GB2485887A

GB2485887A - An anode for the protection of reinforcing steel in concrete

Info

Publication number: GB2485887A
Application number: GB1120100.1A
Authority: GB
Inventors: Michael Henry Bingham
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-11-23
Filing date: 2011-11-22
Publication date: 2012-05-30
Anticipated expiration: 2031-11-22
Also published as: GB201120100D0; GB2485887B; GB201019788D0

Abstract

An anode 4 for use in the cathodic protection or reinforcing steel in concrete is described. The anode system comprises a conductive coating for application onto concrete surfaces where the anode 4 is not in direct contact with the concrete surface 3 but is separated through the provision of an electrically conducting paint coating layer or primary layer 5. The anode 4 is thus in electrical contact with the concrete surface through the primary layer 5. The anode 4 itself may comprise an electronically conducting paint coating layer. The conductivity of the primary layer 5 may be due to the use of water imbibed from the environment of the primary layer 5.

Description

DESCRIPTION

"IMPROVED ANODE FOR CONCRETE"

A BACKUEQUED

Al Introduction

Steel can corrode in concrete where, for example, chlorides destroy the normal steel passivity at small areas of the steel surface. Electrochemical methods are able to halt and prevent this corrosion of reinforcing steel. See, for example, Cathodic Protection of Steel in Concrete', editor P.M.Chess, E&FN Spon 1998, ISBN 0 419 230106.

Application of cathodic protection (CP) to concrete presents problems of designing and installing a suitable anode (an electronic conductor) able to make intimate contact with the concrete. This is necessary because concrete carries the aqueous materials that are the electrolyte of the CP circuit.

Impressed current anode materials in current use include carbonaceous composites.

These contain powdered cokes, carbon blacks, graphites, carbon fibres or combinations of these dispersed within either a polymer or a cement bound matrix.

The polymer bound carbonaceous composites are most frequently thin layer paint coatings, often referred to as "conducting coating anodes" or as "conducting paint anodes". They are typically used at dry film thicknesses from around 100 to 500 microns and generally possess resistivities in the range 0.05 to 1 ohrn.cm. They are applied, using normal painting procedures, directly onto prepared concrete surfaces.

Concrete surface preparation and dryness are very important for adequate adhesion of a conducting-coating anode. Loose, weak or contaminated surface concrete is removed, for example by grit blasting. Organic materials and salts are removed using, for example detergents, steam or high pressure water washing and then drying.

Where in-service failure of conducting coating anodes does occur it might be evident over time-scales ranging down to less than a year. Failure after only a few years service can be attributed to a substantial problem of surface, coating, operation and/or environment. Slow, long-term failure might occur with low or modest anode current densities and appears as localised loss of adhesion to the concrete surface and increased electrical resistance at the coating-concrete interface.

The Inventor has explored aspects of premature failure of conducting coating anodes on concrete and has proven a novel method for limiting early anode failure and prolonging long term anode life. This method is described herein.

A2 Concrete Repair Materials Repairs are made to concrete for many reasons using a wide range of materials. See for example National Research Council of Canada, Technology Update No 59, October 2003.

Reinforced concrete that is to be fitted with CP frequently requires some degree of repair prior to the CP installation. Where repair materials lie within a protective current path it is desirable that current continuity is maintained. A typical specification for such repair materials includes a conductivity requirement e.g. The UK Highways Authority in BA 83/02 requires a conductivity within 50% to 200% of the parent concrete and elsewhere quotes 7-15 Kohm cm for water saturated repair material.

Repair materials are most often sand and cement based and usually incorporate some organic polymer for improved adhesion and mechanical properties. Repair materials are not part of a conducting coating anode, though such anodes are often applied to surfaces where some fraction of the surface has been repaired.

B CONDUCTING COATING ANODES FOR CONCRETE

Bi PriorArt Conducting-coating CP anode systems were pioneered during the early 1970's by Philip Gay e.g. U.S. 3,868,313, for use on steel substrates. They were intended provide cathodic protection andlor anti-fouling in marine use.

A polyamide-epoxy/carbon conducting coating anode for concrete was trialed in Canada and the USA during the late 1970's under the name "Anodon". Proprietary coating Permaguard' SC5/6 is a carbonlchlorinated rubber anode for concrete and was introduced in 1982 (previously licensed to Permarock Products Ltd. Perniaguard is a trademark of The Electric Coating Company Ltd). In this anode a surface treatment with adhesion promoter wash is followed by the anode coating at circa 110-150 microns dry film thickness (dft) and resistivity less than 0.15 ohm.cm.

Adhesion promoters are low viscosity solutions, of coupling agent or surfactant, applied to coat the walls of the concrete surface pores to encourage wetting and penetration by the following conducting-coating anode. Adhesion promoters meld and become integral with the conducting coating anode and are not designed to form a continuous or intact coating layer on or in the concrete surface because this may interfere with current flow from the anode.

Later anode coatings have employed alternatives to the conductive andlor binder components or use water based materials.

Alternative conductive pigments include cokes as in US-4806272, US-4908 157 and US-4632777; short carbon fibres as in US-5069822 and US-53645 11; nickel metal and nickel coated carbon as in US-5069822 and US-53645 11; titanium oxide as in US- 5173215; graphite as in US-6635 192. Alternative binder examples include acrylics as in US-4632777 and US-493 1156; water based urethane resin as in US-5041242; ester resins as in US-4908 157. These systems do not isolate the conducting coating anode from the immediately adjacent concrete surface.

US-53645 11, discloses metal-coated carbonaceous particles for conduction by the anode. Alkaline buffer material is incorporated into the conducting-coating structure hoping to preserve the passive oxide film on the coating metal, by inhibiting the build up of acidity in the coating. However, US-53645 11 does not consider or attempt to reduce the effects of acidity upon the adjacent concrete surface; this system does not isolate the anode processes from the immediately adjacent concrete surface.

EP-059 1775-Al discloses a sprayed zinc metal anode applied over an aggregate-containing primer on the surface of reinforced concrete. The key property of the primer appears to be surface roughness. The primer binders quoted (page 3 lines 3 8-45) are generally taken to be good insulators. The 10-200uM aggregates listed include potentially very good insulators for example plastic powder (page 3 line 28-30). The primer example (page 5) contains a coarse silicon carbide aggregate and it is known that some silicon carbide particles can possess semi-conducting properties, possibly allowing this primer to act as an anode in contact with the concrete surface. However, EP-059 1775-Al does not demonstrate, mention or imply CP current flow through the primer coating from anode to concrete; the test results reported are entirely consistent with CP current flow via the external sodium chloride test solution during immersion, or via salt solution retained on the panel surfaces after immersion.

US-5004562 discloses an electrically conductive polymer composition made by admixing into latex a so! or gel that has been prepared by a heating process. This composition is loosely said to have application in polymer coatings for CF (col 2 line 63, col 8 line 16), but there is no further indication. The reported conductivities were measured using voltages greater than 350 volts with no indication of conduction for voltages below 20 volts.

US-S 650060 discloses an ionically conductive agent interposed between a galvanic anode and concrete The galvanic anode is a self supporting plate, or is supported on an inert substrate (col 5, line 35,36), i.e. it is not a paint coating, nor is it an inert carbon containing anode. Ionic conduction agent is first applied to zinc foil, prior to attachment to concrete (col 12, lines 31, 32, col 13 lines 43, 44). Claims and examples invoke ionically conductive agent only in the form of a pressure sensitive adhesive.

US-663 5192-B 1 discloses a conducting-coating anode, which is claimed to be an electronically conducting microcapillary composite matrix. The coating contains an alumino-silicate, an alumino-hydroxo complex or an aluminium phosphate and uses graphite for conduction. US-6635192-Bl considers acid attack at the concrete surface and suggests that anode life can be extended by the formation of extremely fine porous structures in the pore spaces extending down into the concrete. This system does not isolate the anode processes from the immediately adjacent concrete surface.

WO 98/16670 considered some sprayed metal and conducting paint anodes which showed an increasing circuit resistance with time, where drive voltage required progressive increases to maintain anode current. WO 98/16670 proposes that this resistance increase is caused by drying in the concrete near to the anode.

WO 98/16670 discloses a sprayed metal or conducting paint anode applied to an exposed concrete surface to establish an anode/concrete interface. A burnectant in free flowing form is passed through the anode to become present at or near the anode/concrete interface, to increase the moisture content at the interface and so aid current flow from the anode into the concrete. Additionally, the method allows for the introduction of a lithium salt, through the anode, to reduce alkali-silica attack of some concrete aggregates and the introduction of pH buffers, through the anode, working in the range pH 10-13 to reduce the passivation of zinc anodes.

WO 98/16670 notes that the application of humectant may precede the application of a metal anode but that this is not preferred because there might be interference with the anode-concrete bond. WO 98/16670 does not indicate the extent to which this humectant is absorbed by the concrete, or how any non-absorbed humectant is removed, to yield the exposed concrete surface required by the method.

In the case of a conducting paint, WO 98/16670 does not mention a preceding application of humectant; the paint should be inherently porous and this allows the humectant to be applied to the exposed surface of the conducting paint where it then migrates to the concrete/paint interface. The presence of humectants and water in concrete surfaces, during the application of a conducting paint anode, can lead to early anode disbondment as is well known to coatings practitioners and this is alluded to in WO 98/16670 does not consider or mention any system wherein the anode is deliberately prevented from interfacing with exposed concrete, neither does it consider or mention any system wherein the anode is specifically applied to a surface other than exposed concrete. This system does not isolate the anode processes from the immediately adjacent concrete surface.

B2 Prior Art -The Essential Features

i) The anode material is an electronic conductor in direct and intimate contact with the outermost concrete surface layer.

ii) The electrolyte is carried to the anode surface by the concrete of the protected structure.

iii)Anode processes and acidic species generated at the anode surface make immediate contact with the outermost layer of the concrete.

iv) Conductive-coating anodes are dependent upon the outermost layer of the concrete for mechanical support and for connection to the electrolyte.

v) Drying of the concrete in the region of the anode can increase the circuit resistance.

Drive voltages must be increased to maintain the required anode current. This drying might be mitigated by passing a humectant material through the anode to the anode/concrete interface.

vi) Acid attack of the concrete in contact with the anode might be mitigated by the introduction of a microcapillary alumino complex into the pore spaces of the concrete by incorporating the complex into the anode coating material.

13.1 ObsQrvations By The Applicant -Not Previously Published vii) Water loss from the coating-to-concrete interface will cause any acidic solution located there to become more concentrated and consequently reduce the p11 of the solution in immediate contact with the concrete. Water loss can occur because of, for example, drying in warm, dry weather or electroendosmosis.

Conducting coating anodes can allow relatively rapid loss of water vapour to the atmosphere under some drying conditions.

Electroendosmosis carries water from anode to cathode within a porous structure where the pore walls have acquired a negative surface charge; conversely, a positive pore surface charge transports water in the cathode to anode direction. Various cement matrices may acquire a positive charge on the pore walls under alkaline and mildly acidic conditions. Silica acquires a negative surface charge over a broad pH spectrum.

Acid attack of the outermost surface of concrete produces a zone that is essentially silica and it follows that anode current may tend to drive water away from the anode interface into the concrete.

viii) The movement of hydroxyl and chloride ions from the concrete bulk to the anode concrete interface are diffusion processes whose rates depend upon gradients of concentration and potential. Viewed in cross section, the lowest pH occurs at the anode-concrete interface during current flow. The potential gradient may also direct a drift of cations away from the anode into the concrete (where any cation species is both free to move and in an environment allowing a degree of mobility).

The actual pH at the anode concrete interface is dominated by the balances of rates of arrival of anions such as hydroxyl and chloride ions, and the rates of anode processes.

The various anode processes have rates dependent upon local current density and overvoltage. For a specific situation there is a critical current density where (for example) hydroxyl ion arrival rate is just sufficient to maintain a safe pH at the anode surface. As current density is increased, above the critical, the anode-interface pH reduces below the critical pH for dissolution of the cement component of the concrete; this dissolution may, for example, proceed by migration of aluminium or calcium ions away from the anode surface region at a rate now significant with respect to an installation life of circa 10-20 years.

ix) CP power supplies for conducting-coating anodes are generally of the constant-current type. Where localized anode to concrete failure occurs, the current intended for this locality may instead flow into the surrounding, functional anode area and can accelerate acid-attack there. The effect is progressive and self accelerating and it is

S

retarded by applying supply voltage limits.

x) "Localised current dumping" always occurs to some degree. Conducting coating anodes are used on extended areas which, though separated into discrete groundbeds', display significant point-to-point variations, about the mean, for current density delivered into the concrete. Many factors are involved and some factors are constant whereas others have daily or seasonal variation. Factors include surface temperature, Relative Humidity, wetness (eg dew deposition), salt level in concrete, immediate area of reinforcing steel, anode steel distance, concrete porosity, aggregate fraction etc. Also of note are current density changes across an anode groundbed due to potential drops, along the anode coating, outwards from the current bus and along the current bus from the supply connector, and voltage reduction (localised or extensive) caused by any partial short-circuiting of anode to steel at concrete defects. Note that some writers have referred to the current bus as a "primary anode".

Aggregates exposed at the concrete surface @rior to anode coating) offer two relevant problems because they are non-conducting and do not supply hydroxyl ions. Macroscale differences in degree of aggregate exposure within the same groundbed cause differing current densities, flowing into the OPC-sand matrix, in different zones of the groundbed. Microscale current densities at and around the edges of an aggregate face (on the scale of a few mm) can be significantly higher than the mean current density for the local cement matrix. A degree of aggregate exposure would seem inevitable for most prepared surfaces.

There will always be some variation of current density across the anode surface. As drive voltage and hence average current density is increased, the difference between highest and lowest current density increases.

Localised current dumping effects can be greatly exacerbated by conducting-coating anode materials having A R6 values significantly greater than 1. A R. is the relative change in resistivity from dry to wet, i.e. (resistance whilst immersed in water)/(dry resistance). Some commercial materials have displayed this change to a disturbing degree.

It may be noted that some coating anode compositions display a slow inexorable increase in electrical resistivity; even without current flow, some coatings cease to be electrically viable after 15, 10 or evenS years.

xi) The pH for attack of an Ordinary Portland Cement matrix was investigated.

Acid attack was examined for a specimen cement matrix (OPC/building sandl3O mesh sand 2/1/1, the OPC was Lafarge HP40 to 85 EN 197-1) formed into 1 cm cubes and cured at 100% RI-I for two months at 20°C. Cubes were then immersed in water in individual flasks. Small additions of dilute hydrochloric acid were made each day (or more frequently at first) to bring each flask to a particular pH. A ladder of pHs were used from pH 12 to pH 0.5. The cubes were monitored for loosening of sand particles or, at low pH, for attack depth by penetration of a fine steel probe.

Rate and depth of attack was found to be strongly p11 dependent up to pH 2.5. Cement was degraded to loose sand that acquired a characteristic deep yellow colour. Where attack did occur it was readily detectable at the cement cube surface within less than one day. Figures 1, 2 and 3 illustrate softening depth with pH and time.

Above pH 3.5-4.0 no softening or loosening of sand particles occurred though the test continued (with daily pH adjustment) for over two years. Any loosening of individual sand grains is particularly easy to observe in this test.

This test was a daily cycling of the cement surface between a defined pH and a higher p1-I acquired, on standing for one day, as hydroxyl ions migrated from the cement interior to the surface of the cube. It was clear that time and the rate of acid addition were critical factors for the absence of acid attack of the cement surface. Where sufficient alkalinity was present at the cement surface then small excursions of solution pH to just below 3.5 did not produce acid attack; sufficient alkalinity at the surface required time for hydroxyl ion migration from the bulk to the surface of the cube. Other cement types may have a different pH for susceptibility to attack.

This test demonstrated that there was a critical acid addition rate. Below this critical acid addition rate, the cement surface was not attacked because sufficient bydroxyl ions were migrating from the bulk cement to the surface to maintain a pH greater than 3.5.

Above this critical acid addition rate, hydroxyl migration was too slow to maintain the pH above 3.5 and cement attack occurred. This can be directly translated into the situation where acid is being generated at a cathodic protection anode where the rate of acid generation is a function of anode current density, i.e. when all other factors are constant there is a critical, minimum current density for occurrence of anodic acid-attack of the concrete surface.

B4 Operational Life of Existing In the experience of the Applicant, a good conductive coating anode operating with a current density circa 4-7 mA/sq.M can be satisfactory for very substantially more than years. At 30+ mA/sq.M coating failure is seen in less than 12 months and drive voltages will have become very high. Observed failure involves adhesion loss between coating anode and concrete surface; the concrete outer surface has become weak and friable, and appears as poorly bound sand with a characteristic deep yellow colour; the unsupported anode coating can readily flake away; there might also be evidence of loss of carbon and binder from the coating anode and visible dark stains, bleeding through defects in any decorative top coating.

In practice a limited failure after some years might be acceptable, for example circa 5% of total surface area. For aesthetics a patch-repair, andlor a reduction in current or peak drive voltage, may be all that is required.

There is no simple correlation between the apparent current density and anode life on a real installation. Those areas that operate always below their critical current density might achieve a virtually indefinite life. In the Applicant's experience, as average groundbed current density is increased up to and through 15 to 25 to 35 mAlsq.M, the life to failure falls rapidly because the incidence, extent and rate of localised current dumping increases. Some practitioners in the field draw an inverse correlation of lifetime with total quantity of charge passed. The Applicant believes this to be only of loose value for coating anodes being operated at low to modest average current densities; that the core issue is the time to critical acid-attack of the outermost part of the concrete surface layer, over only part of a groundbed; and that this critical attack can occur in a very short time (months, weeks or sometimes a few days) and might correlate with an event for example hot-weather, drying, or ponding of water; and that this initial attack and disbonding encourages further, progressive localised current dumping elsewhere in a groundbed at a fixed total anode current.

In the Applicant's experience a number of other factors can accelerate anode coating failure of a general or localised nature, including: -Poor anode coating penetration and adhesion to the concrete surface caused by inadequate preparation, contaminants e.g oils, fats, waxes; previous surface treatments, aggressive presence of fungal and algal growth, and materials in the concrete that can migrate to the surface including salt, urea, water. Urea can be a more severe problem than salt, being more readily carried to the surface by water which then evaporates to leave a closed surface (and a characteristic mat of fine urea needles in the worst cases).

Urea is used as a non-corrosive de-ieer on some structures.

-Poor anode coating bond to concrete caused by high concrete porosity (often referred to as suction'). The concrete surface strongly absorbs an excessive fraction of the anode coating binder; the coating to concrete junction becomes excessively rich in conducting pigment and this part of the coating layer becomes underbound and physically weak.

-Short-circuits and partial short-circuits between conducting coating anode and reinforcing steel. Partial short-circuits may exhibit resistance in the range from below ohms to over 200 ohms and can be exceedingly difficult to detect or trace.

It should be noted that rate of failure of conductive-coating anodes and the effects of acid attack of concrete can be reduced via conducting-coating formulation, for example in Permaguard SC5/6. In this anode system, good penetration into the concrete surface delays adhesion loss due to acid attack; localised current dumping is mitigated by optimized polarisation behavior of the anode such that slightly higher current-density requires significantly higher anode overvoltage; and /S ic1 1, i.e. resistivity is not increased by water contact. This anode coating system does not undergo a progressive resistivity increase on externally exposed concrete (30 years as of November 2011).

Q-$IIMMARY OF THE INVENTION According to the present invention there is provided an anode system, for the cathodic protection of reinforcing steel in concrete, wherein the anode is an electronically conducting coating layer that is isolated from direct mechanical contact with the concrete surface and this mechanical isolation is achieved by using a discrete, ionically conducting paint coating layer that is applied to the concrete surface prior to application of the anode and that supports the protective ionic current flow from anode to concrete and that mechanically supports the anode coating; also according to the present invention there is provided an anode system, for the cathodic protection of reinforcing steel in concrete, wherein the anode is an electronically conducting paint coating layer that is isolated from direct mechanical contact with the concrete surface and this mechanical isolation is achieved by using a discrete, ionically conducting paint coating layer that is applied to the concrete surface prior to application of the anode and that supports the protective ionic current flow from anode to concrete and that mechanically supports the anode coating; and also according to the present invention there is provided an ionically conducting paint coating layer, which is applied to the concrete surface prior to the placement of an anode in contact with the coating layer, wherein conduction is solely ionic and this ionic conduction is supported by water imbibed by the coating layer from water in the environment of the coating layer.

The minimum ionic conductivity of the ionically conducting coating must be sufficient to allow at least a minimum cathodic-protection current flow through the coating under a modest potential difference, at the minimum coating thickness, under relatively dry operating conditions. The lowest envisaged current density through the ionically conducting coating is 0.1 mAI square Metre of concrete surface with a potential difference of 10 volts across the coating. This equates to a specific conductivity greater than 1 pS/cm at a minimum coating thickness of 10 microns, when in equilibrium with an atmosphere having a Relative Humidity which is equal to or greater than 7% at 20°C.

Contrary to the existing prior art, the present invention changes the location of the conducting coating anode so that the anode is not in direct contact with the concrete surface; anodic processes do not occur at a concrete to anode-coating interface. Small or short-term, moderate or moderate-term pH excursions below, for example pH 4 might occur at the conducting coating anode surface without influencing the integrity of the outer concrete layer.

The present invention introduces the use of water vapour from the environment as a facilitator of ionic conduction within an ionically conductive paint coating, in a conducting coating anode CP system for concrete, at ambient temperatures. The system provides an ionically conducting paint coating that increases in ionic conductivity by at least a factor of 10 times when moved from equilibrium with a very low Relative Humidity atmosphere of 7% to equilibrium with an atmosphere at 53% Relative Humidity at 20°C. The system also provides an ionically conducting paint coating that increases in ionic conductivity by at least a factor of 15 times when moved from equilibrium with a very low Humidity atmosphere of 7% to equilibrium with an atmosphere at 74% Relative Humidity at 20°C.

It is critical for all aspects of the present invention that the ionically conducting coating possesses longevity in service. Preferably, the ionically conducting coating contains a primary binder. The primary binder is one or more chosen from those binders giving adequate physical properties and durability in a coating for concrete and which retain these properties when exposed to anodic processes in contact with a conducting coating anode. Such primary binders include, for example, epoxies, acrylics, vinyl and vinylidene chlorides, urethanes and siloxanes.

It is critical for all aspects of the present invention that the ionically conducting coating possesses sufficient ionic conductivity. Preferably, the ionically conducting coating contains one or more ionic conduction modifier(s). The ionic conduction modifier may be particulate and be dispersed throughout the ionically conducting coating. Such particulates include montmorrillonite clay, fine particle or colloidal silica, fine particle aluminium oxide, zeolites or ion-exchange particles. Concurrently or alternatively, the ionic conduction modifier may be one or more hydrophilic organic or inorganic molecular segment, molecule or polymer that may be present as a solution, as a separate phase or as a copolymer in one or more primary binders. Such hydrophilic molecular segments, molecules or polymers include vinyl alcohol, vinyl pyrrolidones, hydroxyethylcellulose, ethylene or propylene oxide, amines, irnines, an ion-exchange group or a molecule bearing heteroatoms such as nitrogen, sulphur, phosphorus.

Concurrently or as yet a further alternative, the ionic conduction modifier may be one or more ionisable salts including chlorides, fluorides, borates, cinnamates or benzoates.

It is preferable for some aspects of the present invention that the conductivity of the ionically conducting coating is greater than one thousandth, and more preferable that the conductivity is greater than one hundredth, that of the concrete substrate under the same conditions of humidity. The present system provides an ionically conducting paint coating having an ionic conductivity greater than InS/cm when in equilibrium with an atmosphere of 33% RH at 20°C. The present system also provides an ionically conducting paint coating having an ionic conductivity greater than 1 OOnS/cm when in equilibrium with an atmosphere of 81% RH at 20°C.

It is desirable for some aspects of the present invention that the ionically conducting coating is distinctly more acid resistant than the cementitious surface to which the coating is applied. The present system provides an ionically conducting coating unaffected in a test wherein 2mL of 15% hydrochloric acid solution in water, retained on 8 square centimeters of the surface of the coating (painted on an horizontal, flat, inert support), is allowed to dry over a period of 10 days at 40-60% Relative Humidity at 20°C, in static air. After the 10 days the tested surface is cleaned with water and a stiff bristled nylon brush and dabbed dry with tissue. On inspection the surface is not visibly disturbed or marked, excepting that a modest degree of colouration may occur. Prior to the test the ionically conducting coating must be fully dried and cured over a period of at least 12 weeks at 20-25°C and 40-60% Relative Humidity.

It is desirable for some aspects of the present invention that the ionically conducting coating retards or prevents the pH of water, in the outermost concrete surface, falling below a value at which dissolution or attack of the concrete can occur. The present system provides an ionically conducting coating that buffers above pH 4.0 when contacted by dilute hydrochloric acid.

It is desirable for some aspects of the present invention that the ionically conducting coating modifies the rate and/or direction of electroendosmotic water transport between the concrete surface and the anode. The present system provides an ionically conducting coating that contains material(s) that adopt either negative or positive surface charge as required. Such material(s) include aluminium-containing ceramics such as fine particle size aluminas.

It is desirable for some aspects of the present invention that the ionically conducting coating possesses a level of electronic conductivity, where the electronic conductivity is at a significantly lower level than that required of a conducting coating anode. The slow anodic oxidation of carbon conductors present in the conducting coating anode can be reduced by causing anodic processes to occur other than at the carbon conductors.

Preferably, the ionically conducting coating has an electronic conductivity of less than 0.01 S/cm when in equilibrium with a water-free environment at 20°C. Conveniently the electronically conductive pigment is inert and non-carbonaceous. To this end the ionically conducting coating may contain a dispersion of inert ceramic, electronic conductors such as titanium diboride or titanium oxide, which can provide high levels of electronic conductivity within their particles.

It is desirable, for some aspects of the present invention, to prevent short-circuiting of the conducting coating paint anode to the reinforcing steel at, for example, defects in the concrete surface, or to other metals which may be adjacent to the concrete surface.

To this end the ionically conducting paint coating may be electronically non-conducting and thus prevent the anode coating short-circuiting to reinforcing steel or surface metal, but where electronic conduction is present this electronic conductivity should not exceed 10 -6 S/cm.

It is desirable, for some aspects of the present invention, that the ionically conducting coating is a reservoir or carrier of corrosion inhibiting materials. Such materials can migrate to reinforcing steel or nearby metals to prevent their corrosion, for example during or in the absence of protective current flow. Such inhibitors may also act to reduce the level of CP current necessary for the adequate protection of reinforcing steel.

To this end the ionically conducting coating may contain at least one corrosion inhibitor that may be held in the coating or is free to migrate from the coating.

It is desirable, for some aspects of the present invention that the ionically conducting coating is a reservoir or carrier of hydrophilic material that is able to migrate, or to be carried by the coating solvent, into the concrete surface. Such hydrophilic material acts, for example, to reduce the resistivity of the concrete surface in geographically dry locations.

It is desirable for some aspects of the present invention to present an ionically conducting coating that is within a range of dry film thickness. Dry film thickness might not be readily measured on some concrete surfaces, however the quantity of wet paint applied to a specific area of concrete can be more easily controlled. This known volume or weight of wet paint per unit area can be readily converted to an equivalent dry film thickness' using such data as Specific Gravity and Volume Solids of the wet paint. In accordance with one aspect of the present invention, the system provides an ionically conducting coating having an equivalent dry film thickness up to 20 microns.

In another aspect the system provides an ionically conducting coating having an equivalent dry film thickness between 20 and 60 microns. In another aspect the system provides an ionically conducting coating having an equivalent dry film thickness between 60 and 300 microns. In another aspect the system provides an ionically conducting coating having an equivalent dry film thickness greater than 300 microns.

It is desirable for some aspects of the present invention to present an ionically conducting coating that contains a water scavenging or water reactive material. Where water is present in a concrete surface during and shortly after painting, the adhesion or penetration of a paint coating may be severely inhibited. Water reactive materials are able to enhance the bonding of the coating to a wet or damp surface. The ratio of water reactive material to primary binder may be varied to combat a range of surface water levels at the concrete surface. Improved adhesion may be obtained on some concrete surfaces when the carrier solvent of the ionically conducting coating contains little or no water. Preferably, the ionieally conducting coating contains a water reactive material such as calcium oxide, magnesium oxide, Plaster of Paris or a cement such as Portland cement wherein the primary binder is present at greater than 10% by volume of the dry coating. Alternatively, the ionieally conducting coating may contain a water reactive material such as calcium oxide, magnesium oxide, Plaster of Paris or a cement such as Portland cement wherein the primary binder is present at greater than 25% by volume of the dry coating. As yet a further alternative, the ionically conducting coating may contain a water reactive material such as calcium oxide, magnesium oxide, Plaster of Paris or a cement such as Portland cement wherein the primary binder is present at greater than 45% by volume of the dry coating. Where the ionically conducting coating contains a water reactive material such as calcium oxide, magnesium oxide, Plaster of Paris or a cement such as Portland cement, the carrier solvent of the coating may be a non-aqueous solvent including hydrocarbon solvents such as octane or Xylene, glycol ethers, alcohols or esters.

It is understood that the ionically conducting coating, between anode and concrete surface, may be a composite of one or more discrete paint coatings having one or more different compositions. References to the equivalent dry film thickness of the ionically conducting coating should be taken to mean the equivalent thickness of a particular coating layer component, where a composite coating is employed.

The ionically conducting coating of the present invention has been found to offer benefits when employed with anodes other than carbonaceous conducting coating anodes. In accordance, the present system provides that the ionically conducting coatings may be employed with an anode that is not a carbonaceous conducting paint coating, the anode being separated from direct contact with the concrete by the ionically conducting coating applied to the concrete prior to the placement of the anode.

The ionically conducting coating of the present invention has been found to offer benefits when employed with coating anodes other than organic polymer conducting coating anodes. In accordance, the present system provides that the ionically conducting coatings may be employed with a sprayed metal anode for example zinc, aluminium, magnesium, or alloys of one or more of these, the anode being separated from direct contact with the concrete by the ionically conducting coating.

The ionically conducting coating of the present invention has been found to offer benefits when employed with coating anodes other than carbonaceous organic polymer conducting coating anodes. In accordance, the present system provides that the ionically conducting coatings may be employed with a conducting coating anode pigmented with a conductor more inert than carbon, such as titanium oxide, or galvanically active such as zinc, magnesium, aluminium or an alloy containing one or more of these, the anode being separated from direct contact with the concrete by the ionically conducting coating.

The Invention is illustrated by reference to Figures 6-9.

Figure 6 shows a cross section of a reinforced concrete structure, where 1 is steel reinforcing requiring protection, 2 is the concrete matrix and 3 is the surface of the concrete structure.

Figure 7 shows the same cross section as Figure 6, but with CP anode 4, a conducting coating anode on the concrete surface. Present but not shown is a power supply connected between anode 4 and steel 1.

Figure 8 shows the same cross section as Figure 7, but with an ionic layer 5 on the concrete surface and located between coating anode 4 and concrete surface 3. Present but not shown is a power supply connected between anode 4 and steel 1. Anode 5 may be a conducting paint coating.

Figure 9 shows the same cross section as Figure 8, but here an additional ionic coating layer 6 is located between ionic coating layer 5 and coating anode 4. Ionic layer 6 and ionic layer 5 may be of different compositions. Present but not shown is a power supply connected between anode 4 and steel 1. Anode 5 may be a conducting paint coating.

D -DETAILED_DESCRIPTION OF THE INVENTI

Dl -Ionic Conductiviy Reqjçççj For the ionic layer to function adequately in the embodiments of the present invention it must allow ions to flow to andlor from the anode with an acceptable current density under an acceptable potential gradient. Because the ionic layer is thin relative to the concrete path to steel, the ionic conductivity need be no higher than that of the concrete to which the layer is applied. It has been found that for some arrangements the ionic layer conductivity can satisfactorily be substantially lower than that of the concrete substrate. In some applications the ionic layer conductivity may be below one thousandth that of the concrete substrate and still permit satisfactory CP.

The conductivity of concrete is dependent upon many factors well known to practitioners. The controlling factor is water content and at equilibrium the water content is controlled by the Relative Humidity (RH) of the surrounding atmosphere.

For the ionic layer to function adequately in the present invention it must allow adequate ionic current flow at the RI-I of the concrete environment. Preferably the ionic layer should possess conductivity that is no less than one thousandth, and more preferably no less than one hundredth, that of the concrete substrate at the same Relative Humidity.

For indication, Figure 4 shows the observed relationship between ac 1000Hz conductivity and RH for one specimen cement (solids weight OPC/building sandl30 mesh sand 2/1/1) formed into 1 cm cubes and cured at 100% RH for two months. Two opposing cube surfaces were polished with 240-mesh corundum paper and these two opposing faces were painted with anode grade conductive coating as connection electrodes. Conductivities are shown after the cubes were held at 20°C for 40 days in sealed cells that incorporated saturated solutions of humidity control salts. These illustrative results are not definitive because a large range of concrete compositions, ages and states are met in practice.

Because the ionic coating layers of the present invention are relatively thin layers of large area it is beneficial to consider the data of Figure 4 in the form of resistance through an area of one square metre, at the equivalent thickness of 300 microns, as in Figure 5.

In general, but not always, the ionic coating layer of the present invention is unlikely to be effective at less than 10 microns dry film thickness or circa 10 grams per square metre dry film weight. Some applications may benefit from ionic coating layers greater than 1 mm in dry film thickness. By nature a concrete surface is of varying roughness and porosity and a dry film thickness may require to be deduced from coating weight per square metre, quoted as equivalent' dry film thickness.

Note that the presence of free chlorides in concrete will significantly increase conductivity of the concrete. The conductivity of an ionic layer may be modified by movement and exchanges of ions with the concrete and under the influence of concentration and potential gradients and also anodic processes. A group of cement cubes was prepared to the above mixture including 2% by weight of sodium chloride on the OPC component. The conductance versus RH line was parallel to that of Figure 4 and conductance was generally circa 5 x higher.

D2 -Principal Composition of the Ionic Layer The ionic layer may consist of one or more individual coating layers. Where more than one individual coating layer is present these may have the same composition or have different compositions (for example where the different compositions offer different functions).

An individual ionic coating layer is composed of one or more primary binder(s) and one or more ionic conduction modifier(s); however both functions might be combined in one or more single material as, for example, in copolymer(s). The ionic conduction modifier employs water from the environment of the ionic coating layer to give enhancement of ionic conductivity.

An individual ionic coating layer may also optionally contain one or more acid buffering agents; one or more inert electronically conductive pigments; one or more electroendosmotic transport modifiers; one or more hydrophilic material able to migrate; one or more fungicidal additives; one or more corrosion inhibitors for metal such as steel; one or more depolarising agent for zinc, aluminium, magnesium or their alloys in anode service.

An individual ionic coating paint may optionally contain materials appropriate to the formation of paint structure and its wet application to the concrete surface including, for example, carrier solvent(s); wetting agents and surfactants; structuring agents; colouring agents; curing agents; antifoaming agents. Some of these paint formation materials may themselves contribute to ionic conduction.

D3 Ionic Coating Primary Binder The primary binder is chosen from those able to give mechanical strength to the ionic layer and durability in a coating on concrete and give retention of adequate mechanical properties when exposed to anodic processes in contact with a conducting-coating anode. The primary binder is also subject to the many other requirements as would be sought by a skilled coating technologist. Primary binders should be chosen from organic, inorganic and mixed organic/inorganic materials that fulfill these requirements.

Primary binders may be thermoplastic or thermosetting or a combination of thermoplastic and thermosetting.

Binders which might be employed as a primary binder or as a component of a primary binder include, but are not limited to, flourinated polymers for example polyvinylidene fluoride copolymers; chlorinated polymers, for example polyvinylidene chloride copolymers, and chlorinated polypropylene; acrylics; epoxies; polyurethancs; siloxanes.

D4 -Tonic Conduction Modifiers Ionic conduction modifiers are materials which, when incorporated into the primary binder enhance ionic conductivity in the presence of water, where the water may be present as water vapour at a Relative Humidity greater than zero, Many ionic conduction modifiers are hydrophilic and possess hydrophilic surfaces, zones or polymer segments.

The ionic conduction modifier may be incorporated for example as discrete particles, as a solution in one or more of the primary binder(s), as a component of a separate phase or as part of one or more primary binder(s) molecules in, for example, a copolymer.

Tonic conduction modifiers which might be employed include, but are not limited to, clays for example montmorrillonite clays; silicas of less than 10 microns average particle size for example colloidal silica or diatomaceous silica; aluminium oxides of less than 10 microns average particle size for example colloidal aluminium oxide; zeolites; alcohols for example vinyl alcohols; pyrrolidones for example vinyl pyrrolidones; cellulose for example hydroxyethyl cellulose; ethylene and propylene oxides and glycols; amines; imines; vinyl pyridines; acrylonitriles; oxylates; materials with an ion exchange capacity for example Shieldex TM calcium ion exchange silica; salts for example chlorides, hydroxides, ammonium benzoate, methoxy phenyl pyridinium tetraflouroborate, potassium niccotinate, calcium magnesium carbonate, magnesium hydroxide; polymers presenting a regular or irregular array of nitrogen, sulphur or phosphorus heteroatoms; polymers chosen from the field of ionic polymers'.

D5 -Composingan Ionic LMer The CP systems of the current invention will have application in a diverse range of situations. The formulation of the, or each, ionic layer should be chosen for optimum operation in the chosen situation.

Increasing levels of the appropriate ionic conduction modifier(s), in a given formulation, will generally yield increasing conductivity. However, increasing levels of modifier will often have undesirable negative effects, for example reduced mechanical strength, impaired adhesion of the anode to the ionic layer, excessive swelling or softening by water, or embrittlement. The final chosen formulation should be of adequate conduction and longevity under the expected operating conditions.

Note that conduction measured using simple ac techniques does not differentiate the degree of conduction available to each of the negative and positive ions present, nor the change in conductances occurring over a period of years or the effects of different current density levels over time. It is also important that the anode, where this is a conducting coating, be compatible with and bond adequately to the ionic layer. The function and projected life of an ionic layer should examined using appropriate tests and accelerated tests.

E -EXAMPLES

Example El (Journal no Ji) A commercial acrylic-copolymer was used as the primary binder in a paint coating and this coating was assessed for ionic conductivity in an atmosphere of fixed Relative Humidity. It was accepted that, in the absence of prolonged drying, remaining traces of coalescent solvent might modify and permit some degree of ionic conduction.

Formulation FE! was prepared by stirring.

FE1 weight Primary Binder Rohm & Haas AC339 48.8 Carrier! solvent Water 45.9 Coalescent solvent TexanolTM 1.9 Coalescent solvent Propylene glycol 1.9 Antifoam BeveloidTM 6681 0.3 Thickener Rohin & Haas RM825 100.0 Ionic Coating Layer Conductivity Test Procedure.

One coat of FE! was uniformly brush applied to a tinpiate panel measuring 15 x 10 cm and dried for 2 hours at 45°C. The surface was smoothed using 600 grade silicon carbide papers and wiped clean of dust. The same coating procedure was carried out a further six times to give a total dry film thickness of between 150 and 300 microns. The panel was then further dried at 40°C for 3 days. Prior to the application of each coating layer the test paint was re-homogenised by, for example, stirring.

Masking tape was applied to the FE1 coating such as to leave exposed an area of 10 x 6 cm at the centre of the panel. The dry film thickness of FE! was measured at 25 points uniformly distributed over the exposed area, using a magnetic gauge, and the average taken.

A low resistance, anode grade, conducting coating electrode was brushed over the exposed area of FE!. After 2 hours drying a second coat was applied and the masking tape removed. After overnight drying, wire connections were made to the tinplate base panel and to the conducting coating electrode. The panel was then conditioned at 40°C for 5 days.

This preparation procedure was adopted for subsequent ionic layer conductivity panels because it ensured that the finished coating was defect free, i.e. there were no pinholes, nibs, bittiness, pigment clusters etc crossing completely through the test film to give misleading conductivity measurements. It was judged as sufficient temperature and time to release coalescent solvents etc which, when present, may act as ionic conduction modifiers. (Note that some test coatings may have required greater inter-layer drying times where, for example, a curing process was involved.) The drying oven was located in a room consistently at 40-50% Relative Humidity and 21-25°C.

Test panels prepared as above were mounted in sealed chambers at 2!-25°C wherein the atmosphere was maintained at a chosen Relative Humidity. Ionic conduction within the test panel was measured with an ac conductance meter operating at! K1-Iz and supplying an ac signal of 30 millivolts rms. Alternatively, a 1 KHz conductivity bridge supplying up to 0.6 volts was employed, where for example, low conductivity andlor significant cell emf's were present. The absorption or loss of water by the ionic coating, and hence conductance, would generally reach a steady state plateau within 1-6 weeks.

Plateau values of conductance were converted to resistance and used with thickness and area data to report resistivity as the resistance, in ohms, through one square metre of ionic layer at 300 microns dry film thickness, at a particular Relative Humidity. These resistivity units allow easier interpretation of possible performance in conducting coating anode systems.

Note that testing of this technique, using guard ring arrangements, had confirmed that the conduction pathway was virtually all through the ionic coating and not across the ionic coating surface to the panel edges.

Plateau resistivities can be compared with those of concrete taken from an intended CP structure or, for example, for specimen cement cubes charted elsewhere in this document viz.: Specimen cement at 7% RH 300 Q/M2/300uM Specimen cement at 33% RI-I 25 Specimen cement at 43% RI-I 10.5 Specimen cement at 53% RI-I 5.0 Specimen cement at 74% RH 0.65 Specimen cement at 81% RH 0.30 Specimen cement at 98% RI-I 0.03 Plateau resistivities FE! at 7% RH »= 100,000 Q1M2/300uM FE! at 33% RH 3,867 FE1 at 74% RH 1,205 FE1 at98°/oRH 173 Example E2 (Journal no J1A) Example El was remade incorporating a small quantity of ionic conduction modifier.

FE2 weight Water 5.0 Magnesium oxide 1.0 Sodium chloride 0.1 The three components were continuously stirred together until all reaction between magnesium oxide and water was complete. The slurry was then slowly added with stirring to: FE! 100.0 106.1 Plateau resistivities: FE2 at 33% RH 1,290 Q/M2/300uM FE2at74%RH 379 FE2 at 98%RH 84.4 Example E3 (Journal J2A) Example E2 was remade incorporating colloidal silica ionic conduction modifier, Aerosil 200, by using high speed dispersion.

ff3 weight Primary Binder Rohm & Haas AC339 50.0 Carrier! solvent Water 100.0 Coalescent solvent TexanolTM 2.0 Coalescent solvent Propylene glycol 2.0 Antifoam BeveloidTM 6681 1.0 Ionic conductor Magnesium oxide 1.0 Tonic conductor Sodium chloride 0.1 Tonic conductor Degussa Aerosil 200 11.6 167.7 Plateau resistivities: FE3 at 7% RI-I 2,380 QIM2/300uM FE3at33%RH 1,120 FE3 at 74% RH 5.1 FE3at98%RH 0.09 Example E4 (Journal J3A) Example El was remade incorporating magnesium oxide ionic conduction modifier at higher level than Example 2 and an acid reactive pigment calcium magnesium carbonate (OMYA Microdol H). FE4

Reactant and carrier Water 51.5 Tonic conductor Magnesium oxide 34.9 Acid buffer MicrodolH 28.6 Wetting agent Rohm&Haas Orotan 850E 3.0 The above were high speed dispersed until the reaction of magnesium oxide (into magnesium hydroxide) was completed, and then let down with: Carrier/solvent Water 290.0 Primary binder Rohni&Haas AC339 500.0 Coalescent Texanol 6.0 Coalescent Propylene glycol 6.0 Thickener Rohm&Haas RM825 952.2 Plateau resistivities: FE4 at 7% RI-I 3,300 Q/M2/300uM FE4at33%RJ-l 1,051 FE4at74%RH 610 FE4 at 98%RH 3.2 Example ES (Journal J4A) Example E4 was remade incorporating 800 mesh silica as an ionic conduction modifier.

FES weight Ionic conductor WBB Silica C800 121.2 1934 878.8 1000.0 C800 was dispersed into a fraction of the FE4 and then let-down with the remainder.

Plateau resistivities: FES at 33% RH 1,097 QIM2/300uM FE5at74%RH 322 FES at 98%RH 1.6 Example E6 (Journal J8A) Example E4 was remade incorporating calcined diatomaceous silica, Celite SFSF from World Minerals (UK) ltd.

FE6 weight Ionic conductor Celite SFSF 121.2 FE4 878.8 1000.0 Celite SFSF was dispersed into a fraction of the FE4 and then let-down with the remainder.

Plateau resistivities: FE6 at 33% RH 956 QIM2/300uM FE6at74%RH 132 FE6 at 98%RH 1.2

Example E7

Example E4 was remade incorporating calcium bentonite clay, Steebent CE from 46 Steetley Bentonite and Absorbants Ltd and a corrosion inhibitor for steel. 1 FE7

Reactant and carrier Water 51.5 Ionic conductor Magnesium oxide 34.9 Acid buffer Microdol H 28.6 Wetting agent Rohm&Flaas Orotan 850E 3.0 The above were high speed dispersed until the reaction of magnesium oxide (into magnesium hydroxide) was completed, and then let down with: Carrier/solvent Water 1147.0 Primary binder Rohm&Haas AC339 500.0 11 Coalescent Texanol 6.0 Coalescent Propylene glycol 6.0 Antifoam Silicone emulsion based 3.9 Thickener Rohm&Haas RM825 2.5 Bentonite Steebent CE 143.0 16 Inhibitor triethanolamine benzoate 10.0 1936.4Joumaino J6 The bentonite was dispersed in part of the mix and let-down with the remainder.

21 Plateau resistivities: FE7 at 7% RH 900 Q/M2/300uM FE7 at 33% RH 65 FE7at53%RH 11.7 26 FE7 at 74% RH 0.57 FE7 at 98%RH 0.09 Example E8 (Journal Jl 1) Example E4 was remade incorporating calcium ion-exchange material, ShieldexlM. FE8

As FE7 but replacing 143g Bentonite with l39g of Shicidex.

Plateau resistivities: FE8 at 33% RI-I 871 Q/M2/300uM FE8 at 74% RH 403 41 FE8at98%RH 3.3

Example E9-E17

46 A group of ionic layer coatings were prepared using a primary binder alone, for reference, and with organic ionic conduction modifiers. These ionic conduction modifiers are illustrative of some organic species which might be deployed as, for example, short, medium or long chain polymers or copolymers.

Component FE9 FE1O FEll FE12 FE13 FE14 FE15 FE16 FE17 Jour RO Jour RI Jour R2 Jour 1(3 Jour R4 Jour R5 Jour R8 Jour R9 Jour 1J4 grams grams grams grams grams grams grams grams grams AC339 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 11 DPnB 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Antifoam ** ** ** ** ** ** ** -** ** Water 1.8 10.3 35 120 105 35 25 370 PAC1Ok 12 PEG900 ____ ____ 12 ____ ____ ____ ____ ____ _____ 16 PVPk25 12 ____ _____ --_____ PVA 87-89 12 PEO 100k 12 A500H 12 MP500C1 12 21 H60000 ______ ______ ______ ______ ______ ______ ______ -12 DPrIB Coalescent ex DOW **Antifoam = silicone emulsion Just sufficient added for acccptable antifoaming PAC 10k 50% water solution poygcrylonitri1e ex Aldrich 26 PEG900 Polyethylene glycol 900 ex Aldrich PVP k25 Polyvinyl pyrrolidonek2s ex Aldrich PVA 87-89% Hydrolised Polyyjtyl alcohol ex Aldrich PEO 100k Polyeylene oxide 100k ex Aldrich A50 OH Bayer ion exchang resin. Finely ground for use 31 MP500 Cl Bayer ion exchange resin. Finely ground for use Tylose H6000 Hyclroxyethyl Cellulose ex Twinstar Chemicals Example Ionic Conduction Resistivity Q/M2/300uM 6 Formulation Modifier _______________________________ At 46 days 74% RH at 22-24°C None -Reference 2,110 FE9 ___________________ _________________________ 11 PAC1Ok 154 FE1O ___________________ _________________________ PEG900 425 FEll ___________________ _________________________ PVPI25 21 16 FE12 _______________________ ______________________________ PVA 87-89% Hydrolised 16 FE13 ____________________ __________________________ PEO 100k 98 FE14 ___________________ _________________________ 21 A500H 628 FE1S ___________________ _________________________ MP500C1 435 FEI6 ___________________ _________________________ H6000 2.2 26 FE17 _________________ --Example 18 (Journal no Xl) A coating was prepared using an acrylic copolynier primary binder and ionic 31 conduction modifiers which have been shown to possess a positive surface charge in alkaline and/or neutral and/or acidic aqueous environments. Such modifiers, aluminium oxide (fumed grade from Degussa Ltd.) and magnesium hydroxide in this example, would cause electroendosmotic water transport towards the anode.

36 FE18 weight Primary Binder Rohm & Haas AC339 50.0 Carrier! solvent Water 56.0 Ionic conduction modifier Aluininox C' ex Degussa 14.6 Ionic conduction modifier Magnesium oxide 0.5 Coalescent solvent TexanolTM 0.5 Coalescent solvent Propylene glycol 1.0 Antifoam Silicone emulsion trace Thickener Rohm & Haas RM825 1.2 6 122.8 Magnesium oxide was stirred in part of the water until reaction to hydroxide was complete. Aluminium oxide was added with the remaining water and some AC339 and dispersed. Remaining materials stirred in.

Plateau resistivities: FEl8at33%Rll 974 Q/M2/300uM FEl8at8O%RH 163 16 Example 19 (Journal no Y6) An ionic layer for testing on concrete was prepared using AC339 as the primary binder, with magnesium hydroxide, hydroxyethyl cellulose and bentonite clay as ionic conduction modifiers and calcium magnesium carbonate as an acid buffer (Microdol 100).

21 FE19 Weight Ionic modifier Tylos&'M 100k 7 Solvent Water 278 Fungicide ParmetolTM A26 0.2 Ionic modifier Magnesium oxide 7.5 26 Ionic modifier Bentonite CE 16.3 Acid buffer MicrodolTM 100 9.4 Pigment dispersant OrotanTM 850E 0.6 319.00 31 Magnesium oxide was completely reacted with part of the water. Bentonite and Microdol were dispersed in another part of water using Orotan as a dispersing agent.

Tylose was solubilised in the remainder of the water and then the three parts mixed and let-down with: Primary binder AC339 482 Coalescent Texanol 23.1 Coalescent Dowanol DPnB 7.5 Antifoani Silicone emulsion trace 83 1.6 6 Plateau resistivities: FE19 at 33% RH 982 Q/M2/300uM FEl9at8l%RH 12.4 Example 20 (Journal no Z5) 11 Example of an epoxy based primary binder, cross linked with an amine and siloxane that employs an amine as the ionic conductivity modifier and magnesium hydroxide as an electroendosmotic transport modifier and as an acid buffer. This is also an example of an ionic coating layer where polymerisation takes place after paint application to the target surface.

16 FE2O Weight Electroendosmotic modifier Magnesium oxide 25.5 Dispersing agent Orotan 850 1.0 Carrier/reactant Water 25.0 Disperse and react together. Let-down into: 21 Epoxy resin, ex Huntsman Ltd. PZ756167 294.5 Immediately prior to application, stir in: Amine cross-linker ex Huntsman Ltd. at manufacturer's recommended level. Aradur 39 347.5 Ionic conduction modifier. Amine 26 ex Huntsman Ltd. Aradur 39 277.0 Dow Z6040 siloxane 985.5 Plateau resistivities: 31 FE2O at 7% RH 875 Q/M2/300uM FE2Oat8l%RH 12.6 Example 21 (Journal no HH1) Example of a primary binder which is a copolymer including an ionic conduction modifier. The primary binder is poly (vinyl chloride-co-vinyl acetate), with hydroxy propyl acrylate giving ionic conduction enhancement.

6 VCVAHPA = polyvinyl chloride-co-polyvinyl acetate-co-hydroxy propyl acrylate, supplied by Aldrich Chemicals Ltd. FE21 Weight Primary binder/ionic cond. modifier VCVAHPA 29.3 Dissolve in: 11 Solvent MIBK 37.3 Then add: Solvent Xylene 10.7 Solvent n-Butanol 7.5 Then add: 16 Solvent ex Dow Dowanol PM 1 100.0 Plateau resistivities: FE23 at 7% RH 700 Q/M2/300uM 21 FE23at8l%RH 486 Example 22 (Journal no HH1/B) As Example 21, but also including a solvent soluble salt as ionic conduction modifier.

26 FE22 Weight Solvent soluble salt Methoxy phenyl pyridinium tetraflouroborate 0.7 Dissolve the salt in: Propylene carbonate 7.0 Dowanol PM 2.0 31 Let the solution down into: FE23 3i0 44.7 Plateau resistivities: FE22 at 7% RH 780 Q/M2/300uM FE22at8l%RH 282 Example 23 (Journal no ZZ3) Example of an epoxy based primary binder cross linked with an amine, that 6 employs an amine as the ionic conductivity modifier and bentonite clay as an ionic conductivity modifier, and that displays a high resistance to attack by hydrochloric acid.

11 FE23 Weight ionic conduction modifier Bentonite CE 116.8 Amine ex Huntsman Ltd. Aradur 39 418.4 Carrier Dowanol PM 100.0 Antifoam Silicone emulsion 0.6 16 635.8 Disperse Bentonite into resin and solvent blend.

Immediately prior to use, take 10 grams of dispersion and stir in Epoxy resin, cx Huntsman Ltd. -PZ756167-2.3 grams, adding extra Dowanol PM to achieve satisfactory application viscosity.

Plateau resistivities: FE23at8l%RH 22.3 Q/M2/300uM Example 24 (Journal no AB9) 26 Example of an epoxy based primary binder cross linked with an amine, that employs an amine as the ionic conductivity modifier, and which contains dispersed particles of an inert, ceramic electronic conductor.

FF24 Weight Inert electronic conductor Ebonex 10, from Atraverda ltd 105.0 Ebonex 10 is a titanium oxide of 10 microns average particle size. Prior to use the sample was subjected to extended, vigorous micronisation in a mechanical mill. It was then dispersed into: Amine ex Huntsman Ltd. Aradur 39 20.0 Carrier Water 27.0 Antifoam Silicone emulsion 0.1 Dispersion 152.1 For use, 40 grams of the dispersion above was let down with Epoxy resin, ex Huntsman Ltd. -PZ756/67-2.2 grams.

Plateau resistivities: FE24 was applied as 3 coats to give a total dli of 130 microns. The conduction test panel was cured at 50°C for 24 hours and then conditioned at 110°C for 6 days to drive off all water. Conduction was measured immediately upon cooling to room 11 temperature. Conduction was believed to be entirely electronic.

FE24 at 0 % RH 3.53 Q/M2/300uM Equivalent to a resistivity of 1.2 MQ cm., conductivity 0.85 uS/cm.

16 Example 25

Example of a solvent born ionic coating having an enhanced tolerance and bonding to damp or moist concrete surfaces. FE25

Component Weight 21 Mix to clear solution: Solvent Propylene Glycol Isobutyl Ether 34 Solvent White Spirit 147 Primary Binder Plasticiser Cereclor 65L chlorinated paraffin 69 Primary Binder Pliolite AC8O 68 26 Dispersing Agent Soya Lecithin 2 Disperse into above solution: Water Scavenger Ordinary Portland Cement 304 Water Scavenger/anti Shrink Plaster of Paris (medical) 34 31 Conduction Modifier (silica) Celite 281 diatomaceous 54 Acid Buffer Snowcal 60 calcium carbonate 181 Add with stirring: Primary Binder Structuring Agent Pliolite AC4 17 1 Add as required, then bring to viscosity of 6 poise @10,000/S Solvent White Spirit Plateau resistivities: FE25 at 81% RH 143 Q/M2/300uM

F-EMBODIMENTS OF THE INVENTION

11 Fl Large Concrete Test Panels for Embodiments.

Concrete panels incorporating 0.5% sodium chloride on the OPC component were prepared with a composition (OPC-building sand-sharp sand-lOmm aggregate) in the ratio by weight (3.5-3.3-5.1-4) with just sufficient water added for a workable mix. Panels were cast to 30x50 cm area and 3.5 cm depth, incorporating steel mesh 16 approximately 2-2.5 cm from the flat front surface. Insulated connection wires were included to the steel mesh and exited the rear of each panel.

F1-2 Short-Circuit Modified Large Panels Several panels were modified to be short-circuit' test panels and were prepared 21 with the steel reinforcing sited near the front surface. Shortly after casting, while the concrete matrix was still soft, artificial short-circuit pathways were introduced from the front surface to the reinforcing steel using a fine screwdriver. A tiny area of steel was visible at the base of each pathway. Ten pathways were uniformly distributed across the panel fronts.

Prior to use all panels were aged for 6 months exposed outdoors. Shortly before use the panel front faces and sides were etched with 15% hydrochloric acid, thoroughly rinsed, allowed to dry, then wire brushed and vacuum cleaned. Panel edges were painted with 2 coats of a high build protective coating based on a two pack epoxy, 31 which is known to be extremely water and gas resistant, of extremely high resistivity when wet and which has been used commercially on ship's hulls subject to CP.

Coating, drying and ageing of panels was performed at 15 -25°C, 60-80% RH in a sheltered outdoor location.

36 F2. Small Test Panels for Embodiments Smooth-fronted test panels were made by casting into lOx lSx 1.5cm deep moulds having smooth shiny, FormicaTM linings. Four 12.5cm long, 4mm diameter steel rods were connected by insulated cable and distributed in the central plane, parallel to the front face, using a cement of the mix 1:1:1:1 (OPC:coarse sand:building sand:60 mesh sand using sufficient water for workability). Sodium chloride was 6 added to the mix at 0.3% by weight of OPC. The cement mix was worked in the moulds to release air and ensure a smooth defect-free front surface. The connection wire to the steel rods exited the rear of each panel. These panels were allowed to cure in a sheltered outdoor location for five months, and one to two months prior to use they were tightly abraded on the front surface using 400 mesh emery paper, de- 11 dusted and stored at 20-25°C at 40-60% Relative Humidity. Coating, drying and ageing of panels was performed at 20-25°C, 40-60% RH in an indoor location.

KLPreparation pLCpnducting_Paint Coating Anode for Embodiments.

A natural graphite, 91% carbon, Ceylon origin and obtained from C R Averill Ltd. 16 was dispersed using a high-speed disperser, into a water-based acrylic copolymer emulsion Texicryl 13-002 supplied by Scott Bader Ltd. Note this is not a commercial anode coating formula.

Composition EM3.

2! Wçjght% Texicryl 13-002 40.1 Graphite (Avarc) 43.1 Dispersant Orotan 850E R&H 1.3 Water 14.0 26 Antifoam silicone emulsion Trace Thickener RM825 Rohm&Haas 0.3 Coalescent TexanolTM 12 100.0 31 Graphite PVC = 50% Solids by Weight = 66% Brushed out on melinex film as two coats with 4 hours drying at 45°C between coats and then allowed to dry for two days at 45°C.

Resistivity = 7.1 ohms per square at 280 microns dft (Equivalent to a Specific Conductivity of 5 S/cm) F4. Embodiment 1 -Short-Circuit Prevention.

Two short-circuit panels were taken from Embodiment 11-2 and the defect bearing face of one was given two coats of the coating prepared in Example 19, FE1 9, with 16 hours drying between coats. The total equivalent dry film thickness was estimated to be between 180 and 240 microns. After 2 further days, both panels 6 were coated with conducting coating EM3 prepared in Embodiment 3, to approximately 150 microns dry film thickness on the defect bearing faces, and these were allowed to dry and age for 6 weeks.

The voltage between each conducting coating section and reinforcing steel was measured using a voltmeter of input resistance >10 Mohms. The system with 11 conducting coating alone showed <lmV, and a resistance of circa 2-6 ohms between coating and reinforcing steel, showing that this coating was short-circuited to the reinforcing steel. The panel surface having SF19 interposed between conducting coating and concrete showed 360 mV; various resistors were placed in parallel with the voltmeter terminals and the instant voltage was recorded before significant 16 polarisation drift (which was clearly seen) -Resistance Placed Between Conducting Coating and Reinforcing Steel Voltage 2200 ohms 250mV 21 1000 190 390 113 70 36 26 These test results demonstrated that, with SF19 present, the conduction between conducting coating and reinforcing steel was ionic conduction through the ionic layer and concrete and not due to short circuits at defects.

F5. Embodiment 2 -Veryjow Relative Humidity Qperation.

31 A sample of coating FEI4 of example 14 was thinned with water and applied to the whole front face of a small test panel of 12. The coating was applied to give 8-10 microns dft. After 2 days curing and drying, a water thinned sample of coating FE7 of example 7 was applied to the whole front face, over the FE 14, to give 8-10 microns dft. After two days drying, the outer 2 cms of the front surface edge was 36 masked and anode coating EM3 was brush applied to give 100-130 microns dft. A carbon fibre tape was embedded in one edge of the anode material for electrical connection. The 66 square-cm anode was located centrally on the front face, 2cm from the panel edges, and separated from the concrete by circa 16-20 microns of ionically conducting coating. The panel was dried for 2 weeks at 40-60% RH at 22°C.

6 The panel was mounted in a sealed waterproof container wherein the atmosphere was held at 7% RH at a temperature between 20 and 25°C. An external power supply was arranged to pass a fixed current between anode and steel at an anodic current density of 0.1 mAIM2, i.e. 0.66 isA total anode current. The voltage between anode and steel was measured using a voltmeter having an input resistance of 1010 11 ohms. This voltage was 1.6 at the start of the tests and after 5 months it was less than 10 volts.

P6 Embodiment 3 -Low Relative Humidity Operation.

16 A sample of coating FE12 of example 12 was applied to the whole front face of a small test panel of 12. FE12 acts as a reservoir of hydrophilic material (polyvinylpyrrolidone in this case) able to partially migrate into a concrete surface.

The coating was applied as five coats with intermediate drying, to give 30-40 microns dft. After two days drying, the outer 2 cm edge of the front surface was 21 masked and anode coating EM3 was brush applied to give 100-130 microns dli. A carbon fibre tape was embedded in one edge of the anode material for electrical connection. The 66 square-cm anode was located centrally on the front face, 2cm from the panel edges, and separated from the concrete by circa 30-40 microns of ionically conducting coating. The panel was dried for 2 weeks at 40-60% RH at 26 22°C.

The panel was mounted in a sealed waterproof container wherein the atmosphere was held at 33% RH at a temperature between 20 and 25°C. An external power supply was arranged to pass a fixed current between anode and steel at an anodic 31 current density of 0.1 mA/M2, i.e. 0.66 pA total anode current. The voltage between anode and steel was measured using a voltmeter having an input resistance of 1010 ohms. This voltage was 1.7 at the start of the tests and after 9 months it was less than 10 volts.

36 P7 Embodiment 4 -Intermediate Relativejlumidity Operation.

A sample of coating FE2O of example 20 was applied to the whole front face of a small test panel of 12. The coating was applied as one coat to give 75-90 microns dft. After five days drying, the outer 2 cms edge of the front surface was masked and anode coating EM3 was brush applied to give 100-130 microns dft. A carbon fibre tape was embedded in one edge of the anode material for electrical connection. The 66 square-cm anode was located centrally on the front face, 2cm from the panel edges, and separated from the concrete by circa 75-90 microns of ionically 6 conducting coating. The panel was dried for 2 weeks at 40-60% RH at 22°C.

The panel was mounted in a sealed waterproof container wherein the atmosphere was held at 81% RH at a temperature between 20 and 25°C. An external power supply was arranged to pass a fixed current between anode and steel at an anodic II current density of 2 mAIM2, i.e. 13.2 sA total anode current. The voltage between anode and steel was measured using a voltmeter having an input resistance of 1010 ohms. This voltage was 1.7 at the start of the tests and after 9 months it was less than 10 volts.

16 F8. EmbodimentS -Outdp.q Test Panel A large concrete test panel prepared in Ii was coated with ionically conductive paint coating of Example 19, formula FE 19, over the whole front face. The coating was applied as three separate layers of 110-130 microns dii, allowing 24 hours drying 21 between each layer. The total dry film thickness was estimated to be between 350 and 390 microns. After 4 days further drying, conducting coating anode prepared in Embodiment 3 was applied in two coats to a total dft of 130 microns over the central area of FE 19. The conducting coating anode was not applied within 1.5 cms of the panel edges. A MMO ("mixed metal oxide") coated titanium strip was embedded in 26 one edge of the anode material for electrical connection. The coatings were allowed to dry for 6 weeks prior to exposure to the weather.

After drying of the coatings, the panel was mounted such that the coated test face was semi-vertical and fully exposed to weathering. A constant-current source was 31 connected (positive out to the anode) between the anode coating and the steel cathode and adjusted to 1.01 mA, this being equivalent to an anodic current density of 8 rnA/M2 over the coating anode area of 27x47 em. This test condition was maintained for three years. Drive voltage was measured every 6 hours and the average drive voltage over this period was 2.0 volts, with peaks of 5.4 and 6.0 volts 36 occurring during two periods of prolonged low humidity.

F9. Embodiment 6 -Acid Resistant Ionic Conduciingjy The flat surface of a small test panel of 12 was coated with the paint of Example 23.

The coating was applied in three layers to a total dft of 150-170 microns, with 30 minutes drying at 50°C between coats. The panel was then held at 50°C for 48 hours to allow complete curing. A square of area 8 cm2 was marked on the coating surface and a shallow cup formed about this area by applying multiple layers of waterproof 6 adhesive tape. The panel was mounted horizontally and the 4 mL of 15% hydrochloric acid was introduced into the cup. The acid was allowed to dry off at 20°C in air at 40-60% RH. After 10 days the surface was dry, the tape was removed and the panel surface rubbed with a stiff bristled nylon brush and copious water.

When the surface was examined closely it did not show any mechanical disturbance 11 of the surface where the acid had dried; it was noted that this surface had become stained yellow. The coating was firmly adherent to the cement surface of the panel.

Fl 0. Embodiment 7 -Ionic Layer ContainingJnrt Electronic Conductor is 16 Located Be'een The Conducting Coating Anode and The Concrete Surface A large concrete test panel prepared in Ii was coated with ionically conductive paint coating of Example 23, formula FE23, over the whole front face. The coating was applied at 25-30 microns dft. After 10 hours, the paint coating of Example 24, formula FE24, was applied over the whole front face at 15-25 microns dli. After a 21 day, conducting coating anode prepared in Embodiment 3 was applied in two coats to a total dft of 130 microns over the central area of FE 19. The conducting coating anode was not applied within 1.5 cms of the panel edges. A MMO coated titanium strip was embedded in one edge of the anode material for electrical connection. The coatings were allowed to dry for 6 weeks prior to exposure to the weather.

After drying of the coatings, the panel was mounted such that the coated test face was semi-vertical and fully exposed to weathering. A constant-current source was connected (positive out to the anode) between the anode coating and the steel cathode and adjusted to 0.76 mA, this being equivalent to an anodic current density 31 of 6 mAIM2 over the coating anode area of 27x47 cm. This test condition was maintained for three years. Drive voltage was measured every 6 hours and the average drive voltage over this period was 2.8 volts, with peaks of 6.4 and 8.5 volts occurring during two periods of prolonged low humidity.

36 Fl 1. Embodiment 8 -Ionic Layer is Located Between A Sprayesi Metal Conducting CoatintAnode and The Concrete Surface A large concrete test panel prepared in Ii was coated with ionically conductive paint coating of Example 23, formula FE23, over the whole front face. The coating was applied at 25-30 microns dft and allowed to dry and cure for 3 weeks at 15-25°C. A MMO coated titanium strip was mounted towards one edge of the front face for electrical connection The conducting coating anode was metallic zinc and this was hot sprayed to very approximately 60 microns but was not applied within 1.5 cms of 6 the panel edges as these were masked.

The panel was mounted such that the coated test face was semi-vertical and fully exposed to weathering. A constant-current source was connected (positive out to the anode) between the anode coating and the steel cathode and adjusted to 0.38 mA, this being equivalent to an anodic current density of 3 mA/M2 over the coating 11 anode area of 27x47 cm. This test condition was maintained for three years, Drive voltage was measured every 6 hours and the drive voltage over this period was between 0.7 and 2 volts.

F12.Medium Term Anode System Performance 16 Medium term results, after 8 years of exterior exposure testing, offer some early indications. A large number of substantial test panels employ anode current densities ranging from 6 mAfM2 to over 30 mAIM2, a range of ionic coating layers and a range of conducting coating anodes.

Anode systems of the present invention can show substantial increases in longevity 21 in direct comparison with (the relevant) conducting coating anode used alone and applied directly to the concrete surface.

The relevance of concrete surface carbonation depth could not be included in this work, Carbonation will significantly reduce hydroxyl ion availability and may impair adhesion! penetration of some coatings. Aggressive surface preparation to 26 remove the carbonated layer (which is potentially deep) can produce a high aggregate exposure and has costs. Carbonation remains an important question regarding coating anodes.

F 13 Acknowledgment Development and testing is partially funded by the UK Department of Trade and 31 Industry in a "SMART" project.

Claims

CLAIMS1 An anode system, for the cathodic protection of reinforcing steel in concrete, wherein the anode is an electronically conducting coating layer that is isolated from direct mechanical contact with the concrete surface and this mechanical isolation is achieved by using a discrete, ionically conducting paint coating layer that is applied to the concrete surface prior to application of the anode and that supports the protective ionic current flow from anode to concrete and that mechanically supports the anode coating.
2. An anode system, as in claim 1, where the anode is an electronically conducting paint coating layer.

3 An anode system, as in claims 1 or 2, wherein the ionically conducting paint coating layer conducts by ionic conduction supported by water imbibed from water in the environment of said coating layer.