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02472 PROCEDURE USED FOR ROOT CAUSE ANALYSIS: Pitting in Stainless Steel Tubes Associated with MnO2 Deposition

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Product Number: 51300-02472-SG
ISBN: 02472 2002 CP
Author: Richard W. Lutey and George M. Richardson
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The objective of this study was to develop and implement a procedure for obtaining information useful in defining the root cause of through wall pitting of SS condenser tubes on which a film of MnO2 had formed. Reports in the literature point out that pitting in stainless steel (SS) heat exchanger tubes is an increasing problem. Several mechanisms have been associated with this phenomenon. One mechanism, often overlooked, involves the relationship of deposition of manganese dioxide (MnO2) on the surface of the SS tubes. MnO2 deposition can shift the open-circuit potential (OCP) of SS several hundred millivolts towards the positive or noble direction and increase the cathodic current efficiency. The noble shift in potential can increase the OCP above a critical level, inducing degradation of the tube by pitting. When encountering conditions such as these, it raises the question - Is the deposition of MnO2 a result of microbiological growth and is the degradation microbiologically influenced corrosion (MIC)? The procedure used to answer the question is described and includes three verifying criteria, (1) visual appearance, (2) microbiological involvement at site of pitting, (3) chemical analytical data that confirms the proposed mechanism. An essential factor in defining the root cause of the failure was identifying and characterizing bacteria capable of oxidizing Mn (IV) to MnO2 and its subsequent deposition on metal surfaces.
The objective of this study was to develop and implement a procedure for obtaining information useful in defining the root cause of through wall pitting of SS condenser tubes on which a film of MnO2 had formed. Reports in the literature point out that pitting in stainless steel (SS) heat exchanger tubes is an increasing problem. Several mechanisms have been associated with this phenomenon. One mechanism, often overlooked, involves the relationship of deposition of manganese dioxide (MnO2) on the surface of the SS tubes. MnO2 deposition can shift the open-circuit potential (OCP) of SS several hundred millivolts towards the positive or noble direction and increase the cathodic current efficiency. The noble shift in potential can increase the OCP above a critical level, inducing degradation of the tube by pitting. When encountering conditions such as these, it raises the question - Is the deposition of MnO2 a result of microbiological growth and is the degradation microbiologically influenced corrosion (MIC)? The procedure used to answer the question is described and includes three verifying criteria, (1) visual appearance, (2) microbiological involvement at site of pitting, (3) chemical analytical data that confirms the proposed mechanism. An essential factor in defining the root cause of the failure was identifying and characterizing bacteria capable of oxidizing Mn (IV) to MnO2 and its subsequent deposition on metal surfaces.
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