Sacrificial anode for cathodic protection and alloy therefor

Abstract: An alloy for a sacrificial anode according to a first preferred aspect of the present invention includes about 10% to about 50% of Zn, about 0.03% to about 0.6% of In, and about 0.0005% to about 0.05% of Zr. The balance may be Al and any unavoidable impurities. An alloy according to a second preferred aspect of the present application includes about 10% to about 50% of Zn, about 0.03% to about 0.6% of In, and about 0.05% to about 0.3% of Si. The balance may be Al and any unavoidable impurities. An alloy according to a third preferred aspect of the present invention includes about 10% to about 50% of Zn, about 0.03% to about 0.6% of In, and about 0.02% to about 0.2% of Ce. The balance may be Al and any unavoidable impurities. An alloy according to a fourth preferred aspect of the present invention includes about 10% to about 50% of Zn, about 0.03% to about 0.6% of In, about 0.005% to about 0.1% of Ti, and about 0.001% to about 0.02% of B. The balance may be Al and any unavoidable impurities. An alloy according to another preferred aspect of the present invention includes about 10% to about 50% of Zn and about 0.03% to about 0.6% of In. The balance may be Al and any unavoidable impurities. The present invention also relates to a reinforced concrete structure comprising a cementitious material, metal reinforcement, and a sacrificial anode, the sacrificial anode including an alloy containing Al, Zn and In.

Description: FIELD OF THE INVENTION The present invention relates to an alloy for a sacrificial anode which is suitable for corrosion protection of reinforcement in a structure built of reinforced concrete and to a reinforced concrete structure comprising the sacrificial anode. The invention relates also to a reinforced concrete structure and a method of providing cathodic protection. BACKGROUND OF THE INVENTION Reinforcement in a structure built of reinforced concrete is not substantially corroded because concrete is strongly resistant against alkali. However, the problem of corrosion arises when a reinforced concrete structure is in an environment where salt water may permeate therein. For example, such environments exist when the structure is near the sea or dusted over by chlorides for the prevention of ice accumulation. Most cathodic protection of steel in concrete is done with impressed current systems. Impressed current systems have the inherent need for periodic maintenance which limits their attractiveness to bridge owners. However, the application of impressed current anodes requires that the anode be completely isolated from the embedded steel, otherwise short circuits will occur. Sacrificial anode systems do not have these problems. In an attempt to solve the above-noted problem, use of a zinc alloy has been proposed in a sacrificial anode method which realizes long-term, stable and low-cost corrosion protection. However, a sacrificial anode formed of a zinc alloy has an exceedingly high potential (high positive). A low potential (high negative potential) is one of the important characteristics of a sacrificial anode. Furthermore, pure zinc, aluminum, and aluminum-zinc alloys have been used for sacrificial cathodic protection of steel reinforcing in concrete. All of these alloys have exhibited a phenomenon called passivation while on concrete. Passivation occurs when the pH of the concrete surface decreases below the normally highly alkaline value found in concrete as a result of reactions with carbon dioxide in the air, a process called carbonation, which is a normal process. The effect of passivation is that the current output of the alloy anode decreases to a point which is no longer satisfactory to provide cathodic protection for the steel. These alloys are only satisfactory for use in very wet areas of the structure. SUMMARY OF THE INVENTION The alloys of the present invention do not exhibit the above-identified passivation phenomenon and maintain a satisfactory level of cathodic protection current. Accordingly, the present invention provides an alloy for a sacrificial anode which is suitable for corrosion protection of reinforcement in a structure built of reinforced concrete; namely, an alloy which enables a sacrificial anode formed thereof to have a sufficiently low potential and to cause generation of a sufficiently large amount of electricity. The object of the present invention is an alloy for a sacrificial anode comprising 10 to 50 wt% Zn, 0.03 to 0.6 wt% In, optionally 0.05 to 0.3 wt% Si and at least one of 0.0005 to 0.05 wt% Zr, 0.02 to 0.2 wt% Ce, 0.005 to 0.1 wt% Ti and 0.001 to 0.02 wt% B, the balance being Al and any unavoidable impurities. The present invention also relates to a reinforced concrete structure comprising a cementitious material, metal reinforcement, and a cathodic protection anode, comprising an alloy as defined above, optionally in the form of a sacrificial anode. The present invention further relates to a method of providing cathodic protection to a reinforced concrete structure comprising providing a reinforced concrete structure comprising a cementitious material an metal reinforcement; and introducing a cathodic protection anode optionally in the form of a sacrificial anode into the reinforced concrete structure, and optionally electricially connecting the sacrificial anode to the metal reinforcement, said anode including an alloy as defined above. The present invention also relates to the use of an alloy as defined above as cathodic protection anode for reinforced concrete structure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Unless otherwise specified herein, all amounts indicated are percent by weight. In the alloy according to the present invention, both Zn and In function so as to restrict self dissolution of the alloy thus increasing the amount of electricity generated. If the amount of Zn contained in the alloy is less than about 10%, or if the amount of In contained in the alloy is less than about 0.03%, the above-described function is not sufficiently effected. Also, if the amount of Zn contained in the alloy is more than about 50%, or if the amount of In contained in the alloy is more than about 0.6%, the potential of the anode tends to be too high (too highly positive). In a preferred embodiment, the amount of Zn contained in the alloy is about 10% to about 40%. In another more preferred embodiment, the amount of Zn is about 10% to about 30%. In a more preferred embodiment, the amount of In contained in the alloy is about 0.05% to about 0.5%. In another more preferred embodiment, the amount of In is about 0.1 % to about 0.3%. In the alloy according to the invention, Zr has the same function as Zn and In. If the amount of Zr contained in the alloy is less than about 0.0005%, the function of restricting self dissolution is not sufficiently effected. Also, if the amount of Zr contained in the alloy is more than about 0.05%, Zr is distributed in the grain boundary of the alloy in large grains thus reducing the amount of electricity generated. In a preferred embodiment, the amount of Zr contained in the alloy is about 0.001 % to about 0.01%. In an alloy according to a preferred aspect of the invention, Si has the same function as Zn and In. If the amount of Si contained in the alloy is less than about 0.05%, the function of restricting self dissolution is not sufficiently effected. Also, if the amount of Si contained in the alloy is more than about 0.3%, the potential of the anode formed thereof tends to be too high (too highly positive). In a preferred embodiment, the amount of Si contained in the alloy is about 0.1 % to about 0.2%. In the alloy according to the invention, Ce functions so as to prevent hole-type corrosion of the alloy thus increasing the amount of electricity generated. If the amount of Ce contained in the alloy is less than about 0.02%, the function is not sufficiently effected. Also, if the amount of Ce contained in the alloy is more than about 0.2%, the potential of the anode formed thereof tends to be too high (too highly positive). In a preferred embodiment, the amount of Ce contained in the alloy is about 0.05% to about 0.15%. In the alloy according to the invention, both Ti and B function so as to prevent hole-type corrosion and groove-type corrosion (corrosion occurring in the form of a groove leaving two sides of the groove uncorroded) of the alloy by making the crystals of the alloy microscopic grains instead of large pillars thus increasing the amount of electricity generated. If the amount of Ti contained in the alloy is less than about 0.005%, or if the amount of B contained in the alloy is less than about 0.001 %, the function is not sufficiently effected. Also, if the amount of Ti contained in the alloy is more than about 0.1 %, or if the amount of B contained in the alloy is more than about 0.02%, the amount of electricity generated is reduced. In a preferred embodiment, the amount of Ti contained in the alloy is about 0.01 % to about 0.08%. In another more preferred embodiment, the amount of B is about 0.005% to about 0.01 %. Preferred embodiments of alloys of the present invention as defined above are those comprising the following combinations: 10 to 40 wt% Zn nd 0.05 to 0.5 wt% In, 10 to 30 wt% Zn and 0.1 to 0.3 wt% In, about 10 wt% Zn and about 0.2 wt% In, about 20 wt% Zn and about 0.2 wt% In, about 30 wt% Zn and about 0.2 wt% In, 10 to 30 wt% Zn and 0.05 to 0.5 wt% In, 10 to 30 wt% Zn and 0.05 to 0.5 wt% In and 0.0005 to 0.05 wt% Zr, 10 to 30 wt% Zn and 0.05 to 0.5 wt% In and 0.02 to 0.2 wt% Ce, 0.01 to 0.08 wt% Ti and 0.005 to 0.01 wt% B, and 10 to 30 wt% Zn and 0.05 to 0.5 wt% In and 0.005 to 0.1 wt% Ti and 0.001 to 0.02 wt% B. The following examples illustrate numerous embodiments of the present invention. Preferred Examples 1 through 22 and Comparative Examples A through T Different types of alloys described in Table 1 were dissolved in the air and molded to obtain rod-shaped ingots, each having a diameter of 25 mm and a length of 250 mm. Each ingot sample was used as a sacrificial anode and tested for performance. The test was performed in accordance with "The Method for Testing a Sacrificial Anode" ( The Method for Testing a Sacrificial Anode and its Detailed Explanation, Corrosion Protection Technology , Vol. 31, pp. 612-620, 1982, Japanese Society of Corrosion Engineers, Tokyo, Japan) as follows. Each sample was polished until the surface thereof obtained the roughness equal to that of No. 240 sandpaper and covered with vinyl tape for insulation except for an area of 20 cm 2 of the side surface thereof. Next, an aqueous solution having a composition of 32.0 g/l KCl, 24.5 g/l NaOH, 10.0 g/l KOH and 0.1 g/l Ca(OH) 2 was filled in a one-liter beaker as a test liquid of concrete. Each sample of the alloy was located at the center of the beaker as an anode, and a cylinder formed of stainless steel was located along the side wall of the beaker as a cathode. (The distance between the anode and the cathode was 30 mm.) The anode and cathode were connected to each other via a DC regulated power supply. Electricity was supplied for 240 hours at a constant current density of 0.1 mA/cm 2 at the anode. The amount of electricity generated was obtained by a calculation based on the reduced weight of the sample. The potential of the anode was obtained by measuring the potential of the anode immediately before the electricity supply was stopped and using an electrode formed of silver-silver chloride as a reference. The composition of each sample and the test results are shown in Tables 1 and 2. Preferred Examples or Examples Composition (wt%) Performance Zn In Ce Al Amount of Electricity Generated (A·hr/kg) Potential of Anode (mV vs. Ag/AgCl) Example 1 10 0.05 0.05 Balance 1612 -1555 Example 2 10 0.06 0.20 Balance 1750 -1630 Example 3 10 0.59 0.06 Balance 1773 -1550 Example 4 10 0.53 0.18 Balance 1800 -1440 Example 5 20 0.11 0.15 Balance 1730 -1456 Example 6 20 0.57 0.12 Balance 1850 -1395 Example 7 30 0.08 0.07 Balance 1662 -1303 Example 8 30 0.28 0.20 Balance 1651 -1179 Example 9 50 0.07 0.03 Balance 1660 -1123 Example 10 50 0.06 0.18 Balance 2299 -1081 Example 11 50 0.58 0.18 Balance 2330 -1011 Comp. Ex A 7 0.01 0.01 Balance 579 -1252 Comp. Ex B 7 0.65 0.01 Balance 1100 -950 Comp. Ex C 10 0.02 0.30 Balance 1020 -905 Comp. Ex D 10 0.65 0.35 Balance 1750 -10 Comp. Ex E 30 0.00 0.01 Balance 905 -1047 Comp. Ex F 30 0.70 0.34 Balance 1850 357 Comp. Ex G 50 0.01 0.04 Balance 483 -1050 Comp. Ex H 50 0.70 0.38 Balance 1986 540 Comp. Ex I 60 0.05 0.50 Balance 1984 -100 Comp. Ex J 60 0.60 0.35 Balance 2800 680 Preferred Examples or Examples Composition (wt%) Performance Zn In Ti B Al Amount of Electricity Generated (A·hr/kg) Potential of Anode (mV vs. Ag/AgCl) Example 12 10 0.05 0.005 0.001 Bal. 1612 -1555 Example 13 10 0.06 0.03 0.01 Bal. 1750 -1630 Example 14 10 0.59 0.006 0.001 Bal. 1773 -1550 Example 15 10 0.53 0.08 0.015 Bal. 1800 -1440 Example 16 20 0.11 0.01 0.004 Bal. 1730 -1456 Example 17 20 0.05 0.004 0.004 Bal. 1850 -1395 Example 18 30 0.08 0.007 0.002 Bal. 1662 -1303 Example 19 30 0.28 0.008 0.004 Bal. 1651 -1179 Example 20 50 0.07 0.008 0.004 Bal. 1660 -1123 Example 21 50 0.06 0.005 0.007 Bal. 2299 -1081 Example 22 50 0.58 0.03 0.01 Bal. 2330 -1011 Comp. Ex. K 7 0.01 0.14 0.03 Bal. 579 -1252 Comp. Ex. L 7 0.65 0.13 0.03 Bal. 1100 -950 Comp. Ex. M 10 0.02 0.14 0.03 Bal. 1020 -905 Comp. Ex. N 10 0.65 0.12 0.02 Bal. 750 -10 Comp. Ex . O 30 0.00 0.003 0.0009 Bal. 905 -1047 Comp. Ex. P 30 0.70 0.003 0.0009 Bal. 1850 357 Comp. Ex . Q 50 0.01 0.015 0.0008 Bal. 483 -1050 Comp. Ex . R 50 0.70 0.05 0.009 Bal. 1986 540 Comp. Ex. S 60 0.05 0.004 0.004 Bal. 1984 -100 Comp. Ex . T 60 0.60 0.12 0.03 Bal. 1800 680 An alloy according to the present invention causes electricity generation of an amount as large as 1,500 A·hr/kg or more, and an anode formed of an alloy in accordance with the present invention has a potential as low as -1,000 mV or less. Such an alloy is suitable for corrosion protection of reinforcement in a structure built of reinforced concrete. In use, methods of application of the alloy to structure include thermal spray, but the alloy could also be applied as a sheet or in strips. Arc spray and flame spray are preferred methods of application. For the thermal spray process, the alloy is cast, extruded to a wire form, drawn into wire of a size suitable for the thermal spray equipment, then sprayed onto the surface of the concrete structure. The alloy bonds with the concrete. An electrical connection is made between the steel embedded into the concrete and the anode. For sheet, plate, and strip forms, the alloy can be cast into the structure or mechanically fastened to the structure, then overcoated with a cementitious overlay. Although we do not wish to be bound by any theory, one possible explanation of the invention is the following. Electrical current flows from the anode to the embedded steel in sufficient quantity to cause electrochemical polarization of the steel and subsequent protection of the steel from corrosion by moisture and salts. The present invention also relates to a reinforced concrete structure comprising a cementitious material, metal reinforcement, and a sacrificial anode, said sacrificial anode including an alloy of the invention. Metal reinforcement includes any metal shaped in such a way so as to provide reinforcement to a cement structure in which it is incorporated. For example, the metal reinforcement includes metal grating, metal sheets and metal rods. The metal may be any metal used for concrete reinforcement, but typically is steel. The term cementitious material refers to cement compositions. Generally, a cement is any substance that acts as a bonding agent for materials, or any substance that is set and hardened by the action of water. Nonlimiting examples of a cementitious material include the following: cement, hydraulic cement, Portland cement, gas entrained cement, concretes, mortars, plasters and grouts. This list is intended to be merely illustrative and not exhaustive, and the omission of a certain class of cement is not meant to require its exclusion.

Claims: An alloy for a sacrificial anode comprising 10 to 50 wt% Zn, 0.03 to 0.6. wt% In, optionally 0.05 to 0.3 wt% Si and at least one of 0.0005 to 0.05 wt% Zr, 0.02 to 0.2 wt% Ce, 0.005 to 0.1 wt% Ti and 0.001 to 0.02 wt% B, the balance being Al and any unavoidable impurities. The alloy of claim 1 comprising 10 to 40 wt% Zn and 0.05 to 0.5 wt% In. The alloy of claim 1 comprising 10 to 30 wt% Zn and 0.1 to 0.3 wt% In. The alloy of claim 1 comprising about 10 wt% Zn and about 0.2 wt% In. The alloy of claim 1 comprising about 20 wt% Zn and about 0.2 wt% In. The alloy of claim 1 comprising about 30 wt% Zn and about 0.2 wt% In. The alloy of claim 1 comprising 0.1 to 0.2 wt% Si. The alloy of claim 1 comprising 10 to 30 wt% Zn and 0.05 to 0.5 wt% In. The alloy of claim 1 comprising 0.001 to 0.01 wt% Zr. The alloy of claim 1 comprising 10 to 30 wt% Zn and 0.05 to 0.5 wt% In and 0.0005 to 0.05 wt% Zr. The alloy of claim 1 comprising 0.05 to 0.15 wt% Ce. The alloy of claim 1 comprising 10 to 30 wt% Zn and 0.05 to 0.5 wt% In and 0.02 to 0.2 wt% Ce. The alloy of claim 1 comprising 0.01 to 0.08 wt% Ti and 0.005 to 0.01 wt% B. The alloy of claim 1 comprising 10 to 30 wt% Zn and 0.05 to 0.5 wt% In and 0.005 to 0.1 wt% Ti and 0.001 to 0.02 wt% B. Use of an alloy according to any of claims 1 to 14 as cathodic protection anode for reinforced concrete structures. A reinforced concrete structure comprising a cementitious material, metal reinforcement, and a cathodic protection anode, comprising an alloy as defined in any of the claims 1 to 14. The reinforced concrete structure of claim 16, wherein said anode is a sacrificial anode electrically connected to said metal reinforcement. A method of providing cathodic protection to a reinforced concrete structure comprising providing a reinforced concrete structure comprising a cementitious material and metal reinforcement; and introducing a cathodic protection anode optionally in the form of sacrificial anode into the reinforced concrete structure, and optionally electrically connecting the sacrificial anode to the metal reinforcement, said anode including an alloy according to any of claims 1 to 14.

Inventors: TAKEYA SHOZO, WATANABE KUNIO, TAKEYA, SHOZO, WATANABE, KUNIO
Applicants: CORRPRO CO INC, CORRPRO COMPANIES, INC.
Application year: 1995
Main classification: C22C21
IPC codes: C23F13, C22C21