CADMIUM-FREE ZINC-BASED ALLOY. ITS USE AS A SACRIFICIAL ANODE. A SACRIFICIAL ANODE, AND A METHOD FOR CATHODlC PROTECTION OF CORROSION-THREATENED CONSTRUCTIONS IN AGGRESSIVE ENVIRONMENT

TECFfNICAL FIELD

This invention relates to a new zinc-based alloy; the said alloy for use as the active part of a sacrificial anode; the use of the alloy as the active part of such an anode; a sacrificial anode, the active part of which is an alloy according to the invention; as well as a method for cathodic protection of corrosion-threatened constructions in aggressive environment.

BACKGROUND ART

Know alloys, such as e.g. known from WO 00/26426, can be used for casting anodes of a relatively large size, the order of magnitude of the smallest dimension being several centimeters However, when smaller anodes are needed, the inventors have realised that a tendency of breaking or crack development is observed, especially in connection with machining, e.g. threading or boring, aftei casting of the anodes., Another feature indicated to be essential in this document is that the sacrificial anodes should be free from cadmium, i which poses a substantial risk factor' with respect to loading and contamination of the environment

DISCLOSURE OF THE INVENTION

Based on this prior art it is the object of the present invention to provide new zinc-based cadmium-free alloys, which are suitable as sacrificial anode alloys and which can be cast in relatively small dimensions, such as a smallest dimensioning in the order of 5-10 mm or less. Surprisingly, experiments made by the inventors have revealed that an alloy comprising 04- 0.6% by weight of aluminum, 0.02 - 0.03% by weight of indium, maximum 0.001% by weight of cadmium, maximum 0 005% by weight of iron, maximum 0 005% by weight of copper, maximum 0.006% by weight of lead, maximum 0.0005% of tin, maximum 0.10% by weight of other metals, in particular nickel and thallium, and balance zinc having a purity of

about 99 995 %, makes it possible to cast small sacrificial anodes with a smallest dimension in the oideτ to 5-10 mm or less without the tendency of breaking or crack development in connection with post-machining of the castings Furthermore, experiments by the inventors have shown that a possible change of the content of aluminum to a percentage above 0.6% by weight will induce problems when casting due to a higher viscosity of the molten metal It will be possible to change this viscosity by raising the temperature, which would, however, increase the time necessary for casting and increase the risk of dimensional changes during solidification and cooling of the finished casting. Furtheπnore, such a higher temperature would increase the cost of heating up the molten metal

Correspondingly, a change of the percentage of aluminum to a value below 0 4% by weight would make the alloy brittle and make the casting difficult to perform and possible post- machming of the casting will be difficult due to tendency of breaking of the material which will become brittle

DESCRIPTION OF PREFERRED EMBODIMENTS

An alloy according to the invention, which is preferred on account of its particular advantages regarding casting of small anodes, consists substantially of 0 4 - 0.6% by weight of aluminum, 0 02 - 0 03% by weight of indium, maximum 0 001% by weight of cadmium, maximum 0 005% by weight of iron, maximum 0 005% by weight of copper, maximum 0 006% by weight of lead, maximum 0 0005% of tin, maximum 0 10% by weight of other metals, in particular nickel and thallium, and balance zinc having a purity of about 99 995 %

A particularly piefeπed alloy according to the invention, which with a suitable margin avoids the problems mentioned above, when the aluminum content is less than 0 4% by weight or more than 0 6% by weight, consists substantially of 0.45 - 0 55% by weight of aluminum, 0 02 - 0 03% by weight of indium, maximum 0 001% by weight of cadmium, maximum 0 005% by weight of iron, maximum 0 005% by weight of copper, maximum 0 006% by weight of lead maximum 0 0005% of tin, maximum 0.10% by weight of other metals, in particular nickel and thallium, and balance zinc having a purity of about 99 995 %

The high price of indium makes it advantageous to have a narrower indication of the amount of indium in the alloy and accordingly a more preferred alloy according to the invention

consists substantially of 04 - 0 6% by weight of aluminum, 0 024 - 0 026% by weight of indium, maximum 0 001% by weight of cadmium, maximum 0 005% by weight of iron, maximum 0 005% by weight of copper, maximum 0 006% by weight of lead, maximum 0 0005% of tin, maximum 0 10% by weight of other metals, in particular nickel and thallium, and balance zinc having a purity of about 99 995 %

WO 00/26426 suggests an alloy having a content of aluminum between 0 1 and 1% by weight, but only one of the specific embodiments shown in the table of this document, namely specimen No, 7, has an aluminum content in the naπower range in accordance with the present invention However, this specimen No 7 has an indium content of 500 ppm, which is outside the range provided for the alloy in accordance with the present invention, and furiheirnoie the result of this specimen is indicated to be an anode with a coarse grained surface, which would not be acceptable for a relatively small anode in accordance with the present invention Furthermore, this relatively coarse grained surface indicates a tendency of crack development, as indicated above

This invention also relates to a zinc-based alloy as described above for use as the active part of a sacrificial anode, and the invention also relates to the use of an alloy of the present type as the active part of a sacrificial anode

Finally, the invention relates to a sacrificial anode, the active part of which is a zinc-based alloy of the present type, as well as a method for the cathodic protection of corrosion- thieatened constructions in aggressive environment, in particular smaller components, and this method is characterized in that the constructions aie protected by 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 test having been carried out according to the directions in the 'Det Norske Veritas ¹ Offshore-standard RP B 401; the potentials have been measured vs Ag/AgCl/seawater

Alloy in accordance with the present invention:

AI: 0 4 - 0 6%

In: 0 02 - 0 03%

Cd: Max . 0 001%

Fe: Max 0 005%

Cu: Max 0 005%

Pb: Max 0 006%

Sn: Max , 0 0005% Other metals: Max. 0 10%

and balance zinc having a purity of about 99 995%.

Performance data:

Capacity: 780 Ah/kg

Consumption rate: 1 ,2 kg/A year

Closed circuit potential: -1.03 V vs Ag/AgCl/seawater

Using this alloy it has been possible to cast anodes of diffeient sizes, including relatively small sizes, said anodes being resistant to breakage or crack development and ielatively easy post machining of the castings without tendency to breakage or crack development

Alloy having lower content of aluminum:

Al 0 1-0.4%

Other components as in the above alloy in accordance with the invention.

The performance data were substantially identical to the performance data for the alloy in accordance with the invention, despite the change of content of aluminum, However, with a content of aluminum of 0 4% and 0 1%, the casting becomes brittle and has a tendency to breakage or crack development, especially when post machining the castings. Furthermore, a tendency io ciack development during solidification of the casting has been observed

Aluminum content higher than in accordance with the present invention:

Al 0 6-1 0%

Other components as in accordance with the present invention

Again it can be seen that the performance data are substantially unchanged despite the change of content of aluminum

With this higher content of aluminum, between 0 6 and I 0%, the alloy becomes thick and has a tendency not to fill the comers of the casting cavity This may be changed by using a higher temperature of the casting metal, which however makes the casting more difficult due to higher temperature of the mould and results in longer time for cooling after casting Additionally, a higher temperature will inevitably involve higher shrinkage, i.e reduction of the size of the casting, during solidification, compared to the normally used temperature results Furthermore, higher production costs can be envisaged when using a higher casting temperature

It will be seen that the tested alloys according to the invention substantially meet the requirements according to U S MilI-A-1800 Kl and for a substantial part show even more advantageous values, and furthermore the test results show that small electrodes having a smallest dimensioning in the order of 5-10 mm or less can be cast with said alloy.