Mónica Jazmín Hernández Gayosso

The microbial corrosion of oil and gas pipes is one of the problems occurring in the oil industry. Various mechanisms explaining microbial corrosion have been demonstrated. Commonly, biocorrosion is attributed to sulfate-reducing bacteria. Also, it has recently been reported that microbial species can connect their electron transport system to metal electrodes. In this research, two spore-forming bacteria isolated in different years from a gas pipeline were identified by biochemical techniques and by 16S rDNA amplification, sequencing, and comparison with the NCBI database. Isolates were also compared between them using molecular techniques as the restriction patterns, unique for 16S rDNA (ARDRA), and the profile of the amplified bit from the genomic DNA, using an unspecific primer (RAPD). The results obtained showed that both isolates corresponded to Clostridium celerecrescens with a 99% similarity according to the sequence reported on the NCBI database. Also, the ARDRA and RAPD electrophoretic profiles of both strains were identical, and no plasmids were found in the strains. Thus, it can be settled that this bacterium is persistent in the environment prevailing in gas pipelines. Also, it was demonstrated that the bacterial secretion of organic acids contributes to the pitting and general biocorrosion of API XL 52 steel. The rates of corrosion obtained, approximately after 40 days, were correlated with the presence and metabolic activity of C. celerecrescens on the metallic surfaces.

In this work, X-ray diffraction and Mössbauer spectroscopy techniques were used to characterise and carry out a comparison of the corrosion products obtained during the pigging activities in two sour-hydrocarbon-transporting pipelines located in the Gulf of Mexico: the first one being a gas pipeline and the other one a crude oil pipeline. The results indicate that for these specific conditions, there are differences between the corrosion products formed in each pipeline. Iron sulphides and oxides were the main corrosion products and their presence is directly related to the operating conditions prevailing in the pipelines as well as to the hydrocarbon quality. As for the sour gas pipeline, higher concentrations of greigite and pyrite were observed, whereas more proportions of magnetite, mackinawite and marcasite were found in the sour crude oil pipeline. The most important parameters in the formation of the different types of corrosion products are the water content, hydrogen sulphide concentration and oxygen presence in the system.

During the characterization of sediments formed in pipelines transporting hydrocarbons, the knowledge of the microbiological diversity becomes very interesting, especially when it is related to microbiologically influenced corrosion (MIC). The presence of microorganisms is considered as one of the factors that affect the corrosion processes occurring at the pipeline; therefore, their corrosiveness must be determined. In this way, the identification of new species affecting the MIC processes is still considered relevant. In this work, the effect of Clostridium celerecrescens upon the corrosion of API KL 52 steel was evaluated. This microorganism was isolated and identified from the sediments collected during the inner cleaning procedures of a gas pipeline. The polarization resistance (PR) and electrochemical impedance spectroscopy (EIS) techniques were considered to estimate the microorganism behavior during the corrosion process. The results were complemented with a metal surface analysis, using a scanning electron microscope (SEM). The resistance values induced by the presence of the microorganisms clearly indicated that C. celerecrescens has an effect on the corrosion process occurring at the API XL 52 steel surface.

The Electrochemical Impedance Spectroscopy (EIS), was used to evaluate and compare the behavior of three different corrosion inhibitors: quaternary amine (QA), oleic imidazoline (OI) and amino coco propionic acid (ACP), on the corrosion of mild steel in 3% NaCl solutions, saturated with CO₂ gas, under static conditions. Two pH values: 3.8 and 5.5 were considered. Under the conditions considered in this work, the results indicated film formation at the metal surface, probable due to the formation of iron carbonate. This condition modified the values of the resistance and double layer capacitance of the system. In general terms, the ACP inhibitor exhibited better performance, compared to the QA and OI. During the impedance measurements with OI and ACP, some deviations were observed at high frequencies, indicating probably that the inhibitors adsorption occurs in different way to the QA inhibitor and the amine adsorption is weaker than the other two inhibitors adsorption.