GB2448868A

GB2448868A - Waterproof material on galvanised steel

Info

Publication number: GB2448868A
Application number: GB0707604A
Authority: GB
Inventors: Brian Dunsterville; Richard Penna; Steve Crawley; Rick Simpson
Original assignee: ZINGAMETALL bvba; Winn and Coales International Ltd
Current assignee: ZINGAMETALL bvba; Winn and Coales International Ltd
Priority date: 2007-04-19
Filing date: 2007-04-19
Publication date: 2008-11-05
Also published as: GB0707604D0

Abstract

A water proof material is applied to a zinc layer on carbon steel. Preferably the water barrier layer comprises a fabric impregnated with a material having a specific softening point, or atatic polypropylene in mineral oil. Preferably the laminate also includes a layer of calcium silicate, mineral wool, asbestos fibre, cork, cellular glass or polyurethane foam insulation material, and an outer barrier layer. Preferably the water barrier material adjacent to the zinc is applied by helically winding a double layer flexible tape. Preferably the steel pipe is cleaned and roughened before it is coated with a zinc suspension. Prevents corrosion under insulation of oil pipelines.

Description

A Method for Preventing Corrosion of Substrate The present invention

relates to a method of preventing corrosion of a substrate, particularly a carbon steel substrate that is insulated (commonly termed Corrosion Under Insulation or CUI), for example an insulated pipeline.

Corrosion Under Insulation is a well-known problem in the field of corrosion control. Insulation is used in a variety of different situations and is commonly used on pipelines in oil refineries, offshore installations and other processing situations. Pipelines are clad with thermal insulation for a number of reasons: to protect personnel from hot surfaces, to reduce wasteful heat loss, to ensure full year service even during cold periods, for acoustic deadening, or as part of a fireproof ing system.

A number of different types of material can be used for the purposes of providing thermal insulation including calcium silicate, mineral wool, asbestos fibre, cork, cellular glass and polyurethane foam. These materials are applied to the pipe surface and are externally encapsulated within a weatherproof outer cladding. Over the course of time the cladding can become damaged, deteriorate, or otherwise fail. In the event of a failure in the weatherproof ing integrity of the outer cladding water ingress may occur due to the penetration of rainwater or melt water from snow or ice. Water can also penetrate into the system in the form of water vapour. In addition to these exogenous sources, trace amounts of water could also be present within the insulating materials as applied.

Pipelines are potentially susceptible to CUI when water or

I

moisture is present within the thermally insulating materials. In some cases insulated pipework is subject to temperature fluctuation or cyclic variations in temperatures. This can lead to an accumulation of deposits at the pipe surface and creates highly corrosive conditions.

Also, liquid water may pool at low points within the system and cause particularly severe corrosion of the pipe in these areas.

Historically, low levels of corrosion protection were applied to insulated pipework. Corrosion protection would typically comprise two coats of epoxy primer directly on the metal substrate surface. These paints suffer from two main defects: the thickness of the paint can be variable due, inter alia, to the paint running when applied on a non-horizontal surface, and paint also suffers from pin-hole defects, which allow water to pass through to the substrate surface.

Corrosion under insulation is not easily detected because the presence of insulation conceals the pipe surface. Furthermore, corrosion under insulation can lead to severe metal loss resulting in failure of the pipeline and loss of containment.

In view of these problems, the present inventors sought to develop a method of protecting a pipe surface from corrosion that is effective under severely corrosive conditions, and over a wide range of temperatures. Ideally, the system of protection must be capable of rapid application in order to minimise interruption in the service of the pipeline.

In a first aspect, the present invention provides a covered substrate having on at least part of a surface thereof first and second layers, wherein the substrate comprises a corrodible first metal, the first layer comprises a second metal having a higher electronegativity than the first metal, the first layer is disposed between the substrate and the second layer, and the second layer is substantially waterproof and substantially prevents water passing along the interface between the first and second layers. The first metal preferably comprises iron, more preferably steel, most preferably carbon steel. The second metal preferably comprises zinc.

In a second aspect, the present invention provides a method for preventing the corrosion of a substrate, the method comprising applying a first layer comprising a second metal to a substrate comprising a corrodible first metal, the second metal having a higher electronegativity than the first metal; applying a waterproof second layer to the first layer, the second layer substantially preventing water passing along the interface between the first and second layers and, optionally, applying an insulation layer on the second layer. The first metal preferably comprises iron, more preferably steel, most preferably carbon steel. The second metal preferably comprises zinc. a

In a first aspect, the present invention further provides a covered substrate having on at least part of a surface thereof first and second layers, wherein the substrate comprises carbon steel, the first layer comprises zinc, the first layer is disposed between the substrate and the second layer, and the second layer is substantially waterproof and substantially prevents water passing along the interface between the first and second layers.

In a second aspect, the present invention further provides a method for preventing the corrosion of a substrate, the method comprising applying a first layer comprising zinc to a substrate comprising carbon steel, applying a waterproof second layer to the first layer, the second layer substantially preventing water passing along the interface between the first and second layers and, optionally, applying an insulation layer on the second layer.

"Covered" includes partially covered, i.e. not completely covered, though the substrate may be completely covered if desired.

"Substantially waterproof" means that the second layer will prevent the passage of water therethrough to an extent that the zinc of the first layer and/or the carbon steel of the substrate below will not corrode.

The use of sacrificial metals, e.g. zinc, as a form of protection f or corrodible metals such as steel is known.

Zinc is normally anodic to steel and corrodes preferentially. However, when immersed in water at elevated temperature in excess of approximately 65 C zinc becomes cathodic to the steel and the protective characteristics are negated.

The present invention uses a duplex system where a layer comprising zinc is applied to a substrate, such as a pipe, followed by an outer layer of protective, waterproof material (the second layer). The second layer is adapted such that it prevents water from passing along the interface between the first and second layers. This has surprisingly been found to protect substrates such as pipes from corrosion for longer periods than the methods of the prior art. The second layer isolates the zinc from the corrosive environment and also provides mechanical protection to the first layer. In the event of accidental damage to the second layer and exposure of the substrate surface to water, the zinc will corrode preferentially at temperatures up to 65 C thereby protecting the substrate below. However, when the second layer is damaged at temperatures where the zinc is cathodic, the nature of the second layer limits the effective area of the zinc cathode, which in turn reduces the current density at the anode and thereby reduces the rate of corrosion of the steel, i.e. corrosion of the underlying substrate surface will occur slowly and only in or very near to the region where the second layer is damaged, due to the prevention of water ingress between the first and second layers.

The method can be used for the maintenance or refurbishment of installations and also in new build situations.

S Preferably, the second layer comprises a material having a softening point when measured in accordance with ASTM D 36 Standard Test Method for Softening Point of from C to 250 C, more preferably of from 130 C to 150 CC.

Preferably, the second layer comprises a material having a penetration value, when measured in accordance with IP 50 Cone Penetration, of 30 dmm to 130 dmm, more preferably 40 dmm to 70 drnm. The second layer may comprise a resinous or waxy material. It has been found that, in high temperature situations in which pipes are commonly used, such as in refineries, 100 C is the minimum softening temperature that allows the second layer to remain intact and perform its function.

Ideally, the second layer comprises a material that wets out the substrate. It has been found that material having a softening point and a penetration value within the above ranges performs well in "wetting out" a surface, i.e. contacting a surface and filling any pores and defects along this surface. In the present invention, the material of the second layer should be sufficiently conformable that, on contact with the first layer, it conforms to the shape of the surface to a sufficient degree to prevent water passing along the interface between the first and second layers.

The optimum range of softening point has been found to be in the region of 130 C to 150 CC.

It has been found that a soft and flexible material has advantages over a hard material, such as a layer of paint.

Paint, when dry, generally forms a hard layer across a surface. When used in situations where the temperature varies significantly (e.g. changing frequently from room temperature to temperatures above 100 C), paint often fails in that it cannot sufficiently deform with the underlying substrate and has a tendency to crack and then flake off, allowing water and other substances to then contact the metal below, leading to the metal's deterioration.

The second layer may comprise a tape having a soft and flexible material on at least one surface. The tape is preferably cold applied. Cold application of tapes is known to the skilled person. Cold application means the application of tape without the use of an external heat source. An external heat source may be used to facilitate application but in practice this is normally prohibited due to site health and safety restrictions. The tape may comprise a backing layer or film. Such a backing layer has been found to advantageously facilitate the handling and application of the tape. The backing film also prevents the Soft and flexible material present on and/or in the tape from coming into contact with any insulation that may be applied on top of the tape, e.g. the insulation around a pipe which has thereon a first and second layer as herein described. This prevents cross contamination between the insulation and the second layer.

The second layer preferably comprises a fabric impregnated with a soft and flexible material, preferably a material having a softening point of from 100 C to 250 C (herein termed the soft material). Such a fabric material has the advantage that it is easily applied to a substrate and, due to the matrix of the fabric fibres, can maintain the integrity of the soft material, while still allowing it to conform to the shape of the surface of the first layer and hence form a substantially continuous contact with the first layer, preventing water ingress along the interface between the first and second layers. Preferably the material impregnated into the fabric has an IP 50 Cone Penetration value of 30 dmm to 130 dmm, more preferably 40 dmm to 70 dmm.

The second layer preferably comprises an elastically deformable material. Such a material has the advantage that it will deform as the substrate and/or first layer below deforms, e.g. expanding and contracting with changes in temperature, and maintains continuous contact with the first layer.

The second layer preferably comprises a semi-solid, flexible material, which may comprise a resin or waxy material, that forms a substantially continuous contact along the surface of the first layer. "Semi-solid" means a material that is able to flow, but has sufficient viscosity to be able to maintain its integrity as a layer.

Preferably, the second layer comprises a material containing a polyolef in, preferably in an amount of from 5 to 30 by weight, and a mineral oil, preferably in an amount of 70 to 90 by weight. This material may be impregnated into a fabric, as described above. Preferably, the polyolef in is dispersed in the mineral oil. The polyolefin may comprise polyethylene and/or polypropylene.

Preferably, the polyolef in comprises atatic polypropylene, since this confers a suitable flexibility to the second layer. The mineral oil may be a heavy aromatic petroleum extract of the kind having a kinematic viscosity at 99 C (210 F) of 60 to 70 cs. Suitable mineral oils include, but are not limited to, technical white oil, naphthenic bright stock oil, naphthenic bright stock extract, heavy cylinder oil and aromatic petroleum extract. The second layer may comprise a material described as an adhesive in GB 1 361 970 A and, optionally, a backing material as described in this document.

The first layer preferably comprises at least 95 w/w% zinc, more preferably 96 w/w zinc, most preferably 98 w/w zinc. These are the optimum minimum amounts that have been found to successfully prevent corrosion of the carbon steel.

Preferably, the thickness of the first layer is from 120 pm to 160 pm.

A third layer comprising an insulating material may be disposed on the opposite side of the second layer from the first layer. The insulating material may comprise any suitable material for insulating the substrate, including, but not limited to, one or more of calcium silicate, mineral wool, asbestos fibre, cork, cellular glass and polyurethane foam.

A fourth layer may be disposed between the second and third layers preventing contact of the second and third layers. The fourth layer is preferably a solid, flexible, waterproof material. The fourth layer may be a layer -10 -adhered to the second layer. Prior to application, the second layer and the fourth layer may be adhered, the fourth layer forming a backing to the second layer. For example, the second layer may comprise a fabric in which a material having a softening point of from 100 to 250 C has been impregnated and the fourth layer may comprise a backing sheet disposed on and adhered to one side of the second layer. The backing sheet may comprise a waterproof material, such as a plastic.

A fifth layer may be disposed on the opposite side of the third layer from the second layer, the fifth layer comprising a waterproof material. Such waterproof materials are known to the skilled person in the art of pipe insulation and include, but are not limited to, aluminium and aluminium laminate.

The substrate is preferably in the form of a pipe.

In a preferred embodiment, the present invention provides a covered pipe, wherein the pipe comprises carbon steel and has as a covering on at least part of its outer surface: a first layer comprising zinc and a substantially waterproof second layer comprising a material having a softening point of from 100 to 250 C * and/or an IP 50 Cone Penetration value of 30 dmrn to 130 dmm, wherein the first layer is disposed between the substrate and the second layer, and the second layer substantially prevents water passing along the interface between the first and second layers, and -11 -a third layer comprising an insulating material is disposed on the opposite side of the second layer from the first layer, and, optionally, a waterproofing layer is disposed on the opposite side of the third layer from the second layer, the waterproofing layer comprising a waterproof material.

The second layer is preferably sufficiently soft and conformable to deform during its application such that the formation of voids, especially at any overlap (if the second layer is in the form of a tape wrapped around the substrate), is prevented. The material of the second layer is preferably substantially resistant to alkaline attack and resistant to cathodic disbondment.

The system described is intended for use in situations where temperatures do not exceed 120 C.

As mentioned above, the present invention provides a method for preventing the corrosion of a substrate, the method comprising applying a first layer comprising zinc to a substrate comprising a carbon steel, applying a waterproof second layer to the first layer, the second layer substantially preventing water passing along the interface between the first and second layers and, optionally, applying an insulation layer on the second layer.

Preferably, the surface of the substrate is cleaned and then its surface roughness brought to within Rz 60 jim to 80 pm prior to the application of the first layer.

-12 -Preferably, the first layer is applied in the form of a liquid composition containing zinc particles in suspension, the liquid composition drying and/or curing after application to form a solid layer comprising the second metal. Preferably, the liquid composition contains at least 96 w/w zinc. The liquid composition is preferably applied to the substrate by brush or spray.

The second layer is preferably formed by the application of a flexible tape. Preferably, a flexible material is disposed on at least one side of the tape.

Preferably, a material having a softening point of from 100 to 250 C is disposed on at least one side of the tape. The tape preferably comprises a fabric material impregnated with a flexible material, e.g. a flexible resin, preferably a material having a softening point of from 100 to 250 C and/or an IP 50 Cone Penetration value of 30 dmni to 130 dmm.

The substrate may be a pipe and the tape may be wrapped around the pipe, to which the first layer has already been applied. The tape is preferably wrapped helically.

Preferably at each point along the pipe, at least a double layer of the tape is formed; this can be achieved by ensuring an overlap of at least 55 when wrapping the tape.

The tape may have a backing layer, which is preferably made from a waterproof, flexible material, e.g. a plastic.

Suitable materials for backing layers are known to those skilled in the art.

-13 -A third layer comprising an insulating material may be disposed on the opposite side of the second layer from the first layer.

A waterproofing layer may be disposed on the opposite side of the third layer from the second layer. The waterproofing layer is preferably a hard, durable material.

Such waterproof materials are known to a person skilled in the art of pipe cladding and include, but are not limited to, Aluminium or Aluminium Laminate -14 -

Example

In the following Example, a carbon steel pipe was used.

Zinc Layer (First Layer) The zinc layer coating composition comprised an single-component coating containing a minimum of 96% w/w zinc in a volatile organic solvent (this composition is sold under the tradename Zinga and manufactured by Zingarnetall BVBA of Belgium). Prior to applying the zinc coating composition, the surface of the pipe was blast cleaned. The initial coating of the zinc-containing layer was completed within one hour.

The cleaning and preparation of the pipe surface, and the subsequent application of the zinc-containing layer were as follows: i. the pipe surface was cleaned using a steam-clean or a water-jet in which the water or steam, as applicable, was applied at 15-bar pressure.

ii. abrasive blast to Sa2 (ISO 8501-1) iii. blast-profile was in the range Rz6Opm -80tm iv. the coating comprising zinc was applied by either brush or spray, ensuring that the dry film thickness of the zinc was in the range 120-l6Opm v. the zinc-containing coating was left to cure for a minimum of two hours before the protective layer (the tape) is applied.

-15 -Protective Layer (Second Layer) A cold applied spirally wrapped tape comprising a fabric impregnated and coated with a compound having a softening point in the range 100-250 C and preferably 130- 150 C. The tape is wrapped spirally with a 55% overlap to form a second layer and ensure a double thickness in all areas. The tape used comprised a polyester fabric impregnated and coated with a layer of soft and flexible material as described in GBl36l970 (A) A further insulation layer and optional waterproof cladding could then be applied around the second layer.

The method of the present invention as exemplified above has been found to have the following advantages: A pipe undergoing refurbishment can be rapidly brought back into service; the zinc layer is fast drying and there is no requirement to cure the protective layer.

The protective layer is easily applied with the minimal equipment. Maintenance works can be completed within a single shift.

Unlike liquid-applied coatings, such as paint, the protective second layer (the tape) can have a factory-controlled thickness and is not prone to pinhole defects.

-16 -The tape is flexible, can be easily applied over joints and fittings and will accommodate thermal expansion and contraction.

The tape is easily removable by hand to facilitate routine inspection of the underlying zinc.

The tape limits the effects of the reversal of polarity normally experienced with zinc/steel systems at elevated temperatures.

Claims

-17 -Claims: 1. A covered substrate having on at least part of a

surface thereof first and second layers, wherein the substrate comprises carbon steel, the first layer comprises zinc, the first layer is disposed between the substrate and the second layer, and the second layer is substantially waterproof and substantially prevents water passing along the interface between the first and second layers.
2. A covered substrate according to claim 1, wherein the second layer comprises a material having a softening point of from 100 C to 250 C.
3. A covered substrate according to claim 1 or 2, wherein the second layer comprises a fabric impregnated with a material having a softening point of from 100 C to 250 C.
4. A covered substrate according to claim 2 or 3, wherein the second layer comprises a material having a softening point of from 130 C to 150 C.
5. A covered substrate according to any one of the preceding claims, wherein the second layer comprises a material having an IP 50 Cone Penetration value of 30 dmrn to dmm.
6. A covered substrate according to claim 5, wherein the second layer comprises a material having an IP 50 Cone Penetration value of 40 to 70 dmm.

-18 -
7. A covered substrate according to any one of the preceding claims, wherein the second layer comprises an elastically deformable material.
8. A covered substrate according to any one of the preceding claims, wherein the second layer comprises a semi-solid, flexible resin that forms a substantially continuous contact along the surface of the first layer.
9. A covered substrate according to any one of the preceding claims, wherein the second layer comprises a mineral oil in which atatic polypropylene is dispersed.
10. A covered substrate according to any one of the preceding claims, wherein the first layer comprises at least 98 %wJw zinc.
11. A covered substrate according to any one of the preceding claims, wherein the thickness of the first layer is from 120 pm to 160 pm.
12. A covered substrate according to any one of the preceding claims, wherein a third layer comprising an insulating material is disposed on the opposite side of the second layer from the first layer.
13. A covered substrate according to claim 12, wherein the insulating material comprises one or more of calcium silicate, mineral wool, asbestos fibre, cork, cellular glass and polyurethane foam.

-19 -
14. A covered substrate according to any one of the preceding claims, wherein a fourth layer is disposed between the second and third layers preventing contact of the second and third layers.
15. A covered substrate according to any one of the preceding claims, wherein the fourth layer is a solid, flexible, waterproof material.
16. A covered substrate according to any one of the preceding claims, wherein a fifth layer is disposed on the opposite side of the third layer from the second layer, the fifth layer comprising a waterproof material.
17. A covered substrate according to any one of the preceding claims, wherein the substrate is in the form of a pipe.
18. A covered pipe, wherein the pipe comprises carbon steel and has as a covering on at least part of its outer surface: a first layer comprising zinc and a substantially waterproof second layer comprising a material having a softening point of from 100 to 250 C and/or an IP 50 Cone Penetration value of 30 dmm to 130 dmm, wherein the first layer is disposed between the substrate and the second layer, and the second layer substantially prevents water passing along the interface between the first and second layers, and a third layer comprising an insulating material is disposed on the opposite side of the second layer from the first layer, and, optionally, -20 -a waterproofing layer is disposed on the opposite side of the third layer from the second layer, the waterproofing layer comprising a waterproof material.
19. A method for preventing the corrosion of a substrate, the method comprising applying a first layer comprising zinc to a substrate comprising carbon steel; applying a waterproof second layer to the first layer, the second layer substantially preventing water passing along the interface between the first and second layers and, optionally, applying an insulation layer on the second layer.
20. A method according to claim 19, wherein the surface of the substrate is cleaned and then its surface roughness brought to within a surface roughness of Rz 60 pm to 80 pm prior to the application of the first layer.
21. A method according to claim 19 or 20, wherein the first layer is applied in the form of a liquid composition containing zinc particles in suspension, the liquid composition drying and/or curing after application to form a solid layer comprising zinc.
22. A method according to claim 21, wherein the liquid composition contains at least 96 w/w zinc.
23. A method according to claim 21 or 22, wherein the liquid composition is applied to the substrate by brush or spray.

-21 -
24. A method according to any one of claims 19 to 23, wherein the second layer is formed by the application of a flexible tape.
25. A method according to claim 24, wherein a material having a softening point of from 100 to 250 C and/or an IP Cone Penetration value of 30 dmm to 130 dmm is disposed on at least one side of the tape.
26. A method according to claim 25, wherein the tape comprises a fabric material impregnated with a material having a softening point of from 100 to 250 C and/or IP 50 Cone Penetration value of 30 dmm to 130 dmm.
27. A method according to any one of claims 24 to 26, wherein the substrate is a pipe and the tape is wrapped helically around the pipe, to which the first layer has already been applied.
28. A method according to any one of claims 24 to 27, wherein the tape has a. backing layer.
29. A method according to any one of claims 24 to 28, wherein the tape is cold applied.
30. A method according to any one of claims 19 to 29, wherein a third layer comprising an insulating material is disposed on the opposite side of the second layer from the first layer.

-22 -
31. A method according to claim 30, wherein a waterproofing layer is disposed on the opposite side of the third layer from the second layer.

t8 12873