WO2008110625A1

WO2008110625A1 - Apparatus for measuring sacrificial anode wear

Info

Publication number: WO2008110625A1
Application number: PCT/EP2008/053101
Authority: WO
Inventors: Luigi Pardini
Original assignee: Caen Rfid S.R.L.
Priority date: 2007-03-15
Filing date: 2008-03-14
Publication date: 2008-09-18
Also published as: ITFI20070062A1

Abstract

The object of the present invention is an apparatus for measuring sacrificial anode wear comprising at least one sacrificial anode (10) associated to the material the corrosion phenomenon of which is intended to be evaluated; at least one device (11) adapted to measure voltage and current associated to said sacrificial anode (10); at least one electronic processor (12) associated to said device (11) adapted to measure voltage and current, characterized in that said at least one device (11) adapted to measure voltage and current associated to said sacrificial anode (10) comprises in turn an appropriate supplying device (13) and at least one data logger type device (14) adapted to acquire current and voltage samples from said at least one sacrificial anode (10), store them and make them available for possible processing.

Description

APPARATUS FOR MEASURING SACRIFICIAL ANODE WEAR Field of the invention

The present invention relates to the field of corrosion prevention devices, specifically in the domain of metal boat hulls. State of the art

One of the following three phenomena may occur when a metal is contacted with an aggressive liquid solution (also named electrolyte):

1. Metal corrosion (or oxidation): some atoms on the surface of the metal in direct contact with the electrolyte solution loose one or more electrons, turning into ions (either positively or negatively charged atoms) and then dissolving. This causes the gradual wear of the metal.

2. Metal immunity: the metal is not subjected to corrosion at all, i.e. the number of atoms forming the metal remains constant.

3. Metal reduction: the metal ions in the electrolyte solution are deposited on the surface of the metal itself.

The occurrence of one of these three phenomena depends on the potential E taken by the metal in the surrounding electrolyte with respect to the equilibrium potential Eeq. If E > Eeq, the metal looses ions and oxidizes. If E = Eeq, the metal is not worn and its mass remains constant. If E < Eeq, the metal is subjected to a reduction reaction, thus its mass increases, although in general the increase of mass may be considered negligible. The equilibrium potential Eeq of a metal immersed in an electrolyte may be calculated by means of the Nernst equation: Eeq = EO + K ^* Log(C). EO is the standard potential of the metal, K is a constant and C is the concentration of metal ions in the electrolyte. In order to measure the potential E of the metal in the electrolyte solution, a reference electrode such as the hydrogen electrode (indicated by the acronym SHE) or the silver/silver-chloride electrode (indicated by the acronym AAC) must be used. If the AAC reference electrode is used and the value provided by the voltmeter is 1.3 V, the potential E of the metal immersed in the solution is 1.3 V with respect to the AAC electrode. It is thus apparent that in order to prevent metal corrosion, the potential E of the metal must be lower than Eeq. Cathodic protection is a corrosion prevention technique which may be applied to metal materials contacted with aggressive environments, such as the sea. Such a technique is implemented by making a direct current circulate between an electrode (anode) placed in the aggressive environment and the surface of the metal structure to be protected (cathode). Such a current (consisting of a flux of electrons) causes the lowering of the potential E of the metal under protection, thus reducing corrosion until it stops. Two types of cathode protection are defined according to the method used to circulate the current: galvanic (or sacrificial) anode and impressed current protection. In the so-called impressed current technique, an electromotive force generator is used, the positive pole of which is connected to an appropriate current disperser consisting of an insoluble anode, while the negative pole is connected to the structure to be protected. Being negatively charged, the electrons which flow from the generator onto the metal under protection lower the potential E thereof, so as to either reduce the corrosive process or stop it if E drops below Eeq. These electrons are then neutralized by the cathode reaction which occurs on the surface of the metal. This reaction depends on the solution in which the metal is immersed and, in the case of seawater, is represented by the oxygen reduction reaction by means of which the oxygen atoms dissolved in the sea capture the excess electrons on the surface of the metal. The potential value E at which the corrosion process is blocked is named protection potential and varies from metal to metal. The value of the current, delivered by the generator, which allows the potential of the concerned metal to reach the protection value, is named protection current. Such a current is proportional to two factors: to the concentration of oxygen dissolved in the sea and to the surface of the metal structure intended to be preserved from corrosion. The electrons supplied by the generator to the metal in order to reduce its potential and stop the corrosion process are captured and neutralized by the oxygen atoms in the sea and must be continuously replaced in order to maintain the potential of the metal under the protection value. It is thus apparent that by increasing the concentration of oxygen dissolved in the water, the number of electrons which are neutralized in one second increases and, consequently, the number of electrons which the generator must supply in a second, (that is the protection current value) increases. Similarly, since the electrons which flow from the generator onto the metal are uniformly arranged on its surface, a wider area promotes (and thus increases) their capturing by the oxygen atoms, causing an increase in the protection current. In the so-called galvanic (or sacrificial) anode technique, a galvanic coupling must be established between the metal to be protected and a less noble metal, which is named galvanic anode. In order to obtain the galvanic coupling it is sufficient to electrically connect the metal to be protected to the less noble metal by means of an electric conductor or by reciprocally contacting the surfaces of the two metals. If a metal A is less noble than a metal B, by immersing the two metals in the same electrolyte solution, the potential EA taken by the metal A will be lower than the potential EB taken by the metal B, obviously both potentials are intended measured with respect to the same reference electrode. Both metals oxidize if they are not electrically connected, but if they are connected the following phenomenon occurs. The metal A continues to oxidize but since EB > EA the electrons produced on the surface of the metal A by effect of its oxidation are transferred onto the surface of the metal B. This lowers the potential EB, reducing or even stopping the corrosion process of the metal B, if EB drops below the protection potential value of the metal B. The electrons which flow from the metal A onto the metal B are then neutralized by the oxygen reduction reaction which occurs on the surface of the metal B. The potential EB is subjected to a lowering depending on the EA value. The more the EA is negative, the more EB is reduced. Therefore, in the case of galvanic anode protection, the protection current is delivered by the less noble metal, which for this reason is named galvanic anode or sacrificial anode. If two metals not different enough in terms of oxidation- reduction potential are used, the galvanic anode protection may not be very effective. For example, by using gold as nobler metal and silver as less noble metal, the gold will continue to corrode even if the two metals are electrically connected. This occurs because the silver only slightly reduces the potential E of gold and is not able to make it reach the protection value, thus the corrosion process of gold is only slowed down. In order to fully stop the corrosion phenomenon of gold, a metal even less noble than silver must be used, e.g. copper, which causes a greater decrease in the gold potential. Caution is however needed to prevent an excessive potential reduction of the metal to be protected, otherwise corrosion is stopped but the previously described negative effects are generated. For example, again in the case of the protection of gold, it is not advisable to use aluminium because it excessively reduces the potential of gold. The two foregoing techniques both display advantages and disadvantages: in the case of the impressed current method the advantages are:

- it may be designed for a wide range of voltages and currents;

- each anode may deliver a high current; - the voltage and the current may be varied;

- the protection system may be easily inspected. On the other hand, the main disadvantages are:

- it causes problems of interference;

- it is subject to generator breakdowns; - it requires checks by technical personnel.

In the case of the galvanic anode protection method, instead, the advantages are the following:

- no electromotive force generator is needed;

- no current adjustments are needed; - it is easy to install;

- it does not cause interference problems;

- the anode installation is not expensive. On the other hand, the disadvantages are:

- the delivered current is low; - bare structures require many anodes;

- frequent checks are not possible.

Considering the advantages and the disadvantages of both types of cathodic protection, galvanic anode protection is commonly used on small- to medium-sized boats where the required current is generally low and the number of anodes to be installed is small, while impressed current protection is used for large-sized ships (such as, for example, oil tankers) and underground metal structures. The galvanic anode protection method is simpler and more cost-effective to implement, requires a generally low protection current and a small number of anodes to be installed but it suffers from the major drawback that frequent checks to verify its correct operation are not possible. Indeed, the only way to understand whether the anode protection system is working effectively is to observe the metal parts of the boat which are immersed in the sea. If these are intact, then it means that the protection is effective, but if they are corroded, it means that something in the protection mechanism is not working as it should. Similarly, the sacrificial anodes applied to the hull of the boat must be directly observed in order to verify the wear thereof. This however may only be performed by a scuba diver or by raising the boat above the waterline. Both these operations however may be only occasionally performed and, therefore, there is the risk of realizing that the anode protection system is not working properly when the damages caused by the corrosion are already significant. It is thus apparent the need to make the verification of the wear of the sacrificial anodes and the correct operation of the protection system of the boat possible at any time, for example, by simply observing the monitor of a PC. This will allow to increase the frequency of the checks, greatly reducing the possibility of failure caused by anode protection faults. Summary of the invention

It is the object of the present invention to provide an apparatus for measuring sacrificial anode wear comprising at least one sacrificial anode (10) associated to the material the corrosion phenomenon of which is intended to be evaluated; at least one device (1 1 ) adapted to measure voltage and current associated to said sacrificial anode (10); at least one electronic processor (12) associated to said device (1 1 ) adapted to measure voltage and current, characterized in that said at least one device (1 1 ) adapted to measure voltage and current associated to said sacrificial anode (10) comprises in turn an appropriate supplying device (13) and at least one data logger type device (14) adapted to acquire current and voltage samples from said at least one sacrificial anode (10), store them and make them available for possible processing. Brief description of the drawings Fig. 1 shows a block diagram of the apparatus according to the present invention. Fig. 2 shows a preferred structure of said data logger type device. Fig. 3 shows a first example of the application of the apparatus according to the present invention. Fig. 4 shows a second example of the application of the apparatus according to the present invention.

Fig. 5 shows a third example of the application of the apparatus according to the present invention. Detailed description of the invention The apparatus according to the present invention works to determine the weight variation lost by a sacrificial anode by effect of corrosion. This allows to know when a galvanic anode is worn out, in order to duly replace it, because it is no longer capable of performing its protective function. The corrosion to which a sacrificial anode is subjected depends on the conditions of the environment in which it is located, if these change so does the entity of the corrosion, which is therefore not constant in time. Among the factors which may affect the corrosion speed, there are the concentration of oxygen, the salinity of the sea, the temperature and the presence of possible eddy currents. In order to evaluate the corrosion of sacrificial anodes, the amount of matter that the electrodes themselves have lost in proportion to the protection current they have generated is evaluated. By calculating the integral of the trend of the current delivered by the electrodes in a given time range, an amount of charge Q produced by the galvanic anode in the same time range is obtained. Such a charge Q consists of the electrons released by the atoms of the metal which dissolve, i.e. by the atoms responsible for the corrosion itself of the metal by which the anode is formed. The number of lost electrons is a constant electrochemical feature for atoms of the same kind and is thus always the same for equivalent atoms (i.e. belonging to the same metal), and is named valence (nv). Therefore, by dividing the number of electrons which form the charge Q by the valence nv, the number of atoms that dissolve in the considered time range is obtained. As these atoms are the ones which determine the corrosion of the anode, by multiplying their number by the atomic weight (ma) of the metal forming the anode, the weight variation experienced by the anode in the considered range of time is indeed obtained. Thus, to determine the weight variation of a sacrificial anode in a certain time range, it is necessary to know the current delivered by the anode in the aforesaid range and apply the following formula: Vp = (Q / nv ^*e-)^* ma

Wherein:

Vp = weight variation of the anode in the time range (t1 , t2); Q = charge delivered by the anode in the time range (t1 , t2); nv = valence of the metal forming the anode; e- = charge of the electron ; ma = atomic weight of the metal forming the anode.

In practice, it is not convenient to determine the exact time trend of a magnitude, such as the protection current, which continuously varies, but it is preferable to calculate a finite number of samples. Therefore, instead of computing an integral, the following summation is used to calculate the value of Q:

Q = [(M + I2) / 2] ^* ΔH + [(l2 + l3) / 2] ^* Δt2 + ... + + [(li + li+1 ) / 2] ^* Δti + ... + [(ln-1 + In) / 2] ^* Δtn.

wherein the term Ii is the value of the i-th sample of the protection current in case, while Δti is the time range which elapses between the samples Ii and li+1. The number of samples of current which have been determined in the considered time range is assumed to be equal to n. The error made in the calculation of Q by using the summation instead of the integral decreases as the number of samples of current which are taken in the considered range increases, and thus may be minimized by using an appropriate number of samples as desired. Determining the weight variation of a sacrificial anode is useful to know when the anode is worn out and needs to be replaced, but does not allow to establish whether the protection system is working properly. The fact that a galvanic anode is being worn is a necessary but not sufficient condition to protect the hull of a boat from corrosion. If for any reason, the electrical connection between hull and protection anode is interrupted, for example, the anode will continue to wear, but at this point the hull will also start corroding, because it is not longer protected. Similarly, if the protection current delivered by the sacrificial anode is lower than required by one of the various metal parts of the ship, the corrosion process will only be slowed down but not stopped. As a metal immersed in the sea (or in any electrolyte solution) corrodes only if its potential is higher than a certain precise value, known as protection potential, in order to know whether any of the metal components of a boat is well protected from corrosion, it is necessary to measure its potential and compare it with the protection potential. If the protection potential is the highest, then the component in case is immune from corrosion. In conclusion, the apparatus according to the present invention exploits the measurement and the subsequent processing of samples of current and voltage related to the sacrificial anodes to calculate the weight loss thereof and thus the operating efficiency. Fig. 1 shows a block diagram of the apparatus according to the present invention. There is at least one sacrificial anode 10 associated to the material the corrosive phenomenon of which is intended to be avoided; at least one device 1 1 adapted to measure voltage and current associated to said sacrificial anode 10; at least one electronic processor 12 associated to said device 1 1 adapted to measure voltage and current. Said at least one device 1 1 adapted to measure voltage and current associated to said sacrificial anode 10 comprises in turn an appropriate supplying device 13 and a data logger type device 14 adapted to acquire current and voltage samples from said at least one sacrificial anode 10, store them and make them available for possible processing. Said electronic processor 12, preferably consisting of a personal computer, is equipped with a software program adapted to plot graphs of the trend of said current and voltage samples with respect to time and to process the weight loss of said sacrificial anode. Furthermore, each of said data logger type devices will preferably be installed near said sacrificial anode to be monitored. The presence of a reference electrode is essential in order to measure the voltage on a metal structure immersed in the sea (or in any other electrolyte solution). It must be immersed in the sea and must be arranged as close as possible to the area in which the voltage is intended to be detected, so that the measurement is accurate. Therefore, in order to make the required measurements, two separate tools are generally needed, an ammeter and a voltmeter. It would be appropriate to perform both voltage and current measurements with a single instrument. Indeed, this would considerably reduce the costs of the manufactured product. With reference to accompanying Fig. 2, the preferred structure of said data logger type device is such to include at least one voltmeter 20, at least one A/D converter 21 , at least one memory 22 and at least one microcontroller 23. Said voltmeter 20 is the instrument adapted to make the necessary current and voltage measurements related to said sacrificial anodes according to one of the following preferred embodiments.

In the first embodiment, shown in accompanying Fig. 3, said sacrificial anode 10 is further associated to a reading resistor 30, and to a switch 31 and to a diverter 32. Said reading resistor 30 is connected with a terminal to the metal the corrosion of which is intended to be prevented 33 (e.g. the hull of a boat) and with the other is connected to a first terminal 34 of said switch 31 and to a first side terminal 35 of said diverter 32. The second side terminal 36 of said switch 31 , along with the second side terminal 37 of said diverter 32, are connected to said sacrificial anode 10, arranged near the metal the corrosion of which is intended to be prevented but electrically insulated from it. The voltmeter 20 is connected between said metal the corrosion of which is intended to be prevented 33 (along with an end of the reading resistor 30) and the central terminal 38 of said diverter 32. When no measurements are performed, the switch 31 must be closed so that the sacrificial anode 10 is connected to the metal the corrosion of which is intended to be prevented, which in figure 3 is represented by the hull, while the position of the diverter 32 is not relevant.

In order to measure the current delivered by the galvanic anode, the switch 31 must be closed, while the diverter 32 must have the central terminal 38 connected to its first side terminal 35. In this manner, the protection current crosses the reading resistor 30, generating a voltage drop which is detected by the voltmeter 20. In order to perform the voltage measurement, instead, the switch 31 must be open and the diverter 32 must have the central terminal 38 connected to its second side terminal 36, so that the sacrificial anode 10 is no longer connected to the hull but is connected to one of the terminals of the voltmeter 20, and may thus be used also as reference electrode for the measurement to be performed.

In the second embodiment, shown in accompanying Fig. 4, each of said sacrificial anodes 10 is associated to a single reference electrode 40, also associated to the metal the corrosion of which is intended to be prevented, and to a reading resistor 30 and to a diverter 32. In this case, the reading resistor 30 is connected with one terminal to the metal the corrosion of which is intended to be prevented 33 and with the other to a first side terminal 35 of said diverter 32 and to the sacrificial anode 10. The second terminal 36 of said diverter 32 is connected to said reference electrode 40. The voltmeter 20 is connected between said metal the corrosion of which is intended to be prevented 33 (along with an end of the reading resistor 30) and the central terminal 38 of said diverter 32.

When the diverter 32 is closed on its first side terminal 35, the voltmeter 20 detects the voltage drop on the reading resistor 30 caused by the protection current generated by the sacrificial anode. On the other hand, when the diverter 32 is closed on its second side terminal 36, the voltage with respect to the reference electrode 40 of the metal the corrosion of which is intended to be prevented 33 is measured.

In the third embodiment, shown in accompanying Fig. 5, each of said sacrificial anodes 10 is associated to a single reference electrode 40 - also associated to the metal the corrosion of which is intended to be prevented - to a reading resistor 30 and to a diverter 32. Also in this case, the reading resistor 30 is connected with one terminal to the metal the corrosion of which is intended to be prevented 33 and with the other to a first side terminal 35 of said diverter 32 and to the sacrificial anode 10. The second side terminal 36 of said switch 31 is connected to said common reference electrode. The voltmeter 20 is connected between said metal the corrosion of which is intended to be prevented 33 (along with an end of the reading resistor 30) and the central terminal 38 of said diverter 32. The only difference which respect to the previous case consists in using a single common reference electrode which should be positioned as equally distanced as possible from the used sacrificial anodes. Considering the greater distance which separates the reference electrode from the area on which the voltage is measured, the measurement will be much less accurate with respect to the previous case. Since the voltages and the currents to be measured may vary respectively from a few millivolts to some Volts and from approximately 1 mA to currents higher than 1 Ampere, said A/D converter 21 will preferably have an eleven bit resolution. Said memory 22 is used to store the results of the acquisitions of the A/D converter 21 and its capacity only depends on the number of acquisitions to be made. In order to prevent information loss due to an unexpected power interruption of the data logger, a flash type EEPROM memory is preferably used in the present invention. Finally, said microcontroller 23 will be preferably provided with a serial peripheral interface (SPI) 24 by means of which the communication with said A/D converter 21 and with said memory 22 will be managed. It will be preferably provided with a universal asynchronous receiver-transmitter (UART) 25 for managing the communication with said electronic processor 12, by means of an appropriate serial communication interface 26 preferably based on the RS-485 standard or on the MOD BUS standard.

Claims

1. An apparatus for measuring sacrificial anode wear comprising at least one sacrificial anode (10) associated to the material the corrosion phenomenon of which is intended to be evaluated; at least one device (1 1 ) adapted to measure voltage and current associated to said sacrificial anode (10); at least one electronic processor (12) associated to said device (1 1 ) adapted to measure voltage and current, characterized in that said at least one device (1 1 ) adapted to measure voltage and current associated to said sacrificial anode (10) comprises in turn an appropriate supplying device (13) and at least one data logger type device (14) adapted to acquire current and voltage samples from said at least one sacrificial anode (10), store them and make them available for possible processing.

2. An apparatus according to claim 1 , characterized in that said electronic processor (12) is equipped with a software program adapted to plot graphs of the trend of said current and voltage samples with respect to time and process the weight loss of said sacrificial anode (10) to be monitored.

3. An apparatus according to claims 1 - 2, characterized in that said electronic processor (12) preferably consists of a personal computer.

4. An apparatus according to claims 1 - 3 characterized in that said data logger device (14) is such so as to comprise at least one voltmeter (20), at least one

A/D converter (21 ), at least one memory (22) and at least one microcontroller (23).

5. An apparatus according to claims 1 - 4, characterized in that said at least one data logger type device (14) is installed near said sacrificial anode (10) to be monitored.

6. An apparatus according to claims 1 - 5, characterized in that said sacrificial anode (10) is further associated to a reading resistor (30), to a switch (31 ) and to a diverter (32), said reading resistor (30) being connected with a terminal to the metal the corrosion of which is intended to be prevented (33) and with the other to a first terminal (34) of said switch (31 ) and to a first side terminal (35) of said diverter (32); said second side terminal (36) of said switch (31 ), along with the second side terminal (37) of said diverter (32), being connected to said sacrificial anode (10), arranged near the metal the corrosion of which is intended to be prevented but electrically insulated from it; finally, said voltmeter (20) being connected between said metal the corrosion of which is intended to be prevented (33) and the central terminal (38) of said diverter (32).

7. An apparatus according to claims 1 -5, characterized in that it comprises at least one reference electrode.

8. An apparatus according to claim 7, characterized in that said at least one reference electrode (40) is associated to only one of said at least one sacrificial anode (10) and to the metal the corrosion of which is intended to be prevented.

9. An apparatus according to claims 7 - 8, characterized in that each of said sacrificial anodes (10) is further associated to a reading resistor (30) and to a diverter (32), said reading resistor (30) being connected with a terminal to the metal the corrosion of which is intended to be prevented (33) and with the other to a first side terminal (35) of said diverter (32) and to said sacrificial anode (10); said diverter (32) being connected by means of its second side terminal (36) to said reference electrode (40); said voltmeter (20) being connected between said metal the corrosion of which is intended to be prevented (33) and the central terminal (38) of said diverter (32).

10. An apparatus according to claim 7, characterized in that said at least one reference electrode (40) is associated to each of said sacrificial anodes (10) and to the metal the corrosion of which is intended to be prevented.

11. An apparatus according to claim 10, characterized in that each of said sacrificial anodes (10) is further associated to a reading resistor (30) and to a diverter (32), said reading resistor (30) being connected with a terminal to the metal the corrosion of which is intended to be prevented (33) and with the other to a first side terminal (35) of said diverter (32) and to said sacrificial anode (10); the second side terminal (36) of said switch (31 ) being connected to said common reference electrode; said voltmeter (20) being connected between said metal the corrosion of which is intended to be prevented (33) and the central terminal (38) of said diverter (32).

12. An apparatus according to claims 5 - 11 , characterized in that said A/D converter (21 ) has a resolution of at least eleven bits.

13. An apparatus according to claims 5 - 12, characterized in that said at least one memory (22) comprises a flash type EEPROM memory.

14. An apparatus according to claims 5 - 13, characterized in that said at least one microcontroller (23) comprises a serial peripheral interface (SPI) (24) by means of which the communication with said A/D converter (21 ) and with said memory (22) may be managed among other things.

15. An apparatus according to claims 5 - 14, characterized in that said at least one microcontroller (23) further comprises a universal asynchronous transmitter- receiver (UART) (25) by means of which the communication with said electronic processor (12) by means of a an appropriate serial communication interface (26) is managed among other things.

16. An apparatus according to claim 15, characterized in that said appropriate serial communication interface (26) is based on the RS-485 standard.

17. An apparatus according to claim 15, characterized in that said appropriate serial communication interface (26) is based on the MOD BUS standard.

18. An apparatus according to claims 1 - 17, characterized in that said electronic processor (12) comprises a personal computer.