Effect of cathodic polarization and sulfate reducing bacteria on mechanical properties of different steels in synthetic sea water

Abstract

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 ).

Hydrogen permeation measurements in natural sea environment

“…The means values of S À2 concentration established in the natural sea water for both seasons ( Table 2) were quite close to those measured in the laboratory tests [2,3], especially in the case of summer period.…”

“…It is seen that the hydrogen absorption in natural water was much higher than that in sterile water and in some case even higher than in the inoculated one. This points out that the number of the SRB under the natural sea conditions may approach the number of SRB used in the laboratory tests (10 8 -10 9 SRB in 100 cm 3 of solution [2,3]). …”

“…The reason to promote the hydrogen uptake by structural steels in synthetic sea water inoculated with SRB has been shown [2] to be the production of H 2 S by SRB, the decrease in the near electrode pH, and the increase in polarization current at the similar applied cathodic potential due to the prevention of the cathodic deposits formation. This increase in hydrogen absorption in comparison with sterile water was enough to cause the plasticity loss of structural steels on the application of cathodic polarization, considered to be safe for cathodic protection [3]. However, the laboratory results obtained under artificial conditions of synthetic sea water cannot be directly transmitted to the real situation of marine constructions.…”

“…With the increase in depth, the water pressure increases and hydrogen recombination reactions and the formation of hydrogen bubbles may be inhibited and thus the hydrogen ingress into the metal can be promoted. 3. Movement of water: The movement of water may affect the structure of the gel-like biofilm on the metal surface and hence affects the hydrogen evolution and ingress.…”

“…has been tested under the real conditions of the open Baltic sea water. It seemed quite logical to extend the studies of the SRB effects [2,3] on the study of hydrogen charging under the real sea conditions provided by the laboratory. The design and construction of devices and their application for such investigations were the tasks of the Polish KBN grant [6].…”