Alloy containing bismuth used for a sacrificial anode, and preparation process

Abstract

The present invention is directed to an innovative anodic alloy without Cadmium and containing Bismuth, its preparation and its uses. Preferably, said anodic alloy without Cadmium and containing Bismuth is a Zinc-based alloy. In particular, said alloy has advantageously found application in the cathodic protection of metal structures, for example, boats and / or parts or metal appendages thereof.

Description

[0001] ALLOY CONTAINING BISMUTH USED FOR A SACRIFICIAL ANODE, AND PREPARATION PROCESS

[0002] Technical field of the invention

[0003] The present invention is directed to an innovative anodic alloy without Cadmium and containing Bismuth, its preparation and its uses.

[0004] Preferably, said anodic alloy without Cadmium and containing Bismuth is a Zinc-based alloy. In particular, said alloy has advantageously found application in the cathodic protection of metal structures, for example, boats and / or parts or metal appendages thereof.

[0005] Brief summary of the closest prior art

[0006] Electrogalvanization is a process, developed in the early twentieth century, wherein a layer of zinc, or a metal alloy thereof, is electroplated onto a metal (e.g., iron, steel, aluminium, other metal alloys and the like) to give it good corrosion and temperature protection, and good wear resistance. This type of protection is particularly important for the protection of metals and / or metal alloys subjected to wear and / or continuous operating conditions. A particularly important example of application is represented by the nautical sector, where metal boats (for example, with the hull made of aluminium, steel or metal alloy of various types), or any type of metal appendage of boats with hulls made of composite material and / or carbon are subjected to the severely wearing conditions of the sea and atmospheric agents.

[0007] Technical problem Various types of metal alloys have been developed over the years with the aim of providing the desired durable protection to the metal surfaces subject to particular wear, in particular those described above. However, they have often shown a composition that had a number of disadvantages due, for example, in particular to the presence of more or less high amounts of metals known for their toxicity to humans and / or animals and / or for their negative impact on the surrounding environment.

[0008] In particular, a well-known zinc-based anodic alloy currently commonly used commercially for its excellent anti-corrosion and wear resistance properties is that corresponding to the US Mil Spec. A- 1800 IK, the composition of which is described in the following table: wherein: US MIL Spec. A- 1800 IK corresponds to the specification of the commercial alloy produced by Tecnoseal Foundry S.r.l. at the production plant located in Grosseto Via Senese 193 (Postcode 58100), ITALY; and wherein: NA means Not Available. Unfortunately, this alloy contains, among other components, a non-negligible amount of Cadmium (Cd), a transition metal belonging to group 12 of the Periodic Table of the Elements and chemically similar to the other stable metal of group 12, namely Zinc (Zn). It is, unfortunately, a metallic element notoriously carcinogenic and extremely polluting of the surrounding environment and in fact, for this reason, it has also been specifically included in European Directive 2002 / 95 / EC on the Restriction of Hazardous Substances.

[0009] Many Regulations give indications regarding the maximum Cadmium concentrations that can be contained within the products [for example, DNV RPB 401 and subsequent amendments; EN 16222]; from a study of these Regulations it was found that the amounts used within the alloy in question are on average higher than the limits established by these Regulations:

[0010] Consequently, it would be very useful (it is indeed a strongly felt need in the sector) to have available an innovative anodic alloy for the protective coating of metal surfaces in general, and, specifically, those of the nautical sector, whose formulation is without Cadmium and at the same time allows to provide the same excellent corrosion and wear protection of the aforementioned zinc-based anodic alloy, possibly even better.

[0011] Aim of the present invention is to provide an adequate response to the technical problem highlighted above.

[0012] Summary of the invention

[0013] As part of an environmental policy and in accordance with the will not to go against any rule that imposes restrictions on the use of toxic and polluting substances, the Holder has now unexpectedly found that a new anode consisting of an alloy in which cadmium has been replaced with a so-called post-transition metal (a metal of the p-block, located near the metal-non-metal boundary of the Periodic Table), like, preferably, Bismuth, has provided an excellent response to the technical problem described above.

[0014] In particular, in a preferred embodiment, an object of the present invention is an alloy without Cadmium and containing Bismuth, as described in the appended independent claim.

[0015] Another object of the present invention is also a process for the realization of the above alloy, as described in the appended relative process claim.

[0016] A further object of the present invention is the use of said alloy, as described in the appended relative use claim.

[0017] Further advantages of the present invention can be inferred from the appended dependent claims, as well as from the continuation of the description itself.

[0018] Detailed Description of the Invention

[0019] The process for determining the new alloy consisted in analysing high atomic weight elements that did not have particular indications of toxicity, in order to define a new stoichiometry capable of creating within the crystalline matrix of the zinc the same rupture zones introduced by the cadmium.

[0020] Multiple tests were carried out for the realization of anodic samples with different concentrations of the new element, to define, among other things, the correct anodic mass necessary to protect against the galvanic currents of any metal surface exposed to an electrolyte.

[0021] The choice of the weight percentages fell on those indicated in the following table, based on evaluations also inherent in the workability of the new alloy. As previously mentioned, an object of the present invention is an innovative anodic alloy without Cadmium comprising, among its constituent elements, an effective amount of Bismuth.

[0022] Said amount of Bismuth is included from 0.020% to 0.075% by weight (w:w), with respect to the total weight of the alloy. Preferably, said amount is included from 0.021% to 0.074%; from 0.022% to 0.073%; from 0.023% to 0.072%; from 0.024% to 0.071%; from 0.025% to 0.070%; from 0.026% to 0.069%; from 0.027% to 0.068%; from 0.028% to 0.067%; from 0.029% to 0.066%; from 0.030% to 0.065%; from 0.031% to 0.064%; from 0.032% to 0.063%; from 0.033% to 0.062%; from 0.034% to 0.061%; from 0.035% to 0.060% by weight (w:w), with respect to the total weight of the alloy.

[0023] In a particularly preferred embodiment of the invention, said amount is included from 0.028% to 0.067%; from 0.029% to 0.066%; from 0.030% to 0.065%; from 0.031% to 0.064%; from 0.032% to 0.063% by weight (w:w), with respect to the total weight of the alloy.

[0024] In an even more preferred embodiment of the invention, said amount is included from 0.030% to 0.065% by weight (w:w), with respect to the total weight of the alloy.

[0025] Preferably, said anodic alloy without cadmium is a Zinc-based alloy containing Bismuth in the amounts described above.

[0026] In a particularly preferred embodiment, said anodic alloy without Cadmium and containing Bismuth comprises: wherein: Alloy B corresponds to a particularly preferred exemplary formulation of the anodic alloy of the invention; and wherein: NA means Not Available.

[0027] Cadmium was used as an activator of the zinc-based anodic alloy of the known art described above. Quite unexpectedly, it has now been found that even Bismuth (Bi), despite having chemical characteristics that are very different from Cadmium (in fact it belongs to a different group of the Periodic System), is likewise able to behave as an activator of the alloy of the present invention in the range of weight percentages indicated above.

[0028] Substantially, therefore, the work that has been done involved replacing the element Cadmium (Cd) with a new element (Bismuth (Bi)) which, although demonstrating the same effect as the first one, does not have any contraindication of an environmental nature, as well as in terms of toxicity.

[0029] The functional equivalence, with respect to the alloy of the known technique containing Cd, conferred by the new Bi-component to the anodic alloy of the present invention has been demonstrated by comparing the relevant electrophysical parameters of the two alloys.

[0030] These parameters are essentially the electrochemical open circuit potential, which is the voltage, detected by multimeter, between the anode and a reference probe in Ag / AgCl (silver / silver-chloride) or a Calomel probe; and the nominal electrochemical capacity, which is the peculiarity of the anode (or anodic alloy) to deliver current.

[0031] The methods for evaluating the nominal electrical capacity are described in Annex B of the DNV RP B401 technical standard.

[0032] Comparison of the electrophysical parameters of the two alloys

[0033] The electrophysical parameters of the two alloys are illustrated in the following tables.

[0034] - Alloy of the known art currently marketed, whose activator is Cadmium

[0035] - Alloy B, preferred of the present invention, whose activator is Bismuth The preparation of the alloy subject-matter of the invention is realized in a completely traditional way by carrying out a process comprising the following steps:

[0036] - inserting all the elements that make up said alloy into a suitable container, such as for example a crucible;

[0037] - subjecting the whole to a thermal treatment, at a temperature of about 420 degrees centigrade (°C) that completely melts the mass, by keeping it under stirring for a time sufficient to make it perfectly homogeneous;

[0038] - casting the resulting melted product into moulds or shell machines if the production is by gravity or into die casting machines, and letting it cool.

[0039] Preferably, said resulting melted product is cast into moulds suitable for the realization of sacrificial anodes.

[0040] Consequently, a further object of the present invention is also a sacrificial anode consisting of a Zinc-based alloy, without Cadmium and containing Bismuth according to the present invention. In particular, said sacrificial anode is produced by a process according to what is described above, but it can also be prepared according to any process known in the sector and suitable for the purpose.

[0041] The fundamental advantage of the alloy of the present invention consists in the fact of having available a product that does not contain carcinogenic material, thus allowing to produce sacrificial anodes to make galvanizations under conditions of complete safety for the personnel involved in such processings and also for the environment during use. A preferred use of the alloy of the invention (in particular, Alloy B referred to in the table above) is typically intended for nautical and typically marine applications as already anticipated above. However, among other possible advantageous uses, mention may also be made to those already known for the zinc alloy anodes produced in compliance with the US Mil Spec. 18001K.

[0042] In particular, it is worth reminding, for example, the following additional uses:

[0043] - Cathodic protection of non-painted steel surfaces immersed in sea water.

[0044] - Cathodic protection of painted steel surfaces immersed in sea water.

[0045] - Cathodic protection of aluminium surfaces immersed in sea water.

[0046] - Cathodic protection of painted aluminium surfaces immersed in sea water.

[0047] - Protection of sheet piles for the realization of port infrastructures.

[0048] - Protection of poles for the realization of piers.

[0049] - Protection of sea-water circuits made of CuNi (Cupronickel).

[0050] - Protection against galvanic currents of inboard / outboard engines and / or marine engines in general.

[0051] - Cathodic protection of concrete armatures.

[0052] In addition, being a completely eco-sustainable material, it is possible to reasonably hypothesize a possible use thereof also for fresh and / or drinking water, where, according to the current state of the art, an alloy based on magnesium is used for cathodic protection.

[0053] Industrial applicability

[0054] The present invention has allowed to realize an anodic alloy without Cadmium and containing Bismuth, preferably based on Zinc, which has made it possible to eliminate from the alloy itself a particularly toxic and polluting component (Cadmium), while maintaining the excellent corrosion protection and wear resistance characteristics thereof. In a particularly preferred application, said alloy has advantageously found application in the cathodic protection of metal structures, for example, boats and / or parts or metal appendages thereof.

Claims

CLAIMS1. An anodic alloy without Cadmium comprising an effective amount of Bismuth.

2. An anodic alloy in accordance with claim 1 , wherein said effective amount of Bismuth is included from 0.020% to 0.075% by weight (w:w).

3. An anodic alloy in accordance with claim 2, wherein said effective amount of Bismuth is included from 0.030% to 0.065% by weight (w:w).

4. An anodic alloy in accordance with anyone of the previous claims, wherein, said alloy is a Zinc-based alloy.

5. An anodic alloy in accordance with anyone of the previous claims, wherein said alloy has the following composition:

6. A process for the preparation of an alloy in accordance with anyone of the previous claims, comprising- inserting all the elements that make up said alloy into a suitable container, such as a crucible;- submitting the whole to a thermal treatment, at a temperature that completely melts the mass, for a time sufficient to make it perfectly homogeneous;- casting the resulting melted product into one or more moulds and letting it cool.

7. The process in accordance with claim 6, wherein said melted product is cast into moulds suitable for the realization of sacrificial anodes.

8. A sacrificial anode based on an alloy in accordance with anyone of claims from 1 to 5.

9. A sacrificial anode based on an alloy in accordance with anyone of claims from 1 to 5; said anode being obtained according to a process according to claim 6 or 7.

10. Use of an anodic alloy in accordance with anyone of claims from 1 to 5, in the cathodic protection of metal structures, for example, boats and / or parts or metal appendages thereof.

11. Use of an anodic alloy in accordance with anyone of claims from 1 to 5, for the additional following uses:- Cathodic protection of non-painted steel surfaces immersed in sea water.- Cathodic protection of painted steel surfaces immersed in sea water.- Cathodic protection of aluminium surfaces immersed in sea water.- Cathodic protection of painted aluminium surfaces immersed in sea water.- Protection of sheet piles for the realization of port infrastructures.- Protection of poles for the realization of piers.- Protection of sea-water circuits made of CuNi (Cupronickel).- Protection against galvanic currents of inboard / outboard engines and / or marine engines in general.- Cathodic protection of concrete armatures.

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