GB2091130A

GB2091130A - Corrosion inhibition

Info

Publication number: GB2091130A
Application number: GB8101306A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-01-16
Filing date: 1981-01-16
Publication date: 1982-07-28
Also published as: GB2091130B; EP0059023B1; JPS57145166A; EP0059023A1; DE3270821D1

Description

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GB 2 091 130 A 1

SPECIFICATION Corrosion inhibition

5 This invention relates to corrosion inhibition and in particular to the prevention or inhibition of the corrosion of ferrous metals. The invention also relates to ferrous metal piece parts provided with such corrosion protection.

10 Ferrous metals are employed in the construction of reinforcing member in a variety of building and constructional operations. Typically the metal is used in the form of rods or wires which are embedded in concrete to form a structure having 15 both tensile and compressive strength. Such reinforced concrete structures are employed e.g. in the construction of roadways, bridges, tunnels and buildings. In applications where a high load is to be applied to the reinforced structure it is often pre-20 stressed by applying tension to the reinforcing metal 'skeleton'thus enhancing the load bearing properties of the structure.

A problem associated with all ferrous metal structures is that of corrosion and consequent deteriora-25 tion of the load bearing properties of the structure. This problem is particularly acute when the metal is in contact with or embedded in a building material such as cement or concrete. The commonly used building cements and concretes are strongly alkaline 30 when freshly mixed with water and are highly corrosive in nature. A ferrous material is immediately attacked during the setting process and, as small traces of water remain in contact with the metal and as set concrete is porous so that embedded mate-35 rials are, in the long term, affectively exposed to the water and oxygen in the surrounding air, this attack continues throughout the life of the composite structure. In some cases this can lead to the ultimate failure of the reinforcement.

40 Where composite structures of this type are used in low stress application this failure of the reinforcement can lead to fracture of the structure necessitating eventual replacement which can be difficult and costly. Where the composite structure is used in a 45 high stress application sudden failure of one or more reinforcing members can lead to catastrophic failure with the partial or complete collapse of the structure.

Various techniques for preventing ferrous metal corrosion have been investigated. One of the best 50 known techniques is that of dip coating the metal with a layer of zinc, the process being known as galvanising. This process provides an effective degree of corrosion inhibition but, as the zinc coating is relatively thick, the process is too costly for many 55 applications. Furthermore the period during which the corrosion inhibition is effective is limited in a corrosive environment by dissolution of the zinc which acts as a sacrificial anode. In an attempt to overcome the disadvantages of galvanised ferrous 60 metal it has been proposed to protect the metal surface with a layer of electroplated copper which layer is chemically inert in many corrosive environments. However it has been found necessary to employ a relatively thick, and therefore expensive, 65 copper layer to avoid the risk of pinholes through which accelerated electrolytic corrosion of the ferrous metal can occur. Another reason for employing a thick copper layer is that the rods are often handled in transit and on site in such a way that they are bent. The protection layer must therefore be flexible and/or soft enough to remain continuous after such treatment. Also abrasion damage to the copper layer exposes the underlying metal and again leads to electrolytic corrosion.

Other methods of treatment of ferrous metal surfaces include coating with a plastics surface layer, e.g. an epoxy resin. Such a technique is however relatively expensive and it is also essential to ensure that the ferrous metal is thoroughly dried prior to coating to prevent the entrapment of moisture which will then lead to corrosion.

The object of the invention is to minimise or to overcome these disadvantages.

According to the invention there is provided a ferrous metal body having a corrosion inhibiting surface coating, wherein said coating includes a layer of an unreactive metal in contact with the metal surface and an external layer having corrosion inhibiting properties.

According to another aspect of the invention there is provided a method of inhibiting corrosion of a ferrous metal surface, including applying a coating of an unreactive metal on to the surface, and covering the metal coating with a corrosion inhibiting coating.

Embodiments of the invention will now be described with reference to the accompanying drawings in which:-

Figure 7 is a schematic diagram of a plant for producing corrosion inhibited ferrous metals; and

Figure 2 is a cross-section of a ferrous metal body produced by the plant of Figure 1.

Referring to the drawings, the coating plant shown in Figure 1 is intended for the treatment of e.g. steel rod or wire, or wire products, in continuous lengths. It will however be clear to those skilled in the art that the process can be readily adapted to the treatment of ferrous metals in other forms such as tube and sheet.

The wire or wire product 11 to be treated is fed from a storage reel 12 through an electroplating bath 13 where a flash coating 21 (Figure 2) of a relatively inert metal is deposited on the metal surface. To effect this coating an electric current is fed to the wire product 11 via one or more brush contacts 14, the current being fed via the wire through the electrolytic solution in the bath 13 to an anode 15 of the metal being deposited. The thickness of the electrodeposited coating 21 can be determined by controlling both the rate at which the wire product is fed through the plating bath 13 and the electroplating current. Typically this thickness will be in the range 0.01 to 15 mils although the thickness of the plated layer is not critical.

The deposited metal 21 should be substantially inert in the environment in which the finished product is to be used. Suitable metals for this purpose include zinc, copper, cadmium and chromium. However we prefer to use copper as it is ductile, relatively inexpensive and has a good adhe-

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GB 2 091 130 A

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siori to ferrous metal surfaces.

It will be clear to those skilled in the art that the plating process may include a number of non-electrolytic and/or electrolytic pretreatments of the 5 iron or steel and that many alternative bath compositions and electroplating current/time profiles may be used to optimize the flash plating.

Afterthe electrodeposition stage the wire 11 passes, preferably via a washing and drying station 10 16 where electrolyte residues are removed, to a coating station 17 where the metal coated wire is provided with an outer corrosion inhibiting coating 22 (Figure 2). A wide range of corrosion inhibiting materials may be employed for this purpose, for 15 example a proprietory anti-corrosion paint. However, we prefer a corrosion inhibiting coating wholly or partially comprising a water soluble glass having corrosion inhibiting properties. Typically the glass is a phosphate glass e.g. of the phosphorus pentoxide/ 20 zinc oxide/alumina or the phosphorus pentoxide/ calcium oxide/alumina type, such glasses being described in our published specification No. 23790/ 77 (C.F. Drake 58) and our copending applications no. 7939544 (C.F. Drake-A. Maries-P.F. Bateson 73-2-25 1) and no. 8011297 (A. Maries-P.F. Bateson 3-2). Advantageously the glass is applied as a powder dispersed in a paint resin, but other coating techniques may also be employed. Thus, for example, the glass may be applied as a fine powderfrom a 30 fluidized bedfollowed by fusion ofthe glass to provide a uniform surface layer. Other coating methods will be apparent to those skilled in the art.

Finally the coated wire 11 may be wound on to a storage reel 18 for storage and subsequent ship-35 ment.

The corrosion inhibited ferrous metal products manufactured by the plant of Figure 1 may be employed in a variety of applications in hostile environments where unprotected ferrous metals 40 would be subject to rapid corrosion. Advantageously the corrosion inhibited metal product is in the form of rods or wires which are employed as the reinforcing elements in cement and concrete structures; the corrosion inhibiting glass prevents attack of the 45 ferrous metal during the concrete setting process and continues this protection for an extended period while the structure is in service.

Other applications include the corrosion inhibition of steel tube that is to be employed intheconstruc-50 tion of underground or submarine pipelines e.g. for gas or oil.

The protective mechanism provided by the techniques described herein is thoughtto be as described below, although the invention is in no way limited by 55 this description.

An extremely thin layer of copper if continuous will protect an underlying iron surface from exposure to oxygen and water in the surrounding medium and will not itself be attacked at a significant 60 rate. However, it is difficult if not impossible at the present state ofthe art to produce flash-coatings of a metal, e.g. copper, which are free of pinholes. These pinholes will permit the oxygen and water to contact the iron or steel and rusting will start at these points 65 and spread laterally. These pin-holes however,

occupy much less than 1% ofthe surface of the flash coating. If a corrosion-inhibiting material is available from a second layer then the rusting at the pin-holes can be eliminated. Moreover, other forms of damage 70 to the continuity ofthe metal flash, such as accidental abrasion, can be tolerated as the corrosion-inhibition will provide protection.

In the case of concrete reinforcing rods it will be clear that the main protection is required before and 75 up to the time the rods have been set in the concrete. Pin-holes in the unreactive metal coating will be protected and no further handling-damage is possible. Thus the combination of an unreactive metal and a second anti-corrosive layer results is a saving 80 of both the coating metal and the anti-corrosive material compared with the use of either one alone.

Claims

85 1. A ferrous metal body having a corrosion inhibiting surface coating, wherein said coating includes a layer of an unreactive metal in contact with the metal surface and an external layer having corrosion inhibiting properties.

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2. A metal body as claimed in claim 1, wherein the unreactive metal is copper.
3. A metal body as claimed in claim 1 wherein said corrosion inhibiting layer wholly or partially comprises a water soluble glass.

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4. A metal body as claimed in claim 3, wherein the glass is in the form of a powder dispersed in a paint resin.
5. A metal body as claimed in claim 3 or 4, wherein said glass is a phosphorus pentoxide/zinc

100 oxide/alumina glass.
6. A metal body as claimed in any one of claims 1 to 5, and wherein said unreactive metal layer is an electrodeposited layer.
7. A metal body as claimed in anyone of claims 1 105 to 6, wherein said unreactive metal layer is from 0.01

to 15 mils thick.
8. A metal body as claimed in anyone of claims 1 to 7 and in the form of a wire or a wire product.
9. .A corrosion inhibited ferrous metal body 110 substantially as described herein with reference to the accompanying drawings.
10. A cement or concrete structure reinforced with a wire or wire product as claimed in claim 6.
11. A method of inhibiting corrosion of a ferrous 115 metal surface, including applying a coating of an unreactive metal to the surface, and covering the metal coating with a corrosion inhibiting coating.
12. A method as claimed in claim 11, wherein said unreactive metal is copper.

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13. A method as claimed in claim 11 or 12,

wherein said corrosion inhibiting coating wholly or partially comprises a water soluble glass.
14. A method as claimed in claim 13, wherein said glass is applied in the form of a powder

125 dispersed in a paint resin.
15. A method as claimed in claim 11,12,13 or 14, wherein said metal coating is electrodeposited.
16. A method as claimed in any one of claims 11 to 15, wherein the metal coating is from 0.01 to 1

130 micron thick.

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17. A method of inhibiting corrosion of a ferrous metal surface substantially as described herein with reference to the accompanying drawings.
18. A corrosion inhibited ferrous metal product 5 made by the method of any one of claims 11 to 17.
19. A structural building material containing at least one ferrous metal structural reinforcing member, said reinforcing member having a first unreactive metal coating and having a second coating

10 external to said first coating, said second coating having controlled release corrosion inhibiting properties.
20. The structural building material of claim 19 wherein said first coating is bonded to said reinforc-

15 ing member.
21. The structural building material of claim 19 which is alkaline prior to setting and porous after setting.
22. The structural building material of claim 21 in 20 which the building material is cement or concrete.
23 The structural building material of claim 21 in which the controlled release corrosion inhibiting layer contains a water soluble glass.
24. A ferrous metal structural reinforcing mem-25 ber having a substantially continuous corrosion inhibiting surface coating, wherein said coating includes an inner layer of an unreactive metal firmly adhered to said ferrous metal reinforcing member and an outer layer containing a water soluble glass 30 having controlled release corrosion inhibiting properties.
25. The reinforcing member of claim 24 wherein said inner layer is metallically or chemically bonded to said member.

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26. The reinforcing member of claim 23 in which the glass is a zinc phosphate type glass.
27. The reinforcing member of claim 23 in which the outer corrosion inhibiting layer is a resinous paint containing said glass dispersed therein in 40 finely divided form.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1982.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A1 AY, from which copies may be obtained.