Anode for Cathodic Protection Against Corrosion

The invention relates to an anode having a core of valve metal with an activation layer covering it, for cathodic corrosion protection for a steel reinforcement in concrete, to its use, and to a method for cathodic protection against corrosion.

As a rule, in cathodic protection against corrosion of steel and concrete, a meshlike electrode is applied in planar fashion on an existing component that is to be rehabilitated. A description of this is found for instance in the journal "Metall" [Metal] No. 2, February 1988, Metall-Verlag GmbH Berlin/Heidelberg, Year 42, pp. 133-140, and in the literature cited there.

From PCT Applications WO A 86/06758 and WO A 86/06759, and corresponding U.S. Pat. No. 4,900,410, BENNETT, the use of expanded metals of titanium and other valve metals, or their alloys, as electrodes in cathodic protection against corrosion of concrete is known. The expanded metal, wound into rolls, can be applied to the surfaces to be protected by simply unrolling them; by means of a rhomboid mesh pattern, uniform current distribution with sufficient redundancy is attainable, and by electrocatalytic coating, a current density of 100 mA/m.sup.2 for long-term operation is attainable.

It is difficult to protect a construction that is to be newly built with site-mixed concrete by means of framing by the above method; either the expanded metal acting as the anode must be affixed to the reinforcement by means of insulating spacers, or it must be attached to the finished poured concrete subsequently with dowels and then covered with sprayed concrete.

In the first case, short circuits can easily arise, if the concrete is compacted with vibrating machines; moreover, laying of the sheets of expanded metal can be done only by hand, which is highly time consuming; in the second case, relatively high costs must be expected.

It is also known from European Published Application A 0 147 977 to build up a cathodic protection against corrosion by means of a flexible mesh, acting as an anode, of electrical leads with graphited plastic sheathing; the mesh is secured to the surface of the concrete by coating with ion-conductive material; the ion-conductive material has at least the same ion conductivity as the concrete. It is also possible to use prefabricated slabs with embedded anode leads and connection leads extending out of them.

Vertical, pillarlike structures are protected by being wrapped with meshlike anode material.

Since the application to the surface of the concrete to be protected is done in several layers, this is a relatively expensive method; for vertical structures, the only feasible methods are wrapping them with anode material, or applying prefabricated slabs in which anode material is embedded to the surface of the concrete.

The object of the invention is to devise anodes that can be secured directly to the reinforcement or that can be placed in a reinforcing cage, without the possibility of short circuiting during the pouring and stirring of the site-mixed concrete into the frame; moreover, the anodes should be usable both in the production of finished concrete parts and in framing to produce concrete constructions.

In a preferred embodiment of the subject of the invention, the anode comprises a block of concrete within which is embedded a strip-like expended metal mesh of titanium or titanium alloy, provided with activation coating and poured into ion-conductive material, the anode having the shape of a bar with a round, oval or angular cross section; as the ion-conductive material, cement mortar or concrete are used, the mechanical properties of which are equivalent to that of the concrete to be protected; the cement mortar has at least the same ion conductivity as the concrete to be protected; hereinafter, the cement mortar will also be referred to as concrete. Titanium or titanium alloy leads are extended to the outside at one or both ends of the bar and are electrically connected to the expanded metal mesh, for example by welding. The expanded metal mesh is surrounded by an activation layer, which enables carrying current to the ion-conductive part of the anode by electrocatalytic means. For coating the mesh, metals or oxides of the platinum metal group typically used in the industry are preferably used.

The anode according to the invention can further more be used as a framing element for a concrete frame, instead of the framing boards typically used; it is moreover possible to use the anode according to the invention as a foundation element for producing a finished concrete part.

According to the invention, the economic use of activated titanium expanded metal proves to be particularly advantageous; because of the narrow, elongated form of the anode, not only columns, cross beams, and stairs but also walls or horizontal concrete surfaces can be protected in a simple manner; because of the composite structure of the anode, destruction of the anode when it is installed or when the concrete is compacted does not ensue.

The subject of the invention is described in further detail below in conjunction with FIGS. 1 and 2. FIG. 1 shows an exemplary embodiment of the anode according to the invention, while FIG. 2 shows the use of the anode in the reinforcement cage of a concrete pillar.

According to FIG. 1, the anode 1 is a block which comprises an electronconductive core 2 of rectangular shape and made of expanded metal mesh, and an ion-conductive jacket 3 made of cement-rich mortar; the expanded metal mesh is completely surrounded by the block-shaped ion-conductive jacket 3. In the region of the two short sides 4 of the expanded metal mesh that has titanium or titanium alloy as its core 2, there is a respective bolt 5, 6, serving as an electrical connection, of titanium or titanium alloy, connected to the expanded metal mesh by spot welding. The expanded metal mesh has a planar surface and is disposed with its short sides 4 parallel to the surface diagonals of the short sides 7, 8 of the block 1 and the ion-conductive jacket 3. The bolts 5, 6 extended to the outside in the center of the short sides 7, 8 and are provided with an insulating sheathing. In FIG. 1, for the sake of a better overview, the concrete located beneath the expanded metal mesh in the cutaway portion of the block 1 is identified by reference numeral 3', while the concrete located above the expanded metal mesh is identified by reference numeral 3".

However, it is also possible to use composite bodies having a round or oval cross section, instead of the rectangular profile of the anode.

The production of the anode according to the invention is done in a block-shaped frame; the expanded metal mesh serving as the core 2 is suspended by its two bolts 5, 6 in the frame in such a way that the short sides 4 of the mesh extend diagonally to the rectangular head ends of the frame. After the addition of cement-rich mortar and curing of the mortar, the frame can be removed.

FIG. 2 shows the use of the anode according to the invention in the reinforcement cage of a concrete pillar, in which for the sake of a better overview only a detail of the iron reinforcement 9 is shown. The anode block 1 is firmly attached, simply with band material 10, to two parallel iron reinforcing bars 11 located one above the other; the now-rigid ion-conductive jacket 3 that touches the reinforcing bar prevents any danger of short circuiting between the reinforcing bar 11 and the expanded metal mesh 2. A plurality of such anodes can also be inserted into one reinforcement cage as needed. Next, the filling with site-mixed concrete takes place,see arrows A, FIG. 2; the site-mixed concrete enters into a positive, ion-conductive bond with the ion-conductive jacket 3 of the anode 1. After curing of the concrete to form structure is, the anode via bolt conductors 5,6 and the reinforcement bars 11 are connected to a direct voltage source 16, shown only schematically.

From the exemplary embodiment shown in FIG. 2, it is apparent that applying an external anode protection mesh is extremely difficult; with a narrow pillar, installing anode wires or flexible cables is equally possible only with great difficulty.

A further use of the anode according to the invention is in the production of finished concrete parts, where the anode is introduced into a mold for the finished concrete part and then surrounded by poured-in concrete.

The anode according to the invention can furthermore be used as a framing element for producing reinforced concrete constructions; it proves to be particularly practical that it is possible to provide two slack reinforcements with a corrosion protection system in a single operation, by replacing both the front wall and rear wall of the previously typical frame with board-like anodes or finished concrete parts with anodes cast in according to the invention.