Corrosion behavior of X65 pipeline steel: Comparison of wet-Dry cycle and full immersion

The initial corrosion behavior of X65 pipeline steel subjected to wet-dry cyclic exposure and full immersion were investigated to determine the corrosion behavior of X65 pipeline steel in marine splash zone and below the waterline. The scanning electron microscope (SEM) test results indicated that pitting corrosion occurred under wet-dry cyclic condition whereas general corrosion occurred under full immersion. The statistical results of the measured pit depth and groove depth using 3D measuring laser microscope indicated that both of them follow a lognormal distribution rather than normal distribution. Moreover, the statistical distribution of diameter-to depth ratio of pits also followed a lognormal distribution. The corrosion kinetics under both conditions followed the empirical equation d(t) = beta t(a). The morphology of the corrosion surfaces and the cross-section of rust layer were examined using SEM and the compositions were analyzed using X-ray diffraction, which indicated that the sharp fluctuation of the peak intensity of gamma-FeOOH was caused by its reduction during the wet-dry cycling process.