Effect of Oxygen on Near-neutral pH Stress Corrosion Crack Initiation under a Simulated Tape Coating Disbondment

Abstract:

A test setup that enables investigating the synergistic effects of tape coating disbondment, cathodic protection (CP) and low frequency cyclic loading is used to determine the effect of oxygen on near-neutral pH Stress Corrosion Crack (SCC) initiation. After low-frequency corrosion fatigue tests, results show that surface micro-cracks initiate under the simulated tape coating disbondments at regions with insufficient CP potentials, not more remote locations completely free from CP as commonly believed.  The presence of 1% oxygen in the composition of the purging gas has binary effects on the crack initiation under the simulated tape coating disbondment.  On one hand it increases the size of the surface micro-cracks initiated on the steel surface at locations with some but insufficient CP potentials; and on the other, it reduces the number of surface micro-cracks at regions with no CP because of an extension of the general corrosion region deep inside the simulated coating disbondment.