Corrosion due to ac interference

The effects of alternating currents are much less of a corrosion danger than those of direct currents. Experiments on steel have shown that during the positive half wave [34-37] only about 1% contributes to the dissolution of iron according to Eq. (2-21). The remaining 99% is involved in the discharge of capacitances, of redox systems (e.g., Fe /Fe in surface films) or in the evolution of Oj by 

From the data in Table 2-1 this results in a corrosion rate for Fe of 0.1 mm a for an effective ac current density of 20 A m l Thus only ac current densities above 

50 A m are serious. Frequency has an effect which, however, in the region 16 to 60 Hz is small. Generally the danger increases with falling frequency [34]. 

Even with the superposition of the ac with a cathodic protection current, a large part of the anodic half wave persists for anodic corrosion. This process cannot be detected by the normal method (Section 3.3.2.1) of measuring the pipe/soil potential. The IR-free measurable voltage between an external probe and the reference electrode can be used as evidence of more positive potentials than the protection potential during the anodic phase. Investigations have shown, however, that the corrosion danger is considerably reduced, since only about 0.1 to 0.2% contributes to corrosion. 

The action of effects in the environment and cathodic current densities on ac corrosion requires even more careful investigation. It is important to recognize that ac current densities above 50 A m can lead to damage even when the dc potential is formally fulfilling the protection criterion [40]. 

---