Zinc Alloy for Electrochemical Battery Cans

The present invention relates to an indium- and manganese bearing zinc alloy for electrochemical battery cans.

Such an alloy is known from Chemical Abstracts, Vol. 106, 1987, 36009f. This known alloy contains, besides zinc, indium and manganese, further also lead and cadmium. This known alloy has the drawback to contain the very toxic element Cd.

The aim of the present invention is to provide an indium- and manganese bearing zinc alloy for battery cans, which contains no cadmium and still has a sufficient mechanical strength, processability and resistance to corrosion.

The alloy according to the invention is free from cadmium and it contains 0.001-1% In and 0.005-1% Mn (by % is meant here and hereafter percentage by weight). This alloy may further also contain up to 1% Pb.

The remaining constituents of the alloy according to the invention are either zinc and unavoidable impurities, or zinc, unavoidable impurities and small quantities of other elements, such as for instance 0.005-1% At and/or 0.0005-0.1% REM (REM being a rare earth metal or a mixture of rare earth metals). Unavoidable impurities are the impurities which are present in the zinc and in the elements which are added to the zinc to obtain the alloy. Therefore, the above used expression "The ahoy according to the invention is free from cadmium" should be understood as follows: the ahoy according to the invention does not contain cadmium, unless eventually as an unavoidable impurity.

The indium content shah be at least 0.001% and the manganese content at least 0.005%; otherwise the mechanical strength and/or the resistance to corrosion are insufficient.

The indium content shall not exceed 1%; higher contents impair the processability and increase unnecessarily the price of the ahoy. Advantageously the indium content shall not exceed 0.1% and preferably not 0.05%. An indium content of up to 0.02% is still more preferred and most preferred is an indium content of up to 0.01%. Preferably the indium content shah be at least 0.003%.

The manganese content shall not exceed 1%; otherwise such an amount of intermetallic Zn-Mn compounds is formed that cracks may be produced when the alloy is rolled. Preferably the manganese content is 0.005-0.1% and still more preferably 0.005-0.05%.

If lead is present in the alloy of the invention, its content shall not exceed 1%; at higher lead contents corrosion problems may occur. Preferably the lead content shall not exceed 0.3%. Still more preferably, the lead content shall not exceed 0.05% and most preferably not 0.005%.

The description hereafter of a comparative test proves that the mechanical strength and the resistance to corrosion of a can for electrochemical batteries made from an alloy according to the invention are substantially equivalent to those of a can made from a known alloy containing cadmium.

Starting from thermally refined zinc two alloys were prepared, whose compositions are as follows:

These two alloys were processed into cans in an industrial unit in the traditional way: the cast material was rolled into strips, in which hexagonal slugs were stamped out; and the slugs were extruded into cans. Alloy (1) was processed as easily as alloy (2).

The tensile strength (in MPa) was determined on test pieces cut from the can wall in the extrusion direction; the stretching rate was 1 cm/min. The tensile strength was found to be 187.9 MPa for alloy (1) and 179.4 MPa for alloy (2).

In order to determine the resistance to corrosion use was made of an industrial electrolyte of ZnCl.sub.2. Five cans of each alloy were immersed for 4 days in the electrolyte which was kept at 45.degree. C. After the immersion the loss of weight of the 5 cans as a whole was determined (in gram) and the rate of corrosion was calculated (in microgram per cm.sup.2 and per day).

The results are summarized in table 1.

From these results it appears that the resistance to corrosion of alloy (1) is equivalent to the one of alloy (2). After the immersion these cans were also examined microscopically. One ascertains that both alloys are absolutely equivalent with regard to their resistance to pitting corrosion.

The alloy (1) is an alloy according to the invention. Other typical examples of alloys according to the invention have the following composition:

Zn-0.1% In-0.005% Mn

Zn-0.05% In-0.005% Mn

Zn-0.02% In-0.005% Mn

Zn-0.01% In-0.005% Mn

Zn-0.003% In-0.005% Mn

Zn-0.003% In-0, 1% Mn

Zn-0.003% In-0.05% Mn

Zn-0.01% In-0.05% Mn

Zn-0.01% In-0.1% Mn

Zn-0.02% In-0.05% Mn

Zn-0.02% In-0.1% Mn

Zn-0.05% In-0.1% Mn

Zn-0.05% In-0.05% Mn

Zn-0.1% In-0.05% Mn

Zn-0.1% In-0.1% Mn

Zn-0.1% In-0.005% Mn-0.05% Pb

Zn-0.01% In-0.05% Mn-0.2% Pb

Zn-0.007% In-0.05% Mn-0.025% Pb

Zn-0.005% In-0.05% Mn-0.2% Pb

Zn-0.005% In-0.05% Mn-0.025% Pb

Zn-0.05% In-0.005% Mn-0.01% A1

Zn-0.02% In-0.005% Mn-0.1% A1

Zn-0.01% In-0.005% Mn-0.1% A1-0.01% REM

Zn-0.003% In-0.005% Mn-0.3% Pb-0.01% REM

Zn-0.003% In-0.1% Mn-0.005% Pb-0.01% Al

Zn-0.003% In-0.05% Mn-03% Pb

Zn-0.01% In-0.05% Mn-0.05% Pb-0.02% A1

Zn-0.01% In-0.1% Mn-0.005% Pb

Zn-0.02% In-0.05% Mn-0.005% Pb-0.01% A1-0.001% REM

Zn-0.02% In-0.1% Mn-0.02% Al-0.01% REM

Zn-0.05% In-0.1% Mn-0.02% A1-0.002% REM

Zn-0.05% In-0.05% Mn-0.1% Al-0.005% REM

Zn-0.1% In-0.05% Mn-0.005% Pb-0.002% REM

Zn-0.1% In-0.1% Mn-0.005% Pb-0.005% A1-0.0001% REM

When preparing these alloys one starts from thermally or electrolytically refined zinc and in order to introduce REM one can use either misch metal, an alloy with some 45% Ce, 45% La and 10% other rare earth metals, or an aluminum master alloy with 4% La and 4%Ce.