Enhancement of Alkyldimethylbenzylammonium Chloride and Tributyl Tetradecyl Phosphonium Chloride Biocides using D-Amino Acids against a Field Biofilm Consortium

Abstract

Biocorrosion or microbiologically influenced corrosion (MIC) is a major problem in many industries, especially the oil and gas industry. Biofilms are the culprits of MIC. In this work, D-amino acids were used to enhance two biocides, namely alkyldimethylbenzylammonium chloride (ADBAC) and tributyl tetradecyl phosphonium chloride (TTPC), to treat a tough and corrosive field biofilm consortium on C1018 carbon steel coupons. An equi-mass D-amino acid mixture (“D-mix”) of four D-amino acids (Dmethionine, D-tyrosine, D-leucine, and D-tryptophan) at a total concentration of 50 ppm (w/w) was tested. The cocktails of 60 ppm ADBAC + 50 ppm D-mix and 40 ppm TTPC + 50 ppm D-mix both achieved a 3-log reduction of the sessile cell count of sulfate reducing bacteria (SRB) in the 7-day biofilm prevention test compared with a 1-log reduction achieved by 60 ppm ADBAC and 40 ppm TTPC alone separately. In the 3-hour biofilm removal test that started with mature biofilms on C1018 carbon steel coupon surfaces, the cocktails of 150 ppm ADBAC + 50 ppm D-mix and 100 ppm TTPC + 50 ppm D-mix both achieved a 2-log reduction compared with a 1-log reduction achieved by 150 ppm ADBAC and 100 ppm TTPC alone separately. In all the tests, D-mix alone showed no log reduction. Scanning electron microscope images and confocal laser scanning microscope images supported the results.

Introduction

Biocorrosion, also known as microbiologically influenced corrosion (MIC), is a major problem in various industries, particularly in the oil and gas production industry.¹ In this industry most produced fluids are free of any dissolved oxygen, with some exceptions (e.g., injected waters in some cases). Oxygen is removed using nitrogen sparging and/or oxygen scavengers because it is very corrosive. Even in openair storage tanks, an aerobic biofilm provides a locally anaerobic environment for an anaerobic biofilm to thrive underneath it.

The electrons released by the oxidation of Fe⁰ must be absorbed by an electron acceptor (oxidant) to maintain electroneutrality. In the absence of oxygen, sulfate, nitrate, proton (at low pH) and other chemical compounds can serve as electron acceptors, leading to anaerobic corrosion. However, the reduction of some of these oxidants requires biocatalysis to proceed with an appreciable speed. Sulfate reducing bacteria (SRB) are capable of catalyzing sulfate reduction in their cytoplasm, while nitrate reducing bacteria (NRB) can catalyze nitrate reduction in NRB cytoplasm. They cause Type I MIC, ² which requires the transfer of extracellular electrons from iron oxidation to the cytoplasm. Li et al. ³ pointed out that the cross-cell wall electron transfer process is a limiting step in carbon steel corrosion by SRB. It requires an electrogenic biofilm. Planktonic cells do not perform cross-cell wall electron transfer and thus they do not cause Type I MIC. Type II MIC is caused by corrosive metabolites. ² For example, acid producing bacteria (APB) can secrete organic acids that cause locally low pH underneath their biofilms. Type II MIC’s proton reduction reaction does not require biocatalysis. This is why acid attack can occur abiotically unlike sulfate attack. Because the sessile cell density in a biofilm can be more than 10² times denser than the planktonic cell density in the bulk fluid, the pH underneath the biofilm can be much lower than the bulk-fluid pH, leading to significant acid corrosion. Thus, it is clear that in both types of MIC, biofilms are the culprits.