Control for galvanic corrosion inhibiting coupling
A pipe coupling for joining two electrolyte-conveying pipes of electrochemically dissimilar metals and inhibiting galvanic corrosion resulting from the dissimilarity. A tubular pipe coupling has a pair of axially spaced protection electrodes mounted to its interior wall in electrical contact with the electrolyte. A control electrode is also mounted to the interior wall of the tubular pipe coupling between one of the pipes and the pair of protection electrodes. A current sensing circuit is connected to sense the galvanic current through the control electrode. A controllably variable source of electrical current is connected to vary the current through the protection electrodes. A negative feedback control circuit receives the sensed current as a feedback signal and controls the variable source to increase the electrical current through the protection electrodes and the electrolyte so that the sensed current is reduced thereby reducing the galvanic current in the electrolyte between the pipes. Preferably, the feedback control system has a reference input representing zero current through the control electrode and therefore drives the protection current to a magnitude that nulls the control electrode current.
1. A method for controllably inhibiting the galvanic corrosion resulting from the fluid connection of two electrolyte-conveying, dissimilar, metal pipes electrically connected to a ground, the method comprising:
(a) applying a protection current through the electrolyte between a pair of protection electrodes that are axially spaced from each other, interposed between the dissimilar pipes and in electrical contact with the electrolyte;
(b) sensing the electrical current to ground through the electrolyte and a control electrode in electrical contact with the electrolyte and located between the protection electrodes and one of the metal pipes; and
(c) adjusting the protection current to reduce the magnitude of the sensed current;
wherein the method further comprises forming the control electrode of substantially the same metal as one of the pipes and positioning the control electrode between the pipe having substantially the same metal composition as the control electrode and the protection electrodes.
2. A method in accordance with claim 1 and further comprising adjusting the protection current to a magnitude that reduces the sensed current to substantially zero.
3. A pipe coupling for joining two electrolyte-conveying pipes of electrochemically dissimilar metals and inhibiting galvanic corrosion resulting from the dissimilarity, the pipes being electrically connected to a common ground, one pipe constructed of a relatively more noble metal and the other pipe constructed of a relatively more active metal, the coupling comprising:
(a) a tubular pipe coupling having an interior wall defining an interior chamber adapted at its opposite ends for connection to the pipes and axially conveying electrolyte between the pipes;
(b) a pair of axially spaced protection electrodes mounted to the interior wall of the tubular pipe coupling for electrical contact with the electrolyte;
(c) a control electrode mounted to the interior wall of the tubular pipe coupling axially between one of the pipes and the pair of protection electrodes for electrical contact with the electrolyte, the control electrode being constructed of substantially the same more noble metal as the more noble metal pipe and is positioned between the more noble metal pipe and the pair of protection electrodes;
(d) a current sensing circuit having its input electrically connected between the control electrode and ground;
(e) a controllably variable source of electrical current having its output connected to the pair of protection electrodes; and
(f) a negative feedback control circuit having an input connected to an output of the current sensing circuit to provide a feedback signal and having an output connected to a controlling input of the variable source for applying an electrical current through the protection electrodes and the electrolyte that reduces the magnitude of the sensed current and thereby reduces galvanic current to the pipes.
4. A pipe coupling for joining two electrolyte-conveying pipes of electrochemically dissimilar metals and inhibiting galvanic corrosion resulting from the dissimilarity, the pipes being electrically connected to a common ground, one pipe constructed of a relatively more noble metal and the other pipe constructed of a relatively more active metal, the coupling comprising:
(a) a tubular pipe coupling having an interior wall defining an interior chamber adapted at its opposite ends for connection to the pipes and axially conveying electrolyte between the pipes;
(b) a pair of axially spaced protection electrodes mounted to the interior wall of the tubular pipe coupling for electrical contact with the electrolyte;
(c) a control electrode mounted to the interior wall of the tubular pipe coupling axially between one of the pipes and the pair of protection electrodes for electrical contact with the electrolyte, the control electrode being constructed of substantially the same more active metal as the more active metal pipe and is positioned between the more active metal pipe and the pair of protection electrodes;
(d) a current sensing circuit having its input electrically connected between the control electrode and ground;
(e) a controllably variable source of electrical current having its output connected to the pair of protection electrodes; and
(f) a negative feedback control circuit having an input connected to an output of the current sensing circuit to provide a feedback signal and having an output connected to a controlling input of the variable source for applying an electrical current through the protection electrodes and the electrolyte that reduces the magnitude of the sensed current and thereby reduces galvanic current to the pipes.
5. A pipe coupling in accordance with claim 3 or claim 4 wherein the interior wall of the tubular pipe coupling between the protection electrodes is an electrical insulator.
6. A pipe coupling in accordance with claim 3 or claim 4 wherein the tubular pipe coupling is constructed of an electrical insulator material.
7. A pipe coupling in accordance with claim 3 or claim 4 wherein the control circuit applies a current between the protection electrodes at a magnitude that reduces the sensed current to substantially zero.
8. A pipe coupling in accordance with claim 7 wherein all three electrodes are rings extending annularly around the interior of the tubular pipe coupling.
9. A pipe coupling in accordance with claim 8 wherein the pipes, the rings and portions of the tubular pipe coupling that are axially displaced from the rings all have a substantially cylindrical interior surface of the same internal diameter.
10. A pipe coupling in accordance with claim 9 wherein the control ring has an axial length of at least substantially two inches.
11. A pipe coupling in accordance with claim 9 wherein the control electrode is spaced from the closest protection electrode by substantially one half of said internal diameter.
12. A pipe coupling in accordance with claim 9 wherein the control electrode is spaced from the closest protection electrode by substantially 0.5 inch.
13. A pipe coupling in accordance with claim 9 wherein the control electrode is spaced from the closest pipe by at least two inches.
14. A pipe coupling in accordance with claim 7 wherein the interior wall of the tubular pipe coupling between the protection electrodes is an electrical insulator.
15. A pipe coupling for joining two electrolyte-conveying pipes of electrochemically dissimilar metals and inhibiting galvanic corrosion resulting from the dissimilarity, the pipes being electrically connected to a common ground, one pipe constructed of a relatively more noble metal and the other pipe constructed of a relatively more active metal, the coupling comprising:
(a) a tubular pipe coupling having an interior wall defining an interior chamber adapted at its opposite ends for connection to the pipes and axially conveying electrolyte between the pipes;
(b) a pair of axially spaced protection electrodes mounted to the interior wall of the tubular pipe coupling for electrical contact with the electrolyte;
(c) a control electrode mounted to the interior wall of the tubular pipe coupling axially between one of the pipes and the pair of protection electrodes for electrical contact with the electrolyte;
(d) a current sensing circuit having its input electrically connected between the control electrode and ground;
(e) a controllably variable source of electrical current having its output connected to the pair of protection electrodes; and
(f) a negative feedback control circuit having an input connected to an output of the current sensing circuit to provide a feedback signal and having an output connected to a controlling input of the variable source for applying an electrical current through the protection electrodes and the electrolyte that reduces the magnitude of the sensed current and thereby reduces galvanic current to the pipes
wherein the coupling comprises a longitudinally stacked array of diametric segments, two of the segments forming said protection electrodes and another of the segments being said control electrode, the electrode segments being electrically isolated from the other electrode segments and from said pipes by insulators interposed between the segments, the segments being fastened together end to end.