I think that you can find answer to your question if you will read my papers

Thank you very much dear Dr. Pierluigi Traverso for the answer :D Very much appreciated sir.

Dear Wan,

Depending on environmental condition, combination of several mechanisms can happen. Generally, extracellular polymeric substances (EPS) produced by Sulfate Reducing Bactria have the ability to accelerate corrosion by binding with metal ions. It is also reported in some of literatures that EPS in presence of Cr3+ can also accelerate the corrosion in mild steel in presence of seawater. A dense film may be developed by EPS and sulfide film. What happens here I understand, PH is substantially decreased by SRB(Sulfate Reducing Bactria) on the steel in presence of HS- and produces a severe localized corrosion. It is also possible that galvanic corrosion generated between the anodic metal and cathode iron sulfide products. This is an aggressive phase found which has no hydrogen permeation due to barrier of anti-diffusive effect of EPS. Subsequently, Bactria grows exponentially as atomic hydrogen permeation is stopped by generating substantial hydrogen sulfide. A corrosive SRB biofilm is might have formed on an iron surface and MIC takes place since the sulfate reduction at the cathode will consume the whole droplets of electrons released by iron dissolution at the anode with the help of biocatalyst (SRB behaves like catalyst) and the interface of biofilm and the metal are a place for both anodic and cathodic sites. Possible EPS components may be involve in a colloidal dispersion of insoluble excess corrosion products.

Hope it is helpful for you.

Ashish