Modeling Transport Process and Current Distribution in a Cathodically Protected Crevice

ABSTRACTCrevice corrosion beneath the holidays of a disbonded coating is a common problem on underground pipelines. In low-conductivity soils, cathodic protection has not been able to mitigate the corrosion properly because the coating acts as a shield, blocking the flow of cathodic current to the p...

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Bibliographic Details
Published in:Corrosion (Houston, Tex.) Tex.), 2000-08, Vol.56 (8), p.783-793
Main Authors: Chin, D.-T., Sabde, G.M.
Format: Article
Language:English
Subjects:
Applied sciences
Cathodic coating (process)
Cathodic dissolution
Cathodic protection
Cations
Chemical reactions
Chemical reduction
Chemical speciation
Conductivity
Corrosion
Corrosion potential
Corrosion prevention
Crevice corrosion
Current density
Current distribution
Dissolution
Distribution
Exact sciences and technology
Hydrogen ions
Hydroxyl ions
Ionic strength
Local current
Mathematical models
Metal surfaces
Metals
Metals. Metallurgy
Oxygen
Oxygen reduction reactions
pH effects
Pipelines
Protection
Soil
Steady state models
Submarine pipelines
Transport
Transport processes
Two dimensional models
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Summary:ABSTRACTCrevice corrosion beneath the holidays of a disbonded coating is a common problem on underground pipelines. In low-conductivity soils, cathodic protection has not been able to mitigate the corrosion properly because the coating acts as a shield, blocking the flow of cathodic current to the pipe surface. Cathodic protection changes the chemical environment inside the crevice. Hydroxyl ions (OH­) are produced by an oxygen reduction reaction on the metal surface and increase the pH of solution inside the crevice. OH­ also cause positively charged cations in the soil to migrate into the crevice and, thus, increase the ionic strength and solution conductivity of the crevice solution. In this study, a two-dimensional, steady-state, mathematical model was developed to predict the chemical environment changes and current/potential distribution within a cathodically protected crevice by taking into account the oxygen reduction reaction on the metal surface and the transport of Na+, Cl­, OH­, and dissolved O2 into the crevice. The model assumed that no hydrogen ion reduction or other homogeneous chemical reactions were taking place inside the crevice. Numerical results are presented for the local current density, solution potential, and concentration changes of the chemical species in the crevice for a range of solution conductivity from 10­3 -m to 5 -m. Dissolved O2 mostly was consumed by a cathodic reduction reaction on the metal surface facing the holiday opening. The remaining oxygen diffused into the crevice only to a depth of 4 to 5 times the crevice thickness. The predicted pH and solution conductivity changes agree with published experimental results of cathodic protection in low-conductivity environments.Crevice corrosion beneath the holidays of a disbonded coating is a common problem on underground steel pipelines. Underground gas and oil pipelines are protected against external corrosion by a combined application of coating and cathodic protection. The coatings usually are in the form of fusion-bonded epoxy and polyethylene tapes with an adhesive backing, which are wrapped helically on a pipe surface. Crevice corrosion occurs when coating develops pinholes and ruptures, known as holidays. The development of a holiday is accompanied by the loss of adhesion between the pipe and coating and the formation of a crevice in the surrounding area (Figure 1[a]). Water in the soil then flows through the holiday into the crevice and initiates corrosion on the
ISSN:0010-9312
1938-159X
DOI:10.5006/1.3280581