Sun Cheng

CHARACTERISTICS AND EVOLUTION OF THIN LAYER ELECTROLYTE ON PIPELINE STEEL UNDER CATHODIC PROTECTION SHIELDING DISBONDED COATING

External corrosion and stress corrosion cracking(SCC) have been observed in a thin layer electrolyte under disbonded coating shielding cathodic protection(CP).In this work,characteristic and evolution of the thin layer electrolyte environment(potential,pH,etc.) on pipeline steel under disbonded coatings were investigated by microelectrode technology in a simulating crevice cell at various CP levels and different atmosphere conditions(CO2and O2).The results show that pH of the thin layer electrolyte under the disbondment varies from near neutral to the value as high as 12,depending on the amount of CP current and CO2.CP current on pipeline surface initiates a high pH environment on the steel surface.Due to the CP shielding effect and a high CO2 content,a near-neutral pH environment may exist under disbonded coating.The CP effectiveness and the shielding effect of the disbondment can be estimated by measuring pH of the thin layer electrolyte under disbonded coatings.

Electrochemical Behavior of X80 Pipeline Steel in the Initial Stage of Corrosion in an Acidic Red Soil

Corrosion of X80 pipeline steel in an acidic red soil slurry was studied by means of electrochemical impedance spectroscopy(EIS) and polarization technique. The results show that in a water-saturated red soil, the EIS of X80 steel contains a capacitive arc at high frequency and a capacitive arc from the interface process in frequency region of 10-2~104Hz as well as an inductive loop appears at low frequency exhibiting processes related with adsorption or pitting nucleation;the electrode process was controlled by diffusion process after 30 d exposure; iron oxides residing in the soil tend to enhance the corrosion process of the steel, correspondingly a corrosion mechanism has been proposed.