also measured. After the tests, the concentration of residual hydrogen in samples and the chemistry of the membrane surface were analyzed. The obtained permeation data were compared with the S À2 content in sea water and with the chemistry and appearance of formed deposits. The effect of SRB on the hydrogen permeation through structural steels is dependent on the site of the sample placing and on the sea water temperature. At a lower water temperature, the low hydrogen absorption was measured due to a lower activity of SRB. The highest hydrogen uptake accompanied by the highest content of S in corrosion and cathodic deposits was observed for membranes situated on the sea bed. The electric field of cathodically protected wall did not affect the hydrogen absorption. The hydrogen uptake recorded under the natural sea conditions can be higher than that measured in the laboratory tests, which should be taken into account while servicing marine constructions, especially embedded ones.