HK1185386B - Anode for cathodic protection and method for manufacturing the same - Google Patents

Description

Anode for cathodic protection and method of manufacturing the same

Technical Field

The present invention relates to the field of cathodic protection of reinforced concrete structures and in particular to the design of anodes that are particularly effective in terms of resistance and flexibility per unit length and that are particularly safe to install and handle.

The invention also relates to a method for producing such an anode.

Background

Corrosion phenomena affecting reinforced concrete structures are well known in the art. Steel reinforcement embedded in concrete structures to improve their mechanical properties usually works in an alkaline concrete environment induced passivation environment, but after a period of time, ion migration through the porous surface of the concrete results in local attack of the protective passivation film. Of particular concern is the attack by chlorides, which occur in almost all types of environments in which reinforced concrete structures are employed, and to a greater extent where exposure to brackish water (bridges, columns, buildings sitting in ocean areas), anti-freeze salts (bridges and road structures located in cold climatic zones) or even seawater (for example in the case of piers and docks) occurs. The critical value of chloride exposure, above which the state of passivation of the reinforcing bars is not guaranteed, has been estimated to be about 0.6kg per cubic meter of concrete. Another form of concrete aging is represented by the phenomenon of carbonation, i.e. the formation of calcium carbonate by reaction of the concrete mixture with lime of atmospheric carbon dioxide. Calcium carbonate reduces the alkali content of the concrete (from pH13.5 to pH 9) leaving the iron unprotected. The presence of chloride and simultaneous carbonation represent the worst conditions for preserving the steel reinforcement of the structure. The corrosion products of steel are more bulky than the steel itself and the mechanical stresses due to their formation may lead to phenomena of concrete delamination and cracking, which result in huge losses from an economic point of view other than the safety point of view. For this reason, the most effective method known in the prior art for extending reinforced concrete structures indefinitely (exposure to atmospheric agents, even in the case of relevant salt concentrations) involves cathodically polarizing the reinforcing steel. In this way, the steel reinforcement becomes a site for oxygen cathodic reduction, thereby inhibiting anodic corrosion and dissolution reactions. This mechanism, known as cathodic protection of reinforced concrete, is practiced by incorporating various types of anodic structures onto the concrete, in that connection the steel reinforcement to be protected acts as the counter electrode to the cathode; the current in question, supported by an external rectifier, passes through the electrolyte of the porous concrete comprising the partially soaked salt solution.

Anodes commonly used for cathodic protection of reinforced concrete include a titanium substrate coated with a transition metal oxide or other type of catalyst for anodic oxygen evolution. As far as the substrate is concerned, it is possible to use other valve metals, whether pure or alloyed; but pure titanium is preferred for cost reasons.

European patent EP458951 discloses a mesh-type electrode structure for cathodic protection comprising a plurality of metal strips with electrocatalytic coating, said metal strips having voids of different geometries.

Belts of this type can be manufactured by punching of a solid metal belt or, more commonly, by the traditional method of metal expansion in which the metal sheet is expanded by pressing and punching by a series of knives arranged perpendicularly to the direction of advance of the belt itself. This first step allows to obtain an expanded metal sheet. This sheet is then subjected to a second cutting step suitable for obtaining a band of the desired dimensions. The expanded metal strip is provided with a grid having diamond-shaped voids, wherein the major diagonals of the diamonds are oriented perpendicular to the strip length.

This manufacturing method has the inconvenience of manufacturing a metal strip with the following mesh: the grid has cutting edge protrusions that are automatically formed during the operation of cutting, making these anodes difficult to handle and thus dangerous at the installation stage.

Metal strips with smooth transverse edges are disclosed in canadian patent application CA2078616a 1; by the method described in this document, the resulting strip is provided with a continuous longitudinally extending solid section of a certain width, which section is permanently formed during the manufacturing process and which can be used only for spot welding. However, in today's cathodic protection systems, it is preferable not to weld the strip anodes at all, but to superimpose them directly onto the steel reinforcement, with plastic spacers arranged between them. In this case, the longitudinally extending solid section only loses material, in particular since the solid section is always coated with noble metal during the application of the catalyst layer. However, such a catalyst layer does not work properly on a non-porous structure and affects the calculation of the actual current density applied to the anode structure, thereby complicating the design of the overall cathodic protection system.

Disclosure of Invention

Various aspects of the invention are set out in the appended claims.

According to one aspect, the present invention relates to an anode in the form of a mesh strip for cathodic protection, for example of a reinforced concrete structure, whose edges are substantially free of discontinuities in the form of cutting projections and have a sinusoidal shape, overcoming the inconveniences of the prior art.

For the sake of simplicity, in the context of the present description, reference is made to cathodic protection of reinforced concrete structures; it will be appreciated that the invention may be practiced in the field of cathodic protection in a broad sense, including, for example, cathodic protection of metal can bottoms.

According to another aspect, the invention relates to a method for manufacturing said anode.

According to a further aspect, the invention relates to a cathodic protection system comprising at least one anode in the form of a grid strip, the edges being substantially free of cutting projections.

Some of the most important results obtained by the inventors are presented in the following description, which is provided as an example only and is not intended to limit the invention.

The anode according to the invention comprises an expanded metal strip characterized by a grid with diamond shaped voids having major diagonals oriented along the length of the strip. In one embodiment, the lateral edges of the strip have a sinusoidal profile and are free of cutting projections.

The inventors have surprisingly noticed that anodes for cathodic protection as described above show a significantly reduced ohmic resistance per unit length, for example up to a 4-fold reduction, with respect to anodes of the prior art.

The lower resistance makes it possible to reduce the number of electrical connections, for example in a power grid system, thereby saving material and installation time reasonably.

In one embodiment, the metal mesh strip is made of titanium.

In another embodiment, the metal mesh strip is coated with a catalytic coating comprising a noble metal or an oxide thereof.

In one embodiment, the dimensions of the strip can have a width ranging from 3mm to 100mm, a thickness from 0.25mm to 2.5mm, and a length from 1m to 150 m.

Drawings

For a better understanding of the present invention, reference will be made to the following drawings, which have the purpose of illustrating some preferred embodiments thereof, without limiting the scope thereof.

Fig. 1A shows a top view of a conventional expanded metal anode.

Figure 1B shows a top view of an expanded metal anode according to the invention.

Detailed Description

In detail, fig. 1A shows a top view of a conventional anode with distinguishable cutting protrusions 1 due to a manufacturing method including a cutting step, a rhomboidal geometry with major diagonals 3 of rhomboidal voids arranged in the ribbon width direction and minor diagonals 4 identically arranged in the ribbon length direction.

Fig. 1B shows a top view of an anode according to the invention, in which the rhomboidal geometry of the uncut blunt transverse edges 2, the major diagonals 3 of the rhomboidal voids arranged in the strip length direction and the minor diagonals 4 identically arranged in the strip width direction can be distinguished.

Examples of the invention

Some of the most important results obtained by the inventors are reported in table 1, where the ohmic resistance data of representative anodes of the invention are compared to conventional anodes. The anodes labeled a and B are anodes of rhomboidal geometry conventionally obtained by longitudinal expansion with respect to the displacement direction of the solid metal strip, with the major diagonal of the rhombus oriented perpendicular to the strip length, similar to that depicted in fig. 1A. The anodes labeled C and D are diamond geometry anodes according to one embodiment of the invention, similar to that depicted in fig. 1B.

Anodes C and D are prepared by vertical expansion with respect to the direction of displacement of the solid metal strip which is allowed to travel in the device along parallel rows of knives which expand the solid strip in the vertical direction by pressing and punching. The belt manufacturing is done by means of a last series of knives having blades of a predetermined length longer than the blades of the previous knives, which are suitable for moulding the transverse edges of the belt as depicted in fig. 1B when pressure is applied. In addition to the advantages already explained in terms of conductivity due to the anode geometry, this method has the advantage of providing an expanded metal strip without longitudinally extending solid sections, which, due to the absence of subsequent cutting, do not present any cutting edges and are therefore safer and easier to handle during installation. Furthermore, this method allows advantageously to obtain directly the desired length of the metal strip when the expansion is completed. This production method also allows to obtain a strip of longer length than the conventional method, thus facilitating the installation of larger dimensions, which would require the connection of multiple strips, with a lower solidity of the overall anode system.

From the data reported in the table, it can be noted that the anodes of the invention show an ohmic resistance lower by about 60% for a given width.

TABLE 1

Anode according to FIG. 1A	R-ohm resistance
		A-20mm wide	0.22Ohm/m
B-10mm wide	0.43Ohm/m
		Anode according to FIG. 1B	R-ohm resistance
C-20mm wide	0.088Ohm/m
		D-10mm wide	0.177Ohm/m

The foregoing description is not intended to limit the invention, which may be used according to different embodiments without departing from the scope of the invention, which is defined solely by the appended claims.

Throughout the description and claims of this application, the term "comprising" and variations thereof are not intended to exclude the presence of other elements or additives.

Discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.

Claims (3)

1. A method of manufacturing an anode for cathodic protection in the form of an expanded metal strip having a rhomboidal mesh without longitudinally extending solid sections, the rhomboidal mesh being geometrically arranged with the major diagonal parallel to the length direction of the metal strip, the method comprising the steps of:

-the metal strip travels through an expanding device equipped with at least one row of knives of a first predetermined length arranged parallel to the direction of displacement of the metal strip;

-expanding the metal strip by means of the pressing and punching action of the at least one row of knives;

-forming a transverse edge profile of the expanded metal strip by means of an urging and punching action of a last row of knives having blades of a second predetermined length longer than the first predetermined length.

2. The method of claim 1, wherein the transverse edge profile along the length of the metal strip is free of discontinuities.

3. The method of claim 1 or 2, wherein the metal strip is made of titanium.