Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Metal or metal alloy
Reexamination Certificate
1999-10-29
2001-07-17
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Metal or metal alloy
C205S730000, C205S733000, C205S740000, C204S196010, C204S196060, C204S196110, C204S196120, C204S196150, C204S196160, C204S196240, C204S196260, C324S071200, C324S263000
Reexamination Certificate
active
06261439
ABSTRACT:
TECHNICAL FIELD
This invention relates to systems for protecting structures from galvanic and electrolytic corrosion and more particularly to a system for protecting structures such as underwater cable from electrolytic corrosion caused by stray electrical currents.
BACKGROUND
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.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a cathodic protection system that protects structures from electrolytic corrosion.
It is a further object of the present invention to provide means for measuring stray electrical currents and means for counterbalancing in real time the stray electrical currents to prevent electrolytic corrosion damage.
It is a further object to provide a cathodic protection system which prevents degradation of insulated and polymer protective materials on underwater or buried structures.
These and other objects of the present invention are achieved by a cathodic protection system comprising means for measuring stray electrical currents adjacent a structure to be protected, non-sacrificial anode means located adjacent to the structure, direct current power supply means having a negative terminal connected to the structure and a positive terminal connected to the anode means, and, control means for receiving a signal from the means for measuring and for varying the output from the power supply such that the structure has a controlled charge which renders the structure more negative than the anode means.
By direct monitoring of the stray currents and adjusting the structure so that it remains more negatively charged than the anode means, the stray electrical currents avoid the structure and are directed to the adjacent anode means, thus prevent electrolytic corrosion. Preferably, a reference electrode is provided and located near the structure, with a signal sent to the control means as a control signal, so that the potential difference between the means for measurement and a measurement point on the structure can be determined. Preferably, the means for measurement is a current density sensor which supplies a signal to confirm that the structure remains sufficiently more negative than the anode means for preventing electrolytic corrosion.
REFERENCES:
patent: 4689127 (1987-08-01), McAlister
patent: 5087873 (1992-02-01), Murphy et al.
patent: 5126654 (1992-06-01), Murphy et al.
patent: 5739424 (1998-04-01), Beavers
Bascom, III Earle C.
Chernienko Igor
Fedorov Vladimir
Iossel Boris
Iossel Yuri Ya.
Bell Bruce F.
Iossel Boris
Nims Howes Collison Hansen & Lackert
Sapone William J.
LandOfFree
Cathodic protection system for mitigating stray electric... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Cathodic protection system for mitigating stray electric..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Cathodic protection system for mitigating stray electric... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2568338