Measurements of double layer capacity as a criterion of efficiency of corrosion inhibition on light metals in sea-water and salt solutions with and without cathodic protection

The differential capacity of AlMg-, AlMn- and AlZn-alloys during corrosion in sea-water, with and without addition of inhibitors, was determined by means of a specially made corrosion cell using an impedance bridge. Observations were made on the surfaces of alloys, with and without protective cathodic polarization immediately after immersion and after being allowed to stand in aerated sea-water, for 1, 10, 100, 300 and 1000 d. The values obtained for the differential capacity are compared to those obtained for the corrosion rate. It was found that decreasing differential capacity, also causes the decrease of the corrosion rate. This effect can be produced either by the addition of the inhibitor or by cathodic protection. The correlation between double layer capacity and the efficiency of inhibitors and cathodic polarization is interpreted as due to the formation of passive layers with adsorbed inhibitors and oxide hydrates. Thus, it has been shown that by measuring differential capacity it is possible to determine the inhibitor efficiency and cathodic prepolarization, as well as the rate of corrosion processes.