DK173514B1 - Zinc-based alloy, its use as a sacrificial anode, sacrificial anode and cathodic protection method - Google Patents

Description

in DK 173514 B1

The invention relates to a new, zinc-based alloy, this alloy for use as the effective part of a sacrificial anode, the use of the alloy as the effective part of such anode, a sacrificial anode whose effective part is an alloy of the invention, and -todical protection of corrosion-threatened structures in aggressive environment.

Most of the sacrificial anodes currently manufactured and used for cathodic protection of structures in an aggressive environment include, as the active part, zinc-based or aluminum-based alloys. The zinc-based alloys used today meet the requirements of the US Military Standard (U.S. Mill-A-1800 1 K), which prescribes the addition of cadmium; U.S. Mill-A-1800 1 K includes the following 15 data:

Alloy composition Potential Capacity Efficiency Al 0.10 - 0.50% - 1100 mV 780 Ah / kg 95%

Cd 0.025 - 0.07% 20 Fe max. 0.005%

Cu max. 0.005%

Pb max. 0.006%

Other metals max. 0.10% 25 Residual Zn - purity min. 99.314% (potential measured vs. Cu / CuSO4 reference electrode)

Although zinc alloys of the aforementioned kind have proved suitable as sacrificial anodes, they suffer from the serious disadvantage that they contain, inter alia, in particular the heavy metal cadmium, which represents a significant risk factor with regard to environmental impact and pollution. Thus, alloys of this kind must in the near future be expected to be subject to restrictions beyond those already available. Thus, there is an urgent need to provide non-cadmium-containing, more environmentally friendly zinc-based alloys that are at least or substantially as effective for cathodic protection as the prior art alloys of the aforementioned kind.

The object of the invention is thus the provision of new zinc-based alloys suitable as feranode alloys, which are as far as possible free of environmentally harmful heavy metals such as cadmium, and which are also particularly effective for cathodic protection, in particular of offshore structures, ship hulls, tanks, refrigerators. , pipes, spun walls and the like.

Surprisingly, it has been found that a zinc-based alloy with specific aluminum, tin and possibly indium content substantially fully meets the requirements of US-Mill-A-1800 1 K in terms of potential, capacity and efficiency and for certain alloy compositions In addition, in the case of -15, even more advantageous properties of the kind mentioned are exhibited, in addition, the alloy is cadmium-free and when used for cathodic protection gives rise to a uniform corrosion pattern and no pitting, and thus the object of the invention is achieved with such an alloy.

The invention thus relates to a zinc-based alloy, characterized in that it consists essentially of 0.1 - 1.0% by weight aluminum, 0.005 - 1.0% by weight tin, 0-0.5% by weight indium, not more than 0,0030% by weight lead, iron and / or copper, and not more than 0.0050% by weight lead, iron and / or copper, and other metals, in particular nickel, thallium and cadmium, 30 and residual zinc of a purity of approx. 99.995%.

An alloy of the invention preferred for its particularly advantageous potential, capacity and efficiency, cf. the following test results, consists essentially of 0.1 - 1.0% by weight of aluminum, 0.01-0.0 , 1 wt% cin, 0-0.1 wt% indium, not more than 0.0030 wt% lead, iron and / or copper, and 5 no more than 0.0050 wt% lead, iron and / or copper and other metals, especially nickel, thallium and cadmium, as well as residual zinc with a purity of approx. 99.995%, 10 and this is particularly true of an alloy consisting essentially of 0.1 - 0.2% by weight aluminum, 0.01 - 0.03% by weight tin, 100-300 ppm indium, at most 0 , 0030% by weight lead, iron and / or copper, and not more than 0.0050% by weight lead, iron and / or copper, and other metals, in particular nickel, thallium and cadmium, and residual zinc of a purity 20 of approx. 99.995%.

In practice, it is often preferable to use an alloy that does not contain either indium or only a very small amount of indium. Thereby, a substantial economic advantage is obtained due to the high price of indium, and at the same time nearly as good properties are obtained as with the preferred alloy mentioned above, cf. the following test results. A preferred indium-free alloy or very low indium alloy according to the invention is characterized in that it consists essentially of 0.1 - 0.2% by weight aluminum, 0.05 - 0.10% by weight tin, 0%. 50 ppm indium, not more than 0.0030 wt% lead, iron and / or copper, and 35 no more than 0.0050 wt% lead, iron and / or copper and other metals, especially nickel, thallium and cadmium, and residual zinc with a purity of approx. 99.995%.

From DE Publication No. 2,411.608 5 a zinc-indium alloy for cathodic protection without pollution of the environment is known. The alloy contains 0.005 - 0.02% indium and the rest is high purity zinc. U.S. Patent No. 2,982,705 also discloses a zinc alloy for cathodic protection and contains at least one element selected from indium, thallium, cadmium and tin in an amount of 0.01-3%, the alloy being composed solely of zinc and indium. or of zinc and tin.

In the aforementioned writings there is no mention of aluminum alloy, as is the case in the alloy of the present invention. It has been found that the addition of 0.1 to 1.0% by weight of aluminum has a favorable effect on the structure of the cast anode alloy, which has the advantage that under normal conditions the anode will corrode more, thereby avoiding pitting, which may cause pieces of the anode to fall which cannot therefore be utilized optimally. Furthermore, the aluminum content gives better casting properties, since the anodes, which often have to be machined, do not break by thread cutting and other mechanical machining, whereas zinc without aluminum addition is very coarse crystalline and therefore brittle and breaks very easily.

The invention further relates to a zinc-based alloy as defined above for use as the effective portion of a sacrificial anode, and the invention relates to the use of an alloy of the present invention as the effective portion 30 of a sacrificial anode.

Finally, the invention relates to a sacrificial anode, the active part of which is a zinc-based alloy of the kind disclosed herein, and to a method for cathodic protection of corrosion-threatened structures in an aggressive environment, in particular 35 offshore structures, ship hulls, tanks, refrigeration vessels, pipes and the like. is characterized in that the structures are protected using a sacrificial anode according to the invention.

The invention is further illustrated by means of the following test results obtained with alloys according to the invention, the tests being carried out in accordance with the requirements of Det Norske Veritas' Offshore standard RP.B.401; the potentials are measured vs. CU / CuSO4.

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It will be seen that the tested alloys according to the invention meet substantially the requirements of U.S. Mill-A-1800 K 1 and, to a significant extent, even exhibit more advantageous values.

5

Claims (8)

Zinc-based alloy, characterized in that it consists essentially of 0.1 to 1.0% by weight of aluminum, 0.005 to 1.0% by weight of tin, 0 to 0.5% by weight of indium, at most 0 , 0030% by weight lead, iron and / or copper, and not more than 0.0050% by weight lead, iron and / or copper and other metals, in particular nickel, thallium and cadmium, and residual zinc with a purity of approx. 99.995%. Alloy according to claim 1, characterized in that it consists essentially of 0.1 - 1.0% by weight aluminum, 0.01 - 0.1% by weight tin, 0-0.1% by weight. indium, not more than 0.0030% by weight lead, iron and / or copper, and not more than 0.0050% by weight lead, iron and / or copper, and other metals, in particular nickel, thallium and cadmium, and residual zinc with a purity of approx. 99.995%. 25 An alloy according to claim 1, characterized in that it consists essentially of 0.1 - 0.2 wt% aluminum, 0.01 - 0.03 wt% tin, 100-100 ppm indium, no more than 0 , 0030 wt% lead, iron and / or copper, and not more than 0.0050 wt% lead, iron and / or copper and other metals, in particular nickel, thallium and cadmium, 35 and residual zinc with a purity of approx. 99.995%. DK 173514 B1 An alloy according to claim 1, characterized in that it consists essentially of 0.1 - 0.2 wt% aluminum, 0.05 - 0.10 wt% tin, 5 - 50 ppm indium, no more than 0 , 0030% by weight lead, iron and / or copper, and not more than 0.0050% by weight lead, iron and / or copper and other metals, in particular nickel, thallium and cadmium, 10 and residual zinc with a purity of approx. 99.995%. Zinc-based alloy according to any one of claims 1-4 for use as the active part of a sacrificial anode. 15 Use of an alloy according to any one of claims 1-4 as the effective part of a sacrificial anode. A sacrificial anode, characterized in that its active portion is a zinc-based alloy according to any one of claims 1-4. Method for cathodic protection of corrosion-threatened structures in an aggressive environment, in particular 25 offshore structures, ship hulls, tanks, refrigerated containers, pipes and the like, characterized in that the structures are protected by using a sacrificial anode according to claim 7.