Cleaning compositions for solid surfaces – auxiliary compositions – Cleaning compositions or processes of preparing – For cleaning a specific substrate or removing a specific...
Reexamination Certificate
1999-06-25
2001-02-27
Gupta, Yogendra (Department: 1751)
Cleaning compositions for solid surfaces, auxiliary compositions
Cleaning compositions or processes of preparing
For cleaning a specific substrate or removing a specific...
C510S245000, C510S257000, C510S259000, C510S263000, C510S269000, C510S270000
Reexamination Certificate
active
06194369
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an aqueous preparation for the pickling and activation of aluminum-steel composites prior to electroless dip tinning. Examples of aluminum-steel composites are sliding bearings, bushes, wear disks, dry sliding bearings, etc., for pumps, motors and gear boxes. After pickling and activation, uniform, particularly strongly adhering tin coatings are obtained on the aluminum and steel surfaces of the substrate in a subsequent dip tinning process.
2. Prior Art
EP-A-0 278 752 discloses the tinning of substrates of aluminum alloys by an exchange method using acidic tin salt electrolytes after a pretreatment comprising degreasing and pickling.
Prior art systems comprising degreasing, pickling and tinning give unsatisfactory results when employed on substrates comprising aluminum-steel composites:
the cleaning, pickling and tinning of the steel surfaces and the aluminum surfaces is non-uniform,
the deposition of tin on the steel surfaces occurs non-uniformly and does not give a closed surface,
the adhesion of the tin layer deposited on the aluminum is unsatisfactory.
SUMMARY OF THE INVENTION
The present invention therefore addresses the industrial problem of optimizing the pretreatment of an aluminum-steel composite. In particular, it is an object of the present invention to produce uniform, strongly adhering tin coatings both on the aluminum surfaces and on the steel surfaces of the substrate when using known dip tinning baths.
The abovementioned problem is solved according to the present invention by pretreatment of the substrate using a new pickling/activation solution comprising the following components and additives:
sulfuric acid for pickling steel surfaces,
hexafluorosilicic acid for pickling aluminum surfaces,
surfactants for uniformly wetting the substrate surfaces with the solution,
transition metal cations for optimizing the pickling and activation of the aluminum surface,
nitrate and/or nitrite ions for optimizing the pickling and activation of the aluminum surface.
DETAILED DESCRIPTION OF THE INVENTION
In experiments on the pretreatment by pickling of aluminum-steel composites, applicants unexpectedly determined that a pickling/activation solution comprising sulfuric acid and hexafluorosilicic acid (with the contents of the two mineral acids in each case being matched to one of the two substrate alloys) was significantly more effective than other pretreatments. The sulfuric acid content in the pickling/activation solution of the present invention is preferably from 50 to 150 g/l. The hexafluorosilicic acid content in the inventive pickling/activation solution is preferably from 5 to 25 g/l.
Dilute sulfuric acid is suitable for pickling ferrous alloys but, at temperatures of up to 70° C. and dipping times of a few minutes, it does not attack aluminum to an appreciable extent. Hexafluorosilicic acid, on the other hand, cleans and activates aluminum alloys without significantly attacking iron surfaces. The combination of the two acids fulfills the requirements of a complex substrate structure comprising two alloys having widely different chemical and physical properties. Examples of aluminum-steel composites are sliding bearings, bushes, wear disks, dry sliding bearings, etc., which can be used for pumps, motors and gear boxes.
However, in order to achieve a uniform and strongly adhering tin deposit in the subsequent acidic, electroless dip tinning bath, further additives have to be added to this pickling/activation solution.
To achieve further optimization of the pickling action and, in particular, to achieve additional activation of the aluminum surfaces of the substrate, transition metal cations such as manganese(II), nickel(II) and iron(III) ions have to be added to the acid mixture in concentrations of from 0.05 to 1% by weight. A particularly advantageous effect is obtained using manganese(II) ions in a concentration of 0.1% by weight at a pickling temperature of 40° C. and a pickling time of 5 minutes.
Furthermore, the addition of nitrate and/or nitrite ions has been found to have a positive effect. Additions of alkali metal nitrate and alkali metal nitrite salts such as sodium nitrate, potassium nitrate, sodium nitrite or potassium nitrite in concentrations of from 0.05 to 3% by weight, with an addition of 0.5% by weight of potassium nitrate being optimum, significantly improve the pickling and cleaning results of the pickling/activation solution.
In addition, to achieve a uniform pickling/activation effect, uniform wetting of the surface is necessary. This is achieved in the present invention by addition of wetting agents to the pickling/activation solution. Suitable wetting agents employed in the present invention are essentially all surfactants which are capable of wetting the substrate, yet have sufficient chemical stability in the pickling/activation electrolyte. Particularly suitable wetting agents are those employed in dip tinning baths such as disclosed in EP-A-0 278 752. One example of a wetting agent is polyoxyethylene ether surfactants. The amount of wetting agents is preferably from 1 to 20 g/l.
The present invention accordingly provides aqueous pickling/activation solutions for the pretreatment of aluminum-steel composites prior to tinning in an acidic dip tinning bath, which solutions comprise sulfuric acid, hexafluorosilicic acid, at least one wetting agent, at least one transition metal cation and nitrate and/or nitrite ions. The inventive pickling/activation solutions prepare the substrate surface in such a way that a uniform, strongly adhering tin coating is subsequently obtained.
The amount of transition metal cations, which are, in particular, selected from Groups I, II and V to VIII of the Periodic Table of the Elements, is preferably from 0.05 to 1% by weight. The amount of nitrite ions is preferably from 0.05 to 3% by weight, while the amount of nitrate ions is preferably in the same range.
A further embodiment of the invention encompasses a process for the pickling and activation of aluminum-steel composites, which comprises bringing pickling/activation solutions of the present invention into contact with the composite for a time period of from 1 to 9 minutes at a temperature in the range from 15° to 70° C.
REFERENCES:
patent: 4170525 (1979-10-01), Kirman et al.
patent: 4192722 (1980-03-01), Schardein et al.
patent: 6068001 (2000-05-01), Pedrazzini et al.
patent: 0 278 752 A1 (1988-08-01), None
Eberhardt Jan
Guhl Dieter
Honselmann Frank
Gupta Yogendra
Petruncio John M
Scully Scott Murphy & Presser
Th. Goldschmidt Ag
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