Crevice corrosion beneath the holidays of a disbonded coatings 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 10-3 Ohm.m to 5 Ohm.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.
Modeling transport process and current distribution in a cathodically protected crevice
Modellierung des Transportprozesses und der Stromdichteverteilung in einem kathodisch geschütztem Spalt.
Corrosion, Houston ; 56 , 8 ; 783-793
2000
11 Seiten, 10 Bilder, 2 Tabellen, 32 Quellen
Article (Journal)
English
mathematisches Modell , mathematisches Verfahren , Spaltkorrosion , Pipeline , Beschichtung (Überzug) , Oberflächenschaden , kathodischer Korrosionsschutz , kathodische Reaktion , anodische Reaktion , Elektrochemie , Stoffübergang , Diffusion , Stromdichte , Stromverteilung , elektrische Leitfähigkeit , Erde (Boden)
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