Submarine AC cables generally have a metallic armor, constituted by one or two layers of wires or strips wound with long lay on the outside of the cable. The armor is required in order to sustain part of the mechanical stresses during the laying and recovery of the cable, since the conductor alone as a rule is not sufficient for this purpose. The armor also plays an important role in the conduction of the return current. Moreover, due to the large distance between the cables in usual submarine laying practice, the return current can be very large, approaching the central conductor current value. The armor as a rule is not insulated but is in electric contact with the sea along all the cable length. This is not due to electric reasons, since the e.m.f. between armor and sea is neligible, but to practice ones. In fact it is quite difficult to assure a complete protection along all the armor length, without impairing it during the laying operations, and a single break in the insulation, would be a very strong corrosion point. The contact between the metal which constitutes the armor, galvonized steel in the case of Mexico, and the sea, in presence also of AC electric fields, requires a thorough study of the corrosion problems which might arise. Such a study is given in this communication. It includes an experimental investigation on the behavior of Zn-22 Al and galvanized steel wires in sea water under AC. In particular, it is demonstrated that Zn-22 Al has an optimum corrosion rate of 15.86 mm/year versus 29.98 mm/year for galvanized steel at a fixed alternating current density. A mechanism is proposed to explain this behavior mainly based in the presence of Al as alloy element in Zn-22 Al and the formation of a film, Al2O3xH2O.
Keywords: alternating current, Zn-22 Al, galvanized steel, armor corrosion, sea water, AC corrosion.