Anaerobic pitting corrosion of carbon steel in marine sulfidogenic environments

Abstract

Microorganisms find their way into fuel tanks by multiple means. For example, some fuel tanks in U.S. Navy vessels utilize seawater to replace ballast lost to burned fuel. This introduces a consortium of aerobic and anaerobic fuel-degrading organisms that form biofilms on the tank's inner walls. These biofilms create oxic gradients; allowing aerobic and anaerobic organisms to live in synergistic alliance. The presence and activity of these organisms influence degradation of fuels and corrosion of tank materials. This thesis focuses on corrosion of iron under strict anaerobic (anoxic) sulfidogenic marine environments. Manganese sulfide (MnS) inclusions are known sites of pitting corrosion in carbon steel in aqueous environments. This work investigates pitting corrosion of carbon steel under exposure to sulfate-reducing organisms in anaerobic marine environments. It was found that pitting was initiated in the boundary regions of MnS inclusions at normal pH. Corrosion in general was more pronounced in the presence of a biofilm of sulfatereducing bacteria than in abiotic sulfide medium. It is proposed that the initiation of an anodic reaction leading to the dissolution of the Fe matrix and subsequent pitting of steel in the boundary regions of MnS inclusions is due to strain exerted on the Fe matrix by MnS minerals mixed within the Fe matrix at the interface due to metallurgical processes. We discovered that on the average only 1 out of ~2200 inclusions (or ~2 pits per mm ² area) are activated to initiate pitting corrosion at normal pH. This increases very rapidly and reaches to pitting densities of ~4200 pits/mm ² at acidic conditions (pH <4). The main reason for this is because MnS inclusions start dissolving at low pH conditions via MnS+ 2H + --> H 2S + Mn ²+. This mechanism produces H 2S abiotically, which auto catalyzes the pits to grow along the long (hundreds of microns) micro-wires of MnS inclusions. At certain locations these micro-pits join together to give rise to macroscopic pitting in metals.