Biofilm Formation and Its Stages

Gradual formation of biofilms can change chemical concentrations at the surface of the metal substrate significantly because the physical presence of biofilm exerts a passive effect in the form of restriction on oxygen and nutrient diffusion to the metal surface. 

While a biofilm with a thickness of 100 xm may prevent the diffusion of nutrients to the base of a biofilm, a thickness of just 12 pm can make a local spot anaerobic enough for SRB activity in an aerobic system [35]. Active metabolism of the micro-organisms, on the other hand, consumes oxygen and produces metabolites. The net result of biofilm formation is that it usually creates concentration gradients of chemical species across the thickness of the biofilm [36]. 

It is believed that formation of exopolysaccharidic substances (EPS) could help the fragile bacteria as a survival technique to protect themselves from external factors that could be life threatening to them [20] and, perhaps, increasing then-capacity to absorb more food by expanding their surface area through the EPS. The role of the EPS material in enhancing corrosion has been emphasised [38]. 

Under biofilm, factors such as pH, dissolved oxygen, etc. may be drastically different from those in the bulk solution, resulting in a phenomenon called ennoblement which has been documented for a range of metals and alloys (e.g., stainless steel at various salinities [30, 33, 39]. 

In addition to these mechanisms, there is a mentioning of enzymatic mechanism where hydrogen peroxide (produced as a result of oxidation of glucose) can cause ennoblement of stainless steel, for more details see [42]. 

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