Corrosion protection using a sacrificial anode

Abstract

corrosion protection of steel in concrete is provided by locating an anode assembly including both a sacrificial anode and an impressed current anode in contact with the concrete and providing an impressed current from a power supply to the anode. The impressed current anode forms a perforated sleeve surrounding a rod of the sacrificial anode material with an activated ionically-conductive filler material between. The system can be used without the power supply in sacrificial mode or when the power supply is connected, the impressed current anode can be powered to provide an impressed current system and/or to recharge the sacrificial anode from sacrificial anode corrosion products.

Description

Zn(OH)₄²⁻→Zn²⁺+4OH⁻  (2)
Zn²⁺+2e⁻→Zn   (3)

Theoretically, all the zinc oxide and other zinc ions and zinc corrosion products can be re-deposited on the core as usable zinc for subsequent consumption. In reality, as with rechargeable alkaline batteries, the level of each subsequent recharge is likely to be reduced.

A typical reaction at the impressed current electrode is likely to be:
2OH⁻→½O₂+H₂O+2e⁻  (4) or
H₂O→½O₂+2H⁺+2e⁻  (5)

There is therefore a net balance of the hydroxyl ions which means there is no overall loss in alkalinity within the assembly. There is a net increase in hydroxyl ions at the surface of the zinc anode which is beneficial in accommodating large amounts of the soluble zincate ions once the anode is used again, in galvanic mode, to protect the steel reinforcement. The reaction at the impressed current anode (Eq 4 or 5) involves the production of oxygen gas which needs to escape from the assembly and into the concrete pore structure. The impressed current anode, therefore, should be porous, be in the form of a net or be vented.

A preferred way to employ the anode arrangement herein is to initially set it up as a normal galvanic anode, allowing it to run for a period of say 10-20 years according to exposure conditions. Occasional monitoring will determine when recharging of the anode is required. An external power supply is then used to recharge the anode over a relatively short period, preferably no more than 14-60 days. The anode is then able to produce adequate current for a further period of time, say 5-20 years. The process can be repeated several times until recharging becomes essentially ineffective. If required, the impressed current part of the anode can then be simply used as part of an impressed current corrosion protection system. Protection of the steel reinforcement could therefore be achieved for the whole life of the structure.

The assembly has great flexibility which allows variable application types. For example, a preliminary use of the impressed current part of the anode can deliver an initial high level of charge over a limited period in order to passivate the steel to virtually stop any ongoing corrosion. Alternatively, the impressed current part of the anode can be operated to deliver a cumulative charge to increase the alkalinity of the concrete surrounding the steel and reduce future corrosion and current demand from the galvanic anode. Applied charge of 20,000 to 150,000 and more typically, 70,000 to 120,000 Coulombs per square meter of steel has been shown to be sufficient to passivate the steel. Applied charges of around 700,000 Coulombs/m2 have been effective at re-alkalizing (increasing the pH) of carbonated concrete. The charge required to increase the pH of concrete which is not carbonated will be less than 700,000 Coulombs/m2. This can then be followed by a lower level of galvanic current to maintain passivity of the steel. Using the impressed current anode to deliver the high initial charge is beneficial as this prevents unnecessary consumption and degradation of the sacrificial anode, allows a smaller sacrificial anode to be used and allows the sacrificial anode to provide higher current to the steel after the high initial charge has been passed to the steel by the impressed current anode. Recharging of the anodes can still be carried out if required. Furthermore, additional externally applied current can be delivered via the impressed current anode of the assembly if steel passivity is lost, if the current from the sacrificial anode is not sufficient to polarize the steel or if either the corrosion potential or the corrosion rate of the steel increases above desired levels.

The sacrificial anode may be connected to the steel while the impressed current anode is polarising the steel for the purpose of reactivating or increasing the activation of the sacrificial anode. This can be achieved by increasing the alkalinity at the anode surface which can dissolve zinc oxide corrosion products into soluble zincate ions, according to equation (1), and allow them to dissipate away from the anode surface and allowing better subsequent current flow and improved performance of the anode.

The current flowing to the sacrificial anode may be limited or controlled such that the sacrificial anode is reactivated without necessarily recharging the sacrificial anode.

The assembly also has the capability to operate principally as an impressed current anode with a rechargeable galvanic anode backup for periods when the impressed current anode is off line or is otherwise non-functional. Similarly, the impressed current anode can be available to operate as a backup to the sacrificial anode should the sacrificial anode become non-functional.

In a preferred arrangement, the inert anode may be capable of delivering a high level of current, possibly as high as 1 mA/cm2. The resistance of the electrolyte is preferably therefore as low as possible, so that a gel may be more suitable than a solid. Considerable levels of oxygen gas can be produced during charging which needs to disperse adequately through the anode walls and surrounding concrete.

In order for the anode to be rechargeable, the electrolyte is preferably highly alkaline. This allows high concentrations of Zn(OH)₄²⁻ in solution after the dissolution of zinc which, with supersaturation, is believed to precipitate out as ZnO. These reactions are believed to be as set out in Equations 6 and 7 below, which are essentially the reverse of Reactions 1 and 2.
Zn+4OH⁻→Zn(OH)₄²⁻+2e⁻  (6)
Zn(OH)₄²⁻→ZnO+2OH⁻+H₂O   (7)

Other electrolytes which are not highly alkaline are also suitable as long as soluble or electrochemically mobile zinc ions are present.

Preferably the assembly includes sufficient moisture to be highly ionically conductive and to allow sacrificial anode ions to be mobile during charging or recharging. Humectants, gels and other hydroscopic materials can be beneficial in this regard. In an alternative arrangement, charging or recharging of sacrificial anodes can be improved by applying water or another wetting solution to at least a portion of the structure and or specifically the sacrificial anode to keep it sufficiently conductive during the charging or recharging process.

Testing has shown that zinc can be deposited onto many substrates including; zinc, titanium, copper, brass, 70/30 brass, steel. stainless steel and alloys. As such, partially discharged and fully consumed sacrificial anodes can be regenerated.

Example 1

In one example, a cast zinc anode, 8 cm long with a minimum diameter of 0.7 cm, was located in ZnO/thixotropic paste packed inside a conductive ceramic impressed current anode tube. The zinc paste was made from a solution saturated with LiOH with 2M KOH and 20% ZnO along with carboxymethyl cellulose sodium gelling agent. The paste was packed in the space between the zinc anode and the inner side of the 28 mm tube. Testing has shown that ions can pass through the porous tube walls such that the zinc anode can pass current onto the external steel reinforcing bar even though it is located inside the impressed current anode. Subsequently, charging of the zinc can be accomplished by reversing the flow of ions through the impressed current porous tubular anode by applying an external voltage between the impressed current anode and the sacrificial anode. An applied voltage of around 6-8 Volts resulted in a current of up to 1.6 A to be delivered to the inner zinc anode achieving a total charge/recharge of just under 40,000 Coulombs. Surprisingly, the zinc anode performed better after recharging than it did originally. After charging of the zinc anode, when the zinc anode was reconnected to the steel, the current output and cumulative charge output of the recharged zinc anode through the porous tubular impressed current anode to the steel is increased compared to the original zinc anode. The reasons for this improvement in performance are not fully understood but may relate to an increased surface area of the zinc metal after deposition or to the relative increase of hydroxyl ions at the immediate vicinity of the zinc surface which encourages dissolution of zinc and zinc corrosion products such as zinc oxide and deposition of zinc from zinc corrosion products such as zincate ions (equations 1-3). It is evident, nonetheless, that the current output of the anode after charging is increased.

In FIG. 8 shows an example of an anode apparatus 30 as previously described where the apparatus includes a Cast Zinc Core 31 inside a 28 mm diameter porous conductive impressed current anode 32. An upper end is closed by an attached disk 33 forming a porous form and a lower end is closed by a Porous Fabric Cap 36. Between the core 31 and the cylindrical anode 32 is provided a filler material of LiOH+2M KOH+20% ZnO+carboxymethyl cellulose sodium 35. The core is attached to a steel wire 34 for connection as described above.

FIG. 9 is a graph of current output of the anode of FIG. 8 to steel, a) with the anode as originally made, b) with the anode after a period of charging via the porous conductive impressed current anode.

FIG. 10 is a graph of cumulative charge output of the anode to steel, a) with anode as originally made, b) after a period of charging via the porous conductive tube.

Example 2

An assembly 49 to demonstrate the ability to charge/recharge an anode in situ was constructed as shown in FIG. 11. It consisted of a zinc wire 50 partly immersed in a highly alkaline (7 molar OH—) gel 51. A copper wire connector 53 for the sacrificial anode to be formed in situ was also immersed in the same gel. The gel was contained within a perforated plastic tube 54 lined both internally and externally by a layer of fibre fabric 55 and ionically conductive membrane acting as a separator of the anode and cathode 56. Between the external fabric and the tube a mixed metal oxide (MMO) coated titanium mesh 57 was fixed circumferentially and had a titanium connection wire 58 attached to one side. The whole assembly was encased in a mortar 59 enriched with LiOH.

The anode assembly 49 was cast centrally in a cement mortar prism approximately 80 mm×50 mm×40 mm high ensuring that the whole assembly was encased within the cement mortar 59. As shown in FIG. 12, the prism was then placed in a larger container 61 filled almost to the height of the prism with an alkaline solution 60. An external mesh 62 of MMO coated titanium was placed along the periphery of the container to act as the metal section.

The zinc wire 50 was connected electrically to the external titanium mesh 62. The assembly 49 was then seen to act as a galvanic anode passing current to the external titanium mesh (metal section) and producing zinc corrosion products until all available zinc was consumed.

An external power supply (not shown) was then connected to the internal MMO coated titanium mesh anode 57 within the anode assembly 49 and the copper wire 53 ensuring that the copper was cathodic. Zinc corrosion products from the consumed (corroded) zinc wire 50 were deposited on the copper wire 53 to form a sacrificial anode during this charging process. Subsequent connection of the copper wire 53, now carrying the deposited zinc and the external MMO coated titanium mesh (metal section) allowed current to pass between the charged anode 53 and the metal section 62. The current produced by the charged anode (copper wire with deposited zinc) was comparable to the current produced by the original zinc wire. Comparison of current produced by the original ‘discharge’ of the zinc wire and the zinc which was deposited on the copper wire is shown in Table 1.

TABLE 1
Current output of original zinc wire
and deposited zinc on copper wire
		Current output (mA)
		Maximum	Minimum	Mean
	Original zinc wire	5.47	0.05	0.70
	Deposited zinc on	5.20	0.05	0.51
	Copper Wire

Turning now to FIGS. 13 and 14, a sacrificial anode 100 and an impressed current anode 101 are provided in the ionically conductive covering material 102 where a DC power supply 107 is connected across the sacrificial anode and impressed current anode. This provides a recharging phase which can be carried out during cathodic protection where the sacrificial anode is connected to the steel 108 or as a separate step.

Movement of moisture towards the sacrificial anode 100 from the impressed current anode 101 is obtained during the recharge phase. This is believed to be by the process of electro-osmosis. A simple experiment, as depicted in FIG. 13, demonstrated measurable water movement from the negatively charged electrode 101 to the positively charged electrode 100 embedded in the high alkalinity mortar 102, the latter 100 representing the sacrificial anode material during recharge. Item 103 represents a perforated plastic tube at the anode 101. Added water 104 was detected at the second plastic tube 105.

Table 2 below and FIG. 14 summarise the increase in volume of water observed with time of the recharged anode at a current of 1.5 mA. This application of current thus results in water movement which can replenish the electrolyte around the anode and facilitate better deposition of zinc metal during recharge.

TABLE 2
Moisture movement with time at an applied current of 1.5 mA
between two electrodes embedded in a highly alkaline mortar.
	Current output (1.5 mA)
		Difference in	Volume of water
	Required Drive	height of water	which migrates
	Voltage to	level in two	during application
Time	maintain current	compartments	of recharge current
(days)	(V)	(mm)	(ml)
0	1.94	0	0
1	2.23	0.5	0.03
6	2.19	3	0.19
22	2.04	7	0.44
28	1.98	7.5	0.47

Also shown in FIG. 13 is a separator 106 in the form of a microporous ionically conductive membrane, which is used primarily to limit or avoid dendritic growth of the sacrificial material (zinc metal) during recharge, to restrict zinc deposition to within the contained volume encased by the separator while allowing moisture movement and ionic conductivity through its pores. It is desirable that the pore size and pore distribution of the separator are such that it optimises movement of moisture. It is preferable that it restricts movement of moisture out of the anode-encasing electrolyte into the bulk surrounding electrolyte but allows the beneficial electro-osmotic movement of moisture back into the anode-encasing electrolyte during charging.

As explained previously, the potential difference across the anodes 100, 101 causes ions of the sacrificial anode material to move to the sacrificial anode 100.

Additives 109 are provided in the structure at or adjacent the anode 100 which acts to limit gassing from the sacrificial anode. The additives can be a surfactant, a form of cellulose or can comprise alloying zinc metal with suitable elements such as nickel or indium which is arranged to reduce the hydrogen over-potential significantly and hence limit hydrogen gassing.

The membrane 106 acts for restricting dendritic growth of sacrificial anode material on the sacrificial anode. The membrane separator 106 is located around or adjacent to the sacrificial anode and acts to contain sacrificial material at the sacrificial anode, to avoid dendritic growth of sacrificial anode material on the sacrificial anode beyond the membrane and to allow moisture movement to the sacrificial anode.

In order to provide ions for communication to the anode 100, particles or powder 110 of sacrificial anode material or sacrificial anode corrosion products are provided alone or intermixed with an ionically conductive filler material 111 at or adjacent the sacrificial anode.

As explained previously the potential difference caused by the DC power supply 107 causes an increase in the galvanic current generated by the sacrificial anode subsequent to application of the potential difference relative to that before the application. Also this action acts to increase alkalinity at the surface of the sacrificial anode where the increased alkalinity acts to dissolve zinc oxide corrosion products into soluble zincate ions and allow them to dissipate away from the surface.

The DC power supply is in some cases arranged so as to limit current flowing to the sacrificial anode by the potential difference such that the sacrificial anode is reactivated without recharging the sacrificial anode with additional ions of the sacrificial material.

The application of the DC power supply causes water movement from the negatively charged impressed current anode to the positively charged sacrificial anode. This also can cause an increase in a total surface area of the sacrificial anode material at the sacrificial anode. This can also result in increased quantity of hydroxyl ions at the immediate vicinity of the sacrificial anode.

Claims

1. A method for corrosion protection of a metal section in a cast body of an ionically conductive concrete or mortar material comprising:

providing an anode assembly comprising:

an impressed current anode; and

a sacrificial anode of a material which is less noble than the metal section;

maintaining the impressed current anode electrically separated from the sacrificial anode to prevent direct electrical communication therebetween;

prior to installation, positioning the impressed current anode and the sacrificial anode into an a common assembly, wherein the impressed current anode and the sacrificial anode are fixed at relative positions in the common assembly;

installing the common assembly, while the impressed current anode and said sacrificial anode are at said fixed positions, in contact with the cast body of ionically conductive concrete or mortar material so as to locate the impressed current anode in ionic contact with the cast body of ionically conductive concrete or mortar material and so as to locate the sacrificial anode in ionic contact with the cast body of ionically conductive concrete or mortar material;

providing a dc power supply having a first pole and a second pole; the common assembly including a first electrical connecting components component connected to the impressed current anode and a second electrical component connected to the sacrificial anode; and providing corrosion protection to the metal section by:

providing a connection by the first electrical connecting components component of the first pole of the dc power supply to the impressed current anode;

providing a connection of the second pole of the dc power supply to the metal section; and

providing a connection by the second electrical component of the sacrificial anode to the metal section;

so that the impressed current anode by a current from the dc power supply provides the corrosion protection of the metal section;

and when the dc power supply or impressed current anode is not present functional a current between the metal section and the sacrificial anode provides the corrosion protection of the metal section.

2. The method according to claim 1 wherein a voltage difference between the sacrificial anode and the impressed current anode caused by the dc power supply causes ions of the sacrificial anode material to move to the sacrificial anode so as to regenerate the sacrificial anode.

3. The method according to claim 2 including providing an additive which acts to limiting gassing from the sacrificial anode.

4. The method according to claim 3 wherein the additive is a surfactant.

5. The method according to claim 3 wherein the additive comprises cellulose.

6. The method according to claim 2 including restricting dendritic growth of sacrificial anode material on the sacrificial anode.

7. The method according to claim 2 including causing moisture movement towards the sacrificial anode.

8. The method according to claim 2 wherein there is an ionically conductive membrane separator between the sacrificial anode and the impressed current anode.

9. The method according to claim 8 wherein the ionically conductive membrane separator is located around or adjacent to the sacrificial anode.

10. The method according to claim 8 wherein the ionically conductive membrane separator acts to contain sacrificial material at the sacrificial anode, to avoid dendritic growth of sacrificial anode material on the sacrificial anode beyond the membrane and to allow moisture movement to the sacrificial anode.

11. The method according to claim 1 wherein the sacrificial anode material comprises particles or powder.

12. The method according to claim 1 wherein the sacrificial anode is connected to the metal section while the dc power supply is connected to the impressed current anode.

13. The method according to claim 1 wherein the sacrificial anode is connected to the metal section and the dc power supply is connected to the impressed current anode when the common assembly is installed.

14. The method according to claim 2 including increasing alkalinity at the surface of the sacrificial anode.

15. The method according to claim 14 wherein the sacrificial anode comprises zinc and wherein the increased alkalinity acts to dissolve zinc corrosion products from corrosion of the sacrificial anode into soluble zincate ions.

16. The method according to claim 1 wherein a current flowing to the sacrificial anode in response to a voltage difference between the sacrificial anode and the impressed current anode caused by the dc power supply acts to generate a current flowing to the sacrificial anode and wherein the flowing current is limited to a value such that the sacrificial anode is reactivated without recharging the sacrificial anode with transferring additional ions of to the sacrificial material.

17. The method according to claim 2 including increasing a total surface area of the sacrificial anode material at the sacrificial anode.

18. The method according to claim 1 including increasing a quantity of hydroxyl ions at the immediate vicinity of the sacrificial anode.

19. The method according to claim 1 wherein the sacrificial anode is formed of solid anode material.

20. The method according to claim 1 wherein the sacrificial anode is formed of powdered or finely divided sacrificial anode material.

21. The method according to claim 1 wherein the sacrificial anode comprises zinc oxide.

22. The method according to claim 1 wherein the dc power supply discharges as the current is supplied and additional electrical power is supplied when discharging has occurred.

23. The method according to claim 22 wherein the additional electrical power is supplied by recharging the dc power supply.

24. The method according to claim 22 wherein the additional electrical power is supplied by replacing the dc power supply.

25. The method according to claim 22 wherein the sacrificial anode is connected to the metal section while the dc power supply is connected to the impressed current anode.

26. The method according to claim 22 wherein the sacrificial anode is connected to the metal section and the dc power supply is connected to the impressed current anode when the common assembly is installed.