Pipeline Corrosion and Coating Failure under Alternating Current Interference
Abstract
The results demonstrate that the size of the defect is critical to AC corrosion of steel occurring at the defect base. Generally, circular and triangular defects are associated with the most and least negative direct current (DC) potentials as well as the largest and smallest anodic current densities, respectively. Local pits can be initiated on the steel at sufficiently high AC current densities in both high pH and neutral pH solutions. Mechanistic models are proposed to illustrate the pitting initiation in the presence of AC, where AC can affect passive film or corrosion product layer formed on the steel.
Furthermore, the effect of AC on the properties and performance of epoxy coatings was studied in a simulated soil solution and a conceptual model was developed to illustrate the mechanistic aspect of AC induced coating degradation. The applied CP can be shielded by coating disbondment through monitoring the local potential and solution pH under disbonded coating on a steel.
Then the CP shielding behavior of pipeline coatings, i.e., high density polyethylene (HDPE) and FBE, was investigated. The structure of HDPE does not change upon CP permeating test. Conversely, there are obvious changes of the functional groups in FBE. Water uptakes into the coating occur continuously with time. The CP permeation through FBE coating is time dependent.
Moreover, the effect of AC interference on the CP permeation into the disbonding crevice was investigated. At small AC current densities, AC results in an enhancement of permeation of CP current into the crevice. However, with the increase of AC current density, corrosion product generates and deposits in the solution, blocking the ionic diffusion and CP permeation.
Finally, the effects of AC interference on CP potential and performance were investigated. Results demonstrated that the shift of CP potential depends on the CP level and AC current density. No matter if the DC potential of the steel is shifted negatively or positively upon application of AC, the steel suffers from increased corrosion.
Description
Keywords
Engineering--Mechanical
Citation
Kuang, D. (2016). Pipeline Corrosion and Coating Failure under Alternating Current Interference (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/27361