Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2002-01-23
2003-05-13
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S254000, C148S275000, C148S279000, C427S309000, C427S329000
Reexamination Certificate
active
06562148
ABSTRACT:
DESCRIPTION
The invention relates to a method for the pretreatment of work pieces having a surface made of aluminium or aluminium alloys, for non-cutting shaping and/or the connection by welding or gluing to work pieces which have been pretreated in the same way or pre-coated in another way if appropriate, or to portions made of steel and/or zinc-coated and/or zinc-alloy-coated steel that have been pretreated if appropriate, as well as for a subsequent corrosion-preventing treatment by phosphating, by a chromium-free conversion treatment, by application of primer or by varnishing.
For the chemical surface treatment of metals, for example in preparation for the application of varnishes, glues and plastics, methods are known in which the metal surface is cleaned in the first stage, rinsed with water in the second stage and finally, in the third stage, wetted with an aqueous solution which forms chemical conversion coatings, the liquid film being dried off. As a result of this, there is formed on the metal a thin, non-metallic coating which, in the case of an appropriately selected composition of the treatment liquid and the reaction conditions, can decisively improve the surface quality. Thus, for example, coatings made of varnishes, glues and plastics, if appropriate in the form of foils, can be distinguished by a substantially greater adhesion and a considerably increased protection against corrosion when they are applied to metal which has been pretreated in this way.
Methods of the above-mentioned type work, for example, with an aqueous solution which contains hexavalent chromium, trivalent chromium, alkali ions and silicon dioxide in certain proportions and generates coatings for the electrical insulation, for the corrosion protection and as a wash primer for varnishes and suchlike (DE-AS 17 69 582).
As a result of the presence of hexavalent chromium, these methods have in common the disadvantage that, in particular, precautions are required when applying the coating agent and handling the coated metal.
In order to avoid the disadvantages linked with the use of solutions which contain hexavalent chromium, another category of method for the application of conversion coatings, especially for aluminium surfaces, provides a treatment with compositions based on fluoro-anions of zirconium and/or titanium (U.S. Pat. No. 4,146,670, FR-A-942 789, EP-A-106 389, EP-A-825 280).
Recently, the use of work pieces having surfaces made of aluminium or aluminium alloys has taken on increasing importance in the construction of vehicles. Fundamental reasons for this are the reduction in the vehicle weight, but also the favourable ability of such work pieces to be recycled. In this connection, it is generally usual to make vehicles up from a plurality of portions, which have, as a rule, been shaped beforehand in a non-cutting manner and consist of work pieces having surfaces made of aluminium or aluminium alloys, but also having surfaces made of steel and/or zinc-coated steel and/or zinc-alloy-coated steel. By far the most important forms of connection or joining are welding or gluing.
A corrosion-preventing treatment, which, depending on the nature of the connected portions, can consist of a phosphating treatment, a chromium-free conversion treatment, an application of primer or a varnishing, usually follows the connection of the portions.
In order that a perfect connection of the portions having surfaces made of aluminium or aluminium alloys to each other or to portions made of steel and/or zinc-coated or zinc-alloy-coated steel is guaranteed, it is necessary to ensure that the surfaces of the aluminium or aluminium alloy are free from aluminium oxide or oxides of any alloying constituents of the aluminium that may be present. A pickling treatment which presents itself for this purpose does not achieve the objective, because the aluminium surface becomes coated with a new oxide layer after a very short time.
In order to solve the problem outlined above, the method of EP-B-700 452 provides that, for pretreatment, surfaces made of aluminium or its alloys are, before a second permanent corrosion-preventing treatment, brought into contact with an aqueous solution which contains complex fluorides of the elements boron, silicon, titanium, zirconium or hafnium individually or in a mixture with each other in concentrations of the fluoro-anions of 100 to 4000 mg/l in total and has a pH-value of 0.3 to 3.5. Between the pretreatment and the permanent corrosion-preventing conversion treatment, the portions made of aluminium or its alloys are subjected to a non-cutting and/or cutting shaping process and/or are connected to each other or to portions made of steel and/or zinc-coated and/or zinc-alloy coated steel by gluing and/or welding. The application of the solution, which contains polymers of a certain nature if appropriate, can take place by spraying, immersion or with the no-rinse method, in which case, in the case of the no-rinse method, the wet-film amount is to be between 2 and 10, preferably between 4 and 6 ml/m
2
metal surface. Regardless of the way in which the solution is applied, it is advantageous to dry at temperatures between 40 and 85° C. For the purpose of cleaning, the portions made of aluminium or its alloys are, before the first conversion treatment, cleaned in an acidic or alkaline environment; preferably, further cleaning steps and intermediate rinsings with water and/or with activating rinsing baths take place before the permanent corrosion-preventing treatment.
From the particulars of the concentrations of fluoro-anions in the solutions to be applied on the one hand and the wet-film amount on the other hand, there results, for the case of the application of a solution containing a fluoro-titanate, a layer of 0.06 to 11730 mg/m
2
, preferably 0.12 to 7.04 mg/m
2
(in each case given as titanium metal), and, for the case of the application of a solution containing a fluoro-zirconate, a layer of 0.09 to 17780 mg/m
2
, preferably 0.18 to 7.04 mg/m
2
(in each case given as zirconium metal).
The tests which preceded the conception of the present invention have shown that a plurality of the development possibilities described in EP-B-700 452 do not lead to many advantageous results, in particular with respect to the temporary corrosion protection achieved with the first chemical conversion treatment and the volume resistance which is important for the production of weld joints.
The object of the invention is to make available a method for the pretreatment of work pieces having a surface made of aluminium or aluminium alloys, for non-cutting shaping and/or the connection by welding or gluing to work pieces which have been pretreated in the same way or pre-coated in another way if appropriate, or to portions made of steel and/or zinc-coated and/or zinc-alloy-coated steel and also a subsequent permanent corrosion-preventing treatment by phosphating, by a chromium-free conversion treatment, by application of primer or by varnishing, which method regularly leads to work pieces having a sufficiently low volume resistance and at the same time good adhesion shear-tension values and good temporary corrosion-protection values.
The object is achieved as a result of the fact that the method of the type mentioned in the introduction is developed in accordance with the invention in such a way that the work pieces are
a) pickled with an aqueous, acidic solution containing a mineral acid, by immersion or spraying,
b) rinsed with water,
c) brought into contact with an aqueous, acidic solution which is chromium-free and polymer-free and contains Ti and Zr as complex fluorides in a weight ratio of Ti:Zr of 2:1 to 1:2, in such a way that, after the subsequent drying off, a layer weight of 2 to 15 mg/m
2
(calculated as Ti/Zr metal) results,
wherein, when solutions are brought into contact with the work piece, solutions are used which contain 300 to 700 mg/l fluoride (calculated as fluoro-anion) and have a pH-value of 4.0 to 5.0 in the case of application by immersion or spraying, and contain 5000 to 1500
Bittner Klaus
Möller Peter
Wendel Thomas
Wietzoreck Hardy
Chemetall GmbH
Fulbright & Jaworski L.L.P.
Oltmans Andrew L.
Sheehan John
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