Patent Application
20250146107
The present application claims priority to European Patent Application No. 23208436.8 filed on Nov. 8, 2023, which application is incorporated herein by reference in its entirety.
The present invention relates to an aluminum-based alloy for use in a sacrificial anode and a sacrificial anode produced from the aluminum-based alloy.
Cathodic protection using sacrificial anodes is a well-known technique within a number of areas including ships, vessels and offshore constructions, e.g. oil production platforms, rigs and offshore wind constructions. Sacrificial anodes are made of metal (usually zinc or aluminum) with more negative electrochemical potential than the metals you are trying to protect so the electrons start flowing from them into water and from water into other metals.
The electrochemical process taking place is generally along the following scheme for a ferrous material:
In most environments, the hydroxide ions and ferrous ions combine to form ferrous hydroxide, also known as rust:
It is desired to prevent or at least reduce the deterioration of the ferrous material, as this is the main structural material of the construction in question. Therefore, a sacrificial metal ranking lower in the electrochemical scheme, and therefore being more prone to the electrochemical reaction, is placed in electrical connection with the main construction and protects this main construction from deterioration. The most often used anode materials are aluminum or zinc or alloys comprising aluminum or zinc as the main component, but other materials may be used as alternatives to zinc and aluminum, such as magnesium as well as alloys containing magnesium.
Relevant areas of deployment are numerous; however, constructions in operation in seawater are in particular vulnerable to corrosion and will almost always be protected from such corrosion by means of cathodic protection using sacrificial anodes. Ships, vessels and offshore oil exploitation constructions are well-known examples of such.
The sacrificial anodes should be of a type suitable to the environment of operation, i.e. taking into consideration the chemical composition of the environment and also the temperature. Furthermore, the size of the anode(s) as well as their mutual positioning are relevant to consider in order to provide a satisfactory protection.
Many offshore oil exploitation constructions, pipelines or other equipment are from their very first installation provided with a cathodic protection system using sacrificial anodes, and most often the dimensioning of the anodes is designed for the design lifetime of the oil exploitation construction, meaning that no further exchange of the sacrificial anodes is foreseen. The oil exploitation constructions, pipelines or other equipment are however in many cases kept in operation well beyond the initial design lifetime, meaning that the cathodic protection obtained through the sacrificial anodes will disappear when the anodes are consumed after a certain time. For this situation, there is a need for exchanging or retrofitting the anodes of the offshore oil exploitation construction, pipelines or other equipment with new functional anodes to ensure the cathodic protection for a further life extension of the oil exploitation construction, pipelines or other equipment.
There are a number of ways of ensuring the cathodic protection of the construction. Normally the anodes are mounted directly on the construction with a mutual distance, ensuring the most efficient protection according to the design criteria as mentioned above. Another way of ensuring the correct function is by placing the anodes remotely from the construction to be protected and connecting each anode electrically to the designed connection position of the construction, pipelines or other equipment.
When retrofitting anodes to an offshore construction, pipelines or other equipment the remote positioning of the anodes is often the most effective way of doing this and therefore the preferred way of ensuring the continued cathodic protection.
It is known in the art to provide an anode construction for the purpose of retrofitting anodes to an offshore construction, pipelines or other equipment where the previously known construction comprises a framework with the anodes placed in mutually distanced positions to ensure the proper functioning of the cathodic protection. The anodes are connected to suitable connection points of the construction to be protected by means of suitable wiring. Such previously known anode construction is relatively bulky and therefore difficult to transport from the production site to the actual operational site.
The alloy compositions described herein are designed to have high operating efficiencies to make the alloy as cost-effective as possible, high current output to enable high and long-lasting performance for a given weight of anode (energy density), and optimized operating potential, which will vary depending on the application. An important added benefit is that in the alloys of this invention the content of zinc is very low. The most used commercial aluminum anode alloy is aluminum-5% zinc-0.02% indium. This alloy is specified in MIL-DTL-24779 and has proven to be very effective in worldwide climates to protect a variety of materials including iron, steel, and aluminum piers, ships, off-shore rigs, and bridges among other applications. It is approximately 90% efficient, which is lower than pure zinc, which is about 98% efficient, but much higher than magnesium, which is about 60% efficient.
Unfortunately, zinc is an aquatic toxin and contains residual cadmium from the mining process. Zinc is known to be toxic for marine plants and animals. Consequently, there is a raising demand for alloys in which the content of zinc is very low, but which still provide the same outstanding efficiency, current output and energy density. The alloy of this invention has the potential to replace the aluminum-zinc-indium alloy for use as described above. Moreover, zinc is also more expensive than aluminum. Thus, replacing the amount of zinc in an alloy with aluminum, reduces the cost of producing the alloy. Corrosion protective anodes comprising high amount of aluminum are e.g. disclosed in the patent documents US 2012/0084208 A1 and US 2019/0078179 A1.
Further, sacrificial anodes and especially aluminum sacrificial anodes has shown to have a tendency to be more positive (higher electropotential) compared to the metal to be protected, when deployed in cold waters below 10 degrees Celsius. The anodes thus have a lower performance when deployed in cold sea water.
An object of the present invention is to provide an alloy for sacrificial anodes which is environmental friendly and can be produced in a cost-effective manner and which perform effectively in cold waters such as the waters around Greenland as well as in waters of higher temperatures.
The present invention relates in a first aspect to an aluminum-based alloy for use in a sacrificial anode, characterized in that it comprises:
The term “maximum” indicates that the specified percentage is the maximum content of the metal in the alloy.
The term “balance” indicates that aluminum constitutes the amount of the alloy which is required to reach 100% by weight, e.g. if the combined amount of silicon, zinc, indium, iron, copper, and optional impurities constitutes 0.85% by weight, the amount of aluminum constitutes 99.15% by weight.
The alloy may also comprise elements in form of impurities in an amount of maximum 0.01% by weight of each element. The impurities may e.g. be constituted by magnesium, mangan, chrome, cadmium, tin, boron and other elements. The impurities may originate from the aluminum grade used.
It has surprisingly appeared that the high amount of aluminum combined with the specified amounts of zinc, indium, silicon, iron, and copper provides an alloy for a sacrificial anode with good properties and which is significantly lesser harmful to the environment than sacrificial anodes comprising higher amounts of zinc, such as above 1% by weight or more.
Sacrificial anodes and especially aluminum sacrificial anodes has shown to have a tendency to be more positive (higher electropotential) compared to the metal to be protected, when deployed in cold waters below 10 degrees Celsius. The anodes thus have a lower performance when deployed in cold sea water.
The amount of minimum 0.01% of weight of silicon together with the maximin amount of the remaining specified materials provides an allow which when used in a sacrificial anode, renders the anode especially well performing in colder waters, such as the waters around Greenland.
The specified minimum concentration of silicon together with the defined maximum concentrations arranges an alloy for a sacrificial anode less probe to cold sea water increasing the electropotential of the anode.
The amount of zinc is maximum 0.8% by weight, such as between 0.01-0.5% by weight of zinc. The amount of zinc may constitute between 0.05-0.8%, such as between % 0.08-0.8%, or even between 0.1-0.5%, such as between 0.2-0.5% by weight. The amount of zinc may also vary between 0.3-0.8%, such as between 0.4-0.8%, or between 0.5-0.8% by weight.
In an embodiment of the first aspect, the aluminum-based alloy for use in a sacrificial anode comprises:
In an embodiment of the first aspect, the aluminum-based alloy comprises:
Although the amount of impurities present in the alloy may be larger than the amount of some of the individual elements: silicon, zinc, indium, iron, and copper, the lower presence of these metals, however, improve the electrochemical properties of the alloy, namely an alloy with a sufficiently low electropotential in order to establish a sufficiently current between the structure to protect and the anode.
In an embodiment of the first aspect, the aluminum-based alloy comprises:
The maximum amount of silicon ensures that no overprotection of the structure occurs. Over protection of a submerged structure may occur where the sacrificial anode generates a current which is higher than what is necessary, the anode being to negative. E.g., teel hulls of ships which are overprotected, risk that the paint of the hull rapidly lifts of, due to excessive protection voltage.
Further, excessive cathodic protection accelerates the formation of calcareous deposits including calcium carbonate coral-like structure and formation of alkaline conditions on metallic hulls when there is insufficient flow of water to return the water to its natural pH, which results in accelerated corrosion.
Tests by the applicant has surprisingly shown, that the above-defined values in combination with an amount of silicon between 0.01-0.12% by weight, on one side ensures sufficiently low electropotential of the anode in cold water, and at the same time prevents the anode from overprotecting the submerged structure.
The aluminum in the aluminum-based alloy has a high purity, and in an embodiment of the first aspect the aluminum has a purity of at least 99.0% by weight.
In an embodiment of the first aspect, the aluminum-based alloy according to the invention, the aluminum has a purity of at least 99.5% by weight.
In the aluminum-based alloy according to the invention, the aluminum may have purities in the ranges 99.00 to 99.99% by weight, preferably in the ranges 99.0 to 99.99% by weight, such as purities in the ranges 99.90 to 99.99% by weight.
The aluminum-based alloy according to the first aspect of the invention has an excellent electrochemical efficiency, and in an embodiment the aluminum-based alloy has an electrochemical efficiency above 1500 Ah/kg, preferably above 2000 Ah/kg, such as above 2500 Ah/kg, when tested according to the standard DvN RP B401 (DvN RP B401 Annex B).
Moreover, the aluminum-based alloy according to the invention has low potential (potential vs. Ag/AgCl), and in an embodiment the potential is even lower than −800 mV, such as lower than-1000 mV. Thus, the aluminum-based alloy according to the invention is able to provide a very good cathodic protection to more noble metals, e.g. steel.
The invention relates in a second aspect of the invention to the use of an aluminum-based alloy in a sacrificial anode for protecting metallic constructions in a humid and marine environment, said aluminum-based alloy comprising the features of any of the embodiments of the first aspect of the invention, namely:
Hereby, an alloy for an anode suitable to protect submerged steel structures in colder waters, such as waters having a temperature below 10 degrees Celsius is provided. The given amounts and the amount of silicon between 0.01-0.12% by weight, on one side ensures sufficiently low electropotential of the anode in cold water, and at the same time prevents the anode from overprotecting the submerged structure.
According to the use of the aluminum-based alloy the aluminum may have purities in the ranges 99.00 to 99.99% by weight, preferably purities in the ranges 99.50 to 99.99% by weight, such as a purity in the ranges 99.90 to 99.99% by weight.
The use according to the invention provides sacrificial anodes with an excellent electrochemical efficiency, and in an embodiment the anodes with the aluminum-based alloy has an electrochemical efficiency above 1500 Ah/kg, preferably above 2000 Ah/kg, such as above 2500 Ah/kg, when tested according to the standard DvN RP B401 (DvN RP B401 Annex B).
Moreover, the use according to the invention also provides sacrificial anodes with low potential (potential vs. Ag/AgCl), and in an embodiment of the invention the potential is lower than −1000 mV. Thus, use of the aluminum-based alloy according to the invention provides sacrificial anodes with very good cathodic protection to more noble metals, e.g. steel.
According to a third aspect of the present invention, the above objects and advantages are obtained by:
A sacrificial anode for protecting metallic constructions in a humid and marine environment, said sacrificial anode comprises an aluminum-based alloy according to any embodiments of the first aspect of the invention.
Hereby, an anode suitable to protect submerged steel structures in colder waters, such as waters having a temperature below 10 degrees Celsius is provided. The given amounts and the amount of silicon between 0.01-0.12% by weight, on one side ensures sufficiently low electropotential of the anode in cold water, and at the same time prevents the anode from overprotecting the submerged structure.
The invention will now be described in further details with reference to the drawings in which:
The figures are only intended to illustrate the principles of the invention and may not be accurate in every detail. Moreover, parts which do not form part of the invention may be omitted. The same reference numbers are used for the same parts throughout.
A sacrificial anode according to the invention was produced using the aluminum-based alloy with the approximate composition:
The aluminum-based alloy was shaped into a cylindrical rod constituting a sacrificial anode, and the electrochemical efficiency and the potential were determined.
The sacrificial anode had an electrochemical efficiency above 2500 Ah/kg when tested according to DvN RP B401.
The sacrificial anode had a potential (potential vs. Ag/AgCl) lower than −1050 mV.
Thus, the sacrificial anode provided very good properties in respect of cathodic protection for metallic structures.
| Number | Date | Country | Kind |
|---|---|---|---|
| 23208436.8 | Nov 2023 | EP | regional |
