Effect of pH regulation by sulfate-reducing bacteria on corrosion behaviour of duplex stainless steel 2205 in acidic artificial seawater

Abstract

Sulfate-reducing bacteria (SRB) can regulate environmental pH because of their metabolism. Because local acidification results in pitting corrosion, the potential capacity of pH regulation by SRB would have important consequences for electrochemical aspects of the bio-corrosion process. This study focused on identifying the effect of pH on the corrosion of duplex stainless steel 2205 in a nutrient-rich artificial seawater medium containing SRB species, Desulfovibrio vulgaris. Duplex stainless steel samples were exposed to the medium for 13 days at 37°C at pH ranging from 4.0 to 7.4. The open-circuit potential value, sulfide level, pH and number of bacteria in the medium were recorded daily. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization were used to study the properties of the biofilms at the end of the experiments and the corrosion behaviour of the material. Inductively coupled plasma mass spectrometry was used to measure the concentration of cations Fe, Ni, Mo, Mn, Cr in the experimental solution after 13 days. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDX) were used for surface analysis. The results showed the pH changed from acidic values set at the beginning of the experiment to approximately pH 7.5 after 5 days owing to bacterial metabolism. After 13 days, the highest iron concentration was in the solution that was initially at pH 4 accompanied by pitting on the stainless steel. Sulfur was present on all specimens but with more sulfur at pH 4 in the EDX spectra. EIS showed the film resistance of the specimen at pH 4 was much lower than at pH 7.4 which suggests the corrosion resistance of the stainless steel was better at higher pH. The results of this study suggest that the corrosion process for the first few days exposure at low pH was driven by pH in solution rather than by bacteria. The increasing pH during the course of the experiment slowed down the corrosion process of materials originally at low pH. The nature and mechanism of SRB attack on duplex stainless steel at different acidic environments are discussed.

Keywords: microbiologically influenced corrosion; pH; stainless steel; sulfate-reducing bacteria.

Figure 1.

OCP values of DSS 2205 coupons for the duration of the experiment at different starting pH.

Figure 2.

Dissolved sulfide level versus time data for samples with different starting pH.

Figure 3.

Changes in the concentration of SRB at each pH.

Figure 4.

Variation of pH during exposure time for different starting pH values.

Figure 5.

Concentration of metal ion in different pH solutions.

Figure 6.

Nyquist plot and corresponding Bode plot of each specimen after 3 day's exposure (a,b), and 13 day's exposure (c,d), based on the equivalent electric circuit model given in (a,c).

Figure 7.

Potentiodynamic polarization curve of samples in different pH environment after 3 days (a) and 13 days (b) exposure.

Figure 8.

SEM images of specimens in different pH samples: (a) at pH 4, (b) at pH 5, (c) at pH 6, (d) at pH 7.4 and EDX spectra of specimens in different pH samples: (e) at pH 4, (f) at pH 5, (g) at pH 6, (h) at pH 7.4.

Figure 9.

Pourbaix diagram for Fe with superimposed sulfate reducing process.