SYSTEM AND METHOD FOR CATHODIC PROTECTION BY DISTRIBUTED SACRIFICIAL ANODES

Abstract

A method to reduce the total anode mass of a cathodic protection system by reducing or eliminating the total cathode area is disclosed, the system comprising: a metallic first-layer coating which being anodic to the component or substrate to be protected, bonded to the component or substrate and electrically conductive. A sacrificial anode in the form of a metallic second-layer coating is distributed over the first-layer coating. The second layer coating has an open circuit potential that is equal to the first-layer coating or being anodic to the first-layer coating and to the substrate, the second-layer coating electrically conductive, bonded to the first-layer coating and exposed to the surrounding environment.

Claims

1. A method for cathodic protection of a metal component or substrate, the method comprising the steps of: applying a metallic first-layer coating to the metal component or substate substrate via a first deposition method, the metallic first-layer coating being anodic to the metal component or substrate; and distributing a sacrificial anode over the metallic first-layer coating by applying a metallic second-layer coating via a second deposition method, the sacrificial anode having an open circuit potential equal to the metallic first-layer coating or is anodic to the first-layer coating through a second deposition method.

2. The method of claim 9, wherein the first and the second deposition methods are selected from the group consisting of: hot dip galvanization, co-lamination, co-extrusion, explosion bonding, as well as any deposition method referred to as metal spraying including but not limited to one of detonation spraying, flame spraying, high-velocity liquid fuel spraying, high-velocity air fuel spraying, high-velocity oxygen fuel spraying, plasma spraying, arc spraying, and cold spraying.

3. The method of claim 2, wherein the first and the second deposition methods are the same.

4. The method of claim 2, wherein the first and the second deposition methods are different.

5. The method of claim 1, further comprising depositing an essentially pure metallic aluminium or aluminium alloy to a thickness of 100-300 μm to form the metallic first-layer coating.

6. The method of claim 1, further comprising depositing an aluminium alloy that is anodic to aluminium to a thickness of 200-3,000 μm to form the metallic second-layer coating.

7. The method of claim 1, further comprising feeding a metallic composition comprising aluminium, zinc and indium to a metal deposition process for deposition of a sacrificial anode coating onto the first-layer coating.

8. The method of claim 1, wherein the metallic first-layer coating is an aluminum alloy containing 5% magnesium.

9. The method of claim 1, wherein the metallic first-layer coating consists essentially of pure aluminum.

10. The method of claim 1, wherein the metallic second layer coating is an aluminium-zinc-indium (Al—Zn—In) alloy comprising 2-7% zinc, 0.01-0.05% indium, and the balance of aluminium, having a thickness of at least 200-3,000 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Embodiments of the invention will be further explained below with reference made to the accompanying, schematic drawings. In the drawings

[0059] FIG. 1 illustrates anode demand for bare and coated carbon steel;

[0060] FIG. 2 is a schematic section through a cut out portion of the sacrificial cathodic protection system applied to a metal substrate;

[0061] FIG. 3 is a cross sectional view through a component intended for subsea transport of hydrocarbon fluid;

[0062] FIGS. 4A and 4B are diagrams showing a prior art coating and the new coating respectively, compared to anode protection, in terms of protection current density as a function of exposure time in seawater, and

[0063] FIGS. 5A and 5B are sample photos of a prior art coating and the new coating respectively subjected to electrochemical testing under exposure to seawater.

DETAILED DESCRIPTION

[0064] The use of protective coatings is not exclusively applicable to subsea structures, but also applies generally to buried pipelines or in protection against atmospheric exposure.

[0065] It shall also be noted, that if not otherwise stated, any statement on percentage of constituents given herein refers to percentage by weight.

[0066] In subsea cathodic protection CP, codes like DNV-RP-B401 establish coating degradation factors to account for the normal aging process of different coating systems. While initial coating breakdown factors are typically small, they approach a value of 1 by the end of the service life of the substructure.

[0067] In subsea CP design, the entire surface area of the component must be accounted for in CP calculations because it is cathodic to the conventional anodes. Conversely, surface areas of the structure that have an open circuit potential identical to that of the conventional sacrificial anodes will not be included in the CP calculations.

[0068] FIG. 1 illustrates an example based on a typical DNV-RP-B401 calculation. In this example a 1 m.sup.2 component shall be provided cathodic protection. As shown in FIG. 1, a sacrificial anode has to provide a current of 150 mA to protect bare carbon steel (CS). The application of a non-conductive protective coating, such as paint, reduces the average current demand to 33 mA for the same area of painted CS. Thermally sprayed aluminium (TSA) can further reduce the current demand to 10 mA for same area. As it will be explained below, the distributed sacrificial anode (DSA) of embodiments of the present invention will eliminate current demand from conventional anodes for surface areas that have been converted to DSA areas.

[0069] In FIG. 2, reference number 1 refers to the bulk material of a metallic object forming a substrate to be protected by a cathodic protection system, the system comprising a first-layer coating 2 that is applied to the surface of the substrate 1, and a second-layer coating 3 applied on top of the first-layer coating 2.

[0070] The substrate 1 can be an object of any ferrous or non-ferrous metal that needs either protection from a corrosive environment such as wet soil, water and moist air, or an object which has to be covered to reduce the total anode consumption of a CP system. In subsea applications, the substrate would typically be a component involved in the subsea production and/or transport of oil, gas or water, such as a pipeline, a manifold structure, a pump or compressor part etc., typically having a lumen or passage 4 for transport of fluid through a body of metal 1. The substrate 1 can thus in practise take any form including planar, curved and double-curved shapes, and the coated surface of the substrate can be situated on the exterior or on the interior of the substrate.

[0071] The first-layer coating 2 may comprise any metal or metal alloy that is anodic to the substrate, is electrically conductive and can form a bond to the substrate. The second-layer coating 3 may comprise any metal or metal alloy that has an open circuit potential equal to the metallic first-layer coating or is anodic to the first-layer coating and the substrate, is electrically conductive and which can form a mechanical bond to the first-layer coating. In all cases the metals or metal alloys of the first- and second-layer coatings shall be related in the galvanic series such that the second-layer coating 3 forms a sacrificial anode for the first-layer coating 2 or the substrate, and the first-layer coating shall never be anodic to the second-layer coating

[0072] In embodiments, the first-layer coating 2 contains essentially pure metallic aluminium or aluminium alloy. Pure aluminium or aluminium alloy is, in an embodiment, in this case to reduce the anode demand when compared to the anode demand required to protect the substrate. The pureness of the first-layer coating may be in the range of 85-100% Al. The first-layer coating may alternatively be an aluminium alloy. In embodiments, the first-layer coating 2 contains 99.5% Al, or an Al-alloy containing 5% magnesium (Al5Mg).

[0073] Aluminium alloy anodic to pure aluminium and to the substrate is preferred as the second-layer coating 3. A composition in the second-layer coating 3 is an aluminium-zinc-indium (Al—Zn—In) alloy, although other aluminium compositions that provide corresponding electrochemical properties may constitute an alternative. Other substances that can be combined with aluminium beside zinc and indium in the sacrificial second-layer coating 3 are for example cadmium (Cd), silicon (Si), tin (Sn), manganese (Mn) and titanium (Ti).

[0074] In a Al—Zn—In alloy the zinc may constitute about 2-7% of the composition, indium may amount to about 0.01-0.05%, whereas aluminium constitutes the balance. As will be understood from the aforesaid the composition of the second-layer coating 3 may be similar to that of conventional sacrificial aluminium anodes used to protect subsea components.

[0075] Application of the first- and second-layer coatings to the substrate may include any suitable application process such as co-lamination, co-extrusion and explosion bonding, e.g., wherever this can be permitted with respect to the design of the substrate, such as in connection with planar plates, pipes and rods of continuous radius. For more complicated shapes, metal spraying is the method for deposition of both the first-layer coating 2 and the second-layer coating 3.

[0076] Metal spraying is a general name for several processes in which pure or alloyed metal is melted in a flame or arc and sprayed onto a substrate by means of compressed air or explosion gases. Micrometre-sized droplets of metal are this way created and projected towards the surface of the substrate. By repeating the process, droplets will successively accumulate to form a coating.

[0077] Under the general concept of metal spraying several variations which are suitable for deposition of the first- and second-layer coatings 2 and 3 can be distinguished, such as plasma spraying, detonation spraying, wire arc spraying, flame spraying, high velocity oxy-fuel spraying, warm spraying or hot spraying, cold spraying, e.g.

[0078] A key feature in embodiments of the present invention is the provision of a sacrificial anode coating 3 which is distributed over essentially the entire area of the substrate/component that is exposed to a corrosive environment. The thickness of the second-layer coating 3 is determined by the self-corrosion rate over the service life of the component, whereas the total thickness is a function of the estimated current demand to protect any defect area(s) in the second-layer coating.

[0079] It is assumed that the Al—Zn—In anode composition in the second-layer coating 3 of the embodiment is favoured by a uniform corrosion which affects and reduces the need for layer thickness. A layer thickness in the range of 300-1,500 μm is well within the capacity of metal spraying methods. The range will also cover most applications in terms of service life and operational conditions. It is however within the scope of embodiments of the invention to increase the layer thickness of the second-layer coating 3 up to about 3,000 μm if required, whereas above that range the benefit of weight reduction as compared to fitting of conventional anodes will be less obvious. In any case, the thickness of the second-layer coating 3 should not be less than 200 μm.

[0080] As already has been stated in other parts of the disclosure the thickness of the inner coating 2 is, in an embodiment, within 100-300 μm.

[0081] From the above specification of the cathodic protection system it is appreciated that the second-layer coating 3 constitutes a sacrificial anode mass which is distributed over the substrate and protected component, and which is in direct contact with the environment that surrounds the component. The predominant material in both the first- and second-layer coatings is metallic aluminium which provides electrical conductivity and good bonding properties between the coatings themselves and towards a substrate of ferrous metal and of non-ferrous metal as well.

[0082] The distributed sacrificial anode of the second-layer coating may also be used in combination with conventional sacrificial anodes in case larger areas have to be traditionally coated, e.g. through paint systems or other non-conductive coatings. In all cases embodiments of the invention as claimed provide substantial reduction in anode mass and weight whenever applied in a structure that is subjected to a corrosive environment.

[0083] FIGS. 4A and 4B show that thermally sprayed Al2.5Zn0.02In alloy (DSA) coated on carbon steel differs from thermally sprayed Al5Mg alloy (regular TSA) coated on carbon steel in terms of electrochemical performance in seawater. In this example, the efficacy of DSA vs. that of TSA is illustrated by protection current density vs. time. DSA coated on carbon steel reveals similar or identical behaviour as a traditional cast Al—Zn—In anode coupled to carbon steel in terms of protection current density vs. time, whereas TSA coated on carbon steel reveals a behaviour that differs from that of DSA and cast Al—Zn—In anode coupled to carbon steel.

[0084] Conclusively, FIGS. 4A and 4B show that thermally sprayed Al2.5Zn0.02In alloy (DSA) coated on carbon steel differs from thermally sprayed Al5Mg alloy (regular TSA) coated on carbon steel in terms of electrochemical performance in seawater, in this example illustrated by protection current density vs. time.

[0085] For the experiments, samples designated DSA were prepared by first applying Al—Zn—In-alloy to carbon steel panels by thermal spraying, then samples were cut to size and finally prepared for electrochemical testing by effectively sealing off all carbon steel surfaces of the sample, leaving DSA as the only metallic part of the samples being exposed to seawater. Samples designated TSA were prepared for electrochemical testing in the exact same way as for DSA, except for using Al5Mg alloy in the thermal spraying process. Samples designated CS were prepared from bare carbon steel plates that were cut to size and sealed off as required for attaining the desired surface area ratios of the various couples.

[0086] In the electrochemical experiments performed in fresh, circulated natural seawater, DSA samples were then coupled to CS samples in two different ratios; DSA:CS 100:1 (grey) and DSA:CS 10:1 (blue), simulating different defect sizes (see FIG. 4A).

[0087] Similarly, TSA samples were coupled to CS samples in two different ratios; TSA:CS 100:1 (grey) and TSA:CS 10:1 (blue). For use as reference, Anode samples directly cut from a cast Al—Zn—In-anode were coupled to CS in a ratio of Anode:CS 10:1 (red), see FIG. 4b.

[0088] The resulting protection current densities (mA/m.sup.2) plotted as a function of time (days) in FIGS. 4A and 4B reveal that (i) DSA differs from TSA and (ii) DSA is similar or identical to the conventional Al—Zn—In cast Anode.

[0089] FIGS. 5A and 5B display that after electrochemical testing, carbon steel coupled to thermally sprayed Al2.5Zn0.021n alloy (DSA) differs from carbon steel coupled to thermally sprayed Al5Mg alloy (regular TSA) in terms of both the quantity of calcareous deposits and the level of corrosion on the exposed carbon steel surface. Carbon steel samples, when coupled to thermally sprayed Al2.5Zn0.021n alloy (DSA) and exposed to seawater, show significant build-up of calcareous deposits and no signs of corrosion. In contrast, when coupled to thermally sprayed Al5Mg alloy (TSA) and exposed to seawater, carbon steel samples show that corrosion of carbon steel had occurred.

[0090] Thereby, FIG. 5A illustrates that after electrochemical testing performed as described above, when coupled to thermally sprayed Al2.5Zn0.021n alloy (DSA) and exposed to seawater for 30 days, carbon steel (CS) samples show no sign of corrosion (but a noticeable build-up of calcareous deposits). In contrast, FIG. 5b illustrates that when coupled to thermally sprayed Al5Mg alloy (TSA) and exposed to seawater for 30 days, the carbon steel samples show corrosion of the carbon steel. The area ratios of samples in the photographs are DSA:CS 10:1 and TSA:CS 10:1.

[0091] Although illustrated by way of example, a skilled person will realize that the technical effects and benefits of the cathodic protection system of the present invention is achievable within ranges, whereby modification of the invention within the language and wording of the claims is possible, and that any such modification, also if not literally meeting the claim language, is covered by the scope of protection as defined and afforded by the claims.

[0092] This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.