Yan Maocheng

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.

STRESS CORROSION CRACK INITIATION BEHAVIOR FOR THE X70 PIPELINE STEEL BENEATH A DISBONDED COATING

Stress corrosion cracking (SCC) has become one of the main threats for pipelines safe. The crack initiation is the first step for pipeline SCC. The disbonded coating affects the SCC initiation behavior. In the present paper, the effect of cathodic protection potential on SCC initiation for X70 pipeline steel in the NS4 solution beneath a disbonded coating is studied by a cyclic loading test. The SCC initiation situation was observed at the different distances beneath a disbonded coating area by SEM. The results show that the SCC initiation degree lightens with the increase of distance from the disbonded coating area under the condition of -850 mV cathodic protection potential. When the cathodic protection potential increases to -1000 mV, the SCC initiation degree slightly increases with the increase of distance from the disbonded position. But compared with that under the -850 mV, the crack initiation degree under -1000 mV is lower at the corresponding position. It is shown that the coating disbondment reduces the cathodic protection effect, and in order to obtain the same cathodic protection effect the cathodic protection potential should be appropriately moved in an negative way.