Mechanism of biologically produced ennoblement: Ecological perspectives and a hypothetical model

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Abstract
Literature on biofilm‐assisted ennoblement of sea‐immersed stainless alloys is critically reconsidered in terms of the microbial ecology of biofilm‐formation, ‐succession and ‐dynamics. An evaluation is made of the mechanisms put forth by various authors, taking account of the compatibility of the suggested models with the interfacial solution chemistry data published widely. It is argued on the one hand that the biofilm/metal interface should be strictly anaerobic, thus questioning the applicability of the oxygen reduction catalysis model, while, on the other, the concept of acid biofilm pH is carefully scrutinised. The neutral pH and passivity enhancement model appears most acceptable, and elegantly explains much of the otherwise anomalous test results. Finally, it is derived from the discussion that siderophores (iron chelators), produced by biofilm microorganisms at pHs close to neutral, may act as inhibitors and bring about the passivity improvement.