Friend or Foe? The unintended consequences of L-Cysteine in microbiologically influenced corrosion experiments

Abstract

A key challenge in microbiologically influenced corrosion (MIC) laboratory-based research is the limited comparability of test results generated across different laboratories. This is due, in part, to variations in experimental protocols, including differences in how oxygen levels are controlled in test solutions. L-cysteine, for example, has been widely used to help maintain anaerobic conditions by reducing dissolved oxygen, yet its direct impact on corrosion behavior remains largely unexamined. This study investigates the effects of L-cysteine on MIC of carbon steel by sulfate-reducing bacteria (SRB). Anaerobic immersion tests were conducted to compare corrosion rates, pit depths, and bacterial activity in biotic and abiotic systems with and without L-cysteine. It was found that L-cysteine addition led to markedly higher SRB growth and corrosion rates, even though dissolved oxygen concentrations in solutions tested with and without L-cysteine remained within the same range over time. These results suggest that L-cysteine may enhance MIC through metabolic pathways unrelated to oxygen scavenging, such as increased sulfur availability for sulfide production and/or alterations to the biofilm microenvironment. This work challenges the conventional view of L-cysteine as a simple oxygen scavenger and underscores the need for standardized MIC testing protocols that account for its multifaceted biological and chemical effects.