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The square wave polarisation technique and mathematical/electrochemical models were combined to investigate pitting corrosion of pipeline steel X100 under cathodic protection potential fluctuations with different duty cycles (δ). Mathematical equations were derived to describe the increased potential of the double-charge layer at the steel/solution interface, which increases density from 1100 mm−2 (δ = 10%) to over 6600 mm−2 (δ = 50%). However, an electrochemical model was developed to present the formation of an amorphous corrosion product layer, which reduces the number of local anodic corrosion areas and decreases pitting density when δ is higher than 50%.