BACKGROUND ART

1. Technical Field

This invention is directed to electrodes for use in electrochemical systems which operate at low current densities, particularly cathodic protection systems.

2. Description of Related Prior Art

Impressed current cathodic protection systems are well known in use to protect concrete structures, particularly concrete structures such as support pillars, cross beams, and road decks for bridges containing reinforced concrete. Reinforcing steel bars, commonly referred to as rebars, are used to reinforce concrete structures. It is well known that steel is not corroded in an alkaline medium. The reinforcing bars are very frequently covered with an adherent oxide layer when embedded in the concrete. However, in reinforced concrete contaminated with chloride ion, corrosion of the steel is accelerated with the result that the corroded steel occupies a greater volume than the space occupied by the steel prior to corrosion, eventually creating intense local pressure on the concrete which brings about cracking and eventual spalling of the concrete.

In addition to contamination with chloride ion resulting from the use of common salt to prevent ice formation on the road deck or bridge, it is well known to add calcium chloride to a mixture of concrete before placing in position as a means of lowering the freezing point of the concrete. Such use of calcium chloride increases the rate of corrosion of reinforcing steel

bars in such concrete structures. In addition, concrete structures having reinforcing bars therein may be exposed to salt-laden atmosphere, particularly in marine locations, such that corrosion of the steel reinforcing bars is accelerated. The term "cathodic protection" is defined to include the application of an electrolytic system whereby the electrode potential of the steel reinforcing bars is depressed in order to stop or significantly decrease the corrosion of these reinforcing bars.

It is known from U.S. 5,334,293 that a titanium anode cannot be used in an electrolytic cell, particularly in an electrolytic cell in which during operation of the cell chlorine is evolved at the anode. Such an anode cannot be used in this electrolytic cell as the surface of the titanium anode would oxidize when anodieally polarized and the titanium would soon cease to function as an anode. Coatings comprising ruthenium oxide are disclosed as useful on a titanium substrate to obtain an electrode having a commercially useful lifetime.

Bockris et al. in Modern Etectrochemistry, volume 2, pages 1315 - 1321, Plenum Press, explains the transformation of a metal surface from a corroding and unstable surface to a passive and stable surface as being facilitated by increasing the electrical potential in the positive direction on the metal. As the potential is increased, the current initially increases, reaching a maximum value and then starts shaφly to decrease to a negligible value. The point at which the current sharply decreases is referred to as passivation and the potential at which this occurs is termed the passivation potential.

In the prior art, electrodes particularly for use in cathodic protection systems require electrocatalytic coatings on base metals which are subject to passivation in order to overcome the tendency of such metals to passivate

and cease to function as electrodes. Such coatings are described in U.S. 3,632,498 as consisting essentially of at least one oxide of a film-forming metal and a nonfilm-forming conductor the two being in a mixed crystal form and covering at least two percent of the active surface of the electrode base metal. Similarly, electrodes made utilizing a valve metal substrate are disclosed as requiring one or more layers of a coating contaimng platinum as disclosed in U.S. 5,290,415 and U.S. 5,395,500.

An anode useful in a cathodic protection system to protect the reinforcing steel bars in a concrete structure can consist of a porous titanium oxide, TiO _x where "x" is in the range 1.67 to 1.95, as disclosed in European patent application 186 334 or where "x" is in the range 1.55 to 1.95, as disclosed in U.S. 4,422,917. Other porous materials are disclosed in 186 334 as substitutes for the porous titanium oxide such as graphite, porous magnetite, porous high silicon iron or porous sintered zinc, aluminum or magnesium sheet.

In U.S. 4,319,977, an electrode formed of thin sheets of titanium is disclosed as useful in an electrometallurgical cell. In addition to a metal such as titanium, electrodes consisting essentially of tantalum, niobium, or zirconium are disclosed as useful in the British patent no. 951,766 cited in this United States patent. As described in '977, the titanium electrode is utilized as an anode in a method of electrolytically producing manganese dioxide by immersing the electrode in a solution of manganese sulphate and sulfuric acid and electrolytically depositing the manganese dioxide onto the electrode. Periodically, the manganese dioxide is removed from the electrode.

The novel valve metal electrodes of the invention are particularly suited for cathodic protection systems in which the valve metal electrodes

function as anodes and the reinforcing iron or steel rods in the concrete structure function as cathodes as the result of connecting the valve metal anodes and the iron or steel reinforcing rods to an electrical circuit and impressing a current sufficient to cause the iron or steel rods to act as cathodes. Because it is possible to operate such a cathodic protection system at a low anodic current density, the useful electrical potential in the positive direction on a valve metal anode can be adjusted so as to be well under that required to attain passivation of the valve metal anode. The valve metal anode, accordingly, continues to function as an anode in a cathodic protection system providing that the anodic current density is controlled at a current density up to about 215 milliamps per square meter.

DISCLOSURE OF THE INVENTION

Generally, the valve metal electrodes of the invention are useful as anodes in any electrochemical process where a low system current density is applicable. An anodic current density of up to about 215 milliamps per square meter will allow a useful commercial lifetime of the valve metal anodes of the invention. Specifically, the anodes of the invention are useful in a cathodic corrosion protection system for the cathodic corrosion protection of reinforcing elements made of metals such as iron or alloys thereof, particularly steel bars in reinforced concrete structures whether said structures present a surface structure which is horizontal, vertical, inclined, or an overhead structure. The anodes of the invention can be in the form of a foraminous sheet or mesh, for instance, an expanded metal or a woven mesh. The electrode can be embedded in the concrete or placed in a cut or groove made in an existing concrete structure. The electrode can be applied directly such as by thermal spraying onto the reinforced concrete structure surface. Accordingly, the anodes of the invention are useful in the protection of steel reinforced concrete structures such as bridge decks, parking garage decks, bridge substructures, and building structural members.

Valve metal anodes are especially useful for preventing corrosion of iron or steel reinforcing elements, i.e., steel bars in concrete structures. The reinforcing steel bars serving as cathodes are connected in an electrical circuit with the valve metal anodes. Providing that the anodic current density is low or maintained at up to about 215 milliamps per square meter, a useful commercial lifetime of the valve metal anodes can be expected. Titanium is especially preferred for use as the valve metal anode in a cathodic protection system.

A valve metal for use as an anode in a cathodic corrosion protection system can be pretreated prior to use in order to increase its resistance to passivation. Such pretreatment of the valve metal can take the form of heating the valve metal, generally, at a temperature of about 250°C to about 750°C, preferably, about 250°C to about 450°C, and most preferably, about 250°C to about 350°C.

The use of valve metal electrodes, particularly as anodes in cathodic corrosion protection systems is particularly desirable from an economic standpoint. As set forth above in connection with the discussion of the prior art, it has been thought necessary in the prior art to utilize an expensive anode coating containing a precious metal such as platinum on the surface of a valve metal surface such as a titanium surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a novel valve metal electrode for operation at low current density, particularly, as an anode in a cathodic protection system in which iron or steel rods embedded in a concrete structure are protected against corrosion by connecting valve metal anodes and the iron or steel reinforcing bars in the concrete structure to an electrical circuit and

impressing a current sufficient to cause the iron or steel material to act as a cathode in the circuit.

The valve metal anode can be any shape. It can be, for instance, a metallic sheet placed upon the surface of a concrete structure or a foraminous metal sheet which can be embedded in a concrete structure or a ribbon of metal inserted into a slit cut into the surface of a concrete structure. The anode can be formed on the surface of a concrete structure by a thermal spraying process in which a valve metal is deposited onto a cleaned surface of a concrete structure. In addition, the valve metal anode of the invention also can be used in the form of a mesh or an expanded metal. By use of the word "thermal spraying", it is intended to include arc and plasma spraying processes.

The anode of the invention has an oxide film on the surface thereof and is free of electrocatalytically active coatings which have been applied in the prior art to valve metal electrodes, particularly valve metal substrates for use as anodes in cathodic protection systems. The anode of the invention does not require the apphcation of an electrocatalytic coating precursor and the subsequent activation of said catalytic coating. Surprisingly, it has been found possible to extend the lifetime of a valve metal anode, as determined by exposure of the anode to accelerated testing, by heating the valve metal anode at elevated temperature. Generally, exposure of the valve metal to a temperature of about 250°C to about 750°C for a period, generally, of about 3 minutes to about 5 hours, preferably, about 30 minutes to about 3 hours, and most preferably, about 1 hour to about 2 hours results in a substantial improvement in anode Ufetime, i.e., time before passivation occurs at a given current density. Where the valve metal is apphed to a concrete structure by thermal spraying, no baking of the anode is required to improve

the Ufetime of the anode since the thermal spraying process sufficiently heats the valve metal electrode.

When the anode of the invention is formed in situ by application to a concrete structure by thermal spraying, the concrete surface is desirably cleaned prior to deposition of the anode which is formed in situ thereon. The concrete surface can be treated to dislodge and displace weathered concrete or exposed concrete which has been mechanically abraded or loosened or has disintegrated by atmospheric effects or the like. Sandblasting is a suitable means of cleaning the surface to provide a roughened, pocked or irregular surface which affords mechanical adhesion for the anode coating to be deposited thereon. After sandblasting, it is desirable to remove or cover any bare rebar or other metallic materials connected to the rebar to prevent electrical shorts between the anode and the rebar. Thereafter, prior to any further deterioration of the sandblasted surface, a molten metal is deposited by thermal spraying.

Where the anode of the invention is applied to a concrete structure by thermal spraying, the thickness of the anode coating on the concrete structure is, generaUy, about 0.0025 cm. to about 0.025 cm., preferably, about 0.0076 cm. to about 0.0203 cm., and, most preferably, about 0.013 cm. When the anode is applied to a concrete structure by thermal spraying, the coating can be provided as a continuous layer, as a grid pattern or as a closely spaced particulate deposit.

Subsequent to (1) formation of the anode of the invention in situ on the surface of a concrete structure, or the application of a sheet, mesh, or expanded metal form of a valve metal prior to the pouring of the concrete or (2) the application of a sheet, mesh, or expanded metal form of a valve metal in a cut or groove made in an existing concrete structure, the anode

of the invention is connected to a source of direct current and the circuit is completed by connecting as a cathode the reinforcing elements, i.e., steel bars within the concrete structure. The impressed current is opposite and at least equal to the naturally occurring current which results under normal circumstances. The net result of impressing a direct current which is opposite and equal to the naturally occurring current is to prevent electrolytic corrosion action on the reinforcing steel bars.

Accordingly, the reinforcing bars maintain their integrity over a long period of time. The source of direct current is such as to provide an anodic current density of up to about 215 milliamps, preferably, about 1 to about

160 milliamps, and, most preferably, about 1 to about 110 milliamps per square meter.

Suitable valve metals include titanium, zirconium, niobium, tantalum, and alloys comprising one or more valve metals or metals having properties similar to those of valve metals. Titanium is a preferred valve metal as it is readily available and relatively inexpensive when compared with the other valve metals. Preferably, the titanium is ASTM 265 titanium grade 1 or 2.

It is well known that valve metals exposed to normal atmospheric conditions will inevitably possess a surface oxide layer for example, titanium oxide (Ti0 ₂ ) can be stoichiometric or non-stoichiometric depending upon the conditions of formation of the oxide layer. The anode of the invention is beheved to have a surface oxide layer which is stoichiometric as represented by the compounds Ti0 ₂ , TiO, and Ti ₂ 0 ₃ . As previously indicated, accelerated tests as described above, indicate that the lifetime of the electrode can be substantiaUy extended by activating the electrode at elevated temperatures. It is considered that this process results in the formation of a surface oxide layer which is stoichiometric.

Impressed current cathodic protection systems are well known for the protection of reinforced concrete structures such as buildings and in road construction, and, particularly, in the fabrication of supports, pillars, cross¬ beams, and road decks for bridges. Over the years, increasing amounts of common salt, sodium chloride, have been used during the winter months to prevent ice formation on roads and bridges. The melted snow or ice and sodium chloride in aqueous solution tend to seep into the reinforced concrete structure. In the presence of chloride ion the reinforcing steel rebars are corroded at an accelerated rate such that the resultant corrosion products formed by the oxidation reaction occupy a greater volume than the space occupied by the reinforcing bars prior to oxidation. Eventually an increased local pressure is created which brings about cracking of the concrete and eventual spalling of the concrete covering the reinforcing members so as to expose the reinforcing members directly to the atmosphere. The use of a valve metal without an electrocatalytically active coating thereon as an anode in a cathodic protection system is unexpected in view of the beUef among those skiUed in the art that a titanium anode or an aUoy of titanium possessing properties similar to titanium cannot be used in an electrolytic process as the surface of the titanium would oxidize when anodically polarized and the titanium or alloys thereof would soon cease to function as an anode. For instance, in U.S. 5,334,293, electrocatalytically coated anodes of titanium or an aUoy of titanium are disclosed for use in an electrolytic cell, particularly, for use as an anode in an electrolytic cell in which chlorine is evolved at the anode. The coating utilized usuaUy includes a metal of the platinum group, oxides of metals of the platinum group, or mixtures of one or more metals such as one or more oxides or mixtures or solid solutions of one or more oxides of a platinum group metal and a tin oxide or one or more oxides of a valve metal such as titanium. Similar electrocatalyticaUy coated titanium electrodes are disclosed in U.S. 3,632,498; U.S. 5,354,444; and U.S. 5,324,407.

Thermal spraying of a valve metal or aUoy thereof onto a concrete surface can be accomplished by means including plasma spray application, usuaUy of a particulate form of a valve metal, preferably, a titanium powder. While the metals can be applied in particulate form the metal used to feed the plasma spray can be in a different form such as a wire form. In plasma spraying, the metal to be applied to a concrete substrate is melted and sprayed in a plasma stream generated by heating the metal with an electric arc to a high temperature. Air on an inert gas, such as argon or nitrogen, optionally containing a small amount of hydrogen is utilized to spray the melted metal in a plasma stream. The use of the term "plasma spraying" is intended to be a species of thermal spraying and includes flame spraying and arc spraying.

The spraying parameters such as the volume and temperature of the flame or plasma spraying stream, the spraying distance, the feed rate of particulate metal constituents and the like, are chosen so that the particular metal components are melted and, utilizing the spray stream, are deposited on the concrete substrate while still substantially in melted form so as to provide either an essentiaUy continuous coating or one in which the sprayed particles remain particulate on the concrete substrate thus, having a particulate appearance.

The particulate valve metal employed in plasma spraying, for instance, a titanium powder can have a typical particle size range of 20 - 100 microns, and, preferably, has aU particles within the range of 40 - 80 microns. Particulate valve metals having different particle sizes are equaUy suitable so long as they are readily subject to being applied by plasma spray. The metallic constituency of the particulates can include a valve metal or valve metal oxide or mixtures thereof or valve metal alloys or valve metal alloy oxides or mixtures thereof.

Where not otherwise specified in the specification and claims, temperatures are in degrees centigrade, and parts, percentages, and proportions are by weight.

MODES FOR CARRYING OUT THE INVENTION The foUowing Examples iUustrate the present invention and should not be construed, by implication or otherwise, as limiting the scope of the appended claims.

EXAMPLE 1:

A concrete slab is fabricated having dimensions of 30 cm. by 15 cm. by 10 cm. Sodium chloride is added to the wet concrete mix in order to simulate the conditions found in the preparation of typical reinforced concrete or structures exposed to salt containing environments as a result of the use of salt during winter months on roadways and bridges. A concentration of about 3 kilograms of sodium chloride per cubic meter of concrete is used. Embedded in the concrete slab is a steel bar simulating reinforcing steel bar typical in many concrete bridge structures. The steel bar protrudes at one end of the concrete slab in order to allow connection to an electrical circuit.

The slab is arc-sprayed with titanium formed from a titanium wire having a diameter of 0.16 cm. The spray distance from the concrete slab is about 15 cm. The arc is formed at about 25 volts and 200 amps. The measured thickness of the coating of titanium apphed by arc spraying to the concrete surface is 0.13 cm. The resistance over a 15 cm. span is less than 1 ohm. The concrete block is placed in an atmosphere having a relative humidity of 95 - 100 percent and the titanium arc sprayed coating is connected to a positive post of a regulated power supply. The reinforcing

steel bar is connected to a negative post. The power supply for the circuit is adjusted to give a constant current of 1.0 milhamp which is calculated to be equivalent to a current density on the surface of the titanium anode of 22 milliamps per square meter. The initial cell voltage which is measured between the titanium anode and the steel bar is 1.89 volts and after 114 days of operation the ceU voltage is 1.99 volts indicating that no passivation of the titanium anode has occurred.

EXAMPLE 2:

A concrete slab similar to that described in Example 1 is prepared and the power supply is adjusted to give a constant current of 1.0 milhamp which is equivalent to a current density of 22 miUiamps per square meter on the anode surface. The initial voltage is 1.55 volts and this voltage rose to 1.80 volts after 4 days at which time the current density is increased to 136 milliamps per square meter. The initial voltage is 3.74 volts. After a total of 118 days the voltage is measured at 3.43 volts indicating the absence of passivation of the titanium anode.

EXAMPLE 3:

Eight titanium mesh samples are taken and placed in a solution of sodium hydroxide at 0.6575 molar and potassium hydroxide at 0.1914 molar. The titanium mesh has a geometric surface area of approximately 0.05 square meter per linear meter. The pieces of mesh are cut so that the total surface areas are approximately in the ratio of 1:2:5:10:25:50:100:500. These samples are placed in series with a regulated power supply in an electrical circuit so that the titanium pieces are connected so as to be the anodes in the circuit. Eight nickel sheets each measuring 15 cm. by 5 cm. by 0.16 cm., are also immersed in said solution and connected so as to be the cathodes

in the circuit. The initial voltages on start-up of the 8 cells so formed are recorded as well as the times for a cell voltage increase of 6 volts. It is believed that the time for the cells to show a voltage increase of 6 volts at accelerated current densities is directly related to the expected useful Ufetime of a titanium anode (time to passivation) when connected to an electrical circuit in a cathodic protection system. The results of this experiment are reported in Table I below:

TABLE I Time to titanium passivation as a function of current density

CURRENT DENSITY TIME FOR 6 VOLT (MILLIAMPS PER CELL VOLTAGE SQUARE METER) INCREASE

(HOURS)

2152 59.8

4304 9.75

10760 4.40

21520 0.864

53800 0.216

107600 0.041

215200 0.0175

1076000 0.000833

A power equation was fitted to the points above. The equation is:

Y = 4.079 (IO ,7 ⁷ )\ X v-1.767

where Y is the time in hours and X is the current density in milliamps per square meter. This equation has a correlation coefficient of 0.9975. Based upon this power equation, it is predicted that the titanium mesh anode

utilized in a cathodic protection system for the protection of steel reinforcing bars in concrete at a current density of 110 milliamps per square meter would have a lifetime (prior to passivation) of 1.2 years and at a current density of 22 milliamps per square meter, the anode would have a lifetime of 20.5 years.

EXAMPLE 4;

Seven titanium mesh samples are taken and placed in seven beakers each containing a solution of sodium hydroxide at 0.6575 molar, potassium hydroxide at 0.1914 molar and potassium chloride at 0.4608 molar. The titanium mesh has a geometric surface area of approximately 0.05 square meter per linear meter. The pieces of mesh are cut so that the total surface areas are approximately in the ratio of 1:2:4:8:16:40:100. These samples are placed in series with a regulated power supply in an electrical circuit, so that the titanium pieces are connected so as to be anodes in the circuit. Seven nickel sheets each measuring 15 cm. by 5 cm. by 0.16 cm., are also immersed in said solutions and connected so as to become cathodes relative to the titanium anodes. The initial voltages on start-up of the seven cells so formed are recorded as well as the times for a ceU voltage increase of 6 volts. It is believed that the time for the cells to show a voltage increase of 6 volts, at accelerated current densities, is directly related to the expected useful lifetime of a titanium anode (time to passivation) when connected to an electrical circuit in a cathodic protection system. The results of this experiment are reported in Table II below:

TABLE II Time to titanium passivation as a function of current density

CURRENT DENSITY TIME FOR 6 VOLT (MILLIAMPS PER CELL VOLTAGE SQUARE METER) INCREASE

(HOURS)

538 1080

1076 155

2152 59.8

4304 21.4

8608 8.9

21520 1.2

53800 0.35

A power equation was fitted to the points above. The equation is: Y = 2.926(10 ⁷ ) X ^"1 - ⁶⁸⁵ where Y is the time in hours and X is the current density in milliamps per square meter. This equation has a correlation coefficient of 0.9961. Based upon this power equation, it is predicted that the titanium mesh anode utilized in a cathodic protection system for the protection of steel reinforcing bars in concrete at a current density of 110 milliamps per square meter would have a lifetime (prior to passivation) of 1.3 years and at a current density of 22 milliamps per square meter, the anode would have a lifetime of 18.9 years.

EXAMPLE 5:

Eight samples of titanium mesh previously heated to a temperature of 350°C for a period of thirty minutes are evaluated in accordance with the procedure described in Example 4 at similar current densities. The time in

hours for a 6 volt cell voltage increase at each current density is shown in Table III below.

EXAMPLE 6:

Two samples of titanium mesh previously heated to a temperature of 550°C are utilized in the test procedure of Example 4 such that a current density of 53800 milliamps per square meter and 107,600 milliamps per square meter are placed upon these samples of titanium functioning as anodes in the electrolytic ceUs sirnilar to those described in Example 4. The time in hours for a 6 volt cell voltage increase at each of these current densities is shown in Table III below.

TABLE III

Time for titanium passivation as a function of current density and heating history.

CURRENT DENSITY EXAMPLE 5 EXAMPLE 6 (MILLIAMPS PER (HOURS) (HOURS) SQUARE METER)

1076 483 —

2152 367 —

4304 136 ~

8608 41.4 ~

21520 25.4 ~

53800 11.8 1.6

107600 4.7 1.4

215200 0.62 —

While this invention has been described with reference to certain specific embodiments, it will be recognized by those skilled in the art that many variations are possible without departing from the scope and spirit of the invention, and it will be understood that it is intended to cover aU changes and modifications of the invention disclosed herein for the purpose of Ulustration which do not constitute departures from the spirit and scope of the invention.