GB2194962A

GB2194962A - Cathodic protection of metal surfaces

Info

Publication number: GB2194962A
Application number: GB08621381A
Authority: GB
Inventors: Tian Der Mao
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-09-04
Filing date: 1986-09-04
Publication date: 1988-03-23
Also published as: GB8621381D0; AU7884987A

Abstract

Apparatus for cathodically protecting a metal structure 3, 11 from corrosion comprises (a) a material (e.g. sponge) 1, 12 for retaining an electrolyte in contact with the structure, (b) a permeable cover 2, 13 over the material, (c) an anode 5 in contact with the material but not in direct electrical contact with the structure and means 4 for applying a voltage between the anode and the structure in the case of the impressed current system, and (d) an anode 15 welded to the structure in the case of the sacrificial anode system. The invention may be used to protect the splash or tidal area of any marine structure or vessel since the sea water serving as electrolyte for cathodic protection is retained in contact with the structure. <IMAGE>

Description

SPECIFICATION Improvements in or relating to cathodic protection of metal surfaces The present invention relates to cathodic pro tection of metal surfaces, and, in particular to an electrolyte (such as water) retention system to be used in conjunction with a conventional cathodic protection system for corrosion protection of, for example, the splash and tidal areas of marine vessels (such as the hulls of ships, oil tankers and barges), marine structures (such as buoys and the splash/tidal areas of supports or pilings for jetties, harbours, oil rigs, and off-shore marine crew accommodation platforms) and the rising portions of off-shore oil pipelines.It can also be used to protect existing steel in reinforced concrete decks (such as iron/steel meshes in reinforced concrete decks of bridges, roadways or garages) during snowing seasons in cold countries, where de-icing with salts and/or calcium chloride are necessary for smooth traffic movements.

Marine structures-for example ship hulls, jetties and off-shore structures which are submerged in sea water are highly susceptible to corrosive damage. The exterior surface of a marine structure such as an off-shore structure, may be divided into four different areas: (1) atmospheric area, (2) splash or tidal area, (3) submerged area, and (4) area in the piling section.

The tidal and splash area of marine structures are subject to a combination of seawater turbulence and a high rate of aeration which results in a corrosion rate far in excess of those observed in other areas. Experiments have shown that corrosion rates in the splash or tidal area are about ten to twenty times higher than those in the atmospheric area, and about one and a half to three times higher than those in the submerged area.

Corrosion control for the atmospheric area can be adequately dealt with by protective coatings. Cathodic protection has been suc cessfuliy applied to both the submerged area and the area in the piling section. Cathodic protection is achieved by supplying a current from an external source (either sacrificial anodes or impressed current systems) so that it reverses the natural corrosion currents and ensures that a current is flowing through an electrolyte onto the entire metal surface requiring protection. Although the splash or tidal areas are intermittently wetted by seawater, the cathodic protection is not applicable because of the lack of a reliable electrolyte for conducting current to these areas.

To combat the most severe corrosion rates in the splash or tidal area, the present available methods are to: (1) encase with concrete, (2) clad with copper-nickel alloy sheathings, (3) clad with vulcanised rubber or plastics, (4) wrap with petrolatum tapes, and (5) use epoxy paint or other polymeric resin coatings.

Disadvantages inherent in the existing methods are: (1) they are subject to damage by impact with other objects/particles present in seawater, (2) it is difficult to inspect the splash or tidal area, (3) if epoxy or other polymeric resin paint is used, it is difficult and expensive to repaint, (4) it is difficult to repair damaged paint in seawater, (5) corrosion can be detected only when it is in an advanced stage, and (6) existing methods cannot guarantee a long life-span of from five to twenty five years.

Therefore, an effective and maintenance-free corrosion prevention method for this area is extremely necessary and in fact long overdue.

The present invention provides a system which overcomes or at least mitigates these disadvantages.

According to the first aspect of the present invention there is provided an apparatus for cathodically protecting a structure from corrosion, which apparatus comprises (a) a material for retaining an electrolyte in contact with the structure, (b) a permeable cover over the material, (c) an anode in contact with the material but not in direct electrical contact with the structure in the case of the impressed current system and means for applying a voltage between the anode and the structure; and (d) an anode (such as zinc, aluminium, magnesium or other alloys) welded to the structure in the case of the sacrificial anode system.

According to the second aspect of the present invention there is provided a method of cathodically protecting a structure from corrosion, which method comprises applying, to a surface of the structure to be protected, means for retaining an electrolyte between the metal surface and (a) an anode not in direct electrical contact with the structure and applying a voltage between the anode and the structure in the case of the impressed current system and (b) an anode welded to the structure in the case of the sacrificial anode system.

Thus, the present invention enables the provision of a method to provide protection for the splash or tidal area of any marine structure or vessel with a water retention system so as to retain the seawater which serves as the required electrolyte, thus enabling the cathodic protection system to be used in such areas. Cathodic protection is an established and effective corrosion prevention method and it can be monitored by measuring the electrode potential of the protected structure by determining the e.m.f. between the structure and a suitable reference electrode. The retention system of the present invention comprises an electrolyte retaining material, such as an absorbent material and a permeable, for example, perforated, cover.The absorbent material may contain only one layer of, for example, artificial, sponge or one main layer of sponge plus another thin layer of sponge containing an antifouling agent. The cover (or a netting cover) may be made from a suitable plastics material, fibreglass or rubber sheet.

Anodes and reference electrodes (for the cathodic protection system) can be placed between the two layers of absorbent materials or between the absorbent material and the cover in the present invention. The whole electrolyte retaining system is then fixed firmly onto the splash or tidal area of a marine structure which will be protected as a cathode in a cathodic protection system (using either a sacrificial or an impressed current system).

The present invention can make use of a soft artificial sponge (such as those made from polyurethane, polyethylene, polyvinyl chloride, PVC/nitrile or any other flexible foam plastics material) as an absorbent material to retain seawater in the electrolyte retaining system.

The absorbent material together with its perforated cover can be easily added onto the splash or tidal area of any marine structure or vessel either on land or at sea. For maintenance purposes, these can be easily removed or renewed. As the artificial sponge is flexible and soft, it will not be damaged by impact from other objects or particles present in seawater. Therefore, the combination of this simple electrolyte retaining system with a conventional cathodic protection system will not only eliminate or mitigate the disadvantages of the present existing methods mentioned above, but also ensure as far as possible that the splash or tidal area is free from corrosion for any specified period required by the users.

The present invention can also be applied to the cathodic protection of steel in reinforced concrete structures, particularly in northern latitudes where de-icing salts and/or calcium chloride are widely employed. The corrosion of these steels is mainly due to the contamination of the concrete by chloride ions resulting from such de-icing salts-either applied directly to the surface of the structure, as in the case of bridge decks and roadways, or brought in by automobiles, as in the case of garages. Studies have shown that the corrosion product can occupy over 2.2 times the original volume of steel, causing severe internal pressure of up to 330 kg/cm2. Such pressure results in the concrete spalling and cracking. To overcome this corrosion problem, cathodic protection with the aid of an electrolyte retaining system will be a suitable solution.

For a better understanding of the present invention and to show how the same may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a schematic view of an offshore marine structure, Figure 2 shows a partially sectional view of an embodiment of the present invention (using the impressed current system), Figure 3 shows a partially sectional view of an embodiment of the present invention applied to a circular pipe (using the sacrificial anode system), Figure 4 shows a partially sectional view of the present invention, applied to the hull of a ship (using the impressed current system), Figure 5 shows diagrammatically the application of the invention to beam pilings (using the impressed current system), and Figure 6 shows a partially sectional view of the invention adapted to a reinforced concrete deck.

Referring now to the drawings, Fig. 1 illustrates the four different areas of the external surface of a marine structure. Fig. 2 shows an electrolyte (in this case, sea water) retaining system consisting of an absorbent portion 1 (in this figure only one layer of artificial sponge is shown) and a perforated or netting cover 2. A metal structure surface 3 is protected with an impressed current cathodic protection system. The D.C. current is supplied from an external power source 4, which may be a rectifier, a potential-controlled rectifier or any other D.C. producing device. Small point or pancake type anodes 5 (or wire type anodes) are placed between the absorbent portion 1 and cover 2 and are connected to the positive terminal of the external power source 4. The metal structure is welded at a point 6 and is connected to the negative terminal of the external power source 4.Reference electrode(s) 7 (such as a zinc electrode or a Ag/AgCI or Cu/CuSo4 reference electrode or a calomel electrode) are also placed between the absorbent portion 1 and cover 2. A voltmeter 8 is used to measure the potential difference between the structure 3 and the reference electrode 7. The most common criteria for achieving complete cathodic protection is to suppress the iron/steel structure potential to -850 millivolts respective to an Ag/AgCI reference electrode. Thus, the corrosion control of the metal structure can be monitored with the proper adjustment of the transformer/rectifier or an automatic potentialcontrolled rectifier. The absorbent portion 1 which is a soft, flexible artificial sponge can retain sea water as the required electrolyte in the cathodic protection system. Because the voltmeter 8 is used as a monitoring device to check whether the metal structure is under fully cathodic protection, it can be placed at any convenient location and it can be used either continuously in a monitoring system or whenever it is necessary.

Referring now to Fig. 3, there is shown an electrolyte (in this case sea water) retaining system being used to protect a rising pipeline with a sacrificial anode cathodic protection system. The outer surface of a pipe 11 is covered with an absorbent portion 12 (it may contain one or two or more layers of artificial sponge) then a perforated or netting cover 13.

A bracelet (or other shaped) sacrificial anode 14 (such as zinc, aluminium or magnesium) is placed between the absorbent portion 12 and the cover 13. An electric-conducting strip (or rod) 15 is fixed to the sacrificial anode and is welded to the outer surface of the pipe 11 underneath the absorbent portion 12. The continuous measurement of the potential of the protected metal surface against a reference electrode to check whether the metal surface is properly and fully protected may not be necessary for a sacrificial cathodic protection system. However, occasional checking and a proper design of the number and distance among the sacrificial anodes are important.

Fig. 4 illustrates the application of an electrolyte retaining system (consisting of an absorbent portion and a perforated or netting cover) on a ship's hull using an impressed current cathodic protection system. The external D.C. power source is of the potential-controlled rectifier type.

Fig. 5 shows the application of an electrolyte retaining system used in conjunction with a cathodic protection system, to protect the splash/tidal area of "H" beam pilings under a pier, jetty or an off-shore marine structure. A simple transformer-rectifier is used for the impressed current cathodic protection system.

Referring now to Fig. 6, there is shown a water retention system used to protect existing steel reinforced concrete structures, for example an iron/steel mesh inside a concrete bridge deck. The concrete bridge deck 21 which is reinforced with an iron/steel mesh 24, is covered with an absorbent portion 22 (in this case, it contains only one layer of artificial sponge) and then a perforated cover 23 (made from rubber sheet, fibre-glass or plastics material). The D.C. power source 25 should be kept at a convenient and safe place. The small point or pancake type anodes 26 (or wire type anodes) are placed between the absorbent portion 22 and the cover 23, and are connected by cables to the positive terminal of the external power source 25. The iron/steel mesh 24 is welded to a cable which is connected to the negative terminal of the power source 25. Reference electrodes 27 are also placed between the absorbent portion 22 and cover 23. A voltmeter 28 (which may be placed together with the power source 25 at a convenient and safe place) is used to measure the potential difference between the iron/steel mesh 24 and the reference electrode 27 as a monitoring system. Thus, the iron/steel mesh inside the concrete bridge deck can be fully cathodically protected with an impressed current system. It is important to make sure these anodes 25, reference electrodes 27 and connecting cables must be properly protected (such as laying inside the cutting holes or slots on top of the concrete bridge decks with absorbent portion underneath or embedded in new concrete) to prevent any possible damages caused by movement of heavy vehicles.

Claims

1. An apparatus for cathodically protecting a metal structure from corrosion, which apparatus comprises (a) a material for retaining an electrolyte in contact with the structure, (b) a permeable cover over the material, (c) an anode in contact with the material but not in direct electrical contact with the structure and means for applying a voltage between the anode and the structure in the case of the impressed current system, and (d) an anode welded to the structure in the case of the sacrificial anode system.

2. An apparatus according to Claim 1, wherein the material is an open sponge.

3. An apparatus according to Claim 1, wherein the material comprises a first layer of open sponge and a second layer of open sponge coated with an anti-fouling agent.

4. An apparatus according to Claim 2 or 3, wherein the open sponge is made from a flexible foam plastics material.

5. An apparatus according to Claim 4, wherein the flexible foam plastics material is polyurethane, polyethylene, polychloride or polyvinylchloride/nitrile.

6. An apparatus according to Claim 3, 4 or 5, wherein the anti-fouling agent is a compound of copper, mercury or tin.

7. An apparatus according to any one of Claims 2 to 6, wherein the thickness of the open sponge is in the range of from 0.2 cm to 50 cm.

8. An apparatus according to any one of the preceding claims, wherein the permeable cover is made from plastics material, fibreglass or rubber.

9. Apparatus for cathodically protecting a metal structure substantially as hereinbefore described with reference to and as shown in Figs. 1 and 2, 3, 4, 5 or 6 of the accompanying drawings.

10. A method of cathodically protecting a structure from corrosion, which method comprises applying to a surface of the structure to be protected, means for retaining an electrolyte between the surface and an anode not in direct electrical contact with the structure and applying a voltage between the anode and the structure in the case of the impressed current system; and an anode welded to the structure in the case of the sacrificial anode system.

11. A method according to Claim 10, which further comprises monitoring the voltage applied between the reference electrode and the structure to ensure that the structure is cathodically protected as fully as possible.

12. A method according to Claim 10 or 11, wherein the means for retaining electrolyte comprises an open sponge and a permeable cover according to any one of Claims 2 to 8.

13. A method according to Claim 10, 11 or 12, wherein the means for applying a voltage between the anode and the structure in the case of the impressed current system comprises a device for producing a DC current.

14. A method according to Claim 10, 11, 12 or 13, wherein the electrolyte is seawater or chloride containing aqueous solutions resulting from the de-icing process.

15. Any novel feature or combination of features described herein.

15. A method of cathodically protecting a structure from corrosion, substantially as hereinbefore described with reference to Figs. 1 and 2, 3, 4, 5 or 6 of the accompanying drawings.