ABSTRACT

A strain of sulfate reducing bacteria (SRB) was isolated from the inner deposits of a pipeline that transports sour gas in the Marine Region of Mexico. The sample was collected from the 36 Atasta - NohochA pipeline. The strain was identified by 16S rRNA sequential analysis as Desulfovibrio sp. The physiological kinetics in Posgate C media showed sulfate consumption and 30 mM sulfide production up to 250 h. The corrosion rate was evaluated with API XL 52 probes by weight loss and three electrochemical techniques: Rp, EIS and electrochemical noise. When Desulfovibrio sp. was inoculated in the Posgate C media, a reduction in the corrosion rate vs. time was measured by Rp and EIS. This behavior was attributed to the formation of a FeS film and biofilm, which form an effective diffusion barrier. On the other hand, the electrochemical noise measurements indicated that a localized corrosion process took place on the surface sample, which was confirmed by SEM analysis.

1.0 Introduction

Microbiologically influenced corrosion, MIC, is the deterioration of a metal by corrosion processes that happen directly or indirectly as a result of the metabolic activity of the microorganisms1. The prevention of MIC problems constitutes a great challenge for the petroleum industry. Further, the detection, evaluation and control of the problems originated by the activity of microorganisms are particularly difficult in petroleum production processes. There is a recognized need for technology to detect, evaluate and control MIC in a continuous manner. However, basic theory about MIC mechanisms2,3,4 and the technologies to evaluate MIC are in the development process.

Because microorganisms modify the rate of the electrochemical reactions, electrochemical techniques can be used to evaluate MIC5. These methods are useful since the behavior of the corrosion process at the metal/electrolyte interface is a direct function of the mechanisms and electrochemical kinetics. These techniques can be quick, precise and non-destructive tools in the evaluation of the corrosion process. For MIC studies6, the most consistent and useful information can be obtained from the combination of different electrochemical techniques, such as: corrosion potential, Ecorr; redox potential; polarization resistance, Rp; electrochemical impedance spectroscopy; EIS; and electrochemical noise, EN.

In the present work several electrochemical techniques were used for MIC evaluation using API XL-52 steel probes, with microorganisms isolated from the inner deposits of a pipeline that transports sour gas in the marine region of Mexico.

2.0 Experimental

The bacterial sample was collected from the inner deposits of 36 diameter pipeline Atasta-Nohoch A, that transports sour gas; in the Marine Region of Mexico. The deposits were collected under an oxygen-free atmosphere during cleaning of the inner pipeline. The bacterial sample was inoculated in API RP-38 medium7. The strain was isolated in plates with agar API RP-38. The isolation was carried out using an anaerobic chamber with a nitrogen-hydrogen atmosphere at 37ΒΊ C. In order to verify the strain purity, it was inoculated in solid and liquid nutritient media and solid and liquid glucose-peptonesulfate media. The microscopic morphology was determined in fresh preparations by differential illumination and phases contrast. The identification of the isolated microorganisms was performed by polymerase chain reaction, PCR, followed by sequential analysis of 16S rRNA gene8. The physiological characterization was carried out in an electrochemical cell in Postgate C medium during 10 days (figure 1). The bacterial growth was measured through the most probable number, MPN, of sulfate reducing

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