NO791808L - HEAT RECOVERABLE ITEM. - Google Patents

Description

Method for protecting a metal substrate against

corrosion and heat recoverable subject to application

by the procedure

The present invention relates to heat-restorable or heat-recyclable objects and in particular methods for their use in protecting substrates against attack from the surrounding environment.

Heat-restorable objects, especially heat-shrinkable objects, have now found widespread use in many areas where insulation, sealing and encapsulation are required. Generally, these objects are restored or recovered, by heating, towards an original shape from which they have previously been deformed, but the term "heat recoverable", as used here, also includes objects which, when heated, assume a new shape, even if they are not previously been deformed.

In the most common embodiment, such articles comprise a heat-shrinkable sleeve made of a polymeric material with elastic or plastic memory, as described for example in US Patent Nos. 2,027,962, 3,086,242 and 3,957,372. As it is pointed out, for example, in US patent no. 2 027 962, the original, dimensionally heat-stable shape can be a transition shape in a continuous process in which, for example, an extruded tube is expanded in a hot state into a dimensionally heat-unstable shape, but in other cases a dimensionally heat-stable shape produced in advance is deformed subject to a dimensionally thermally unstable form in a separate step.

In other items, as described for example in British Patent Document 1 440 524, an elastomeric component such as an outer tube component is held in a stretched state by means of another component, such as an inner tube component which on heating weakens thereby allowing the elastomeric component to recover .

Heat-shrinkable sleeves are used for many purposes, especially when connecting and terminating wires, cables and pipes. However, there are also other applications where it is desirable to use a binding, insulating or protective heat recoverable component for elongated objects such as cables and pipes where the ends are not accessible, or, if they are accessible, where it is not desired to disconnect or move these . For such applications, so-called "enveloping sleeves" have been developed. Generally speaking, these are heat-restorable sheets which can be wrapped around the substrate to form a substantially tubular sleeve, and which are usually provided with fasteners to retain them in the wrapped state during restitution. Such fastening devices are normally mechanized and may consist of, for example, rigid hoops, pins or channel-shaped parts that cooperate with suitably designed molded or extruded projections at the overlapping edges of the heat-restorable plate. Different types of fastening devices are described, for example, in US Patent 3,379,218 and British Patents 1,155,470, 1,211,988 and 1,346,479. In other cases, however, the plate can be held in the wound state during the recovery of an adhesive which, in in some cases, can be applied on site.

Such heat-restorable sleeves or sheathing sleeves, which may possibly be provided with a coating of a fusible, adhesive material or other sealing material such as a grease or a mastic as described for example in British patent documents 1 033 959 and 1 116 878, have been used for many purposes , among other things for corrosion protection of metal substrates such as steel pipes and lead caps in electrical cables. Until now, however, such heat-restorable products have been used due to their ability to physically prevent degrading components in the environment from coming into contact with the metal surface. The protection thereby achieved is useful for many purposes, but there is a growing need for an effective method of corrosion protection that can be used under difficult conditions.

Two particular areas where such a need exists are combating stress corrosion and cathodic protection of underground structures. Stress corrosion is the result of crack propagation caused by corrosion in components, especially in metal structural components, e.g. steel, which is exposed to external stresses or which has not been freed from internal residual stresses. The metal failure caused by corrosion under these circumstances is sometimes called stress corrosion cracking and is a widespread form of corrosion failure.

An area of particular interest is the use of tension anchors, for example in tunnel construction and road construction in mountainous areas.

In the latter case, for example when a road is laid along a mountainside, there is a danger that the mountainside above or below the road will collapse and destroy the road. To prevent the development of cracks in the rock face and subsequent rock falls, the rock is put under pressure. This is achieved using tension anchors.

The principle of tension anchoring basically involves embedding one end of metal rods in the rock and then

stretch the bars to approx. 80% of their outer limit and attach them to the mountainside while they are still stretched. This compresses the mountainside, and cracks and the like cannot form or propagate.

Tension anchors are of two general types, foundation tension anchors and rock tension anchors. Basic tension anchors are normally 20 - 25 m long and have a diameter of 22 to 30 mm. The steel grade used is usually 105/85 or similar. A hole of somewhat larger diameter is drilled in the rock, and a casing pipe is inserted into this. The anchoring rod is then contained in this casing pipe. The free length of the anchoring rod, i.e. the section between the fixed anchoring zone (in the rock) and the anchoring head (on the rock wall) must be protected from corrosion. Stress corrosion of these rods is a serious problem. In the current systems, cement injection or grease injection is used to protect the rod and to keep degrading elements away from the space between the casing pipe and the rod.

The other type of tension anchor, the rock tension anchor, is much shorter, as it is only approx. 5 - 6 meters long, and has a diameter of approx. 20 mm. The steel grade used is usually one with a low yield strength, such as grade 80/65. No casing pipe is used around these rods, and an epoxy coating is usually used for corrosion protection.

The most important corrosion protection systems for ground tension anchors suffer from the weakness that they are injection systems. It is difficult to achieve complete coating of the rods due to entrapped air (and with this associated moisture). Enclosed air pockets can cause corrosion, as the oxygen and moisture in the air pockets can form a tiny corrosion cell due to inhomogeneity in the steel surface. Corrosion takes place, and the moisture and oxygen are used up. Due to the difference in water vapor pressure between the inside and the outside, diffusion of moisture and oxygen into the pocket takes place, whereby the corrosion process gets new nourishment and so-called "penetration corrosion" takes place. Moreover, cracking in the protective cement, for example due to movements in the ground or vibrations, can also lead to the ingress of moisture and consequently to corrosion. As far as rock tension anchors are concerned, the epoxy coating can be easily damaged during handling on site.

Underground structures have usually been protected against corrosion by having a protective coating applied to them. Recently, however, the method of cathodic protection has been adopted, and this has quickly become an important method. In its basic features, cathodic protection involves applying an electromotive force

force (EMF), on e.g. about. -1 volt, on the construction, so that it becomes cathodic in relation to the surroundings, for example the rear. This can be achieved by using auxiliary anodes for applying the electromotive force to the structure or by using so-called sacrificial anodes, whereby the corrosion of the structure is avoided by the anodes being degraded.

However, cathodic protection is rarely used alone, because the currents required are very large and tend to produce an excess of alkalinity. Moreover, it cannot provide protection against purely chemical corrosion attacks. Simultaneous application of non-conductive adhesive coatings is unfortunately also relatively ineffective, because the applied voltage causes the adhesive to release the steel at the end of the coating or in places where the coating has been accidentally damaged and the steel has been exposed. Because of this failure in adhesiveness, it becomes necessary to increase the amperage in order to maintain the cathodic protection, which in turn can give an unwanted excess of alkalinity.

With the help of the invention, a method for protecting a metal substrate against corrosion is now provided, whereby a heat-restorable object is restored around the substrate. The method is distinguished by the fact that a heat-restorable object is used which is provided in the direction of restitution with a covering or a lining of a sealing material which contains a corrosion inhibitor suitable for the substrate.

More particularly, the invention provides a method for protecting a metal substrate against corrosion, in which a heat-restorable object is restored around the substrate in order to insulate it. In this embodiment, the method is distinguished by providing the heat-restorable object in the direction of restitution with a cover or a lining of a sealing material containing a soluble, corrosion-inhibiting compound with the ability to passivate the metal in the substrate.

With the help of the invention, heat-restorable objects suitable for use in the above-mentioned methods are also provided, and in particular a heat-restorable object which, in the direction of restitution, is provided with a covering or a lining of sealing material which contains a soluble, corrosion-inhibiting compound with the ability to passivate a metal surface.

It will be understood that the present invention is applicable to heat-restorable objects in general, including heat-expandable objects, and that it is particularly applicable to heat-shrinkable plates, tapes, sleeves, casing sleeves, caps and end caps which are used for a number of purposes. It is also to be noted that the sealing material can be applied to the heat-restorable material as a coating or as a separate lining which is placed in contact with the heat-restorable material. For certain applications, moreover, the sealing material can advantageously be applied in situ to the substrate, to the heat-restorable material or to both, immediately before the restitution takes place. It will likewise be understood that the heat-restorable item may include one or more parts that are not restorable and/or that the sealing material does not necessarily need to be applied over the entire extent of the item. The present invention is therefore not limited by the heat-restorable object's specific physical structure or by the manner in which it is produced.

The corrosion-inhibiting compounds, hereinafter referred to as corrosion inhibitors, which are used in connection with the invention, are preferably used in the form of finely divided particles which are distributed evenly in the sealing material. In some cases, for example when protecting aluminum and copper substrates, the corrosion inhibitors can be materials, especially organic compounds, which are absorbed or adsorbed by the metal surface and form a barrier below the surface or a surface barrier against corrosion attack. However, the corrosion inhibitor is preferably one which is soluble, and which prevents corrosion by passivating the metal surface. Thus, the preferred embodiments of the present invention are based on the surprising realization that the protection against ingress of moisture achieved by means of the heat-restorable material and the physical and chemical properties of the sealing material make it possible to use corrosion inhibitors that could not be used in the previously proposed anti-corrosion coatings due to their solubility.

The term "soluble", as used here, means that the corrosion inhibitor will dissolve in an aqueous corrosive medium in an effective corrosion-inhibiting concentration, so that it can passivate the metal surface. Generally, therefore, the corrosion inhibitors will have a water solubility of at least 0.05 g/100 ml, preferably at least 0.1 g/100 ml and preferably at least 5 g/100 ml, at 20° C, but some corrosion inhibitors, such as zinc phosphate, can, to despite the fact that they are only very slightly soluble in pure water, dissolve to a sufficient extent below the prevailing pH values in corrosive environments, to have an effective passivating effect. When the invention is used for applications where cathodic protection is used, however, the corrosion inhibitor is preferably one that is soluble in water, e.g. one with a solubility of at least 5 g/100 ml, especially at least 30 g/100 ml, for example at least 50 g/100 ml, because there has been shown to be a very surprising connection between the corrosion inhibitor's water solubility and its ability to prevent cathodic removal of the adhesion. Moreover, it is believed that the effectiveness of the preferred corrosion inhibitors must be attributed at least in part to the density with which they are taken up in an aqueous corrosive medium when such penetrates through the sealing material, and that they are thus able to move towards the substrate dissolved in the medium and to passivate the metal before any significant corrosion has taken place.

There is obviously no upper limit to the solubility as far as the inhibitory effect is concerned, and the inhibitors used can have a solubility of the order of 100 g/100 ml. However, it will be understood that the protection provided by the heat-restorable plate material is of extremely great importance if such inhibitors are used, and that these may be suitable for certain applications in that they can be quickly leached from the sealing material.

The preferred corrosion inhibitors, which are ionizable in water, are defined as "passivating" in that they electrochemically increase the surface potential of the metal to a more positive value as defined by ASTM G15-7 6 and/or in that they react chemically with ions formed on cathodic points: of the metal surface, so that a continuous, insoluble, protective layer is formed which insulates the surface against corrosion. These two mechanisms, which are usually referred to as anodic inhibition and cathodic inhibition respectively, are different from the physical barrier produced by absorption or adsorption of certain organic inhibitors in that a specific electrochemical and/or chemical reaction with the metal surface occurs, and are likewise different from the action of certain metal particles, e.g. zinc powder, as sacrificial anodes.

Among anodic inhibitors that can be used according to the invention, certain chromates, molybdates and nitrites (which form oxidizing anions) and other oxygen-containing anions, such as e.g. phosphates, tungstates, silicates, benzoates and borates. Cathodic inhibitors include magnesium and calcium salts, which form relatively insoluble protective hydroxide films. In addition, it is believed that certain inhibitors such as polyphosphates are inhibitory by both mechanisms, and in other inhibitors, such as magnesium croronate, they

individual ion types a combined inhibition effect.

Although a professional in the field of corrosion protection

should be able to select suitable, soluble, passivating inhibitors with support in the above remarks and taking into account the demands made, especially with regard to the nature of the substrate, it is assumed that the following remarks will nevertheless be of help.

The inhibitor should preferably be one that is present in a relatively finely divided form, so that it can be easily mixed and dispersed evenly in the sealing material without affecting the sealing material's viscosity properties. Moreover, it should preferably have a relatively low density, so that it does not separate out in the sealing material and impair its homogeneity.

The inhibitors used in connection with the invention must of course be present, in sufficient quantities, so that they can prevent practically all corrosion. The minimum amount required will depend on the nature of the corrosion inhibitor and on various other factors, but in typical cases the inhibitors will be used in amounts of up to approx. 30% by weight of the sealing material, especially in an amount of from 0.25 to 25% by weight, and most preferably in an amount of from 5 to 15% by weight.

It will also be understood that the inhibitor must not react with harmful consequences with any other component of the sealing material or of the heat-restorable material so that this is consumed and/or so that other properties of the heat-restorable object are affected in an undesirable way. In addition, it must of course be able to withstand the heating to which it will be exposed in the recovery operation, which means that it must preferably be stable at temperatures from 90 to 140°C.

For certain applications it would also be preferable

that the inhibitor does not create an excessively alkaline environment during the prevention of the corrosion. The need to avoid an excess of alkalinity is believed to be particularly great in applications where cathodic protection is used, because it has been shown that cancellation of the adhesion occurs particularly easily in an alkaline environment, and corrosion inhibitors that allow or produce a high pH value of f .ex. 10 or more, are unsuitable for such applications.

It will be understood that the inhibitor must be of such a nature that it does not adversely affect the sealing material's ability to moisten, i.e. to form a satisfactory bond to the metal substrate or to. the heat-restorable plate material.

Particularly suitable soluble, passivating corrosion inhibitors for use in connection with the invention can be selected from the chromates, molybdates, nitrites, phosphates/tungstates, silicates, benzoates and borates of the alkali metals, the alkaline earth metals and zinc, including mixtures of such compounds. These preferred compounds are at least partially inorganic and capable of forming ions in aqueous media.

The following specific compounds and mixtures of compounds are particularly suitable corrosion inhibitors for application of the invention to iron and steel substrates, sodium chromate, sodium molybdate, sodium nitrite, sodium benzoate, sodium borate, trisodium orthophosphate, potassium dihydrogen orthophosphate, disodium hydrogen orthophosphate, magnesium cronate, magnesium hydrogen orthophosphate and zinc chromate.

As far as lead substrates are concerned, such as the capsules in telephone cables etc, the corrosion is determined to a large extent by the low solubility of lead salts and by their amphoteric nature (as these properties are exhibited to a stronger or weaker degree by zinc, copper, tin, cadmium, magnesium and nickel ). Corrosion is accelerated by certain acids and alkalis and is usually at a minimum in the pH range from 6 to 9. Suitable corrosion inhibitors for lead substrates therefore include silicates, carbonates, sulphates and phosphates which passivate the lead or act as buffers thereby keeping the pH within above area and thus electrochemically prevents the anodic dissolution reaction.

When the present invention is used to prevent stress corrosion, e.g. when a heat-restorable sleeve is crimped around a tension anchor of the type described above to provide corrosion protection, it is important that no components of the heat-restorable article contain halide, sulfur or nitrate groups which can themselves cause stress corrosion cracking in steel.

It will be understood that the corrosion inhibitors used

in connection with the invention, can be bundled together with other filling

substances that can also help protect the metal substrate against corrosion. In this regard, it has been found that certain lamellar fillers such as mica and talc, which are relatively inexpensive and present as plate-like particles, can be advantageously incorporated together with the corrosion inhibitors. It appears that the presence of these materials brings significant advantages because the materials form an effective barrier against the rapid propagation of moisture due to the length of the paths that the moisture is forced to follow in order to penetrate the sealing material and thus at the same time increase the moisture's absorption of corrosion inhibitors.

The heat-restorable material can be made from any of the polymers known in the art to be useful for the production of heat-restorable items. Generally, the plate material will have a constant composition throughout. In some cases, however, laminates of two different polymers that are bonded or fused together can be used. Among suitable polymers can be mentioned, for example, polyolefins, especially polyethylene, copolymers of ethylene and vinyl acetate, copolymers of ethylene and ethyl acrylate; chlorinated and fluorinated polymers, especially polyvinyl chloride, polyvinylidene fluoride and polymers containing units from vinylidene fluoride, hexafluoroethylene and chlorotrifluoroethylene; and rubbers such as ethylene/porpylene rubber, chlorinated rubbers, e.g. "Neoprene" and silicone rubbers, which can be used in admixture with a crystalline or glassy polymer such as an olefin polymer. All the named materials can optionally be cross-linked, e.g. by irradiation and/or by chemical means.

For the above-mentioned reasons, the polymeric material preferably does not contain groups that would in themselves be able to cause corrosion on the metal substrate, and for this reason some of the above-mentioned halogenated polymers may be unsuitable for certain applications, especially in cases where the heat-restorable object is to be used to prevent stress corrosion in steel. Furthermore, because many of the corrosion-inhibiting reactions lead to the formation of hydroxyl groups, the polymer is preferably one that is resistant to attack by hydroxyl groups. The heat-restorable sheet should also preferably be sufficiently thick to provide the finished encapsulated article with the necessary degree of physical strength and likewise to provide a thick, impermeable barrier to prevent substantial ingress of moisture. The plate material will therefore in typical cases have a thickness of at least 0.5 mm, preferably at least 1 mm, and most preferably at least 2 mm, after the restitution.

The surface of the heat-restorable component that comes into contact with the substrate is provided, as mentioned above, with a coating or a lining of a sealing material containing the corrosion inhibitor. Because the sealing material comes into direct contact with the metal substrate, it is particularly important that none of its components contain groups that could produce corrosion (such as the above-mentioned halide, sulfur or nitrate groups when the substrate is made of steel). Moreover, for most applications, it will preferably not contain components that will be exposed to attack by hydroxyl groups. Furthermore, it is important that the sealing material's flow and wetting properties are such that its adhesion to the metal substrate is maintained, so that the danger of entrapping air with consequent penetration corrosion is avoided. Although it is desirable, as is known per se, to add one or more fillers to reduce the costs of the sealing materials, the amount that can be added is limited by the need to maintain a suitable viscosity and suitable sealing properties. Generally, the total filler (including the corrosion inhibitor but not any tackifiers, etc.) should not be greater than 30% by weight, typical fillers, in addition to the above, being calcium carbonate and wood flour. The viscosity of the sealing material (measured according to ASTM 1084", method B) is preferably between 100 and 2000 poise, especially between 200 and 600 poise, at 160°C.

Taking into account the above remarks, the sealing material can be chosen from among the sealing materials known in the art. Thus, this can be a hot melt adhesive. Particularly suitable hot melt adhesives include, for example, polyamides, ethylene/vinyl acetate copolymers and terpolymers (with or without incorporated wax) and polyester. Such materials are described, for example, in British patent specification 1 440 810.

For other applications, the sealing material can be a curable adhesive which has melting and flowability during recovery but which will not be hot-meltable afterwards. Among such materials can be mentioned, for example, the fast-hardening epoxy resins and other similar heat-hardening adhesives.

For many applications, however, the sealing material is preferably a mastic, as the term "mastic" here includes, among other things, viscous, water-resistant, macromolecular materials that exhibit both viscous and elastic response to stresses. These materials will usually have a cohesive strength of approximately the same order of magnitude as their adhesion force and are suitable for filling cavities and spaces, at least at the installation temperature, so that a seal is formed against contaminants such as moisture, dust, solvents and other liquids. They are usually monotonic, i.e. they undergo no appreciably abrupt decrease in viscosity when heated to, for example, 300° C. Typically (as described in ASTM 1146) they will exhibit at least second-order cohesion blocking (and preferably second-order adhesion blocking to metal substrates) at a temperature between room temperature and the material's crystalline melting temperature or range' or glass transition temperature or range. They are preferably tacky at room temperature, in that they are capable of forming an adhesive bond of measurable strength immediately after the adhesive and the metal substrate are brought into contact with each other under low pressure (cf. I Skeist, Handbook of Adhesives, Reinhold Publishing Co. (1962)).The properties of such materials, which usually comprise mixtures of elastomeric and/or thermoplastic polymers, usually cannot be predominantly attributed to some'certain single component but is a result of the constituents' different properties.

Among suitable elastomers from which the sealing materials used according to the invention can be produced, mention may be made, for example, of naturally occurring rubber, such as e.g. SMR (Malaysian Rubber Bureau); synthetic rubbers, such as acrylic rubbers, e.g. "Hycar 4021" (B.F. Goodrich Chem. Co.) and "Krynac 882" (Polysar Rubber Services Inc.); chlorosulfonated polyethylene, ■ e.g. "Hypalon 20" (duPont); epichlorohydrin gums such as "Herclor C" (Hercules); ethylene-propylene copolymers, such as "Epcar 306" (B.F. Goodrich); ethylene-propylene-diene terpolymers, such as e.g. "Royalene 522" (Uniroyal Chem. Inc.) "Nordel 2522" and "Nordel 1320" (DuPont); fluoroelastomers, such as "Viton CIO"

(DuPont); isobutylene-isoprene(butyl) rubbers such as "Exxon Butyl 065" (Exxon Chem. Co.), isoprene-acrylonitrile rubbers such as "Krynac 833" (Polysar), nitrile rubbers such as "Hycar 1052-30"

(Goodrich), polybutadiene rubbers such as "Ameripol CB220" (Goodrich), polychloroprene rubbers such as "Baypren 112" (Mobay) and "Neoprene AD" (DuPont), polyisobutylene rubbers such as "Vistanex LMMH"

(EXXon), polyisoprene rubbers such as Shell isoprene rubber 3 05 (Shell), silicon rubbers such as Baysilicon rubber HV6 (Mobay), styrene-butadiene rubbers such as "Kraton 1102" (Shell), urethane rubbers such as "Adiprene C" (DuPont), and thermoplastic elastomers such as polyester rubbers, e.g. "Hytrel 4055" (DuPont), ethylene-vinyl-acetate elastomers, e.g. "Vynathene EY 903" (U.S.I. Chem.) and ethylene/

acrylic elastomers such as "Vamac N123" (DuPont), the polyolefins marketed under the trade names "Telcar" (B.F. Goodrich) and "Ren-Flex" (Renplastics), the styrene-butadiene copolymers marketed under the trade name "Solprene" (Philips Petroleum) and the styrene-polyolefin copolymers marketed under the trade name "Elexar" (Shell Chemicals).

Particularly preferred elastomers for use in the sealing materials used according to the invention include polyisobutylenes, ethylene/propylene terpolymers and modified butyl rubbers.

The sealing material may contain one or more tackifying agents, the amount of which can advantageously be up to 50% by weight, preferably from 5 to 40 parts by weight, especially from 10 to 20 parts by weight, in order to improve and maintain the adhesion of the sealing material to the metal substrate. Aromatic tackifiers are particularly preferred because of their natural stability. Suitable tackifying agents include e.g. terpene phenol resins such as "Piccofyn A100" (ex Hercules Incorporated), hydrocarbon resins such as "Escorez 1102" (ex Exxon Corp.), resin derivatives such as "Staybelite ester 10" (ex Hercules Inc.), wood resins such as "Vinsol" (ex Hercules Inc.) and polyketone resins. Some of these, besides other applicable tackifiers, are described in German specification 23 47 779. Terpene-phenolic resins are particularly preferred. Other suitable materials can be taken out by a specialist in the area.

For many applications of the present invention, it will be advantageous that the nature of the sealing material is such that the heat-restorable object becomes self-repairing. This means that if the external heat-restorable jacket breaks or is punctured, the sealing material should under the influence of. the residual pressure exerted by the restorable part floats and thereby fills the fracture site or hole that has occurred, preferably within 24 hours at -20° C. For this reason and to improve the absorption of corrosion inhibitor in moisture that diffuses through the sealing layer, the sealing material is preferably mainly non-crystalline, with a crystallinity not higher than 30%, preferably not higher than 20%. For certain applications, however, it may be advantageous to have a certain degree of crystallinity to improve the sealing material's flow properties in cold addition, so that it is prevented from seeping out from the underside of the heat-restorable part. Thus, a crystalline ethylene/vinyl acetate copolymer, e.g. "Elvax 420" (DuPont) is advantageously incorporated in some cases. Other thermoplastic polymers that can be advantageously incorporated include ethylene/ethyl acrylate copolymers such as "DPD 9169" (Union Carbid), ethylene-methacrylic ester copolymers such as "Vamac 124" (DuPont), ethylene-acrylic acid copolymers such as "EAA 435" (Dow Chem. Co.), ethylene acrylic acid acrylic ester thermopolymers, such as "Lupolene A2910 MX" (BASF Chemie NV), and terpolymers of ethylene, vinyl acetate and an organic acid, such as e.g. "Elvax 4310" (DuPont). Blends of any of the above thermoplastic polymers may be used where appropriate.

The sealing materials can be prepared in the usual way and can be applied by means of conventional coating methods or, in some cases, by simultaneous extrusion with the heat-restorable material. In certain cases, they can also be applied from a solution, but such methods are not among those preferred when carrying out the invention. The thickness of the sealing layer before the restoration will usually be at least 1 mm.

As can be seen from the above explanation, the invention will find application in many situations where it is necessary to protect metal substrates against corrosion, and that it will be particularly useful for additional protection of underground structures that are cathodically protected against corrosion through the application of a small, negative potential. according to a preferred aspect of the invention, there is provided a method for using a structure against corrosion, in which a small, negative electric potential is placed over the structure, and in which the structure is also protected by using a heat-restorable object that has a coating or lining of a sealing material containing a corrosion inhibitor suitable for the metal substrate, in particular a soluble, passivating corrosion inhibitor.

When applying the invention to tension anchors, the rods for anchoring in the ground or in rock can be easily protected on site by means of a heat-shrinkable sleeve before they are placed in the rock. Damage resulting from handling is thereby reduced to a minimum, and the bars can always be easily visually inspected before they are installed.

The following examples illustrate the invention. Parts

and percentages are on a weight basis, unless otherwise stated.

Example 1

This example illustrates various sealing materials which are suitable for use in connection with the invention.

Sample plates were produced by pressing sealing materials as indicated below onto 152.4 mm 4 76.2 mm recently sand-blasted steel plates at 80° C. The layers of sealing material were approx. 1.65 mm thick.

After the plates had been produced in this way,

a T-shaped piece cut out at the top of each plate. The width of the T-shaped piece was approx. 7.94 mm, and the T's post extended down over approx. 3/4 of the plate's length. In this way, a significant part of the length of the coated plate was subjected to assessment.

The specimens were then stored in a salt fog for 40 days, after which they were removed for examination.

The sealing materials were as follows:

The following corrosion inhibitors were tested in both

materials A and B.

Dicyclohexyiammonium nitrite

Zinc chromate

Zinc orthophosphate

Magnesium hydrogen orthophosphate

Strontium orthophosphate

Sodium chromate

Sodium molybdate

Sodium nitrite

Sodium benzoate

Potassium dihydrogen orthophosphate (38%) I

Disodium hydrogen orthophosphate (62%)J

Trisodium orthophosphate

All of the above-mentioned inhibitors proved to give satisfactory results in the tested sealing materials, indicated by the fact that there was no sign of either corrosion or cancellation of the adhesion between the layer of the sealing material and the substrate.

Other suitable test methods are specified in ASTM-G8, ASTM-G42 and ASTM-G19.

Example 2

Cathodic de-adhesion studies were performed at room temperature on various sealing materials applied as inner coatings to a heat-shrinkable polyethylene sleeve ("PS" marketed by Raychem Corporation). The materials contained 47% polyisobutylene, 47% atactic polypropylene wax and 6% of the corrosion inhibitors listed below. A comparison sample contained 6 parts by weight of carbon black instead of the corrosion inhibitor. The investigation was carried out by crimping the sleeves around a cathode in the form of a sandblasted steel tube with a diameter of 50 mm, and the sealing material was exposed at three points by drilling holes of dimate 9.5 mm through the sleeve and the seal.

The anode was a carbon rod, and the electrolyte used was a solution of 1% sodium chloride, 1% sodium sulfate and 1% sodium carbonate in distilled water. The protected cathode was maintained at -1.5 volts relative to a saturated calomel electrode for 30 days, after which the mean diameter of the non-adherent area around the holes was measured. The general procedure was as described in ASTM methods G8 and G4 2. The results are listed in the following Table I.

It will be seen that the soluble inhibitors showed a markedly increased resistance to loosening of the adhesion, while the relatively insoluble materials were usually rather ineffective in this respect. This shows that in order for an inhibitor to be effective in preventing loosening of the adhesion, it should preferably be sufficiently soluble that it easily dissolves in water which diffuses through the sealing material and is thereby quickly transported to the surface of the steel.

Example 3

Stretch marks became. protected with heat-shrinkable polyethylene sleeves provided with an internal coating of various sealing materials and, according to DIN 50021, exposed to a salt shower for up to 40 days. The protective sleeve was then removed and each individual bar was examined for pitting caused by penetration corrosion.

The examination with regard to the loosening of the adhesion was carried out by cutting a slit 25 mm x 2 mm in the sleeve before the test and by examining the non-adherent area around the slit after the test.

Certain sealing materials were also tested using a method of immersion in brine.

The materials and the results obtained are listed below.

It will be seen that the materials A-H containing corrosion inhibitors in accordance with the invention provided much better protection against corrosion than the typical commercial sealing materials I - M. Furthermore, it will be seen that significantly less loosening of the adhesion took place, and that in some cases (where the dimensions of the non-adherent areas is smaller than the dimensions of the gap) "self-repair" took place, i.e. the sealing material during the test flowed into the gap and tended to fill it.

[In cases where two or three specimens were tested with respect to a given sealing material, the results are marked (a) , (b) , etc. ]

Example 4

The following sealant was used as an inner layer on a pipe repair casing (WRSM marketed by Raychem Corporation). The sleeve is crimped around a steel tube and tested according to the salt shower method according to DIN 50021.

No signs of corrosion or significant breakdown of adhesion can be seen after 40 days. This preferred sealant contains mica as a laminar filler to enhance the corrosion inhibiting action of the disodium hydrogen phosphate. "Elvax 4310" is a crystalline polymer incorporated to give the sealant desirable flow properties.

A similar sealing material has the following composition:

The material is applied to a WRSM sleeve and gives similar results when tested as indicated above. In this material, talc is used as a laminar filler to prevent corrosion protection.

Example 5

The following sealing materials A-G can be used with good effect as corrosion-inhibiting sealing materials in the form of coatings or liners on the inside of heat-shrinkable components such as TPS sleeves, WRSM sheathing sleeves and end caps, which are marketed by Raychem Corporation.

Although the present invention is particularly described with reference to steel and lead substrates, it will be understood that it can also be used for corrosion protection of other metals. For example, suitable inhibitors for aluminum include mixtures of silicates and nitrates of the alkali and alkaline earth metals and zinc.

In general, it will be understood that the provision of the protective, heat-restorable plate material makes it possible to use soluble passivating inhibitors which are particularly effective in that they are easily absorbed by moisture as this passes through the layer of the sealing material and is thus transported with the moisture to the metal surface, so that they can immediately act to prevent corrosion.

Claims (65)

1. Method for protecting a metal substrate against corrosion, in which a heat-restorable object is restored around the substrate, characterized in that where a heat-restorable gj <*> unit is used, one is provided in the direction of restitution with a covering or a lining of a sealing material containing a suitable corrosion inhibitor for the substrate. 2. Method according to claim 1, characterized in that the corrosion inhibitor used is one which is soluble and which is able to passivate the surface of the metal substrate. 3. Method according to claim 1, characterized in that the corrosion inhibitor used is one that can be absorbed by or adsorbed on the surface of the metal substrate to form a protective barrier. 4. Method according to claim 1, characterized in that a metal is used as a corrosion inhibitor which can act as a sacrificial anode to prevent corrosion. 5. Method for protecting a metal substrate against corrosion, in which a heat-recoverable object is recovered around the substrate, characterized in that where heat-restorable object is one which, in the direction of restitution, is provided with a covering or a lining of a sealing material containing a corrosion inhibitor with the ability to passivate the surface of the metal substrate. 6. Method according to claims 1-5, characterized in that one that is heat-shrinkable is used as a heat-restorable object, and that the sealing material consists of an internal coating or an internal lining. 7. Method according to claim 6, characterized in that a sleeve, an encasing sleeve, a plate, a band, a cap or an end cap is used as a heat-restorable object. 8. Method according to claims 1-7, characterized in that a finely divided, particulate solid which is dispersed in the sealing material is used as a corrosion inhibitor. 9. Method according to claims 1-8, characterized in that a sealing material whose crystallinity is not higher than 30% is used. 10. Method according to claim 9, characterized in that a sealing material whose crystallinity is not higher than 20% is used. 11. Method according to claims 1-10, characterized in that the corrosion inhibitor is used in the sealing material in an amount of up to 30% by weight. 12. Method according to claim 11, characterized in that the corrosion inhibitor is used in the sealing material in an amount of from 0.25 to 25% by weight. 13. Method according to claim 12, characterized in that the corrosion inhibitor is used in the sealing material in an amount from 5 to 15% by weight. 14. Method according to claims 1-13, characterized in that a corrosion inhibitor is used whose solubility in water at 20° C is at least 0.05 g/100 ml. 15. Method according to claim 14, characterized in that a corrosion inhibitor whose solubility in water at 20° C is at least 0.1 g/100 ml is used. 16. Method according to claim 15, characterized in that a corrosion inhibitor whose solubility in water at 20° C is at least 5 g/100 ml is used. 17. Method according to claim 16, characterized in that a corrosion inhibitor is used whose solubility in water at 20° C is at least 30 g/100 ml. 18. Method according to claim 17, characterized in that a corrosion inhibitor is used whose solubility in water at 20° C is at least 50 g/100 ml. 19. Method according to claims 5-18, characterized in that a corrosion inhibitor selected from chromates, molybdates, nitrites, phosphates, tungstates, silicates, benzoates and borates of the alkali and alkaline earth metals and zinc and mixtures of any two or more such is used connections.. 20. Method according to claims 5-18, characterized in that iron or steel is used as the metal substrate, and that the corrosion inhibitor is selected from among sodium chlorate, sodium molybdate, sodium nitrite, sodium benzoate, sodium borate, trisodium orthophosphate, potassium dihydrogen orthophosphate, disodium hydrogen orthophosphate, magnesium hydrogen orthophosphate, magnesium chromate, zinc chromate and mixtures of which any two or more such compounds. 21. Method according to claims 5-18, characterized in that lead is used as the metal substrate, and that the corrosion inhibitor is selected from among silicates, carbonates, sulfates and phosphates of alkali and. alkaline earth metals and zinc, and mixtures of any two or more such compounds. 22. Method according to claims 5-18, characterized in that aluminum is used as the metal substrate and that the corrosion inhibitor is selected from mixtures of silicates and nitrates of the alkali and alkaline earth metals and zinc. 23. Method according to claim 5-22, characterized in that a corrosion inhibitor is used which is at least partially ionizable in water. 24. Method according to claims 1-23, characterized in that a sealing material is used which also contains a lamellar filler. 25. Method according to claim 24, characterized in that mica is used as lamellar filler. 26. Method according to claim 24, characterized in that talc is used as lamellar filler. 27. Method according to claims 1-26, characterized in that a mastic is used as sealing material. ' 28. Method according to claim 27, characterized in that a sealing material based on polyisobutylene, an ethylene-propylene terpolymer or a butyl rubber is used. 29. Method according to claims 1-26, characterized in that a hot-melt adhesive is used as sealing material. 30. Method according to claims 1 - 26, characterized in that a fast-hardening epoxy resin is used as sealing material. 31. Method according to claims 1-30, characterized in that a sealing material with a viscosity in the range from 100 to 2000 poise at 160° C is used. 32. '- Method according to claim 31, characterized in that a sealing material with a viscosity in the range from 200 to 600 poise at 160° C is used. 33. Method according to claims 1-32, characterized in that a small, negative electric potential is placed over the metal substrate to provide cathodic protection against corrosion. 34. Method for protecting a metal structure against corrosion, characterized by placing a small, negative electric potential over the structure and protecting the structure by using a heat-restorable object with a coating or a lining of a sealing material containing a corrosion inhibitor suitable for the metal. 35. Method according to claim 33, characterized in that a soluble, passivating inhibitor is used as corrosion inhibitor. 36. Method for protecting a tension anchor against corrosion, characterized in that the tension anchor is covered with a heat-shrinkable object provided with a coating or a lining of a sealing material containing a corrosion inhibitor suitable for the metal in the tension anchor, and is heated to release the heat-shrinkable object to be recovered around the tension anchor. 37. Method according to claim 36, characterized in that a soluble compound capable of passivating the tension anchor's metal surface is used as a corrosion inhibitor. 3.8. Heat-restorable object with the ability to protect a metal surface against corrosion, characterized in that it is provided in the direction of restitution with a coating or a lining of a sealing material containing a soluble, passivating corrosion inhibitor. 39. Heat-restorable object according to claim 38, characterized in that it is heat-shrinkable, and that the sealing material is arranged as an internal coating or an internal lining.. 40. s Heat-restorable object according to claim 38 or 39, characterized in that it is a sleeve, an enveloping sleeve, a plate, a band, a cap or an end cap. 41. Heat-restorable object according to claims 38 - 40, characterized in that the corrosion inhibitor is a finely divided, particulate solid which is dispersed in the sealing material. 42. Heat-restorable object according to claims 38 - 41, characterized in that the crystallinity of the sealing material is not higher than 30%. 43. Heat-restorable object according to claim 42, characterized in that the crystallinity of the sealing material is not higher than 20%. 44. Heat-restorable object according to claims 38 - 43, characterized in that the corrosion inhibitor is present in the sealing material in an amount of up to 30% by weight. 45. Heat-restorable object according to claim 44, characterized in that the corrosion inhibitor is present in the sealing material in an amount of from 0.25 to 25% by weight. 46. Heat-restorable object according to claim 45, characterized in that the corrosion inhibitor is present in the sealing material in an amount of from 5 to 15% by weight. 47. Heat-restorable article according to claims 38-46, characterized in that the solubility of the corrosion inhibitor in water at 20° C is at least 0.05 g/100 ml. 48. Heat-restorable object according to claim 47, characterized in that the solubility of the corrosion inhibitor in water at 20° C is at least 0.1 g/100 ml. 49. Heat-restorable object according to claim 48, characterized in that the solubility of the corrosion inhibitor in water of 20° C is at least 5 g/100 ml. 50. Heat-restorable article according to claim 49, characterized in that the solubility of the corrosion inhibitor in water at 20° C is at least 30 g/100 ml. 51. Heat-restorable article according to claim 50, characterized in that the solubility of the corrosion inhibitor in water at 20° C is at least 50 g/100 ml. 52. Heat recoverable object according to claims 38 - 51, characterized in that the corrosion inhibitor is selected from chromates, molybdates, nitrites, phosphates, tungstates, silicates, benzoates and borates of the alkali and alkaline earth metals and zinc, and mixtures of any two or more such connections. 53. Heat-restorable object according to claims 38 - 51, characterized in that the corrosion inhibitor is selected from among sodium chromate, sodium molybdate, sodium nitrite, sodium benzoate, sodium borate, trisodium orthophosphate, potassium dihydrogen orthophosphate, disodium hydrogen orthophosphate, magnesium hydrogen orthophosphate, magnesium chromate, zinc chromate and mixtures of any two or more such compounds. 54. Heat-restorable article according to claims 38-51, characterized in that the corrosion inhibitor is selected from among silicates, carbonates, sulfates and phosphates of the alkali and alkaline earth metals and zinc, and mixtures of any two or more such compounds. 55. Heat-restorable article according to claims 38 - 51, characterized in that the corrosion inhibitor is selected from mixtures of silicates and nitrates of the alkali and alkaline earth metals and zinc. 56. Heat-restorable object according to claims 38 - 55, characterized in that the corrosion inhibitor is at least partially ionizable in water. 57. Heat-restorable article according to claims 38-56, characterized in that the sealing material also contains a lamellar filler. 58. Heat-restorable article according to claim 57, characterized in that the lamellar filler is mica. 59. Heat-restorable article according to claim 57, characterized in that the lamellar filler is talc. 60. Heat-restorable object according to claims 38-59, characterized in that the sealing material is a mastic. 61. Heat-restorable object according to claim 60, characterized in that the sealing material is based on polyisobutylene, an ethylene-propylene terpolymer or a butyl rubber. 62. Heat-restorable object according to claims 38-59, characterized in that the sealing material is a hot-melt adhesive. 63. Heat-restorable object according to claims 38 - 59, characterized in that the sealing material is a fast-hardening epoxy resin. 64. Heat-restorable object according to claims 38 - 63, characterized in that the viscosity of the sealing material is in the range from 100 to 2000 poise at 160°C. 65. Heat-restorable article according to claim 64, characterized in that the viscosity of the sealing material is in the range from 200 to 600 poise at 16.0° C.