Patent Application
20050121113
The invention relates to a corrosion protection method for application in connection with the production of finished metallic components from organically precoated metal sheets. In the course of such production, areas of uncoated metal which inevitably come about in the course of cutting, forming and joining the precoated metal sheets are provided with a corrosion protection coat which serves at the same time as a tie coat for the subsequent finish.
Components joined from metal sheets, such as vehicle bodies, housings of domestic appliances or pieces of metal furniture, for example, may be put together from metal sheets which at that point do not have a coating providing permanent corrosion protection. In a process sequence which embraces a plurality of stages a coating providing permanent corrosion protection, composed of a conversion coat and a finish coat, can be produced after the metallic components have been assembled. One known example of this is the process sequence of phosphating and finishing, such as is customary in auto making, for example. In this sequence the phosphating itself is only one step in a treatment sequence which in general embraces not only cleaning and rinsing stages but also pre-phosphating activation, the phosphating itself, and, frequently, a post-phosphating passivation. This is followed by a plurality of finishing stages. Pretreatment prior to finishing, then, requires a plurality of treatment steps, which in turn necessitate a correspondingly extensive and therefore expensive pretreatment plant. Additionally phosphating gives rise to wastes which contain heavy metal and require disposal, at great expense.
Apart from phosphating there are other methods known of producing a coat known as a conversion coat, which protects the underlying metal from corrosion and constitutes a tie coat for a subsequent finishing coat. A “conversion coat” is a coat on a metal surface which is formed by “conversion treatment”, involving exposure to a “conversion solution”, and which contains not only elements from the metal surface but also from the conversion solution. Typical examples are phosphate coats or chromating coats. Besides phosphating and chromating methods there are further methods known of conversion treatment, using for example conversion solutions based on complex fluorides of boron, silicon, titanium or zirconium. Mostly these complex fluorides are employed together with organic polymers. Examples of conversion treatments of this kind are specified in DE-A-101 31 723 and the references it cites. None of these alternative methods, however, has yet been able to displace phosphating as a pre-finishing pretreatment in auto making.
In principle it would be more advantageous from an economic and environmental standpoint to produce metallic components from material which had already been precoated by the manufacturer of the metal strips (or coils) and which following assembly needed only to be cleaned and finished. Waste associated with the pretreatment would then arise in a centralized fashion, at the premises of the metal strip producers, and not in a widely dispersed fashion, at the premises of the metal strip processors. Accordingly there are already precoated metal strips being offered on the market. These strips may be prephosphated, i.e., may carry a phosphate coat but no further coating based on organic polymers. In the automobile and domestic appliance industries the metal strips processed are increasingly coming to include those which have already been provided with a corrosion protection coat at the premises of the strip manufacturer. Materials of this kind are known, for example, under the name Granocoat®, Durasteel®, Bonazinc® and Durazinc®. They carry a thin organic coating over a conversion coat, a chromating or phosphating coat for example. The organic coating is composed of polymer systems such as epoxy or polyurethane resins, polyamides and polyacrylates, for example. Solid additives such as silicas, zinc dust and carbon black enhance the corrosion protection and make it possible by virtue of their electrical conductivity to carry out electrical welding and electrolytic finishing of the metal parts that are coated with coats having a thickness of from about 0.3 to about 10 μm, preferably up to about 5 μm. The substrate materials are generally coated in a two-stage operation in which first of all the inorganic conversion coat is produced and subsequently in a second treatment stage the organic polymer film is applied. Further information on this can be found in DE-A-100 22 075 and the references it cites.
Metal sheets provided in the strip process with a coating based on organic polymers are therefore already being used in part in the construction of vehicle bodies, domestic appliances and pieces of furniture. In this context it is in auto making that the most stringent requirements are imposed in terms of corrosion protection and adhesion of a subsequently applied finish material, since vehicles are subject to the greatest corrosion stresses. At the present time there are no vehicle bodies produced exclusively from organically precoated metal sheets. Instead this material is used at best together with nonprecoated metal sheets to construct the vehicle bodies. The assembled bodies therefore at present still pass through the usual pretreatment process prior to finishing; that is, they are subjected to the complex operating sequence of phosphating.
The phosphating process could in principle be replaced by a less complicated pretreatment process if the vehicle bodies were produced exclusively from organically precoated metal substrate. For that purpose, however, it is necessary to solve the problem that the assembly of bodies from organically precoated metal sheets inevitably gives rise to areas in which the organic coating is damaged or completely missing. This is the case, for example, at cut edges, at weld spots and in abraded areas.
For reasons of improved corrosion protection effect it is common in vehicle construction for use to be made of organically precoated metal substrates of the kind where the metal substrate used is electrolytically galvanized or hot-dip-galvanized steel. With organically coated metal substrates of this kind, however, the aforementioned areas with a damaged organic coat are particularly difficult to treat, since they differ in terms of their electrochemical potentials and their chemical reactivity from the customary metal surfaces. In such damaged areas there are generally fractions both of the steel substrate (i.e., iron) and of the zinc coating alone. There may be a high local area ratio of steel (iron) to zinc, such as a ratio of >9:1, for example. This is the case in particular for cut edges, which represent a cross section through the coated steel substrate. The corrosion conditions are different from the other conditions on the homogeneous surface at these boundary regions which combine zinc and iron. Depending on the local ratio of zinc to iron in the exposed metal areas, a different electrochemical potential is established between the potentials of zinc and iron. Furthermore, machining of the bodies gives rise to abraded areas which exhibit special conditions and therefore particular electrochemical potentials. This is because the abrading operation gives rise to an activated interface of steel (iron) with finely divided reactive zinc.
The object addressed by the invention, as part of an operation for producing finished metallic components which have been put together from metal sheets precoated with organic polymers, is to provide a simpler method than phosphating that allows corrosion protection and finish adhesion to be produced at the aforementioned sites of damage.
The present invention in a first aspect provides a method of producing a finished component comprising metal parts made of galvanized steel, said method comprising
All metal parts of the component are therefore to be composed of organically precoated galvanized steel. In addition to these metal parts, however, the component may further include constituents made of plastic, such as may be the case, for example, in auto making. For producing, say, a vehicle body, therefore, the metallic components of organically precoated galvanized steel may be joined to parts of plastic.
The expression “galvanized steel” embraces steels galvanized by the hot-dip method and steels galvanized electrolytically. It further embraces alloy-galvanized steels, where the coating may be composed, for example, of a zinc-nickel alloy or a zinc-aluminum alloy. The steels may have been heat-treated after galvanizing, such that an iron-zinc alloy was formed at the boundary layer between steel and zinc.
The joining of the metal sheets to form the component in substep a) may take place in accordance with the usual techniques known in the prior art, such as by adhesive bonding, flanging, riveting, crimping and/or welding, in particular by electric welding. In addition to cutting and/or punching in substep a), joining by means of welding, owing to the associated damage to the coating based on organic polymers, leads to there being further sites on the component that are not covered by the coating based on organic polymers. These sites too are passivated in substep c), in the same way as the metal areas which come about as a result of abrading. The method of the invention is especially suitable for producing components comprising organically precoated metal sheets which have a coating based on organic polymers with a thickness in the range from 1 to 10 μm, the coating comprising electrically conductive particles as well as the organic polymers. On the basis of these features of the organic coating the components can be joined by electric welding. Examples of coatings of this kind are contained in DE-A-197 48 764, DE-A-199 51 113, DE-A-100 22-075 and also in the references each of them cites. As mentioned as an introduction, metal strips (or coils) having such coatings are available commercially under a variety of trade names.
The passivating coat produced in substep c) should therefore not constitute a conventional zinc phosphate coat, since the present objective is to use a process sequence which is shorter and hence more economic than zinc phosphating. A zinc phosphate layer is not formed if the treatment solution does not simultaneously contain at least 0.3 g/l zinc ions and at least 3 g/l phosphate ions (as phosphoric acid or any protolysis stage thereof).
In substep c) the joined component can be contacted in a variety of ways with the acidic aqueous treatment solution: for example, by being immersed in the treatment solution or being sprayed with the treatment solution. After this step the water can be rinsed off, but need not necessarily be so. In other words, the method can be employed as a rinse or as a no-rinse process.
The treatment of substep c) is not a passivation of a prior principal conversion-coat-forming operation; instead, it is the only treatment step after the components have been assembled that produces a passivating coat on the bare metal sites.
The process sequence of the invention can be employed in particular in connection with the production of vehicle bodies, domestic appliances or pieces of furniture or a part of each thereof.
The aqueous treatment solution in substep c) preferably has a pH of at least 2, in particular at least 2.5, to not more than 5, in particular to not more than 4. At pH values which are lower than this there is increasingly a risk of excessive pickling attack and of damage to the coating based on organic polymers. At pH values of more than 5 the pickling attack becomes increasingly so weak that the passivating coat which forms is inadequate. In actual practice it will be appreciated that the transitions are in each case fluid.
In substep c) it is possible to use chromium-free acidic aqueous treatment solutions which are known in the prior art for the large-area treatment of uncoated metal parts or metal strips. It is preferred to use a treatment solution containing in total at least 0.01 g/l, preferably at least 0.025 g/l, and up to 10 g/l, preferably up to 1 g/l, in particular up to 0.5 g/l, Ti and/or Zr and/or Si ions and also containing at least an amount of fluoride such that the atomic ratio of Ti to F and/or of Zr to F and/or of Si to F is in the range from 1:1 to 1:6, and which additionally contains at least 0.005 g/l, preferably at least 0.01 g/l, and up to 20 g/l, preferably up to 1 g/l, organic polymers. The stated Ti, Zr and/or Si ions may be used completely in the form of hexafluoro complexes such as, for example, the hexafluoro acids, or their salts which are water-soluble within the stated concentration range, such as the sodium salts, for example. In that case the atomic ratio is 1:6. It is also possible, however, to use complex compounds in each of which there are less than six fluoride ions joined to the central elements Ti, Zr or Si. These compounds may be formed automatically in the treatment solution when not only hexafluoro complexes of at least one of the central elements Ti, Zr or Si but also at least one further compound of one of these central elements are added to said treatment solution.
Examples of suitable such further compounds include nitrates, carbonates, hydroxides and/or oxides of the same central element or of another of the three stated central elements. The treatment solution, for example, may comprise hexafluorozirconate ions and also (preferably colloidal) silica (SiO2) or reaction products thereof. Unreacted silica may be in suspension in the treatment solution. A treatment solution of this kind can also be obtained by using hydrofluoric acid or its salts (acidic salts where appropriate) together with such compounds of Ti, Zr and/or Si as are able to form fluoro complexes therewith. Examples are the aforementioned nitrates, carbonates, hydroxides and/or oxides. It is preferred in total to use an amount of Ti, Zr and/or Si as central metal, and an amount of fluoride, which are such that the atomic ratio of central metal to fluoride is less than or equal to 1:2, in particular less than or equal to 1:3. The atomic ratio may also be less than 1:6 if the treatment solution contains a greater amount of fluoride, in the form for example of hydrofluoric acid or its salts, than is required stoichiometrically to form the hexafluoro complexes of the central metals Ti, Zr and/or Si. By way of example the atomic ratio may be as low as 1:12 or 1:18 or even lower, if a corresponding excess of fluoride is used, in other words two or three times or even more times the amount required to form the hexafluoro complexes in full.
In this context it is possible to use treatment solutions comprising combinations of ingredients that are known in the prior art: for example, in accordance with U.S. Pat. No. 5,129,967, a treatment solution which comprises at least the following components in water:
Preference, however, is given to treatment solutions in which the organic polymers are selected from homopolymers and copolymers of vinylpyrrolidone. Treatment solutions of this kind are described in DE-A-100 05 113 and DE-A-101 31 723.
Where, therefore, a treatment solution comprising vinylpyrrolidone copolymers is used in the method of the invention, these copolymers may include, further to vinylpyrrolidone, one or more other monomers. They may therefore be present, for example, as copolymers of 2 components or as copolymers of 3 components (i.e., terpolymers). In addition it is possible to use mixtures of homopolymers and two-component copolymers, homopolymers and terpolymers or two-component copolymers and terpolymers.
Examples of suitable homopolymers or copolymers of vinylpyrrolidone include the polymers listed in Table 1 and/or polymers of the monomers indicated therein. Copolymers of vinylpyrrolidone with monomers containing caprolactam groups or imidazole groups are particularly preferred.
*International Specialty Products
The preferred treatment solutions described above preferably have a temperature in the range from 20 to 45° C., in particular from 30 to 40° C. This treatment solution is contacted with the cleaned joined component for a time in the range from 1 to 5 minutes, in particular from 2 to 3 minutes.
The treatment solutions to be used ought by definition to be free from chromium. It is further preferred in this context for the acidic aqueous treatment solution to contain no transition-group metals (transition metals) other than metals from transition group 4 of the Periodic Table (in the form, for example, of complex fluorides of Ti and/or Zr). This simplifies the treatment of wastewaters obtained.
In substep e) the metallic component pretreated in substep c)—with or without intermediate rinsing as substep d)—can be coated with a finish material which is customary for the contemplated use. This finish material may be selected, for example, from a dip-coating material, an electrodeposition material or a powder coating material.
In a second aspect the present invention provides a finished component comprising metal parts made of galvanized steel which is obtainable by the method described above. As elucidated above this means that all of the metal parts of the component are composed of organically precoated galvanized steel. In addition to these metal parts, however, the component may also include constituents made of plastic, such as may be the case in auto making, for example.
Through the method of the invention it is possible to obtain finished metallic components such as, for example, vehicle bodies, domestic appliances, furniture or parts of each thereof that meet all of the present requirements as regards appearance, corrosion resistance and paint adhesion. The exclusive use of organically precoated raw material allows the necessary chemical treatment to be shortened significantly at the premises of the component manufacturer. This, for the component manufacturer, provides economic and environmental advantages, since a less complicated pretreatment plant is sufficient and since the volume of wastewater loaded with chemicals is smaller.
(The Granocoat® products specified in the examples are coating systems for galvanized steel based on organic polymers and conductivity pigments, as elucidated in more detail in the description above. These products are described in patent applications DE-A-100 22 075 (Granocoat® ZE) and DE-A-100 22 075 (Granocoat® S).)
Operating sequence (dip application):
Test: SAE J 2334 test 80 rounds
Operating sequence (dip application):
Test: SAE J 2334 test 80 rounds
Operating sequence (dip application):
Test: SAE J 2334 test 80 rounds
Operating sequence (dip application):
Test: SAE J 2334 test 80 rounds
Operating sequence (dip application):
Test: SAE J 2334 test 80 rounds
Operating sequence (dip application):
Test: SAE J 2334 test 80 rounds
◯ no corrosion products
□ few corrosion spots
X red rust products
The test results show that with the shorter process sequence of the invention the results achieved at least match those achieved with a zinc phosphating process. In terms of tendency the results in accordance with the method of the invention are in fact better than those obtained with a zinc phosphating process.
| Number | Date | Country | Kind |
|---|---|---|---|
| 103 53 149.1 | Nov 2003 | DE | national |
