NO343826B1 - Treatment process for concrete - Google Patents

Description

Description

TREATMENT PROCESS FOR CONCRETE

Technical area

This invention relates to the electrochemical treatment of reinforced concrete to protect it from degradation arising from corrosion of the steel. More specifically, this invention relates to a hybrid electrochemical treatment to arrest corrosion of steel reinforcement and subsequently prevent corrosion initiation.

Background technology

Corrosion of steel in reinforced concrete is a major problem. Both permanent and temporary electrochemical treatments have been used to stop this problem. This involves sending a current through the concrete to the steel from an installed anode system. In all cases, the steel becomes the cathode in the electrochemical cell that is formed. In impressed current electrochemical treatment, the anode is connected to the positive terminal and the steel is connected to the negative terminal of a DC power supply. In sacrificial electrochemical treatment, the protective current is provided by corroding sacrificial anodes directly connected to the steel.

Persistent or long-term electrochemical treatments are installed with the intention of maintaining the treatment for the foreseeable future. The electrochemical treatment period will typically be measured in years. A well-known family of persistent or long-term techniques is cathodic protection. It includes impressed current cathodic protection, sacrificial cathodic protection, periodic cathodic protection and cathodic prevention. In these techniques, a long-lasting or permanent anode supplies a small current to the steel reinforcement. Average current densities expressed per unit area of steel surface typically range from 2 to 20 mA/m<2> to arrest existing degradation, and 0.2 to 2 mA/m<2> to prevent initiation of degradation. The current can be pulsed, but the average applied currents are typically within the ranges above. The current may be adjusted from time to time with adjustments based on an analysis of performance data.

Temporary or short-term electrochemical treatments are installed with the intention of interrupting the treatment in the foreseeable future. The electrochemical treatment period will typically be measured in days, weeks or months. Temporary treatments designed to arrest reinforcement corrosion include chloride extraction (US 6027633) and realkalization (US 6258236). In these systems, a temporarily installed anode system is used in conjunction with a temporary DC power supply to deliver a large current of the order of 1000 mA/m<2>expressed per unit area of steel surface for a short period (typically less than 3 months) to the steel reinforcement .

Anodes are electrodes that support a net oxidation process. Anodes for concrete structures can be divided into inert anodes or sacrificial anodes. They can be further divided into anodes embedded in a porous matrix or anodes attached to the concrete surface so that they are exposed and accessible, as well as discrete or non-discrete anodes. Anodic systems that include an anode and a supporting electrolyte can be divided into temporary and long-term anode systems. A summary of the differences is provided in the following sections.

Inert anodes prevent anode consumption. They have been used in most electrochemical treatments, the principle exception being sacrificial cathodic protection. The most important anodic reaction is the oxidation of water which produces oxygen gas and acid. The acid attacks the cement mass in concrete. As a result, the current density of inert anodes tends to be limited to less than 200 mA/m<2> expressed per unit area of anode surface. A widely used anode system is a titanium mask coated with mixed metal oxide (MMO) embedded in a cement coating on the concrete surface (US 5421968). A discrete porous titanium oxide anode required to deliver higher anode current densities up to 1000 mA/m<2> of anode surface has also been used (US 6332971).

Sacrificial anodes are consumed in the process of supplying the protection current. The most important anode reaction is dissolution of the sacrificial metal. As a result, the life of sacrificial anodes is limited. Sacrificial anodes have been used as embedded (buried) discrete anodes in sacrificial cathodic protection systems (WO 9429496) and a mask with a cover in sacrificial cathodic protection (US 5714045). However, the use of built-in sacrificial anode systems is prevented by the need to replace the anodes at the end of their life. Sacrificial anode systems have also been attached directly to the concrete surface (US 5650060) and are available to facilitate anode replacement. The use of sacrificial anodes in an impressed current role is prevented by the more rapid consumption of the anode in this role. However, surface applied anodes are easy to replace and this type of system has been used as an impressed current anode at anode current densities typically less than 25 mA/m<2> (US 5292411).

Discrete anodes are individually separated compact anodes that are usually embedded in holes in the concrete or installed in places where remedial repairs to the concrete have been made. A description of discrete anodes is given in US 6217742. Embedded discrete anodes are strongly attached to the concrete, and attachment failure is less common for discrete anodes than for the non-discrete anodes used on concrete surfaces.

US6419816 describes a process that uses an electro-osmotic treatment to strengthen concrete as a first treatment and immediately follows this with a cathodic protection at a relatively low voltage.

GB2389591 describes an anode assembly to be inserted into an opening between a section of reinforced concrete and another fixed structure. The anode assembly includes a sacrificial anode and a non-sacrificial electrical conductor.

Temporary anode systems are typically attached to the concrete surface to deliver short-term, high-current temporary electrochemical treatments, and are removed at the end of the treatment period, which is typically less than 3 months. Temporary anodes are surrounded by a temporary electrolyte, such as a liquid contained in a tank or an electrolytic material, such as saturated cellulose fiber, which is easily removed at the end of the treatment process (US 5538619). A high drive voltage with a high volume of electrolyte is generally required to support the high current output. In contrast, long-term anode systems intended to deliver a protective current over several years are firmly attached to the concrete, and can be embedded in voids in the concrete to improve anode attachment.

Account of the invention

Technical problem

Imprinted current cathodic protection with is the most proven of the existing methods for stopping chloride-induced corrosion of steel in concrete. However, it requires a high level of maintenance compared to other inspection or maintenance requirements for reinforced concrete structures. In addition, impressed current cathodic protection systems are commissioned after all delaminated and spalled concrete areas have been repaired and then only protective current densities mainly below local steel corrosion rates since cathodic protection currents that are high at start-up have detrimental effects resulting from the generation of acid and gas on some anode systems . While low current densities eventually stop corrosion, corrosion-induced damage continues to occur until the corrosion process is stopped.

Temporary electrochemical treatments stop the corrosion process quickly and have no maintenance requirements after the initial treatment. However, a significant level of chloride sometimes remains and there are concerns regarding the duration of such treatments in chloride-containing environments. Furthermore, the duration of the treatment can last several months and access to the treated surface is limited during this time.

Sacrificial cathodic protection is not always considered to be sufficiently powerful to stop corrosion. However, the process requires little maintenance and it is a reliable process that can be used in a preventive role.

The problem solved by this invention is the effective delivery of powerful electrochemical protective treatments of corroding steel in concrete to stop corrosion and achieve long-term durability of the protective effects with minimal maintenance requirements and minimal breakdowns during system installation.

More specifically, the above problems are solved by an application of an anode and a DC power source, as well as an anode suitable for carrying out this application, to protect steel in concrete as stated in the appended patent claims.

Technical solution

An analysis of available data provides strong evidence that electrochemical treatments applied to reinforced concrete arrest corrosion by restoring alkalinity at corroding sites using a relatively small amount of charge.

Existing electrochemical treatments can therefore be improved by dividing the treatment into two phases; namely a short initial treatment with high current to quickly stop corrosion which minimizes further damage, and a substantially long-term preventive treatment with low maintenance requirements to maintain passivity and ensure durability. A simple composite treatment node capable of delivering both initial high current, short-term electrochemical treatment to stop corrosion and mainly the long-term low current treatment to prevent later corrosion initiation is presented. To deliver the high current initial treatment, the composite treatment anode is capable of delivering very high anode surface current densities at low and safe DC voltages. In order to achieve a lasting long-term preventive treatment, the composite treatment anode is used in a cathodic preventive role, preferably connected to the steel as a sacrificial anode.

The composite treatment anode is based on the use of a sacrificial anode metal in a temporary role with high impressed current. An observation leading to the development of the composite treatment anode was that an aluminum alloy sacrificial anode metal can deliver current densities above 10000 mA/m<2> (expressed per unit anode area) of the anode surface at very low and safe DC voltages that are not sufficiently positive to induce gas evolution even when the sacrificial anode is embedded in a porous material in a cavity formed in the reinforced concrete. This is possible because the anodic reactions occur easily on sacrificial anode metals when compared to anodic reactions occurring at inert anodes with applied current.

Compact discrete anodes with very high current density can therefore be embedded in the concrete to limit the breakdown caused during the short high applied current treatment. A short high applied current treatment moves corroding sites from locations on the reinforcing steel to mounted sacrificial anodes because hydroxide is produced on the steel causing the pH to rise and aggressive ions, such as chloride and sulfate, are drawn from the concrete to the sacrificial anode. The anode can then be used as an activated sacrificial anode to maintain steel passivity.

Accordingly, the present invention provides, in a first aspect, a method of protecting steel in concrete comprising using an anode and a DC power source and a temporary impressed current treatment and a low current preventive treatment, wherein the temporary treatment with impressed current is a high current treatment that uses the DC power source to drive the current of the anode to the steel to improve the environment of the steel, and the low current preventive treatment is applied to prevent the initiation of steel corrosion after applying the temporary treatment with impressed current. The same anode is used in both treatments and the anode includes a sacrificial metal element which undergoes sacrificial metal dissolution and as its main anodic reaction.

Another observation leading to the development of several treatment technologies was the high charge density of aluminum alloy anodes. Four aluminum alloy anodes of 100mm long and 15mm in diameter have sufficient charge to supply approximately 500mA for a week, and 1mA for 50 years in their impressed current and sacrificial anode functions. The high charge density in some sacrificial anodes means that long lives are achievable from small sacrificial anodes embedded in concrete. This alleviates the concern regarding the cost of replacing the anodes embedded in porous materials at the end of their service life.

The inclusion of a connection detail for the impressed current anode on a compact discrete sacrificial anode alleviates the risk of corrosion of the connector when the discrete sacrificial anode is used as an impressed current anode. Forming the sacrificial anode metal around an impressed current anode which can be used in an impressed current cathodic preventive role after the sacrificial metal has been consumed can also be used to extend the life of the treatment.

Beneficial effects

The anodic reactions occurring on a sacrificial metal occur more easily than the anodic reactions occurring on an inert anode, requiring less driving voltage and generating less acid and less gas. This enables a short high-current electrochemical treatment to be delivered much more easily. The application of a high current to a steel cathode in an electrochemical cell stops rapid corrosion of the steel and minimizes further corrosion damage. Aggressive ions in the concrete are drawn to the anode by the applied current treatment. The combination of these aggressive ions and the sacrificial metal forms a sacrificial anode which is activated without the addition of other activating chemicals to the concrete. Connecting this sacrificial anode directly to the steel provides a simple method of applying a continuous, preventive treatment to prevent future corrosion initiation. The corroding areas are effectively moved from the steel to the installed anode during the initial treatment. Incorporating an anode system into the concrete allows the concrete surface to be used while the high impressed current electrochemical treatment is applied.

Description of figures

The invention is now further described with reference, by way of examples, to the figures, in which:

Fig. 1 shows a schematic diagram for the use of an anode in a hybrid sacrificial electrochemical treatment with impressed current;

Fig. 2 shows the experimental arrangement used to determine an anode potential-current relationship.

Fig. 3 shows the potential current relationship determined on an aluminum alloy anode and a titanium anode coated with mixed metal oxide (MMO);

Fig. 4 shows the current density driven by an aluminum alloy anode in an aggressive environment using a DC power supply in Example 1;

Fig. 5 shows the galvanic current density provided by an aluminum alloy anode resulting from an initial treatment with impressed current in Example 1;

Fig. 6 shows the current density driven by 25 aluminum alloy anodes in a mild environment using a DC power supply in Example 2; and

Fig. 7 shows the galvanic current density provided by 25

aluminum alloy anodes resulting from an initial applied current treatment in Example 2.

Embodiment of invention

Mechanism of electrochemical protection

Electrochemical treatments applied to steel in concrete include cathodic protection and prevention, periodic cathodic protection, chloride extraction and realkalization. The protective effects induced by these treatments are a negatively driven potential shift that prevents the dissolution of steel to form positive iron ions (corrosion), the removal of chloride ions from the steel surface that makes the environment less aggressive to passive films on steel, and the generation of hydroxyl ions at the steel surface that stabilizes the formation of passive films on steel. The traditional understanding of electrochemical treatments of reinforced concrete is that different treatments depend on different protective effects. In this understanding, the basis of cathodic protection is the achievement of a negatively driven potential shift. Realkalization of carbonized concrete requires the generation of a reservoir of hydroxide around the steel. Chloride extraction requires the removal of chloride ions from the concrete. Intermittent cathodic protection relies on changing the environment of the steel either by removing chloride or by generating hydroxyl ions to prevent steel corrosion for a short period while the protection current is interrupted.

It has been argued that the protective effects of a negative potential shift are negligible and most electrochemical treatments used to arrest ongoing corrosion of steel in concrete achieve this by inducing open circuit steel passivity by extracting chloride and generating hydroxyl ions at the steel. While this observation is still debated in the case of cathodic protection (see discussion and reply in Jorunal of Materials in Civil Engineering, 13(5) 396-398, 2001) the comparison and analysis of the available evidence suggests that a protective effect probably has a dominant effect on the success of all electrochemical treatments applied to steel in atmospherically exposed concrete. This dominant protective effect is the increase in pH at the steel/concrete interface.

Atmospherically exposed concrete is concrete that has been periodically allowed to dry out so that the cathodic reaction kinetics (the reduction of oxygen) on the steel is weakly polarized (oxygen reduction occurs easily). In this environment, steel is usually protected by a passive film, and breakdown of the passive film is in principle induced by chloride contamination or carbonation of the concrete cover. Steel passivity is indicated by a positive open circuit (no applied current) potential. The open circuit potential is the result of the combination of the potential of an iron electrode with the potential of an oxygen electrode. Passive steel has an open circuit potential that tends towards the potential of the more positive oxygen electrode. As the passive film breaks down, the open circuit potential approaches the more negative iron electrode. An open circuit potential must not be confused with a driven potential. Whereas a positive open circuit potential indicates the presence of an intact passive film on the steel which drives the steel potential to more positive values by applying an external source of force which increases the force inducing iron to dissolve as positive iron ions causing passive film breakdown and hence corrosion .

In the case of chloride-induced corrosion, the local dissolution of iron at weaknesses in the passive film is followed by reaction with water to produce iron oxide and hydrogen ions. The positive charge of the hydrogen ions is balanced by the negative charge of the chloride ions, and the localized generation of hydrochloric acid occurs. This local reduction in pH destabilises the passive film and results in an accelerating and spreading corrosion process often called pitting corrosion. Chloride ions do not directly destabilize the iron oxides that form the passive film. This is an indirect result of a local pH reduction.

Carbonation-induced corrosion is also caused by a decrease in concrete pH that occurs as a result of the reaction of carbon dioxide and water with alkalinity normally present in concrete. The generation of hydroxide in steel is widely accepted as the protective effect relied upon in the application of realkalization of carbonized concrete. This is a less severe treatment than chloride extraction, and its application to arrest chloride-induced corrosion would offer some practical advantages. A typical realkalization treatment would require the application of 600 kC/m<2> (168 Ah/m<2>) or 1 A/m<2> for one week (expressed per unit of steel surface area) to realkalize a significant portion of the carbonized concrete covered. This can be compared to the charge density of approximately 3600 kC/m<2> (1000 Ah/m<2>) applied in a typical chloride extraction treatment.

Evidence that the generation of hydroxide by the steel is also the most important protective effect of electrochemical treatments applied to chloride-contaminated concrete arises primarily from the relatively low applied current densities and charge densities that induce open-circuit steel passivity in chloride-contaminated concrete.

In a laboratory study on periodic cathodic protection applied to steel in heavily chloride contaminated specimens exposed to an aggressive simulated marine environment (Glass, Hassanein and Buenfeld, Corrosion Science, 43 (6) 1111-1131, 2001) an open circuit steel potential shift to more passive potential values clearly evident after 6 months when the steel received integrated protective current densities of 6 and 40 mA/m<2> (expressed per unit of steel surface area). This positive potential shift indicates that the steel has become passive. This conclusion was supported by photographs of the specimens obtained after 12 months of periodic cathodic protection showing that continued corrosion in the control specimens and the specimens receiving a less severe treatment caused corrosion-induced cracking, while specimens producing open-circuit steel potentials typically of passive steel remained intact. Further analyzes of these data indicate that, in the case of specimens receiving an integrated protective current density of only 6 mA/m<2>, steel passivity was induced with a charge of less than 100 kC/m<2> (less than 28 Ah/m <2>).

Strong evidence that the relatively small charge density required in practice to arrest chloride-induced corrosion comes from an analysis of extensive data obtained in both field and laboratory studies of cathodic protection. Design current densities with cathodic protection will typically be up to 20 mA/m<2>, and cathodic protection systems are usually ordered at lower current densities. However, it is common to achieve large potential shifts with such relatively small current densities after less than 50 days of cathodic protection. Large potential shifts at small applied current densities are only possible on passive or nearly passive steel (Glass, Roberts and Davidson, Proc.7th Int. Conf. Concrete in Hot and Aggressive Environments, October 2003, Volume 2, pp. 477 – 492, 2003) , and clear evidence for such induced passivity has been produced under laboratory conditions (Glass, Roberts, and Davidson Corrosion 2004, NACE, Paper No. 04332, 2004). The charge equivalent of a protection current of 10 mA/m<2>applied for 50 days is less than 50 kC/m<2>. This would be a more typical charge density required to induce steel passivity on a repaired concrete structure and is extremely small compared to the charge applied in a typical chloride extraction treatment (3600 kC/m<2>).

The importance of the generation of hydroxyl ions by the steel is also supported by the observation that this induction of open circuit steel passivity has been achieved by using current densities with cathodic protection that are significantly lower than the localized steel corrosion rates. Average corrosion rates of 0.02mm steel section loss per year and localized corrosion rates greater than 0.1mm per year are not uncommon in chloride contaminated concrete. These equate to corrosion current densities of approximately 20 and 100 mA/m<2>. However, design current densities with cathodic protection are almost always less than or equal to 20 mA/m<2>, and applied current densities are constantly lower than the design current densities (BS EN 12696: 2000).

Two other factors further add to this surprising observation. First, the applied protective current is not effective in directly reducing the corrosion rate in atmospherically exposed concrete. The technical reason for this is that the cathodic reaction kinetics is weakly polarized (occurs easily) in this environment. Second, current flows preferentially to the more positive cathodes rather than the corroding anodes in natural corrosion cells formed in concrete. It has been shown that even in arrangements where geometry and environmental resistivity variations favor current distribution to the corroding steel, a modest applied current flows preferentially to the passive steel (Glass and Hassanein, Journal of Corrosion Science and Engineering, Volume 4, Paper 7, 2003).

Under these conditions, it is very unlikely that the applied current will result in the extraction of any chloride from the corroding anodic sites. To achieve a reversal in the direction of the local current at a corrosion site, sufficient current must be applied to drive the potential to a value that will be more negative than the open circuit potential of an isolated corroding site that is not connected to any passive steel . At the most modest applied protection current densities typically used in cathodic protection of reinforced concrete, a net anodic current will always leave places with high corrosion activity. However, realkalization of such sites is still possible because a pH concentration gradient between the surrounding concrete and the corroding sites will provide an additional force to move hydroxyl ions to the corroding sites. This combines with the electric field imposed by the cathodic protection system which weakens the strong electric fields that maintain the high concentration of hydroxyl ion gradients to allow the pH to rise. As the pH increases, the process of establishing actively corroding sites on the steel is reversed until a point is reached where insoluble iron oxides are the most stable corrosion product and the steel passive film is redone. The process of realkalizing the corroding sites to achieve steel passivity in open circuit can be called pit realkalization.

The above analysis suggests that the range of charge densities applied to reinforced steel in concrete to induce steel passivity to open circuit may be an order of magnitude below that previously claimed to be necessary for temporary electrochemical treatments standing alone in US 6322691. Less aggressive environments require less charge .

Charge densities as low as 30 kC/m<2>may be sufficient on repaired concrete structures, a charge of 100 kC/m<2>has been shown to induce steel passivity in heavily chloride contaminated concrete in simulated marine exposure conditions in the laboratory and 600 kC/m<2> 2>seems to be sufficient to realkalize not only the corroding places (pit realkalization), but a significant part of the concrete cover in the realkalization of carbonized concrete.

Improve the electrochemical treatment process

A number of factors can be taken into account when evaluating methods for improving technologies for the electrochemical treatment of reinforced concrete. These include:

- the rate at which the corrosion process is stopped,

- the charge density necessary to stop corrosion,

- the duration of the treatment, and

- the maintenance requirements for the process.

It has been mentioned above that the relatively low charge densities can be used to restore steel passivity. A temporary electrochemical treatment process to stop corrosion can therefore be substantially less intensive than the very intensive temporary electrochemical treatments that are sometimes used. In particular, the period for a temporary electrochemical treatment can be reduced. Therefore, a temporary electrochemical treatment can be used for less than 3 months, and preferably less than 3 weeks. However, the duration of a short-term treatment will be disputed despite the immediate reductions in corrosion rate. Such an initial treatment would be more acceptable if a supplementary long-term corrosion prevention treatment was used.

An improved treatment process would therefore be a hybrid electrochemical treatment, in which an initial charge density sufficient to arrest corrosion and induce open circuit steel passivity was applied and followed by low maintenance cathodic preventive treatment to prevent any subsequent corrosion initiation. It would be advantageous to use the same anode system in both the powerful impressed current treatments to stop corrosion, and in the later low maintenance treatment to maintain steel passivity.

Two examples of such two-step electrochemical treatments include:

- briefly driving a high current of a sacrificial anode to passivate the steel and then connecting the sacrificial anode directly to the steel to provide a low sacrificial current preventive cathodic treatment, and

- applying a voltage to an inert impressed current anode clad with a sacrificial metal element where the sacrificial metal element initially facilitates a high anodic reaction rate associated with a high protective current density to passivate the steel, and when the sacrificial metal is consumed the impressed current anode continues to provide a cathodic preventive treatment with low impressed current.

The average current applied during an initial electrochemical treatment with applied current will typically be at least an order of magnitude greater than the average current mainly applied during the preventive treatment with low current. The low current preventive treatment will usually involve the delivery of an average current density of less than 5 mA/m<2>, and more than 0.2 mA/m<2> to the steel surface.

Treatment technology

The present invention provides, in a first aspect, a method of protecting steel in concrete which includes using an anode and a DC power source, and a temporary impressed current treatment and a low current preventive treatment, wherein the temporary treatment with impressed current is a high current treatment that uses the DC power source to drive the current of the anode to the steel to improve the environment of the steel, and the low current preventive treatment is used to prevent the initiation of steel corrosion after using the temporary treatment with impressed current and the same anode is used in both treatments, and the anode includes a sacrificial metal element that undergoes sacrificial metal dissolution as its main anodic reaction.

In another aspect, the present invention provides an anode for protecting steel in concrete comprising a sacrificial metal element with an impressed current anode connector, wherein the anode is a compact discrete anode and the sacrificial metal element is less noble than steel, and the impressed current anode connector includes a conductor with at least one junction point where the conductor remains passive at potentials less positive than 500 mV above the potential of the reference potential of copper/saturated copper sulfate, and the conductor is substantially surrounded by the sacrificial metal element over a portion of its length to form an electrical link that conducts electrons between the conductor and the sacrificial metal , and the connection point is on a portion of the conductor extending away from the sacrificial metal element where the conductor can be conveniently connected to another conductor.

In a third aspect, the present invention provides use of the anode described in the second aspect of the present invention in the method described in the first aspect of the present invention.

In a fourth aspect, the present invention provides for the production of an activated sacrificial anode embedded in a chloride-contaminated concrete structure which includes providing a path for electrons to travel between a conductor and a sacrificial metal element less noble than steel, forming a cavity in the concrete structure and embedding the sacrificial metal element in a porous material containing an electrolyte in the cavity which leaves part of the conductor exposed to provide a junction point and provide a path for electrons to flow between a positive terminal of a DC power source and the conductor, driving a high current of the sacrificial metal to draw chloride ions present in the concrete to the surface of the sacrificial metal to activate the sacrificial metal and disconnect the DC power source from the conductor.

In a fifth aspect, the present invention provides a method of protecting steel in concrete which includes a temporary electrochemical treatment with a high applied current to improve the environment of the steel followed by a preventive treatment with a low current to prevent the initiation of steel corrosion in which an anode is used in the temporary impressed current treatment, and the same anode is used in the low current preventive treatment, and the anode includes a sacrificial metal element undergoing sacrificial metal dissolution as its main anodic reaction, and the anode is connected to the positive terminal of the DC power source in the temporary impressed current treatment, and the anode is connected to the steel to provide a path for electron conduction from the sacrificial metal element to the steel in the low current preventive treatment.

An example of the preferred hybrid electrochemical treatment is illustrated in Fig. 1. A sacrificial metal element [1] is embedded in a porous material [2] containing an electrolyte in a cavity [3] formed in concrete [4]. The sacrificial metal element is connected to the positive terminal of a DC power source [5] using an electrical conductor [6] and electrical coupling [7]. An impressed current anode connection detail is used to connect a sacrificial metal element [1] to the electrical conductor [6]. This preferably involves the formation of the sacrificial metal element around a portion of a conductor [8] which remains passive during the applied current treatment. The conductor [8] provides a suitable connection point [9] away from the sacrificial metal to facilitate a connection to another electrical conductor. The negative terminal of the power supply [5] is connected to the steel [10] using an electrical conductor [11] and a connector [12]. While the power supply is connected to the anode and the steel, electrical connection [13] is not made.

Initially, a large short-term impressed current is driven from the anode assembly [1, 8] to the steel [10] for a short period using the DC power source [5]. In the process, oxygen and water [14] are converted to hydroxyl ions [15] on the steel. This neutralizes the acid corrosion sites and promotes repair of the protective passive film on the steel. In addition, aggressive ions, such as chloride ions [16], are drawn from the concrete into the porous material [2] around the anode. The local environments around the embedded steel and around the embedded anode are modified by this brief treatment with applied current. The changes mean that the local environment at the steel supports steel passivation while the environment at the anode maintains sacrificial anode activity. The corroding sites are effectively moved from locations on the steel reinforcement to the installed sacrificial anode. At the end of the impressed current treatment, a prolonged low power cathodic preventive treatment can then be applied using the same anode.

It is preferred to disconnect the power supply [5] by electrical connections [7] and [12], and to connect the remaining sacrificial anode metal directly to the steel through electrical connection [13]. The activated discrete sacrificial anode formed by the temporary impressed current treatment is then used in a long-term sacrificial cathodic preventive role to maintain steel passivity. This is preferred because the current output of sacrificial anodes is more reliable than that of a DC power supply and is, to some extent, self-adjusting with more aggressive environments leading to higher sacrificial anode current output. Furthermore, monitoring is not critical to sacrificial anode system function and can be tailored to compliment end-user requirements for the protected structure. A simple method of monitoring performance uses non-destructive potential mapping techniques to determine whether only the regions of anodic activity are located at locations where the discrete sacrificial anodes are embedded.

The links [7, 9, 12, 13] and the conductors [6, 8, 11] are all electron-conducting links or conductors in that they provide a path for the electrons to travel on. They may be referred to as electronic connectors or electronic conductors. The conductors will typically be wires or electrical cables. These conductors and junctions are different from ionic conductors or ionic junctions. The electrolyte in the concrete [4] provides an example of an ionic coupling between the sacrificial metal element [1] and the steel [10]. To achieve sacrificial cathodic protection or prevention, both an electronic coupling and an ionic coupling between the sacrificial metal element and the steel are required.

The DC power sources [5] for the short high current treatment include a DC power supply or a battery powered by a main line. It is an advantage if the connection between the anode and the positive terminal of the power supply is kept as short as possible to minimize the corrosion risk of this connection.

The preferred anode includes a compact discrete sacrificial metal element with an imprinted current anode connector. Compact discrete anodes can be embedded in cavities formed in reinforced concrete. This improves the bond between the anode and the concrete structure. Examples of such voids include holes up to 50 mm in diameter and 200 mm in length that can be formed by core drilling or drilling, as well as longer pipe channels up to 30 mm in width and 50 mm in depth that can be cut into the concrete surface. When the cavities are hole-shaped by drilling, it is preferred to keep the diameter below 30 mm. A number of anodes will typically be distributed over the concrete structure to protect the embedded steel.

The impressed current anode connector is used to connect the anode to the positive terminal of the DC power source. All metals connected to the positive terminal of a DC power source are at risk of becoming anodes if they come into contact with an electrolyte in the surrounding environment, and therefore need to be protected from anodic dissolution if this is not intended. Existing compact discrete sacrificial anodes for reinforced concrete are provided with connecting details consisting of a non-insulated steel or galvanized steel wire that relies on the sacrificial operation of the anode to protect the connecting wire. These links will undergo induced anodic dissolution and corrode along with the sacrificial metal when the anode is operated as an impressed current anode.

A junction detail for impressed current in a compact discrete sacrificial anode can be achieved by forming the sacrificial metal element around a portion of a conductor that includes a second portion that provides a junction point and remains passive when the anode is driven to positive potentials by the power supply. A passive conductor is one on which no significant metal dissolution takes place, and therefore has no visible corrosion-induced breakdown since its potential is driven to positive values. The conductor and sacrificial metal element will be driven to positive potentials during the initial treatment with applied current that is generally more noble (positive) than the reference electrode for copper/saturated copper sulfate, and may be more noble than 500 mV, or even 2000 mV above the reference electrode for copper/saturated copper sulfate. Copper and steel do not naturally remain passive at these positive potentials when in contact with an electrolyte.

The example in Fig.1 shows a sacrificial metal element [1] which is formed around part of a conductor [8] with a second part extending beyond the sacrificial metal and providing a connection point [9]. In order to obtain a passive conductor, an inert conductor which is naturally passive in contact with an electrolyte at the anode potentials that arise in treatment with applied current can be used. Alternatively, the conductor may be isolated from ambient electrolyte by the presence of the surrounding sacrificial metal element and the presence of a layer of insulation on the portion of the conductor that extends beyond the sacrificial metal element to form the junction. The preferred connection detail involves distributing the sacrificial metal element around a portion of an inert titanium wire providing a connection point on an exposed portion of the titanium wire away from the sacrificial metal element to suitably connect the titanium wire to another electrical conductor. This can be another titanium wire or an insulated electrical cable that facilitates the connection of the anode to the positive terminal of the DC power source.

An inert conductor may derive its corrosion resistance from one or more materials, examples of which include carbon, titanium, stainless steel including stainless steel alloys of nickel-chromium-molybdenum, platinum, tantalum, zirconium, niobium, nickel, nickel alloys including hast alloy, monel and inconel . The conductors may be made of these materials or protected with inert coverings of these materials.

Titanium is a preferred material because it is readily available and it resists anodic dissolution over a wide range of potentials.

The use of an inert impressed current anode as the conductor around which the sacrificial metal element is formed allows the anode to be used as an inert impressed current anode in a preventive cathodic impressed current role when the sacrificial metal element around the inert anode is consumed. This extends the functional life of the anode system. Examples of inert impressed current anode include metal oxide coated titanium, platinized titanium and platinized niobium.

The inert anode conductor will, in theory, be surrounded by a porous metal oxide or salt resulting from the dissolution of the sacrificial metal. This provides a layer that maintains a pH gradient between the inert anode and the surrounding concrete that limits acid attack on the surrounding concrete. It also provides a route through which any gas generated at the anode can escape. These characteristics will allow an inert anode core to be operated at current densities above the limits normally imposed on the use of such anodes when making direct contact with cement mortar or concrete.

A conductor, such as steel, can be made passive by using an insulating material to separate the conductor from the electrolyte in the surrounding environment. This prevents corrosion-induced breakdown of the portion of the conductor not protected by the sacrificial metal when the anode is used in its impressed current role. In this case it is preferred to extend the insulation either into the anode metal or over the surface of the anode metal where the conductor enters the anode metal. This is to maintain the separation of the conductor from the electrolyte in the surrounding environment as the sacrificial anode metal dissolves around the conductor. It is preferred to isolate all cable connections between the anode and the positive terminal of the DC power source from the electrolyte in the surrounding environment.

The sacrificial metal is preferably less noble than steel. Examples include zinc, aluminum or magnesium or alloys thereof. An aluminium-zinc-indium alloy is preferred. Aluminum has a high charge density and therefore an advantageous life-to-volume ratio. The alloying elements promote anode activity which is further promoted by the presence of chloride contamination in the surrounding environment.

The most important anodic reaction occurring at a sacrificial metal anode is the dissolution of the sacrificial metal. This oxidation reaction occurs much more easily than the oxidation of water to produce acid and gas, which is the most important anodic reaction occurring on an inert impressed current anode. Greater anode current densities can therefore be delivered at low drive voltages from sacrificial metal elements. The dissolution of the sacrificial metal produces a metal salt. The production of gas can be avoided and the only acid produced is the result of the secondary hydrolysis reaction of the metal salt. This secondary reaction will be limited. The minimum pH value is determined by the equilibrium between the metal salt, the acid present (which determines the pH) and the metal oxide. The problems associated with the production of acid and gas that often occur on impressed current anodes can be avoided by the use of a sacrificial metal element in the anode. In this way, current densities greater than 200mA/m<2>and preferably greater than 1000 mA/m<2>expressed per unit area of anode surface can be achieved on a built-in anode without significant degradation of the surrounding concrete occurring.

The preferred placement of sacrificial anode materials in the past has been on concrete surfaces where they are accessible and easily replaced. However, loss of adhesion to the concrete substrate and rapid drying of the concrete surface in the absence of moisture limit the performance of surface anodes. These problems can be overcome by embedding the sacrificial metal anodes in a porous material in voids in the concrete. The porous material holds the anode in place while its porosity also holds the electrolyte and provides space for the products of anode dissolution. To adapt the products for anodic dissolution, it is preferred that the porous material has "putty like" properties, including a compressive strength of less than 1 N/mm<2>, and preferably less than 0.5 N/mm <2>, and contains compressible void space.

A characteristic of the use of a sacrificial metal in an impressed current role is the ease with which any accidental shorting of anode steel (contact between the anode and the steel that provides a path for electrons to flow directly from the anode to the steel) can be resolved. This is because the sacrificial metal preferentially corrodes at locations of dissimilar metal short circuits to generate a metal oxide that breaks the direct short circuit.

An advantage of using an embedded sacrificial metal anode is the high impressed current density that can be delivered by the anode. The magnitude of the current was determined by determining the anodic polarization behavior (anode current output as a function of anode potential) of an aluminum alloy anode embedded in mortar in a hole in concrete, and comparing this polarization behavior with that determined on a titanium inert anode coated with mixed metal oxide (MMO) (mixed metal oxide) in the same environment.

An aluminum alloy was distributed around an MMO coated titanium wire to produce a sacrificial anode with an exposed aluminum surface of 2180 mm<2>connected to a length of exposed titanium wire. The aluminum alloy was US Navy specification MIL-A-24779(SH). A 1.0mm<2>copper core clad cable was connected to the exposed titanium wire. The copper-titanium connection was maintained in a dry environment above the concrete.

An inert anode was fabricated using a short length of MMO coated titanium ribbon connected to a 1.0mm<2> copper core clad cable. The connector was insulated and the exposed MMO coated titanium surface measured 1390mm<2>.

The polarization behavior (potential-current ratio) of the aluminum and MMO-coated titanium anodes was determined using the experimental setup shown in Fig.2. A concrete block [20] measuring 300mm long by 140mm wide and 120mm deep was distributed using dry 20mm all inclusive graded aggregate (0 to 20mm), ordinary Portland cement (OPC) and water in the proportions 8: 2:0.95 by weight, respectively. Sodium chloride was dissolved in the water before mixing the concrete to contaminate the concrete block with 3% chloride (expressed as a percentage by weight of chloride ions to cement).

While the concrete was still flowing, two holes [21] 22 mm diameter and 90 mm deep were formed 200 mm apart in the concrete block by pressing a rigid plastic tube into the concrete. A steel rod [22] with a diameter of 10.5 mm and a length of 140 mm was located in the concrete in the middle between the two holes. It extended 40mm above the concrete surface. The ends of two flexible Luggin capillary tubes [23] with an internal diameter of 2 mm were located in the concrete midway between each hole and the steel bar. Two additional steel rods [24] were embedded 100 mm apart and equidistant from the holes [21] to be used as counter electrodes in the test. Clad copper core cables were connected to the exposed ends of the steel rods.

After the concrete had hardened, the rigid plastic tubes were removed, the aluminum anode and the MMO-coated titanium anode were centrally located in separate holes [21] and the remaining space in the holes was filled with a gypsum-based finishing mortar to leave an incision in the surface above the anode. The mortar was left to harden to form a rigid porous material. The Luggin capillary tubes [23] were filled with a conductive gel made by heating with stirring, a mixture of agar powder, potassium chloride and water in the proportions 2:2:100 by weight, respectively. The gel-filled Luggin capillary tubes extended into small containers [25] containing a saturated copper sulfate solution. A piece of light ground copper [26] was placed in each container to form two copper/saturated copper sulfate reference electrodes. A copper core cable was connected to the copper of the reference electrode with and the connection was insulated.

A potentiostat and function generator [27] was used to control and vary the potential of the anode relative to the potential of the reference electrode by sending current from the counter electrodes to the anode under test. A separate test was carried out for each anode. An anode and its nearest copper/saturated copper sulfate reference electrode were connected to the working electrode (WE) and reference electrode (RE) terminals of the potentiostat/function generator, respectively [27]. A 5 ohm resistor [28] and a relay switch [29] were connected between the counter electrodes and the counter electrode terminal (CE) of the potentiostat/function generator. Clad copper core cables [30] were used in all the connections. The testing took place indoors at a temperature between 7 and 15 ̊C.

The incisions in the mortar above the anodes were periodically moistened.

The measurements included the anode current, the current-on potential of the anode measured relative to the reference electrode while the current was flowing and the instantaneous-off potential of the anode measured between 0.02 and 0.07 seconds after interrupting the current every instant from the anode for a period of no more than 0.15 seconds using the relay switch [29]. The instantaneous-off potential of the anode is a corrected potential, in which the geometry-dependent voltage drop between the anode and the reference electrode induced by the current is subtracted from the current-on anode potential. These measurements were recorded using a high impedance data logger which also controlled the relay switch. The potential of the anode was initially controlled at a value close to its natural potential in the absence of any current. The controlled potential was then increased at a rate of 0.1 mV/s to approximately 2000 mV relative to the reference electrode to obtain the polarization behavior.

Fig. 3 shows the current density output from the aluminum anode and MMO-clad titanium anode as a function of their current-on potentials and instantaneous-off potentials measured relative to the reference electrode 10 days after casting the concrete. The current density on the axis is expressed as current per unit area of anode surface and is plotted against the potential in mV relative to the reference electrode of copper/saturated copper sulphate on the axis. When the on-potential of the aluminum node increased to 2000mV, the current density of the aluminum increased to 16000 mA/m<2>, and the instantaneous off-potential of the aluminum increased to 1000mV. In contrast, the current of the MMO-coated titanium anode was only significant since its potential was increased above 1000 mV. At an on-potential of 2000 mV, the current density of the MMO-coated titanium anode approached 3000 mA/m<2> and its instant-off potential was 1400 mV.

The aluminum was therefore able to generate much higher current densities at lower anode potentials. In fact, the current density delivered by the aluminum anode was greater than 10,000 mA/m<2>when its instantaneous-off potential reached the potential of the copper/saturated copper sulfate reference electrode.

The comparison of the anodic polarization characteristics of an aluminum anode with those of an MMO-clad titanium anode in this example demonstrates the significant advantages to be gained by using embedded sacrificial metals in an impressed current role. The use of a sacrificial metal embedded in a porous metal in a hole in the reinforced concrete allows impressed current anode densities to be achieved that are significantly greater than any of those achieved using existing impressed current anode technology with the same driver voltage.

One issue concerning the use of a sacrificial metal anode embedded in a porous material used on reinforced concrete is the life of the anode. The life of the hybrid anode is related to its size and current output. A typical size for a long life is calculated using the following assumptions:

- A current of 500 mA/m<2>applied to the steel for one week will change the local environment at the steel to induce steel passivation.

- An average protection current of 1 mA/m<2>will maintain steel passivity and prevent corrosion initiation for the next 50 years.

- The installation of 4 anodes per square meter will achieve a reasonable current distribution.

- A discrete aluminum alloy anode is used with a density of 2700kg/m<3>, a charge density of 2980 Ah/kg and an efficiency of 93%.

A current of 500 mA for 7 days followed by 1 mA for 50 years is equivalent to a charge of 522 Ah, or 140 Ah per anode. The properties of the sacrificial metal indicate a useful charge of 7458 Ah per liter of anode metal and a 130 Ah anode can be obtained with an anode volume of 0.0174 litres. This can be achieved with an anode that is 15 mm in diameter and 100 mm in length. The installation of four anodes of this size for every square meter of steel surface in a concrete structure is a relatively simple task.

As mentioned above, 500 mA/m<2>applied to the steel for one week will be more than sufficient to induce changes in the environment leading to steel passivity in most cases. A cathodic prevention current density of 1 mA/m<2> is in the middle of the expected range of current density for cathodic prevention given in BS EN 12696:2000. This calculation shows that it is practical to use built-in sacrificial anodes in a hybrid electrochemical treatment, and to achieve a long service life.

The invention will now be further described in the following examples.

Example 1

An anode 15mm in diameter and 100mm long comprising a rod of aluminum alloy known to US Navy specification MIL-A-24779(SH) which was distributed around a titanium wire to facilitate electrical connection to the aluminum was embedded in lime putty in a 25mm diameter by 130mm deep hole in a concrete block. The basic arrangement is shown in Fig.1. The concrete block measuring 380 by 220 mm was made using graded all included in one 20 mm aggregate and ordinary Portland cement in a ratio of 8:1. The ratio of water to cement was 0.6, and 4% chloride ion by weight of cement was added to the mixture by dissolving sodium chloride in the mixing water. A sheet of steel with a surface area of 0.125m<2> was included in the concrete block. The lime putty was produced by slaking and storing burnt lime, and was taken from a producer of lime putty and lime mortar. The hole in the concrete block containing the lime putty and the anode had been left open to the air. The concrete block was stored in a dry indoor environment and the temperature varied between 10 and 20C.

The anode and steel were connected to a 12 volt DC power supply for a period of 13 days during which a charge of 65 kC was delivered from the anode to the steel. The current density delivered by the anode in the first 11 days is given in Fig.4. During most of this time, the current delivered by the anode was greater than 5000 mA/m<2>.

At the end of the impressed current treatment period, the DC supply was removed and the anode was connected to the steel. The galvanic current of the anode was measured using a resistance of 1 ohm as a current sensor in connection between the anode and the steel. The current density delivered by the anode operating only in a galvanic mode for the next 40 days is given in Fig.5. During most of this period, the current density delivered by the anode was between 500 and 600 mA/m<2>.

It may be noted that the presence of 4% chloride in the concrete represents a very aggressive environment leading to a very high current output of the sacrificial anode when operating in both impressed current and galvanic modes.

Example 2

Twenty-five aluminum alloy nodes 15mm in diameter and 100mm long described in Example 1 were embedded in a concrete column containing steel reinforcement with a steel surface of 3.2m<2>. The column was protected from rain and moisture and was very dry, but was within sight of the sea and exposed to airborne chloride pollution. The anodes were installed by drilling 25mm holes 180mm into the concrete, partially filling the holes with a mixture of lime putty and 10% polystyrene, and finally pressing the anodes into the putty until they were fully embedded in the putty. The anodes were evenly spaced over the column, and were located between the reinforced steel bars.

The anodes were connected to the positive terminal with a 12 volt DC power supply and the steel was connected to the negative terminal for a period of 8 days, during which time a charge of 67 kC/m<2> was delivered to the steel surface. The current density delivered by the anode during this period is given in Fig. 6. The current delivered by the anodes varied between 4500 and 1500 mA/m<2>. After this initial treatment, the holes containing the anodes were sealed with a standard cement mortar repair material.

At the end of the impressed current treatment period, the DC supply was removed and the anodes were connected to the steel. The galvanic current of the anodes was measured by using a 1 ohm resistor as a current sensor in the connection between the anodes and the steel.

The current densities delivered by the anodes operating in a purely galvanic manner for the next 30 days are given in Fig.7. The galvanic current density delivered by the anodes was between 80 and 150 mA/m<2> which is equal to a protective current on the steel surface between 3 and 5 mA/m<2>.

Very dry conditions represent a relatively non-aggressive environment, and both the impressed anode current density and the galvanic anode current density were low compared to the data obtained in Example 2. However, the galvanic current delivered to the steel as a preventive treatment is relatively high for cathodic prevention , especially in this environment. The remaining life of the sacrificial metal in the anodes delivering 3mA/m<2>to the steel, assuming an anode efficiency of 70%, was calculated to be 28 years, and will be longer if the average cathodic preventive current density stabilizes at a lower value.

Industrial application

The industrial use of the presented technology relates to methods and products for stopping and preventing corrosion of steel in reinforced concrete structures.

Advantages of the presented technology include rapid prevention of steel corrosion, short on-site processing time, no usual long-term maintenance, easy installation and self-correction of accidental anode to steel short circuits. Standards applicable to this technology include BS EN 12696:2000 (cathodic protection of steel in concrete) and prCEN/TS 14038-1 (electrochemical realkalization and chloride extraction treatments for reinforced concrete).

Claims (25)

Patent claims 1. Application of an anode and a DC power source to protect steel in concrete, where the application includes: applying a current from the anode to the steel using a DC power source to temporarily supply the applied current treatment means to improve the environment of the steel stopping steel corrosion, and characterized by a subsequent provision from the same anode to the steel of a preventive treatment with a low current designed to prevent the initiation of steel corrosion, in which the temporary treatment with applied current is a treatment with a high current in relation to the preventive treatment with a low current, and where the anode comprises a sacrificial metal element undergoing sacrificial metal dissolution as its main anodic reaction, in which the low current preventive treatment is achieved by providing a path for electron conduction from the sacrificial metal element to the steel. 2. Application as stated in claim 1, in which the treatment with temporarily applied current is less than 3 months in duration. 3. Application as stated in claim 2, in which the treatment with temporarily applied current is less than 3 weeks in duration. 4. Application as stated in claims 1 to 3, in which the temporary impressed current is at an anode current density greater than 200 mA per square meter anode. 5. Application as stated in claim 4, in which the temporary applied current is at an anode current density greater than 1000 mA per square meter anode. 6. Use as stated in one of claims 1 to 5, in which the average current in the treatment with temporarily applied current is at least an order of magnitude greater than the average current in the preventive treatment with low current. 7. Application according to one of claims 1 to 6, in which the preventive treatment with low current comprises applying a current of less than 5 mA per square meter of steel to the steel 8. Application according to one of claims 1 to 7, where the preventive treatment with low current has low maintenance in relation to the applied current cathodic protection. 9. Application according to one of claims 1 to 8, where the preventive treatment with low current is a long-term treatment. 10. Application as stated in claim 9, in which the anode is embedded in a porous building material attached to the concrete for long-term use. 11. Application as stated in claim 10, in which the anode is a compact, discrete anode embedded in a porous material in a cavity formed in the concrete. 12. An anode adapted for use according to one of claims 1 to 11, comprising - a sacrificial metal element with a connection detail for impressed current anode characterized in that - the anode is a compact discrete anode designed to be embedded in the concrete, and the sacrificial metal element is less noble than steel, - and the connection detail for impressed current anode includes a conductor with at least one connection point where the conductor is protected from anodic dissolution and remains passive at potentials at least as positive as 500 mV above the potential of the copper/saturated reference potential, and the connection point is on a part of the conductor that extends away from the sacrificial metal element where the conductor may be suitably connected to another conductor. 13. An anode as set forth in claim 12, wherein the compact discrete anode is small enough to fit into a drilled or drilled cavity in concrete 50mm in diameter and 200mm in length. 14. An anode as set forth in claim 12, wherein the compact discrete anode is small enough to fit into a slot cut into a concrete surface 30 mm wide and 50 mm deep. 15. An anode as set forth in one of claims 12 to 14, wherein the sacrificial metal element is formed around the conductor over part of its length. 16. An anode as set forth in any one of claims 12 to 15, wherein the conductor remains passive at potentials at least as positive as 2000 mV above the potential of the copper/saturated copper sulfate reference potential. 17. An anode circuit as set forth in any one of claims 12 to 16, wherein the conductor is an inert conductor which remains passive when exposed to an electrolyte. 18. An anode as set forth in claim 17, wherein the inert conductor is titanium. 19. An anode as set forth in claim 17, wherein the inert conductor is an inert impressed current anode. 20. An anode as set forth in claim 19, wherein the inert impressed current anode is selected from the list including metal oxide coated titanium, platinized titanium, platinized niobium. 21. An anode as set forth in any one of claims 12 to 16, wherein the conductor remains passive because it is isolated from the electrolyte in the environment by a layer of insulating material. 22. An anode as set forth in claim 21, wherein the insulating material extends into the body of the sacrificial metal or extends over a portion of the sacrificial metal surface when the conductor enters the sacrificial metal. 23. An anode as set forth in any one of claims 12 to 22, wherein the sacrificial metal element includes aluminum or zinc or magnesium or an alloy thereof. 24. A combination of an anode as set forth in one of claims 12 to 22 and a porous embedding material adapted to embed the anode into a cavity formed in the concrete. 25. A combination according to claim 24, in which the porous build-in material has a compressive strength of less than 1 N/mm<2>