Cathodic protection systems are known. These systems provide protection by utilizing either sacrificial anodes in electrical contact with a metal to be protected, or non-sacrificial anodes connected to the metal with a direct current applied to the metal and anode, to neutralize the damaging galvanic effects.
Cathodic protection systems are used for buried metal structures as well as underwater structures. A particular area of concern is underwater power cables. These cables are typically laid upon or buried beneath the sea bed and are exposed to seawater which is an aggressive medium that can cause corrosion damage and limit cable life.
To resist the corrosion effects caused by exposure to seawater, such cables are typically insulated with a plastic jacket, most commonly made of polyethylene. In addition, such cables may carry internally, steel armor wires to protect from physical damage.
Referring to FIG. 1, a cross section of a typical power cable is shown. This has an oil duct 1, a copper conductor 2, a conductor screen 3, an insulation layer 4, an insulation screen 5, a lead sheet 6, a multiply polymer tape 7, a copper return conductor 8, a second polymer tape 9, a second copper return conductor 10, a polymer jacket 11, a layer of polypropylene yam 12, galvanized steel and zinc armor 13 and a polypropylene yarn covering 14.
In the design of such cables, it was expected that the zinc component of the armor cable would act as a sacrificial anode to provide cathodic protection from galvanic corrosion. The particular concern was corrosive effects on the steel cable. However, it was discovered that rather than galvanic corrosion of the protective steel armor, there is a significant, previously unknown, corrosive effect that could impact cable life.
It was discovered that corrosion protection needs to be considered not only to protect the metal in the cable, but in addition to protect the plastic protective jacket. Such jackets are typically produced of polyethylene, and usually doped with conductive material such as carbon. When subjected to electrolytic currents, these conductive materials leach out and/or dissolve from the protective jacket, leaving voids that fill with seawater. If allowed to continue, seawater could penetrate the jacket and begin an attack on the copper conductors. This type of corrosive penetration is illustrated in FIG. 2.
Once identified as a potential path for shortening cable life, attention turned to the conditions under which this would occur. It was determined that such corrosion would occur in areas where current caused by electrolytic effects leaves the structure, in an area known as the anode zone. As illustrated in FIG. 3, an underwater cable 1 will pass electrolytic currents in a way which establishes a cathode zone at one end of the cable where it leaves the sea floor and an anode zone at the other end. Since the polarity of electrolytic current is important, AC currents do not generally cause electrolytic corrosion. Thus, the fact that it was a power cable was not a probable cause of such electrolytic corrosion. Upon further investigation it was discovered that stray DC currents from various sources which travel through the earth and water, enter the cable to seek a path to ground. Such stray currents may arise from the passage of electric trains in an area near where the cable is located, from welding operations, from geomagnetic induced currents as a result of tide action, and even from other cathodic protection systems which utilize DC current to protect other structures. Such stray electric currents are quite variable over time, and thus, a cathodic protective system which utilizes a fixed current, as is typically used in conventional cathodic protection systems, would not protect against these variable electrolytic effects as there is no capability for adapting to the variation in current density.
Existing technology for detecting and measuring electrical currents under water is to use a pair of reference electrodes spaced at some distance and then to measure the voltage potential between these electrodes. This approach requires that very stable reference electrodes be used. A reference electrode with good stability is characterized by having a relatively constant voltage over the expected operating range of current density to be detected. A problem with reference electrodes in general is that they all have a self potential shift depending on the type of reference electrode used. The magnitude of this potential shift is on the order of several milli-volts. The potential shift limits sensitivity of the reference electrode system because it is impossible to detect a voltage potential between a pair of electrodes less than the potential shift of the electrodes. For the purposes of controlling cathodic protection systems, this limitation can present a problem. In principal, a technique to remove the potential shift of the electrodes would involve revolving electrodes in the water in the plane parallel to the electric field being measured. Practically, this is difficult to do in water while maintaining sufficient electrode spacing to provide good sensitivity.