A Comparison between Mechanical Behavior of Mild Steel in Mixed and Pure Cultures of SRB and IRB by Using Slow Strain Rate Testing (SSRT) Method

Stress corrosion cracking (SCC) is a major problem in industry, When a material susceptible to corrosion is put into service in an environment that is corrosive and tensile stresses are applied on the material, stress corrosion cracking can happen. While SCC has been known to cause corrosion for a relatively long time, so :far no experimental study of the effect of the bacteria associated with microbiologically influenced corrosion (MIC) such as sulfate reducing bacteria (SRB) and iron reducing bacteria (lRB) on SCC has been reported. This is especially true when it is also considered that research about dual effects of lRB on corrosion (both corrosion enhancing and protective) has been started recently. The "purity" of cultures used in these series of experiments was decided upon by considering the morphology ofthe species observed and not their genetic structure. While these tests and experiments were carried out on mild steel, stainless steel 316L and duplex stainless steel SAF2205 samples, this paper reports the results ofSCC tests done by slow strain rate testing (SSRT) method on only mild steel samples exposed to both microbial (biotic) and non-microbial (abiotic) environments. The biotic environment consisted of (1) a natural mixed culture of SRB, IRB and other unidentified microorganisms, (2) pure culture of SRB and (3) pure culture of lRB. The abiotic environment was synthetic seawater (3.5% NaCI). The tests in the abiotic environments were repeated in both sterile andnon-sterile environments to ensure that the chemicals used for sterility did not have inference on the corrosion failure, Further, to :find a possible mechanism, SSRT tests in abiotic environment were electrochemically subjected to hydrogen charging. In terms of time to failure (T0, all the samples used in biotic cultures failed sooner than abiotic cultures, the Tc appears to have decreased by -75% when the SSRT tests were conducted in pure lRB or SRB environments, as compared to the Tc in abiotic synthetic seawater. In mixed biotic culture, Tf decreased by 600!o in comparison with that in the abiotic solutions. With hydrogen charging, Tf has been decreased by -70% in comparison with synthetic seawater and about 30% in comparison with mixed culture biotic environment. These findings, which verify the effect ofhydrogen charging on accelerating of cracking, also provide evidence in support of hydrogen-assisted mechanism of SCC of mild steel.

Keywords:

lRB. SRB , Mild Steel , Stress Corrosion Cracking