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98654 THE INFLUENCE OF COATING DISBANDMENT ON THE CORROSION OF COATED REINFORCEMENT - A NUMERICAL MODEL

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Product Number: 51300-98654-SG
ISBN: 98654 1998 CP
Author: R.A. Cottis
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This paper describes a sirnplified, ab initio numerical model of the oxygen transport and electrochemical reaction kinetics associated with a hole in a coated reinforcement bar. The model examines the influence of area of disbandment of the coating around the hole on the corrosion rate of the exposed steel. The significance of coating disbandment is found to depend on the concrete permeability. An important conclusion is that experiments in which the coated bar is coupled to uncoated bar can be expected to give markedly different results, as the uncoated bar (assuming that it is passive) will provide a large area cathode that negates many of the assumptions made in this model. It is also clear that coupling between coated and uncoated bar provides a very severe test of the coating integrity. The thin solution layers in the crevice are able to transfer the current produced by oxygen reduction efficiently to the anodic site at the holiday in the coating. It appears that for nominally dry concrete the oxygen permeability of the epoxy coating and the radius of disbandment of the coating will have an important influence on the total metal loss occurring, and it should have a low oxygen permeability and disbond as slowly as possible. The area of hole in the coating will be important when processes in and around the hole are rate-controlling, including situations when there is very widespread disbanding and/or the coating has a very high oxygen permeability. Corrosion occuming as a result of oxygen transport through the hole will be important when the coating is not disbonded and/or has a very low oxygen permeability.
This paper describes a sirnplified, ab initio numerical model of the oxygen transport and electrochemical reaction kinetics associated with a hole in a coated reinforcement bar. The model examines the influence of area of disbandment of the coating around the hole on the corrosion rate of the exposed steel. The significance of coating disbandment is found to depend on the concrete permeability. An important conclusion is that experiments in which the coated bar is coupled to uncoated bar can be expected to give markedly different results, as the uncoated bar (assuming that it is passive) will provide a large area cathode that negates many of the assumptions made in this model. It is also clear that coupling between coated and uncoated bar provides a very severe test of the coating integrity. The thin solution layers in the crevice are able to transfer the current produced by oxygen reduction efficiently to the anodic site at the holiday in the coating. It appears that for nominally dry concrete the oxygen permeability of the epoxy coating and the radius of disbandment of the coating will have an important influence on the total metal loss occurring, and it should have a low oxygen permeability and disbond as slowly as possible. The area of hole in the coating will be important when processes in and around the hole are rate-controlling, including situations when there is very widespread disbanding and/or the coating has a very high oxygen permeability. Corrosion occuming as a result of oxygen transport through the hole will be important when the coating is not disbonded and/or has a very low oxygen permeability.
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