ABSTRACT

The microbial induced corrosion (MIC) is a very dangerous process, which affects the oil industry, particularly the hydrocarbons extraction, transport and storage. The activity and microorganisms growth at the pipelines steel cause surface modifications, which can induced a more complex corrosion process. MIC evaluation has been normally based upon microbiological tests, and just a few references mention alternating methods, such as electrochemical techniques, which can be used as criteria for their evaluation.In this work, different electrochemical laboratory techniques were used, in order to determine the corrosion behavior of a microbial consortium, obtained from the Atasta ? Nohoch gas transporting pipeline, in the southeast of Mexico. The bacteria population growth was found to be different for sessile and plancktonic microorganisms. Moreover, long incubation times were required to reach the maximum concentration of sessile bacteria. The electrochemical techniques used in this study: EN and PR, exhibited a similar tendency on the corrosion rate behaviour with time, and values above 0.3 mmy were observed at the end of the experiments. The experiments were complemented with surface analysis. Scanning electron microscope observation of APIXL52 steel coupons, exposed to the consortium action, revealed bacteria presence, as well as a damaged steel surface. A type of localized corrosion was observed on the metal surface, and it was associated to the bacteria effect.

INTRODUCTION

The microbiologically influenced corrosion (MIC) is a very dangerous process, which affects the oil industry, particularly during the hydrocarbon extraction, transport and storage1-3. The activity and microorganisms growth at the pipelines steel may cause surface modifications, which induce a more complex corrosion process. In general, the study of the MIC has been based upon microbiological tests and gravimetric techniques, and just few references mention alternating methods, which can be used as criteria for their evaluation. The use of electrochemical techniques4-6 for the evaluation of MIC, such as polarisation resistance (PR) and electrochemical noise (EN), has been considered during the corrosion rate determination and this is an interesting area which requires more research, in order to implement and standardize their laboratory experiments and field application. Most of the study involving electrochemical techniques and MIC processes has been done considering short experimental times. Under these conditions, mainly low corrosion rates can be observed. Usually, the experimental time is established considering several aspects, such as: Kinetics growth of microorganisms in the electrolyte (plancktonic microorganisms), Sulphate consumption Sulphuric acid production However, a basic characteristic on the MIC process frequently is not considered: biofilm formation on the metal surface. Kinetics growth is different for plancktonic microorganisms and those established at the metal surface (sessile microorganisms). Once the biofilm is formed, the corrosion damage on the metal surface should be dependent mainly on the sessile microorganisms, as the plancktonic microorganisms are not in contact with the metal surface. In this study, the PR and EN techniques were used to evaluate the corrosion rate of carbon steel in presence of plancktonic and sessile microorganisms, in order to determine the influence of both: plancktonic and sessile microorganisms, on the MIC processes.

EXPERIMENTAL

Microorganisms cultivation

The consortium used in this work was taken from a gas pipeline located in the Marine Region of Pemex, in México, during the inner cleaning procedure. The collected samples were inocul

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