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(11) | EP 0 087 288 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Bath and method for black chromate plating of zinc and cadmium surfaces |
| (57) An aqueous bath for black chromate plating of a surface of zinc, zinc alloy or cadmium
alloy containing sulphate ions, 5 to 40 g/l of hexavalent chromium, 0.25 to 30 g/l
of copper ions, 0.1 to 5 g/l of silver ions and/or molybdic ions, 30 to 120 g/l of
acetic acid and 0 to 6 g/l of formic acid. Such surfaces plated by the method using
the said bath produces a shiny black adherent coating giving good protection against
corrosion of the substrate. |
[0001] The present invention relates to new bath permitting, by chromium plating, to obtain a shiny black coating on surfaces of zinc, or zinc-or cadmium-containing alloys.
[0002] The protection of surfaces of zinc, or of zinc-or cadmium-containing surfaces by chromium-plating said surfaces, has already been described.
[0003] The solutions used to this effect contain essentially chromium (for example in sodium or potassium dichromate and or chromic acid form), sulphate ions and, in some cases a metallic compound giving rise to the formation of a suitably coloured chromate deposit.
[0004] Amongst the metallic compounds that can be used, there are the silver salts (for example silver nitrate) which lead to the formation of a shiny black chromium plating film, which looks good but of which the resistance to corrosion is affected by metallic pollutions (zinc, iron, etc.).
[0005] However, the baths usable for obtaining these silver-containing coatings are rather expensive to prepare and to maintain. For this reason, attempts have been made to find compounds of metals other than silver which, when used in chromium-plating baths, could give black, shiny and corrosion-resistant coatings.
[0006] A chromium-plating bath has recently been described which contains besides chromium (6) and sulphate ions, a copper salt and a mixture of acetic acid and formic acid. Unfortunately, when using this kind of bath, the deposit obtained is dull, this making it less interesting at the industrial level.
[0007] It is the object of the present invention to overcome this drawback by proposing a method for chromium-plating surfaces of zinc, or of zinc or cadmium alloys, the said method producing a shiny black coating with good adherence to the support and ensuring a good protection against corrosion of the support, which method consists in placing the support to be chromium - plated in contact with a bath containing:
5 - 40 g/l , expressed in Cr, of hexavalent chromium,
sulphate ions,
0.025 - 40 g/l of cupric ions,
0.1 - 5 g/l of ions selected from silver and the molybdic ion,
30 - 120 g/l of acetic acid,
and 0 - 6 g/l of formic acid.
[0009] It will be noted:
- that the quantities of chromium ions to be used are substantially the same as those
currently used in the chromium-plating baths,
that sulphate ions are used, as known,. the effect of these ions being, all other
things being equal, to improve the plating speed and also the adherence of the deposits
to the support, especially when said sulphate ions are added in the form of sodium
sulphate and sulphuric acid; said sulphate ions can also be added in the form of a
copper sulphate for example,
- and that cupric ions will be used in any case, these ions producing a chromium-plating of a very pleasant colour,
- that the shiny finish of the deposits is obtained due to the presence, with the cupric ions, of another ion selected from silver or the molybdic ion.
[0010] It is known that the use of silver has already been recommended, but the simultaneous use of copper ions and silver ions has never been described. When simultaneously using these two ions, the operation will take place, according to the invention, in the presence of preferably 0.025 - 0.2 g/l of copper ions and preferably
0.1 - 0.2 g/l of silver ions. It is inside these ranges of concentration that the best results are obtained.
[0011] The use of molybdic (
) ions with the copper ions, also produces according to the invention, a shiny black deposit with excellent anti-corrosion properties. The simultaneous use of copper ions and molybdic ions implies that fairly large quantities of these ions are used; the best results are obtained when operating with concentrations of between 15 and 30 g/l of copper ions and between 0.15 and 5 g/l of molybdic ions (
);
- that in all cases (whether using the copper-silver combination, or the copper-molybdate combination or the copper-silver-molybdate combination) the baths will contain acetic acid; the concentrations of that acid are variable and can be between about 30 and about 120 g/l. Advantageously, they will be between 65 and 90 g/l,
- that, finally, in the special case of a copper-molybdate bath, it is advantageous to use, in addition to acetic acid, a certain quantity of formic acid, the preferred concentration of this acid being about 3 g/1.
[0012] The conditions of operation, to obtain the chromium-plating according to the invention can be the conditions conventionally used for chromium-plating zinc or cadmium surfaces. These conditions are for example:
- a pH obviously acid, between 0.5 and 4,
- a temperature approaching the ambient temperature, i.e. between about 15 and 40°C,
- a period of contact of the piece to be treated and the bath, which can vary, depending on the Deaths, between 10 seconds and 10 minutes,
- with or without stirring.
[0013] The pieces to be treated according to the invention have of course a coating of black and shiny chromate which is very adherent and constitutes a good protection against corrosion.
[0014] It is however possible to further improve the properties of these coatings, and in particular in certain cases, the shiny finish of the deposit, and its efficiency as a protector against corrosion, by submitting the piece thus coated to a post-rinse with a phosphating bath. The usable phosphating baths are known per se and have already been recommended for certain applications, such as for example for polishing certain deposits. It is considered that in the case of the deposits according to the invention (copper-silver or copper-molybdates) such rinsing baths are specifically efficient and especially advantageous in that they improve the protective effect of the coatings. Amongst the baths usable, the following are given by way of example and non-restrictively:
- baths containing up to 5% by weight of phosphoric acid,
- baths'containing between 5 and 20 g/l of sodium hexametaphosphate,
- baths containing between 0.5 and 15 g/l of monosodium phosphate and between 0.5 and 15 g/l of monopotassium phosphate.
[0015] As indicated hereinabove, the exact composition of phosphating baths can vary to large extents, but very good results have always been obtained with the special baths indicated hereinabove.
[0016] The conditions required for this phosphating treatment are usually as follows:
- ambient temperature, i.e. between 15 and 40°C,
- duration between 10 seconds and 5 minutes, in general about one minute.
- with or without stirring.
Example 1
[0020] The piece to be protected was dipped into this bath, at ambient temperature (20-27°C) with air-stirring of the bath, and for a duration of 2 minutes.
[0021] The resulting shiny black piece was then rinsed and dipped into a phosphating bath containing 12 g/l of sodium hexametaphosphate, for one minute at 20°C, without stirring.
Example 2
(i) sulphate ions;
(ii) 5 to 40 g/l of hexavalent chromium;
(iii) 0.025 to 30 g/l of copper ions;
(iv) 0.1 to 5 g/1 of silver ions and/or molybdic (
) ions;
(v) 30 to 120 g/1 of acetic acid;
(vi) 0 to 6 g/l of formic acid.