CA1171772A

CA1171772A - Composite outer coating for tubular metal members and its method of application

Info

Publication number: CA1171772A
Application number: CA000379828A
Authority: CA
Inventors: Rene Marchal; Alain Sibue
Original assignee: Pont a Mousson SA
Current assignee: Pont a Mousson SA
Priority date: 1980-06-18
Filing date: 1981-06-16
Publication date: 1984-07-31
Also published as: FR2485156A1; IT8167837A0; DD159807A5; ES8205049A1; ZA814039B; JPS5731563A; GB2078331B; FR2485156B1; CH645450A5; KR840001750B1; EG17588A; GB2078331A; MX160083A; DE3123895C2; ES503150A0; IT1144241B; KR830006397A; DE3123895A1

Abstract

ABSTRACT OF THE DISCLOSURE

The outer surface of a ferrous metal pipe is covered with epoxy or polyester resin weighing between 50 and 500 g/m2.
Before it hardens the resin layer is covered by a wound on second layer of woven or non-woven fabric impregnated with still wet cement mortar applied to the fabric before winding. The fabric fibres may be polypropylene, polyvinyl chloride, polya-mide or polyester. The cement mortar can be portland, aluminous or slag cement and, with respect to the cement, contain between 20 and 50% by weight of water and between 0 and 100% by weight of sand. The second layer is then covered with a sealing layer which can be bitumen, tar or an emulsion of resin.

Description

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The present invention relates to a composite outer coating for tubular metal members as well as to its method of application and in particular it relates to a coating for tubular ferrous metal members.
Coatings are known based on cement intended for protect-ing ferrous metal pipes (of cast iron or steel) from ths corrosive action of fluids or the surrounding earth in which they are buried.
These coatings are quite satisfactory when they are loca-ted inside the pipes, since the dome effect contributes to main-taining adhesion to the support whilst preventing the formation of cracks.
The same is not true when the coating is placed on the outer surface of the pipe, where it is exposed to impact or ovalisation which may occur during storage, transportation or handling. The elimination of the dome effect thus makes it possible for the coating to become detached from its support or, under the least unfavourable circumstances, allows cracking which enables the aggressive elements of the surrounding medium to penetrate as far as the metal support.
In addition, it is known that a coating of cement on a metal pipe constitutes an "active" coating. In contrast to a "passive" coating generally constituted by a layer of inert - material, which forms a barrier with regard to external elements, but which, whatever the thickness, is subject to damage exposing the support and in the vicinity of which no further protection is provided, an "active" coating~employs chemical reactions between the coating and its suppoxt. This is the case with the cement mortar which causes passivation of the iron by virtue of the highly alkaline medium constituted by said mortar. In order that this protection has maximum efficiency, it is necessary that in addition to low porosity and e~ually low permeability, the mortar:

- lS and above all remains in intimate contact with the metal ~ :' ~ t~77,~' structure to be protected, - is free from damage, flaws or cracks extending as far as the metal.
If these conditions are not fulfilled, macropiles of corrosion, also frequently called "macroelements" are formed due to the presence of various types of damage such as damage through to the metal surface, cracks, detachment of the coating, which by facilitating local depassivation of the iron due to a drop in the pH and access to depassivating substances, will localize iso-lated anodic sites within cathodic plates constituted by the passivated surfaces where the mortar remains in initmate contact with the metal structure. Henceforth, since the ratio of the cathodic surface to the anodic surface is generally high, the discharge of the current which is established in these macropiles may be high when active depolarization of the cathodic surface is possible.
Thus, cracking of the cement coating increases these phenomena.
The present invention intends to provide an outer coat-ing containing cement for tubular metal members which adheres firmly to these members and ensures effective protection of these members.
It relates to a composite outer coating for tubular fer-rous metal members, characterised in that it comprises, in contact with the metal member, a layer of a composition based on hardened epoxy resin or based on hardened polyester resin, a covering of a layer of fibres impregnated with a hydraulic binder disposed on this layer and an outer sealing layer.
Furthermore, the applicant has discovered that a coat-ing of this type provides clearly more effective protection than a coating of the same type but in which the covering of a layer of fibres impregnated with a hydraulic binder is replaced by a layer of hydraulic binder reinforced with glass fibres applied by .spraying.

~ ~t772 The invention also relates to a method of coating tubular ferrous metal members, according to which a layer of a resin-based composition which hardens at ambient temperature and is chosen from epoxy resins and polyester resins is applied to the members, then immediately thereafter a layer of fibres impregnated with a hydraulic binder is wound around said members and the arrangement is then coated by spraying with a sealing layer and the coating is left to harden.
The coating according to the invention has excellent ad-hesion to the metal support and makes it possible to reduce the formation of macroelements very considerably.
Within the scope of the invention, all types of cement are suitable (Portland cement, aluminous cement or slag cement), the choice being yuided by the nature of the aggressive medium in which the protected structure is intended to be placed and on the double condition that: 1) they allow easy application and thus have a consistency in relation to the quantity of water to be added and the possible adjuvantsi 2) the product obtained protects the metal support, which assumes an alkaline medium.
A quantity of water comprised between 20 and 50% by weight with respect to the cement is added to the cement, the lower limit being determined by inadequate fluidity which impedes good impregnation of the layer of fibres; beyond 50% the mixture becomes to fluid. Advantageous proportions are between 30 and 35% water.
An addition of sand is optional, in the proportion by weight with respect to the cement of 0 to 100%; when it is added, fine sand having a grain size of less than lmm is used, taking into account the desired final thickness of the coating and preferably, less than 0.5 mm.
Within the scope of the invention, the term layer of fibres is meant to include woven textile materials as well as non-woven textile materials, such as felts.

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Generally, all felts serving for impregnation may be suitable, on condition that they are not too dense since the cement mortar must be able to penetrate thoroughly. Similarly, woven fabrics which are suitable should also have a sufficient mesh in order that the mortar is able to impregnate the fabric but not too great a mesh in order to retain the cement mortar.
Substances forming the textile material may be chosen from poly-propylene, polyvinyl chloride, polyamide or polyester fibres.
The coating is applied in the following manner.
The support to which the hardenable layer of resin is applied is prepared in order to be clean, i.e. free from deposits such as fatty substances or rust, but in the case of cast iron pipes, an oxide skin is not troublesome and shot-blasting is thus needless. The layer of hardenable xesin is deposited by any suitable means, by brush or preferably by spraying, with the sole condition that its viscosity is appropriate and adapted to the chosen system of application. The quantity of resin to be used depends on the roughness of the support and it is advantageously comprised between 50 g/m and 500 g/m . As soon as the pipe is covered in the resin, one proceeds with the winding of one or more layers of fibres impregnated with cement mortar, in order that the hardening or cross-linking reaction of the resin takes place at the same time as the setting of the cement mortar, with a view to ob-taining good adhesion of the two layers.
The layer of fibres, which are either natural or synthetic, which is wound around the pipe coated with fresh resin, is previous-ly coated with cement paste or cement mortar, by being passed through a trough containing the mixture, which causes absorption and entrainment of the material. One then proceeds with gauging of the impregnated layer. The thickness at the end of the gauging operation depends on the thickness of the final coating.

The winding is ad~antageously carried out in a helical lP'~7~
manner with possible overlapping of the turns from 0 to 5~ and so that a certain tension exists after positioning. A winding of this type has advantages with respect to the spraying of cement mortar containing fibres since, in this case, the jet of cement is able to disturb the coating of fresh resin, which reduces the thickness of the resin locally.
A last layer of coating is applied by spraying to the arrangement, which is still preferably fresh, in order to form a sealing layer opposing carbonation of the mortar and thus keeping its pH in the akaline range. This layer also prevents a premature separation of the water during hardening. Possibly, said layer may be applied after 24 or 48 hours and it thus solely provides its barrier effect favourable for maintaining the pH. It may be con-stituted by bitumen, tar or an emulsion of resin and its minimum thickness is 100 ~.
The coating is then left to harden, either at ambient temperature or in an oven in order to obtain accelerated hardening.
The total thickness of the final coating is of the order of several mm, for example between 3 and 8 mm depending on the number of lay-ers and the intended objective.
As a variation, the ferrous metal pipe may be zinc-coated by spraying or galvanization. The direction of the current dis~
charged by the aforedescribed macroelement will thus be reversed, which does not constitute a danger with regard to attacking the pipe, but results in a consumption of the zinc. The presence of the layer of resin which in this case preserves all its properties of adhesion, again intervenes by its barrier effect on the reduc-tion of the discharge of the macroelement.
The results of tests revealing the advantages provided by the present invention are given hereafter.
1 - Test for simulating a macroelement In order to test a macroelement in the vicinity of an _ 5 _ ~ -7~,~
interruption of the protection, according to the accompanyin~
drawing, which is a wiring diagram of an apparatus measuring the intensity of the current of the macropiles, an artificial macro-element is formed in the following manner.
A layer 2 of epoxy resin and cement mortar 3 is placed around a portion of ferrous metal tube 1, then an electrode 4 of a metal identical to that of the tube 1 is inserted, which electrode is insulated from the tube by a layer of resin 5. Two electric wires 6,7, respectively connected to an ammeter 8 are connected in-side the tube 1 and the electrode 4. The two ends of the tube 1are then each closed off by a cover 9 of insulating resin, then the arrangement is partially immersed in a bath 10 of conducting elec-trolyte contained in a container 11, the electrolyte being a 1.65 g/l solution of sodium chloride. The respective surfaces of the electrode 4 or anode and of the coated metal member 1 (cathode) are 0.4cm2 and 600cm2. Partial immersion of the coated cyl-ndrical portion is intended to simulate the conditions of maximum activity of the macroelement, thus promoting diffusion of oxyaen to the metal surface below the cement mortar impregnated with e~ectrolyte by capillarity. In fact, total immersion would slow down the diffu-sion of oxygen and the discharge of the macroelement would be lower. Likewise, the tube member has not received an outer seal-ing layer so as not to impede the diffusion of oxygen.
Different types of coating have been applied to a portion of metal tube and in each case the discharge of the macroelement was measured. In these tests, the thickness of the cement coating was approximately 4 mm.
The results obtained are given in Table 1.
It is apparent that the coating according to the invention provides a considerable improvement of the protection.

2 - Adhesion Test Measurements of the adhesion were carried out in the ~7~77~

following manner.
A circular portion of coating having a diameter of approximately 4 cm is cut out until the support is reached, then a metal disc serving to strip the coating from its support is stuck to the portion of coating by means of an epoxy adhesive (Prochal).
The adhesion corresponds to the stripping force. Tests were carried out on a coated pipe having an outer diameter of 118 mm, after two days of controlled hardening, then after 15 days in the atmosphere at ambient temperature.
In these tests, the thickness of the cemer.t coating was approximately 4 mm.
The results obtained are given in Table 2.

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It can be seen from reading Table 2 that the epoxy layer increases the adhesion of the coating to the metal: it becomes greater than the tensile strength of the coating itself. Further-more, the adhesion is better when the coating contains a covering of felt than in the case of fibres sprayed with the mortar.
3 - Cracking Test A cracking test under ovalisation is carried out, which consists of crushing in a press a metal ring provided with its outer coating of cement mortar having a thickness of approximately

4 mm and of observing the cracking of the coating depending on the ovalisation, defined as the relative variation of the vertical diameter under load A d/d in %.
The tests were carried out on the same tubes as in the preceding test.

. _ Presence of Ovalisation at which superficial the first the first shrinkage cracks superficial cracks Sample before applying cracks under reaching the load load appear metal appear (invention) No 1.4 approx. 7 (invention) No 2.5 approx. 7 .

comparative) No 1.4 approx. 3.5 . 10 ..
'CmPaFative) Yes 1 _ I approx. 3.

It will be noted that the coating according to the inven-tion cracks less easily under vertical load than coatings not con-taining a covering of layers of fibres.

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4 - Adhesion test under shearing An adhesion test was carried out under the action of a shearing force exerted parallel to the metal/coating interface.
In this example, a section of coated cylindrical metal tube is placed below a press comprising a split metal ring, in order to be able to adjust the diameter thereof. The ring, whereof the inner diameter is slightly greater than that of the metal tube, so that it corresponds substantially to the level of the resin/cement inter-face, is driven onto the cylindrical section and the forces exerted are raised.
The coating of the samples is constituted by a layer of resin whereof the thickness varies and of a polyester felt having a weight of 150 g/m2 impregnated with cement mortar containing for one part by weight cement 0.5 parts of fine sand and 0.35 parts by weight water. The thickness of the impregnated felt is approx-imately 4 mm.
All the results are given in Table 4.

. _ . . . . .
Resin Type Thickness Force Stress Sample 2 2 _ (g/m ) (N) (N/mm ) 11 _ _ 9,000 0.48 12 epoxy* 170 36,000 1.9 13 epoxy* 380 36,000 1.9 14 epoxy** 230 56,000 3 epoxy** 350 38,000 2 * Icosit 285 Resin (Lechler) ** Eskadur Resin It is apparent that the layer of resin considerably im-proves the adhesion of the coating.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A composite outer coating for tubular ferrous metal members, comprising a bonding layer in contact with a said metal member, said bonding layer being of a hardened epoxy resin -or a hardened polyester resin, and said bonding layer being covered by a layer of fibres impregnated with hydraulic binder and an outer sealing layer.

2. A coating as claimed in claim 1, in which the bonding layer has a weight of 50 to 500 g/m .

3. A coating as claimed in claim 1, in which the layer of fibres is a felt.

4. A coating as claimed in claim 1, in which the layer of fibres is a polyester felt having a weight of approximat-ely 150 g/m2.

5. A method of applying a coating as claimed in claim 1 to a tubular-ferrous metal member, comprising applying to the member a layer of a composition having a resin base which hardens at ambient temperature chosen from epoxy resins or polyester re-sins, immediately thereafter a layer of fibres impregnated with a hydraulic binder is wound around said member, the arrangement is coated by spraying with a sealing layer, and the coating is left to harden.

6. A method as claimed in claim 5, in which the layer of fibres is impregnated, by soaking, with a hydraulic binder con-taining 20 to 50% by weight water, then gauged and would helically around the member coated with resin.