Method and apparatus for protection of a metallic object in an electrically conductive environment

Abstract

 A method and an apparatus for protecting a coated metallic object (2) in an electrically conductive environment, an indefinite interfering voltage source (10) being shielded from the said object by applying a controllable D.C. voltage between an auxiliary electrode (18) in the near field of said interfering voltage source and a contact pad (9) on the said indefinite interfering source (10). A measuring voltage level (16) for controlling the D.C. voltage is being sensed (17) in the near field of the metallic object. The apparatus comprises a high electric current source (12) supplying the controllable D.C. voltage.

Description

[0001] The invention relates to a method for protecting a coated metallic object in an electrically conductive environment, the said metallic object being a considerably better electric conductor than the said environment.

[0002] Further the invention relates to an apparatus for carrying out the said method.

[0003] A method as specified in the preamble is well known. An example of a conventional method is a method for cathodic protection wherein the metallic object is connected at one or more spots with a D.C. voltage source being inserted into the environment and electrically contacting same. The object of the thus connected D.C. voltage source is to compensate at least the electrochemical potential of the metal body of the object in the environment. Apparently the known method is deficient in case in the environment leakage currents occur which result in fluctuations of the electric field in the environment. Such fluctuations can put temporarily the cathodic protection in an inoperative condition. In particular cathodic protection gets lost in case the leakage currents have a high intensity and can enter into the metallic object that in its turn conducts the leakage current to the spot or spots where a D.C. voltage source is connected for cathodic protection. The thus diverted leakage current might wholly annul the compensating effect of the D.C. voltage source, and worse, it might contribute to the corrosion in the environment of the metal body of the object.

[0004] The conditions revealing this drawback of the conventional method arise in particular, in case the coated metallic object is buried adjacent a rail track equipped for electric trains. The return current of the traction motors carried by a rail can be an interfering voltage source causing a leakage current in the ground near the track. The high electric current in traction motors (at 1500V D.C. voltage up to 3000A) can cause leakage currents through a cathodic protected metallic object buried in the vicinity which currents may attain an intensity of tens of amperes. A D.C. voltage source comprised in the cathodic protection provides in quiescent conditions for a compensation with a voltage potential in the order of 1200 mV with regard to a copper/ copper sulfate element in case the metallic object is made of steel. Such a compensation voltage potential is readily annulled by a leakage current of 10A from the rail which is the interfering voltage source.

[0005] According to the present invention the above drawback is eliminated in that an indefinite interfering voltage source having a leakage field in the said environment is shielded from the coated metallic object by applying a controllable D.C. voltage between an auxiliary electrode in electrical contact with the said environment in the near field of said interfering voltage source and a contact pad on the said indefinite interfering voltage source, the controllable voltage being adjusted to a quiescent voltage level by a measured voltage level being sensed in the near field of the metallic object.

[0006] The leakage field displays in the near field of an indefinite interfering voltage source such as a railtrack for electric trains, a fixed directional pattern. To the contrary the strength of the electric field is subject to fast and large fluctuations. In such a situation according to the invention the auxiliary electrode is positioned at the spot where the potential of the leakage field of the indefinite interfering voltage source has an extreme value.

[0007] With regard to the large and fast fluctuations furthermore an apparatus for carrying out the method is characterized by a first connector means for high D.C. currents, a high current reversible and controllable D.C. source comprising a controlling means, a second connector means for high currents, and an electrode for high currents, to be connected in this order to the indefinite interfering voltage source, a series circuit arrangement of a measuring electrode body, a connector means for low measurement currents and a measurement voltage sensor and a reference voltage source, said measurement voltage sensor and said reference voltage source are capable to establish a differential voltage in the controlling means, said differential voltage being the controlling voltage for direction and power of the reversible and controllable D.C. source.

[0008] A preferred embodiment of the apparatus is characterized in that with an A.C. voltage supply to the reversible and controllable D.C. source having a frequency of 50 Hz the adjustment of the reversal of polarity of said D.C. source takes less than 0.04 s and the slope di/dt of the current control amounts to more than 50 A/s per 100 mV measured voltage difference while the resistance across the interface between the auxiliary electrode and the environment amounts to 10 ohm.

[0009] The slope amounts to maximum 200 A/s per 100 mV measured voltage difference with a 50 Hz AC voltage supply of the D.C. current source. The slope can be steeper in accordance as the frequency of the A.C. supply voltage is increased.

[0010] The invention is now elucidated by the description of an embodiment referring to a drawing.

Fig. 1 is a schematic view of a steel pipeline crossing a rail track and a second pipeline.

Fig. 2 is a diagram of an embodiment of an apparatus according to the present invention.

[0011] In fig. 1 a site is shown where a rail track 1 is crossing a coated steel pipeline 2. A second pipeline 7 is crossing the pipeline 2 at some distance from the crossing with the rail track. Leakage currents 3 leave the return rail 10 of the rail track 1 during the passage of an electric train and find a ready diversion in the pipeline through cracks in its coating. At the spot where the leakage current finds entrance blisters may arise, presumably due to the development of hydrogen gas. In view of cathodic protection-the steel body of the pipeline 2 the spot 4 thereon is connected via a D.C. voltage source 5 to an electrode 6. Thus the electrode 6 turns into an exit for the electric leakage currents in the pipeline, this effect in addition being amplified by the presence of the second pipeline 7 crossing pipeline 2 and as it were,sucking the exiting leakage current 8.

[0012] The D.C. source 5 is shown as an adjustable element, as it concerns a conventional method for cathodic protection providing an adaptation in the course of time to the local field conditions.

[0013] Fig. 2 is a diagram of an embodiment of an apparatus for carrying out the method according to the invention.

[0014] For the sake of simplicity in fig. 2 the rail track 1 and the coated pipeline 2 crossing the rail track 1 are shown in one plane. In figure 2 the arrangement for cathodic protection of the coated pipeline 2 comprising in series connection a connector means 21, a D.C. voltage source 22, a second connector means 23 and an electrode means 24, is known per se. According to the invention one end of a connector means 11 is connected electrically conducting to the contact pad 9 at a return rail 10 in the rail track 1 and the other end thereof to a reversible and controllable D.C. source 12. The D.C. source 12 is connected via a connector means 13 to an inert auxiliary electrode 18 in an electrode field 19. The two connector means 11 and 13 are suitably selected for high current intensities and also the D.C. source 12 can supply a relatively high electric current.

[0015] The measuring voltage for controlling the output current of the D.C. source 12 is sensed via two connecting leads 14 and 15 across a sensing resistor 16 which is connected in between the steel body of pipeline 2 and an exposed electrode 17 immediately adjacent to the exterior of the coating of pipeline 2. This current measurement is preferred over a voltage measurement at the same location.

[0016] As the potential of the leakage field of the interfering voltage source, that is the return rail 10, can vary both positively and negatively, it is necessary that the D.C. source 12 is reversible. It is found that the necessary arrangement of the D.C. source 12 requires an adjustment period of the reversal of polarity which is shorter than 0.04 s. Furthermore it is found that the slope di/dt of the current control of the D.C. source must amount to more than 50 A/s per 100 mV measured voltage difference. In an embodiment of the D.C. source 12 with rectifiers of an AC supply voltage having a frequency of 50 Hz a maximum slope of 200 A/s can be reached.

[0017] In an electrode field 19 an auxiliary electrode 18 is selected such that the leakage field around the return rail 10 is at least entirely eliminated on the spot of the pipeline 2. Hereto the auxiliary electrode is positioned on the spot where the potential of the leakage field of the indefinite interfering voltage source has an extreme value.

[0018] The invention is elucidated in the foregoing with an embodiment wherein a cathodic protected metallic object is exposed to leakage currents from an interfering source.

[0019] However, metallic objects not being provided with cathodic protection may be corroded by leakage currents. The method according to the invention and the apparatus for carrying out said method which is actually dynamically insulating the interfering source from the metallic object, also provide the desired protection in this case.

Claims

1. A method for protecting a coated metallic object (2) in an electrically conductive environment, the said metallic object (2) being a considerably better electrical conductor than the said environment, CHARACTERIZED IN THAT an indefinite interfering voltage source(10) having a leakage field in the said environment is shielded from the coated metallic object (2) by applying a controllable D.C. voltage (12) between an auxiliary electrode (18) in electrical contact with the said environment in the near field of said interfering voltage source (10) and a contact pad (9) on the said indefinite interfering voltage source, the controllable voltage being adjusted to a quiescent voltage level by a measured voltage level (16) being sensed (17) in the near field of the metallic object (2).

2. A method according to claim 1, CHARACTERIZED IN THAT said auxiliary electrode (18) is positioned on the spot where the potential of the leakage field of the indefinite interfering voltage source (10) has an extreme value.

3. Apparatus for carrying out the method according to claim 1 or 2, CHARACTERIZED BY

a first connector means (11) for high D.C. currents,

a high current reversible and controllable D.C. source (12) comprising a controlling means, a further connedtor means (13) for high currents, and an electrode (18) for high currents, to be connected in this order to the indefinite interfering voltage source, a series ciruit arrangement of a measuring electrode body (17), a connector means (14, 15) for low measurement currents and a measurement voltage sensor (16), and a reference voltage source, said measurement voltage sensor and said reference voltage source are capable to establish a differential voltage in the controlling voltage for direction and power of the reversible and controllable D.C. source (12).

4. Apparatus according to claim 3, CHARACTERIZED IN THAT with an A.C. voltage supply to the reversible and controllable D.C. source (12) having a frequency of 50 Hz the adjustment of the reversal of polarity of said D.C. source (12) takes less than 0.04 s and the slope di/dt of the current control amounts to more than 50 A/s per 100 mV measured voltage difference while the resistance across the interface between the auxiliary electrode (18) and the environment amounts to 10 ohm.

Drawing