Rotating Ring-Disk Studies on the Impact of Superimposed Large Signal ac Currents on the Cathodic Protection of Steel

Abstract

Corrosion experiments with dc and ac polarisation were carried out in neutral aerated solutions using a rotating ring-disk electrode (RRDE) with a mild steel (ST 1.0037) disk and a platinum ring electrode. Combined with solution analysis and weight loss measurements, the influence of superimposed large signal ac currents on the anodic iron dissolution was studied by analysing the ac current signal at the ring electrode i R as a function of the frequency (f: 0, 5 ,16 2/3, 30, 50 Hz), the effective ac current density of the disk (i D,eff : 1, 5, 10, 15, 20 mA cm -2 ) and different cathodic dc protection current densities (i = D : -0.1, -0.5, -1.0 mA cm -2 ). Measurements were performed in sodium sulphate solutions (pH 5) and in systems where a defined carbonate layer is formed on the iron disk electrode (0.3 M NaCI, CaCO 3 + CO 2 , pH 5.8). The time average ring current density, which is proportional to the dissolution rate of the iron disk, was found to increase with increasing ac current density but decreased with increasing frequency f and cathodic dc protection current density i = D . After iR-drop correction of the maximum disk potential a unique current density potential curve of iron dissolution can be obtained, which fits all data obtained independent of the frequency and ac amplitude applied. This polarisation curve provides to determine a threshold potential for cathodic protection under ac polarisation conditions.