GB2474084A

GB2474084A - Impressed current cathodic protection (ICCP)

Info

Publication number: GB2474084A
Application number: GB0917880A
Authority: GB
Inventors: Barry Charles Torrance; Richard Perkins
Original assignee: AISH TECHNOLOGIES Ltd
Current assignee: AISH TECHNOLOGIES Ltd
Priority date: 2009-10-13
Filing date: 2009-10-13
Publication date: 2011-04-06
Also published as: GB0917880D0

Abstract

An open loop ICCP system for a marine structure 22 comprises a plurality of anodes 30 attached to the structure 22 and a DC power supply 32 connected to the anodes 30 by way of power cables 36 to deliver a voltage thereto. A control unit associated with the power supply 32 is operable to set said voltage to a low fixed level. A monitoring unit 34 powered independently of the power supply 32 is operative to monitor said voltage and disconnect the power supply 32 from the anodes 30 if said voltage exceeds said fixed level by a predetermined margin. With low voltage operation the system cathode potential can be maintained to be no more negative than —850 mV, whereby substantially no hydrogen is generated at the cathode, so avoiding embrittlement and the hazardous build-up of gas. The system allows sacrificial anodes to be replaced by impressed current anodes in substantially enclosed spaces, as between inner and outer hull casings of submarines or within seawater piping systems, thereby reducing lifetime costs.

Description

I

!mpressed Current Cathodic Protection This invention concerns impressed current cathodic protection (ICCP) for marine structures, particularly but not necessarily exclusively for use in substantially enclosed spaces such as ballast tanks or between inner and outer hull casings of submarines or within seawater piping systems.

Corrosion, as of the hull of a marine vessel in sea water, is essentially an electrolytic process: the water functions as an electrolyte and a corrosion current flows from the metal of the hull. Cathodic protection systems work by setting up an electrical current to oppose the corrosion current.

There are two basic kinds of cathodic protection system -sacrificial anode systems and impressed current systems.

In a sacrificial anode system there is attached to the hull of the vessel an anode which is galvanically more active in the water than the metal of the hull. The attachment of the sacrificial anode to the hull sets up an electrical protection current that forces the hull structure to become cathodic and thus resistant to corrosion, whilst the sacrificial anode corrodes in preference to the hull. Aluminium, zinc and magnesium and alloys thereof are common materials for a sacrificial anode for use with a steel hull and the anode-to-structure potential that drives the protection current is typically in the range 0.8-1.1 V, depending on the anode material.

Those skilled in the art will appreciate that in use the material of the sacrificial anode is indeed progressively sacrificed' and will eventually be totally depleted and no longer effective. It is therefore necessary to replace sacrificial anodes regularly during the structure's lifetime. This can be a very costly process on large structures, for example on ships, which have to be taken out of service for a significant period of time for replacement to take place.

An impressed current cathodic protection (ICCP) system avoids the recurring cost and downtime penalties of the sacrificial anode system. In the ICCP system, current from a direct current (DC) power source is supplied to an anode to oppose the corrosion current. In this system the anode is made from a metal that is substantially inert with regard to corrosion and so remains intact and effective throughout the lifetime of the structure to which it is attached.

Heretofore ICCP systems have used a voltage much higher than the anode-to-structure potential of the sacrificial anode system -maybe as much io as 30 V. This means that fewer impressed current anodes are required to protect a specific area of the structure (albeit with increased risk in topographically complex structures that some areas will be in the shadow' of the protection current and therefore unprotected). Importantly, however, higher voltage can cause the generation of hydrogen at the cathode. This may be in the form of hydrogen gas, creating a risk of fire and/or explosion in substantially enclosed spaces, or hydrogen ions absorbed in the structure and causing embrittlement of welds, high-stress steel or other hard materials used in the structure.

It is an object of the present invention to provide an ICCP system that does not generate substantial quantities of hydrogen.

United States Patent US 5 225 058 describes an ICCP system for use with reinforced concrete structures and intended to avoid both reinforcement over-protection and hydrogen embrittlement. This system includes a power supply unit and a control unit that sets a minimum protection potential (-0.9 V or -1.1 V) and adjusts the voltage, supplied to an anode in relation to feed voltage and anode potential.

United States Patent US 6 346 188 describes an ICCP system for inhibiting oxidation of a reinforcing, member disposed within a cementitious structure. This system uses a relatively low voltage power supply (a 1.5 V battery) but it is not arranged to prevent the generation of hydrogen nor otherwise concerned with embrittlement.

According to a first aspect of the present invention there is provided an ICCP system for a marine structure, which system comprises an anode attached to the structure, a DC power supply connected to the anode to deliver a voltage thereto, a control unit operable to set said voltage to a low fixed level and a monitoring unit powered independently of said power supply and io operative to monitor said voltage and disconnect the power supply from the anode if said voltage exceeds said fixed level by a predetermined margin.

The term low' as used herein in relation to the voltage setting of the invention means a voltage which is very much smaller than the 30 V or so typical of conventional ICCP systems heretofore. The particular significance of setting the voltage to a low level is that an ICCP system embodying the invention system can thereby be operated without causing hydrogen to be generated at the cathode, thus avoiding the risk of embrittlement and, in substantially enclosed spaces, the hazardous build-up of hydrogen gas. It is preferred that said fixed level be fixed so low that substantially no hydrogen is generated at the cathode by operation of the system.

As reported in Design Of Duplex Stainless Steel Subsea Equipment Exposed To Cathodic Protection, Det Norske Veritas DNV-RP-F1 12, Draft Issue April 2006, laboratory testing shows a risk of hydrogen induced stress cracking (HISC), as a result of hydrogen embrittlement, in duplex stainless steel subjected to elevated stress under cathodic protection with a cathode potential beyond (that is, negatively greater than) .-850mV relative to a silver/silver chloride (Ag/AgCI) reference electrode in sea water.

Further, Kim S-J, Jang S-K and Kim J-l in Electrochomical Study of Hydrogen Embrittlement and Optimum Cathodic Protection Potential of Welded High Strength Steel, reported in Metals and Materials International, Vol II, No 1, 2005, found the optimum potential for cathodic protection without hydrogen embrittlement to be between -770mV and somewhat above - 850mV relative to a saturated calomel reference electrode.

In view of these findings the ICCP system of the present invention is configured and arranged so that during operation of the system cathode potential is not greater than -850 mV, and preferably in the range -750 mV to -850 mV.

The monitoring unit may include a reference electrode (such as an Ag/AgCI electrode) and operate by measuring cathode potential of the system with respect to the reference electrode.

The system may comprise a plurality of said anodes.

The invention is particularly but not necessarily exclusively applicable to protecting marine vessels against corrosion, and its application to a marine vessel constitutes a second aspect of the invention.

In such application the ICCP system of the invention may be arranged to resist corrosion within a substantially enclosed space of the vessel such as a ballast tank or, where the vessel is a submarine, between inner and outer hull casings thereof. The system may include display means operative to display outside the enclosed space an operating measure such as cathode potential of the system.

In a third aspect the invention extends to a method of repairing a structure such as a marine vessel equipped with a plurality of sacrificial anodes configured and arranged to resist corrosion of the structure, which method comprises replacing each sacrificial anode of said plurality with a substitute anode of an ICCP system according to the invention.

The invention will now be described by way of example only with reference to the accompanying schematic drawings in which -Figures 1 and 2 illustrate differences between a sacrificial anode and an equivalent anode of an ICCP system embodying the invention, such as might be used in a tank or between casing space; Figures 3 and 4 illustrate differences between a sacrificial anode and an equivalent anode of an ICCP system embodying the invention within a seawater piping system; Figure 5 illustrates deployment of sacrificial anodes for resisting corrosion within a substantially enclosed space; Figure 6 illustrates an ICCP system embodying the invention and designed to replace the sacrificial system of Figure 5; Figure 7 is a table showing indicative through-life costs of cathodic protection systems; Figures 8 to 10 illustrate a test rig arranged to compare a sacrificial anode with a low voltage impressed current anode of an ICCP system according to the invention; Figures 11 and 12 show block diagrams of the sacrificial anode and the impressed current anode on the test rig of Figure 4; Figures 13 and 14 graphically compare the dynamic resistance of the sacrificial anode and the impressed current anode as tested; and Figure 15 is a graphical plot of cathode potential against cathode area.

Referring first to Figure 1, this shows a sacrificial anode 10. The sacrificial anode 10 is of generally cuboid form, having a length of 400mm, a breadth of 150mm and a depth of 30mm. Formed of Al/Ga alloy, the sacrificial anode 10 weighs 4kg. Thus with a nominal 2000AH/kg, the sacrificial anode can deliver an electrical protection current of 9OmA over 10 years.

Figure 2 shows an impressed current anode 12 designed to be equivalent to the sacrificial anode 10 of Figure 1. Like the sacrificial anode 10, the impressed current anode 12 weighs 4kg and is powered from a power supply (not shown) to deliver an electrical protection current of 9OniA.

However, because the impressed current anode 12 is not sacrificed in use, it does not have a limited life, and can last as long as the structure it is protecting.

Figure 3 is a cut-away view of part of a seawater piping system 13. The io sacrificial anode 14 of Figure 3 is intended to protect the interior surface of the piping system 13. As well as needing regular replacement, the anode 14 obstructs the flow of water through the piping system 13, increasing turbulence, noise, and erosion.

Figure 4 shows an impressed current anode 16 designed to be equivalent to the sacrificial anode 14 of Figure 3, but it is embedded in an insulator within the pipe wall 13 so as to be flush with the pipe wall and thus does not impede the flow of water.

Hitherto, in enclosed or semi-enclosed spaces such a ballast tanks of marine vessels or between inner and outer casings of a submarine having a double hull structure, sacrificial anodes have been used almost universally as a means of providing cathodic protection. ICCP systems have not generally been used in such spaces because of the hazard arising from the generation of hydrogen as a result of the relatively high voltage (-30V) conventionally used to power ICCP systems.

Between-hull and other substantially enclosed spaces are almost always of complex form and therefore require the deployment of many anodes.

Such a layout is shown stylised in Figure 5, in which a plurality of sacrificial anodes indicated at 20 are shown spread out within a space of irregular shape indicated in broken lines at 22.

Those skilled in the art will be well able to devise a layout for the sacrificial anodes 20 which provides protection throughout the space 22 without shadowing. They will also know, however, that the progressive depletion of the anodes 20 in use means that they will need to be replaced regularly, probably at intervals of no more than three years. Replacing these anodes is generally difficult (because the space 22 is substantially enclosed), dangerous (because the space 22 is often confined, possibly awash, and may io well contain electrical apparatus) and costly (in both manpower and downtime).

Figure 6 shows how the space 22 can be protected by means of an ICCP system instead of the sacrificial anode array of Figure 5. The sacrificial anodes 20 are replaced (on a one-for-one basis and in the same locations) by equivalent impressed current anodes 30. The substitute impressed current anodes 30 (arranged in two groups, although it will be understood that there may be more groups, or only one) are driven by a DC power supply 32 with a pre-settable output (not detailed in Figure 6) predetermined to provide equivalent or better protection from corrosion than the sacrificial anodes 20, and without the risk of a high potential at the cathodes causing the generation of hydrogen. A separate, independently powered monitoring unit 34 is provided to ensure that the pre-set output is shut down if its voltage diverges from a predetermined range under fault conditions not detected by the power supply 32.

Power from the power supply 32 is delivered to the anodes 30 by way of power lines 36 and connecting nodes 38. The pre-set output of the power supply 32 is held precisely at the required level by sensing the node potential and communicating this information to the power supply 32 by way of sense lines 40. If the node potential moves out of a prescribed band then the power supply 32 shuts down. It will be noted that the power supply 32 and its associated cables 36 are separate from the monitoring unit 34 and its associated cables 42.

The ICCP system of Figure 6 operates as an open loop system. Thus the output voltage is set to a preselected level, and any changes in parameters such as cathode potential are not fed back (as in a closed loop system) to adjust the output magnitude. This open loop system is operationally more io robust than a closed loop system because the closed loop system relies for its operation on feedback devices any failure of which will disable the system and thus remove the corrosion protection needed; and those skilled in the art will appreciate that corrosion protection systems must operate reliably for very long periods, because repair is difficult and expensive.

In the present system the output voltage is set at a level calculated to provide a cathode potential in the range -750mV to -850mV, which.provides corrosion protection in the space 22 without causing hydrogen to be generated, thereby avoiding hydrogen embrittlement and the hazardous build-up of hydrogen gas.

Because the impressed current anodes 30 do not deplete in operation, they do not need to be replaced at regular intervals like the sacrificial anodes of Figure 5. Of course the installation costs for the impressed current system of Figure 6 are likely to be higher than for the sacrificial anode system of Figure 5. Through-life costs of an impressed current system are, however, significantly lower, as illustrated by the table shown in Figure 7, which also indicates figures for an impressed current system with low profile anodes and therefore lower mass and reduced equipment cost.

Figure 7 also indicates potential weight savings, which could save additional fuel costs in the case of a ship. Those skilled in the art will further appreciate that the fixed weight of impressed current anodes may facilitate submarine design, where the continuously reducing weight of sacrificial anodes may introduce design complexities.

Figure 8 shows a test rig comprising a tank 50 containing a 1000cm2 plate 52 exposed to a 21 ppt salt solution, with an array of reference electrodes 54. The test rig of Figure 8 was used to compare a zinc sacrificial anode 56 shown in Figure 9 with an impressed current anode 58 shown in Figure 10.

io Each of the anodes 56 and 58 was circular, with a diameter of 15mm and an exposed area of 177mm2.

Figure 11 is a block diagram illustrating the test of the sacrificial anode and Figure 12 is a block diagram illustrating the test of the impressed current anode.

In the tests, the following parameters were examined: Equivalence of the dynamic (slope) resistance of each anode; and Ability of the fixed-voltage impressed current anode to protect a wide range of surface areas within a protection potential range of -750 to 950mV (where there is no hydrogen risk criterion) and -750 to -850mV (where there is a hydrogen risk criterion).

Figures 13 and 14 are plots of voltage against current and show the dynamic resistance (dV/dl) of the sacrificial anode 56 and the impressed current anode 58 respectively.

The so-called ?eterson Formula' (see Waidron L J & Peterson M H, US Naval Research Laboratory Report 4891, Feb 1957) provides an empirically derived prediction of the resistance R of an anode of surface area A exposed to sea water of density p as:

R-

0.58A°757 Thus the predicted resistance of a circular anode with surface area 177mm2 is 46Q. The measured resistances (dV/dI) were 35«= for the sacrificial anode 56 and 521k for the impressed current anbde 58. Within the limits of the experimental method, this is a satisfactory equivalence.

Figure 15 plots reference electrode potential against cathode area and shows the potential at cathodes of various areas, under the influence of the 177mm2 impressed current anode 58 operating at a fixed voltage in the 21 ppt salt solution. The results indicate that the anode 58 would protect an area up to 225 cm2 of steel (where there is no hydrogen risk criterion) and between 55 and 225 cm2 (where there is a hydrogen risk criterion and potential is therefore limited to -850mV).

Claims

Claims 1. An ICCP system for a marine structure, which system comprises an anode attached to the structure, a DC power supply connected to the anode to deliver a voltage thereto, a control unit operable to set said voltage to a low fixed level and a monitoring unit powered independently of said power supply and operative to monitor said voltage and disconnect the power supply from the anode if said voltage exceeds said fixed level by a predetermined margin.
2. An ICCP system as claimed in claim I wherein the system includes a reference electrode configured and arranged to monitor the cathode potential of the system with respect to the reference electrode.
3. An ICCP system as claimed in claim 2 wherein the reference electrode is an Ag/AgCI electrode.
4. An ICCP system as claimed in any preceding claim wherein said fixed level is so low that substantially no hydrogen is generated at the cathode by operation of the system.
5. An ICCP system as claimed in claim 4 configured and arranged so that during operation of the system cathode potential is less negative than -85OmV.
6. An ICCP system as claimed in claim 5 wherein during operation of the system cathode potential is in the range -750 mV to -850 mV.
7. An ICCP system as claimed in any preceding claim, which system comprises a plurality of said anodes.
8. An ICCP system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.
9. A marine vessel including an ICCP system as claimed in any preceding claim.
10. A marine vessel as claimed in claim 9 wherein the ICCP system is arranged to resist corrosion within a substantially enclosed space of the vessel.
11. A marine vessel as claimed in claim 10 wherein the substantially enclosed space comprises a ballast tank of the vessel.
12. A marine vessel as claimed in claim 10 or claim 11, which vessel is a submarine and said space is between inner and outer hull casings thereof.
13. A structure including an ICCP system as claimed in any of claims I to 10 wherein the substantially enclosed space is within a piping system carrying seawater or the like.
14. A marine vessel or structure as claimed in any of claims 10 to 13, which system includes display means operative to display outside the enclosed space an operating measure of the system.
15. A marine vessel or structure including an ICCP system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.
16. A method of repairing a structure equipped with a plurality of sacrificial anodes configured and arranged to resist corrosion of the structure, which method comprises replacing each sacrificial anode of said io plurality with a substitute anode of an ICCP system as claimed in claim 7.
17. A method of repairing a structure substantially as hereinbefore described with reference to and as shown in the accompanying drawings.