Localized Dissolution of Millscale-Covered Pipeline Steel Surfaces

Electrochemical impedance spectroscopy, corrosion potential measurements, and surface analysis by scanning electron microscopy/energy-dispersive x-ray spectroscopy (SEM/EDX) and Raman spectroscopy were used to investigate the localized dissolution of millscale-covered pipeline steel surfaces. The porous millscale originally present on the pipe surface exerts an influence on the corrosion of the pipeline and may contribute to the eventual onset of stress corrosion cracking (SCC). Three regions in the corrosion potential-time plot were observed after exposure to an aqueous environment, corresponding to the initial attempts at breakdown of the millscale, coupling of the dissolution of the underlying steel to reductive dissolution of the millscale, and active corrosion of the steel at the base of pores in the film supported by water reduction either on the metal or on the millscale surface. The corrosion rate increases as the dissolved carbon dioxide (CO₂) concentration increases. Changes in the solution resistance, polarization resistance, and pore resistance are related to the corrosion kinetics and growth of the pores. The porous structure of the millscale increases the possibility of local separation of anodic and cathodic sites, which would promote localized corrosion at the base of pores. These stress-raising pits eventually could act as precursor sites for the initiation of SCC.