The Study Of Localized Corrosion Mechanism Of Typical Engineering Metal Material In Seawater Containing Microorganism

Sulphate-reducing bacteria(SRB) and iron-reducing bacteria(IRB) are considered as the major corrosion bacteria in marine environment. In this thesis, corrosion behavior of typical engineering metal materials(Q235 carbon steel, 304 stainless steel and copper) in seawater containing SRB and IRB were investigated through wire beam electrode(WBE), electrochemical methods and surface analysis techniques. Corrosion mechanism models based on the results of research are proposed. This provides a great theoretic foundation for the prevention of marine microbiologically influenced corrosion(MIC). The main contents in this thesis are shown as follows:(1) The influence of SRB on copper corrosion in seawater was studied. The results demonstrate that SRB adhere onto copper surface to form biofilm due to that cuprous sulfide and EPS provide a barrier against copper toxicity. In SRB growth cycle, corrosion rate is related to metabolic activity. Corrosion rate of copper increases in exponential growth and stationary phases, and decreases in death phase. Especially in exponential growth and stationary phases, SRB metabolism decreases anodic zone area and initiates localized copper corrosion. Surface analysis results of localized area indicate that SRB may obtain electrons from copper surface directly through conductive nanometer wires, which promotes cathodic process and progress of localized corrosion. Extracellular polymeric substances(EPS) of SRB were extracted. Results indicate that copper corrosion is inhibited by EPS during short period of exposure. However, protective film of copper is destroyed due to complex reaction of Cu+ ions and EPS for a long time of immersion,which results in localized copper corrosion.(2) The heterogeneous corrosion behavior of Q235 carbon steel and 304 stainless steel in seawater containing SRB were investigated. Results indicate that heterogeneous corrosion of Q235 steel initiates during early stage of immersion in sterile medium, and is inhibited by accumulation of corrosion products in late period of immersion. In SRB media, current distributes homogeneously as a result of absorption of EPS during the early stage of immersion. However, current distribution map presents a electrochemical characteristic of big cathode and small anode during late stage of immersion, which promotes localized corrosion of Q235 carbon steel. A loose and porous biofilm structure can help to promote corrosion of Q235 carbon steel on anodic area. This is the major factor leading to inhomogeneous electrochemical distribution on Q235 carbon steel surface. After 30 days of immersion in SRB medium, current distributes inhomogeneously on 304 stainless steel surface, and anodic area is fixed. Cathodic process on cathodic area is promoted by attachment of compact biofilm, and passive film on anodic area is destroyed due to heterogeneous biofilm.(3) The study of IRB induced Q235 carbon steel corrosion demonstrates that IRB attach onto Q235 carbon steel surface and form a heterogeneous biofilm, which accelerates the corrosion rate of Q235 carbon steel. Localized corrosion is the main corrosion form after 10 days of immersion in IRB medium. Metabolism of IRB in biofilm can reduce the solid Fe(III) oxide to soluble Fe(II) ions. This process destroys protective film on Q235 carbon steel surface, results in formation of electrochemical characteristic of big cathodic area and small anodic area, and promotes localized corrosion of Q235 carbon steel.