ABSTRACT

Corrosion of pipes made of stainless steel in the process water distribution system of a leather processing plant was investigated by various analytical and electrochemical methods. The attack was identified as MIC by manganese oxidizing microorganisms. The preferential attack at the heat tinted welds is explained by the potential shift caused by biomineralized MnO2, preventing repassivation.

INTRODUCTION Stainless steel (SS) is the material of choice in many systems carrying fresh water. In the passive state, the material exhibits practically no corrosion rate, resulting in a virtually unlimited lifetime, eliminating the need for further corrosion protection and keeping the water free of undesired corrosion products. However, passivity is vulnerable to breakdown by chloride ions, possibly inducing phenomena like pitting or crevice corrosion. Besides the chloride concentration, the corrosion potential determines the probability of corrosion initiation and repassivation, respectively.

Normally, the corrosion potential is determined by the oxygen dissolved in the water. However, additional redox systems may influence the potential. Biofilms at the metals surface may provide such redox systems and a potential shift in the anodic direction is frequently denoted .Ennoblement.. In particular, manganese oxidizing microorganisms (MOMOs) have been identified as a group of organisms that may cause .Ennoblement. by biomineralizing dissolved Mn2+ as manganese oxides and hydroxides in a higher oxidation state, i.e. MnO2 and related compounds1,2. In the present view3, the role of such biomineralized MnO2 in corrosion processes is governed by its well known abiotic _ electrochemical properties. It may act as a strong oxidant by itself and, moreover, serve as a catalyst for oxygen reduction.

A number of case histories related to MIC by MOMOs have been reported so far4-8. In continuation of our work, a recent case from a leather processing plant is presented here. The results of various investigations will be reported and discussed below with respect to the practical consequences.

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