Patent Application
20050145303
A. Field of the Invention
The present invention relates to a multiple step, low temperature, low sludge-producing, low phosphate, corrosion-prevention pretreatment process suitable for simultaneous processing of steel, zinc, and aluminum substrates.
B. Description of the Related Art
For many reasons, such as weight, rigidity or recyclability, aluminum is increasingly used in vehicle construction. As used herein, the expression “aluminum” refers not only to pure aluminum but also to aluminum alloys whose main component is aluminum. Examples of commonly used alloying elements are silicon, magnesium, copper, manganese, chromium and nickel. The total proportion by weight of these alloying elements in the alloy normally do not exceed 10%. Whereas engine and gear parts, wheels, seat frames, etc. already contain large amounts of aluminum, the use of aluminum in bodywork construction is presently still restricted to parts such as hoods, rear trunk lids, inner door parts and various small parts as well as truck cabins, side walls of transporters or attachments to minivans. Overall, worldwide less than 5% of the metal surface of automobile bodies is made of aluminum. The increased use of aluminum in this sector is being intensively investigated by the aluminum and automobile industries.
Today many automobile parts are made from a variety of materials, including but not limited to aluminum, magnesium, steel, and zinc. Preferably, such prefabricated metal components are provided with a phosphate coating. The aim of phosphating metals is to produce firmly adhering metal phosphate layers on the metal surface that improve the corrosion resistance, and, in conjunction with paints or other organic coatings, contribute to a substantial improvement of the coating adhesion and resistance to creepage under corrosive stress.
Such phosphating processes have been known for a long time. For the pretreatment before painting, especially before electro-dipcoating, low zinc phosphating processes are used. It is well known that zinc phosphate conversion coatings, particularly those of the “low zinc” type, are capable of producing excellent corrosion-protective undercoatings for subsequent painting. It has been generally regarded in the prior art that two important characteristics of a “low zinc” phosphating liquid composition are a phosphate concentration of at least 5 grams per liter of composition, this unit of concentration being hereinafter usually abbreviated as “g/l”, more preferably at least 10 g/l, and a weight ratio of phosphate-to-zinc concentrations that is at least 10:1. A basic parameter in these low zinc phosphating baths is the weight ratio of phosphate ions to zinc ions, which is normally above 8 and may reach values of up to 30.
Generally only one phosphating installation is available in an automobile factory. Therefore, it is necessary to be able to carry out phosphating operations at such factories in a mixed operation on automobile components made entirely of steel, entirely of aluminum, entirely of zinc, or a combination of steel, aluminum, and zinc in varying proportions.
However, in practice it has been found that in the joint phosphating of surfaces of aluminum, zinc, steel and/or galvanized steel substrates, technical compromises have to be made regarding the composition of the phosphating baths. Aluminum ions released from the aluminum surface by the etching and pickling action act as a bath poison for the phosphating solution and interfere in the formation of zinc phosphate crystals on iron (steel) surfaces. The dissolved aluminum must therefore be precipitated or masked by appropriate measures. For this purpose free or complex-bound fluoride ions are normally added to the phosphating baths.
The fluoride ions mask the aluminum ions by complex formation and/or precipitate these ions as hexafluoroaluminates of sodium and/or potassium if the solubility products of the corresponding salts are exceeded. Furthermore, free fluoride ions usually lead to an increased etching attack on the aluminum surfaces, with the result that a more or less closed and sealed zinc phosphate layer can form on the latter.
The joint phosphating of aluminum structural portions with those of zinc, steel and/or galvanized steel thus has the technical disadvantage that the phosphating baths have to be very accurately monitored as regards their fluoride content. This increases the control and monitoring work involved and may require stocking and metering of fluoride-containing solutions as separate replenishment solutions. Also, the precipitated hexafluoroaluminate salts increase the amount of phosphating sludge and raise the cost of its removal and disposal.
It is well known that zinc phosphate conversion coating processes produce a solid byproduct called “sludge” in addition to the desired solid conversion coating on the metal being phosphated. In order to continue using a liquid conversion coating composition, sludge eventually has to be removed from the bath and disposed of in an approved landfill site. Sludge reduction is desirable because the number of available landfill sites for disposal of this byproduct is dwindling and known recycling alternatives through chemical treatment are not economical at this time.
A typical zinc phosphating bath includes phosphate ions, divalent metal ions, hydrogen ions, and an oxidizing compound such as nitrite or chlorate as the process accelerator. The mechanism of the reaction involves acid attack on the substrate metal, iron (steel or zinc) in this instance, at micro anodes and deposition of phosphate crystals at micro cathodes. It also involves the liberation of hydrogen and the formation of phosphate sludge. Changes in the accelerator can affect the amount of sludge formed. For example, the lower the amount of nitrite accelerator, the greater the amount of sludge formed during the process. U.S. Pat. No. 5,900,073, discloses how varying the process conditions of a phosphate bath effects the amount of sludge produced by the bath.
It is also desirable to provide high-quality, phosphate coatings at a lower operating temperature than has been generally used heretofore to obtain highly corrosion-resistant undercoats for paint. Such lower temperatures would reduce the energy costs associated with production of such coatings.
Simultaneous phosphating of composite metal structures that contain aluminum and/or aluminum alloy in addition to steel and/or galvanized steel portions has been proposed in the art. For example, the in International Patent Application No. WO 99/12661 proposes such a process for simultaneous phosphating of composite structures made of steel and aluminum. Despite rectifying the need to make technical compromises in the simultaneous phosphating process, WO 99/12661 requires pretreating such composite structures with a zinc phosphate bath containing high levels (5 to 40 g/l) of phosphate and low levels (0.01 to 0.2 g/l) of nitrite accelerator, and at a high temperature of 20° C. to about 65° C.
German Patent Application No. DE 197 35 314 also sets forth a process for simultaneous phosphating of composite structures. Like WO 99/12661, however, DE 197 35 314 requires pretreating such composite structures with a zinc phosphate bath containing high levels (14 g/l) of phosphate, and at an even higher temperature of 40° C. to 70° C. DE 197 35 314 fails to discuss use of a nitrite as an accelerator.
Thus, both processes set forth in WO 99/12661 and DE 197 35 314 create excessive amounts of undesirable sludge due to the high phosphate and/or low nitrite content of the phosphate bath, and operate at undesirably high temperatures which increases the energy costs of the process.
Accordingly there exists a need for pretreatment processes for composite structural parts or substrates, for example automobile bodies that contain besides aluminum portions, zinc portions, and/or steel and/or galvanized steel portions, that reduces the sludge produced by the process, the temperature at which the process operates, and the need for monitoring the process. The result of the overall pretreatment should be the formation of a conversion layer on all exposed metal surfaces that is suitable as a corrosion-preventing paint substrate, especially before a cathodic electro-dipcoating.
The present invention solves the needs of the related art by providing a process for the chemical pretreatment, before an organic coating, of composite metal structures that contain aluminum or aluminum alloy portions together with zinc or zinc alloy portions, and steel, galvanized steel and/or alloy-galvanized steel portions, such that the phosphate level and the operating temperature of the zinc phosphate bath is reduced, and the nitrite accelerator level of the bath is increased. The process of the present invention reduces the sludge produced by the process, the temperature at which the process operates, and the need for monitoring the process. Furthermore, the pretreatment process of the present invention forms a conversion layer on all exposed metal surfaces that is suitable as a corrosion-preventing paint substrate, especially before a cathodic electro-dipcoating.
In accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a process for chemical pretreatment, before an organic coating, of a composite metal structure that contains at least one aluminum or aluminum alloy portion, at least one zinc or zinc alloy portion, and at least one steel, galvanized steel or alloy-galvanized steel portion, the process comprising: (I) treating the composite metal structure with a zinc phosphating solution having less than 5 g/l of phosphate ions and more than 0.2 g/l of nitrite ions, the zinc phosphating solution forming a surface-covering crystalline zinc phosphate layer having a coating weight in the range from 0.5 to 5 grams per meter squared (hereinafter abbreviated as g/m2) on the zinc or zinc alloy portions, and the steel, and galvanized and/or alloy-galvanized steel portions, but without forming a zinc phosphate layer on the aluminum or aluminum alloy portions; and (II) subsequently, with or without intermediate rinsing with water, contacting the composite metal structure with a treatment solution that does not dissolve more than 60% of the crystalline zinc phosphate layer formed on the zinc or zinc alloy portions, and the steel, galvanized and/or alloy-galvanized steel portions, but does produce a conversion layer on the aluminum or aluminum alloy portions.
The stipulation that no zinc phosphate layer is to be formed on the aluminum portions in the treatment step (I) is to be understood to mean that no closed and sealed crystalline layer is formed and that the mass per unit area of any deposited zinc phosphate does not exceed 0.5 g/m2. In order to satisfy this condition, the phosphating baths may be arbitrarily formulated as long as specific conditions for the fluoride concentration are observed. Such fluoride concentration conditions may be found in European Patent No. 0 452 638 B1. According to this disclosure the concentration of free fluoride ions, measured in g/l, should satisfy the condition that, at a specific temperature T (in ° C.), it lies above a value of 8/T. Since, however, within the scope of the present invention no zinc phosphate layer should be formed on aluminum in the phosphating step (I), at a specific temperature T (in ° C.) the concentration of free fluoride ions (in g/l) in the phosphating solution must be below 8/T.
The following detailed description of the invention does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof.
Except in the claims and the operating examples, or where otherwise expressly indicated to the contrary, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description and claims, unless expressly stated to the contrary: percent, “parts of”, and ratio values are by weight; the term “polymer” includes “oligomer”, “copolymer”, “terpolymer”, and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; specification of materials in ionic form implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole, and any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to the objects of the invention; and the term “mole” and its variations may be applied to ionic, chemically unstable neutral, or any other chemical species, whether actual or hypothetical, that is specified by the type(s) of atoms present and the number of each type of atom included in the unit defined, as well as to substances with well defined neutral molecules.
As used herein, the term “aluminum” refers to pure aluminum as well as aluminum alloy, the term “zinc” refers to pure zinc and zinc alloy, and the term “steel” refers to pure steel, galvanized steel, and alloy-galvanized steel.
The present invention is drawn broadly to a multiple step, low temperature, low sludging, low phosphate, corrosion-prevention pretreatment process for simultaneous processing of substrates containing steel, zinc, and aluminum. Generally, the process is a two-step pretreatment process having conventional intermediate rinsing and cleansing steps. The first pretreatment step produces little or no zinc phosphate coating on the aluminum, while forming complete, uniform zinc phosphate coatings on the steel and zinc. The second pretreatment step imparts corrosion protection for the aluminum, while not reducing, and preferably improving, the corrosion prevention properties of the zinc phosphate coating applied to the zinc and steel. In the second step, the nature and concentration of the solutions should be chosen so that on the one hand a layer is reliably formed on the aluminum surfaces, but on the other hand the crystalline zinc phosphate layers formed on the steel and zinc surfaces are not excessively damaged.
The process is particularly intended for use in automobile manufacturing. In automobile manufacturing, car bodies or car body parts that contain structural portions of aluminum and/or its alloys in addition to structural portions of zinc and/or its alloys, and steel and/or galvanized steel are subjected to a conversion chemical pretreatment before they are painted. In this connection a cathodic electro-dipcoating is conventionally used at the present time as the first painting stage. The process according to the invention is particularly suitable as a pretreatment for this stage.
A. The First Process Step
The zinc phosphate solution used in the first pretreatment step preferably has a low phosphate ion (PO4−3) concentration, preferably less than 5 g/l of phosphate ions, and a high nitrite ion concentration, preferably greater than 0.2 g/l of nitrite ions, and more preferably greater than 0.3 g/l of nitrite ions. The low phosphate, high nitrite conditions reduce the amount of sludge produced by the operation of the solution. Furthermore, the zinc phosphate solution preferably operates at low temperatures, preferably between 20° C. and 40° C., and more preferably between 30° C. and 35° C. The zinc phosphate solution may comprise many components and be formulated in at least three different manners, as long as the low phosphate ion concentration, high nitrite ion concentration, and low temperature conditions are met.
1. First Type of Zinc Phosphate Solution
In accordance with one aspect of the present invention, the zinc phosphate solution may comprise components as set forth in International Patent Application No. WO 01/32953, the disclosure of which is herein incorporated by reference in its entirety, except where inconsistent with the low phosphate, high nitrite, and low temperature conditions.
In accordance with WO 01/32953 and the present invention, the first type of zinc phosphate solution preferably consists essentially of, or more preferably consists of, water and the following components:
Optionally, the first zinc phosphate solution may also include one or more of the following components:
2. Second Type of Zinc Phosphate Solution
In accordance with a second aspect of the present invention, the zinc phosphate solution may comprise components as set forth in U.S. Provisional Patent Application Ser. No. 60/274,106, the disclosure of which is herein incorporated by reference in its entirety, except where inconsistent with the low phosphate, high nitrite, and low temperature conditions.
In accordance with Ser. No. 60/274,106 and the present invention, the second type of zinc phosphate solution preferably consists essentially of, or more preferably consists of, water and the following components:
One or more of the following components may also be present in the second phosphate solution:
If the compositions of the first and second zinc phosphate solutions have an initial pH value lower than 3.80±0.03, they have positive Free Acid points which are quantitatively defined as equal to the number of milliliters (hereinafter usually abbreviated as “ml”) of 0.100 N strong alkali required to titrate a 10 ml sample of the composition to a pH value of 3.80±0.03. If the initial value of pH of the first and second zinc phosphate solutions is higher than 3.80±0.03, they have negative Free Acid points, which are defined as the negative number with the same absolute value as the number of ml of strong acid required to titrate a 10 ml sample of the composition to a pH of 3.80±0.03. If the first and second zinc phosphate solutions have a pH of 3.80±0.03, they have 0.0 points of Free Acid. In addition to containing the above-noted components, a working composition for the first and second phosphate solutions preferably has a Free Acid value that is at least, with increasing preference in the order given, −1.0, −0.5, 0.0, 0.10, 0.20, 0.30, 0.40, or 0.49 points and independently preferably is not more than, with increasing preference in the order given, 3.0, 2.5, 2.0, 1.90, 1.80, 1.70, 1.60, 1.50, 1.40, 1.30, 1.20, 1.10, 1, 0.9, 0.8, 0.75, 0.7, 0.65, 0.6, or 0.55 points.
The presence of nickel cations in a composition for the first and second phosphate solutions is preferred, unless the anti-pollution laws where the composition is used make the presence of nickel impractical economically. In such an instance, the presence of copper cations is alternatively preferred, unless they too are economically impractical because of pollution.
The presence of fluoride containing anions in a composition for the first and second phosphate solutions is generally preferred, especially when phosphating aluminum under most conditions. When phosphating steel or zinciferous surfaces such as galvanized steel, all of the fluoride present is preferably complex fluoride, but when phosphating aluminum, some of the fluoride is preferably present as “free fluoride,” a characteristic of the composition that can be measured by a fluoride ion sensitive electrode in contact with the composition and electrically connected to a reference electrode also in the same volume of composition, as known to those skilled in the art. Complex fluoride is preferably supplied to the first and second zinc phosphate solutions by at least one of tetrafluoroboric acid, hexafluorosilicic acid, hexafluorotitanic acid, hexafluorozirconic acid, and salts of all of these acids. At least for economy, hexafluorosilicic acid is most preferred. When free fluoride is needed or desired, it is preferably supplied by hydrofluoric acid and/or ammonium hydrogen fluoride.
The presence of nitrate in the first and second zinc phosphate solutions is preferred, and independently the nitrate is preferably provided at least in part by nitric acid, although nitrate salts may also be used. When nitrate is used, it preferably is present in a ratio to phosphate that is at least, with increasing preference in the order given, 0.20:1, 0.25:1, 0.30:1, 0.37:1, 0.39:1, 0.41:1, 0.80:1, 1.2:1, 1.6:1, or 1.9:1 and independently, at least for economy, preferably is not more than, with increasing preference in the order given, 30:1, 20:1, 10:1, 5:1, 3:1, 2.5:1, 2.2:1, or 2:1. The major identified reason for a preference for the presence of nitrate in at least the above ratios to phosphate is an improved resistance to corrosion after painting in such tests as GM 9540P, particularly on cold rolled steel.
Nitrite is used as the accelerator for the first and second phosphate solutions, because of its high technical reliability and effectiveness at a low concentration. When nitrite is used as the accelerator, its concentration preferably is at least 0.2 g/l, and more preferably at least 0.3 g/l. Because nitrite is subject to fairly rapid decomposition in acid solutions, it preferably is not added to a phosphating composition until shortly before it begins to be used and therefore preferably is not included in make-up or replenisher concentrates.
If the convenience of a single package replenisher is preferred, hydroxylamine in one of its stable bound forms is preferred as the accelerator for the first and second zinc phosphate solutions. Salts of hydroxylamine with any strong acid are generally stable enough in compositions according to the invention to be practically included in single package concentrates, with the sulfate being particularly preferred at least for economy. Oximes can also serve as a suitable source of hydroxylamine. Irrespective of the specific source, when hydroxylamine is used as the accelerator in the first and second zinc phosphate solutions, the concentration, measured as its stoichiometric equivalent as hydroxylamine, preferably is at least, with increasing preference in the order given, 0.20, 0.25, 0.30, 0.33, 0.36, or 0.39 g/l and independently preferably is not more than, with increasing preference in the order given, 1.5, 1.0, 0.90, 0.80, 0.85, 0.80, 0.75, 0.70, 0.65, or 0.61 g/l.
A phosphating process using either the first or second zinc phosphate solutions can be accomplished by contacting a suitably prepared substrate with either solution. Any method of achieving contact may be used, with one of immersion and spraying generally being preferred, depending on the size and the complexity of the shape of the surface to be phosphated, as generally known in the art. Consistent phosphating results are generally obtained when, and it is therefore preferred that, the temperature of the phosphating composition is controlled while it is in contact with the surface being phosphated. The second zinc phosphate solution preferably operates at low temperatures, preferably between 20° C. and 40° C., and more preferably between 30° C. and 35° C.
The mass of the phosphate coating formed can be determined by methods known in the art. This characteristic of a process according to the invention is generally reported as “coating weight”, which is defined as the mass of the coating in g/m2. For predominantly ferriferous surfaces such as cold rolled steel, the coating weight preferably is at least, with increasing preference in the order given, 0.50, 0.60, 0.70, 0.80, or 0.86 g/m2 and independently preferably is not more than, with increasing preference in the order given, 5.0, 4.5, 4.0, 3.5, 3.3, 3.0, 2.8, or 2.6 g/m2. For predominantly zinciferous surfaces such as all types of galvanized steel, the coating weight preferably is at least, with increasing preference in the order given,0.50, 0.60, 0.70, 0.80, 0.90, 1.00, or 1.10 g/m2 and independently preferably is not more than, with increasing preference in the order given, 7.0, 6.5, 6.0, 5.5, 5.0, 4.5, 4.1, or 3.8 g/m2.
The time of contact between the first and second zinc phosphate solutions and the composite substrate in a process according to the invention is generally not at all critical if the desired coating weight is achieved, presumably because the rate of formation of the coating is much faster at the beginning of contact of a fresh metal surface with the first and second zinc phosphate solutions than after even a thin phosphate coating has initially formed. As a general guideline, when contact is by immersion, the contact time preferably is at least, with increasing preference in the order given, 0.2, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, or 1.9 minutes and independently preferably is not more than, with increasing preference in the order given, 30, 20, 15, 10, 5, 3.0, 2.7, 2.5, 2.3, or 2.1 minutes. When contact is by spraying, the contact time preferably is at least, with increasing preference in the order given, 0.05, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, or 0.95 minutes and independently preferably is not more than, with increasing preference in the order given, 10, 7, 5, 4.0, 3.5, 3.0, 2.5, or 2.1 minutes.
Before being contacted with either the first or second zinc phosphate solutions, a composite substrate to be phosphated in a process according to the invention is preferably cleaned, rinsed, and activated by any of the means known for these purposes in the art. Similarly, after the desired time of contact between either the first or second zinc phosphate solutions and a composite substrate has been completed, the substrate is preferably removed from contact with the phosphating composition, rinsed with water, and further treated.
3. Third Type of Zinc Phosphate Solution
In accordance with a third aspect of the present invention, the third zinc phosphate solution may comprise components and operate as set forth in United Kingdom Patent Application No. GB 2 199 047 A, the disclosure of which is herein incorporated by reference in its entirety, except where inconsistent with the low phosphate, high nitrite, and low temperature conditions.
In accordance with GB 2 199 047 A and the present invention, the third type of zinc phosphate solution contains nitrate, fluoride, less than 5 g/l of phosphate, and 1.5 to 2.5 g/l of zinc. The weight ratio of zinc to phosphate (Zn/PO4) is preferably 0.08 to 0.21 and the free acidity value is from −0.1 to 0.1 when the amount of zinc is 1.5 g/l, is from 0.1 to 0.5 when the amount of zinc is 2.5 g/l, and is at an interpolated intermediate value at intermediate amounts of zinc. The solution also includes an accelerator, preferably nitrite ions having a concentration of greater than 0.2 g/l and more preferably greater than 0.3 g/l, to accelerate the dissolution of steel from the steel surface and to promote the deposition of Phosphophyllite. With this solution it is possible to obtain good coatings at low temperatures, such as between 20° C. and 40° C., and more preferably between 30° C. and 35° C.
Before treatment with the third zinc phosphate solution, the substrate should be cleaned using a conventional cleaning process that is used prior to zinc phosphating. The cleaned surface is also preferably subjected to conventional surface conditioning. The cleaned and conditioned metal surface is then treated with the third zinc phosphate coating solution. Treatment is preferably by dip, conducted preferably at temperatures between 20° C. and 40° C., and more preferably between 30° C. and 35° C. The third solution is a zinc phosphate coating solution that contains nitrate and fluoride. Particularly important parameters of the solution are its concentration of zinc, the inclusion of an appropriate amount of an accelerator that is additional to the nitrate that is in the solution, and the relationship between its free acidity and the amount of zinc.
The amount of zinc is generally above 1.5 g/l, which makes it possible to increase the speed of depositing the appropriate phosphate coating on the sheet steel or other surface. If the amount of zinc is less than 1.5 g/l, a phosphate coating of the desired weight cannot be formed, and if the amount of zinc is greater than 2.5 g/l, the coating becomes too heavy and it is hard to obtain a high P/P+H ratio. Paint film adhesion and corrosion are consequently worse.
The free acidity chosen is dependent upon the zinc concentration, wherein the optimum free acidity increases as the zinc concentration increases. As a result, the pH of the third zinc phosphate solution can be increased and this broadens the range of conditions at which satisfactory precipitation of zinc phosphate will occur. In particular, the free acidity should be from −0.1 to +0.1 when the amount of zinc is 1.5 g/l, should be from 0.1 to 0.5 when the amount of zinc is 2.5 g/l, and at intermediate amounts of zinc the free acidity should be at interpolated intermediate values.
If the zinc and free acidity values are chosen in the manner indicated, the resultant coating may have an unsatisfactorily low ratio of P/P+H. For instance, the ratio is preferably at least 0.7 and most preferably at least 0.8. A supplemental accelerator is therefore included in order to accelerate the dissolution of iron from the steel surface and to facilitate the deposition of phosphophyllite on to the surface in the coating. In this way the reactivity of the process is such that the resultant phosphate coating gives high quality film performance. In particular, it is preferred to include a nitrite accelerator in an amount of greater than 0.2 g/l of nitrite ions, and more preferably greater than 0.3 g/l of nitrite ions.
The ratio Zn/PO4 should be between 0.08 and 0.21. If the value is below 0.08 the tendency to form a phosphate coating is seriously worsened, and if the value is above 0.21 there is no advantage, and the crystal size may become too coarse. Preferably the crystal size is mainly in the range of 2 to 5 μm.
The presence of fluoride contributes to the uniform etching of the steel surface as well as the densification of the phosphate crystals. The fluoride may be introduced as simple fluoride or as complex fluoride. The amount of fluoride is preferably from 0.5 to 1.5 g/l. If the amount is below 0.5 g/l, etching tends to be non-uniform and densification of the phosphate coating crystals may be poor. If the amount is above 1.5 g/l, the phosphate coating tends to become too thin and the performance properties may be difficult to achieve.
In addition to containing zinc, phosphate, nitrate, fluoride and accelerator, all as described above, it is particularly preferred that the third zinc phosphate solution contain nickel and a small amount of ferrous iron, and manganese.
The inclusion of nickel contributes to the densification of the phosphate coating crystals and results in improved corrosion resistance and paint adhesion. The amount of nickel is preferably from 0.5 to 1.5 g/l. If the amount is below 0.5 g/l, there is inadequate improvement in the density of the phosphate coating crystals, and in the corrosion resistance and paint adhesion. Amounts above 1.5 g/l, are non-economical as they do not provide any improvement.
Ferrous iron serves to elevate the pH value of the third zinc phosphate solution at which it starts to precipitate, and thus facilitates the formation of the phosphate coating. The amount of ferrous iron is preferably in the range 2 to 20 mg/I. Amounts below 2 mg/l inadequately increase the pH at which precipitation formation occurs, and results in retarded formation of the phosphate coating. If the amount is above 20 m/l, there is a tendency towards the formation of iron phosphate sludge and the destruction of the balance of the treatment solution.
The inclusion of manganese results in the improvement of the secondary adhesion of the paint film after water soaking and so is particularly preferred when such properties are required. If the amount of Mn2+ is above 1 g/l, the formation of the phosphate coating may become harder to achieve and, in particular, its rate of formation may be reduced. If the amount is below 0.2 g/l, there may be no benefit, and so preferably the amount of manganese is from 0.2 to 1 g/l.
The preferred process using the third zinc phosphate solution is for the formation of a phosphate conversion coating on composite structures which have been cleaned and surface conditioned. In this process the composite structures are dip-treated with the third zinc phosphate solution containing less than 5 g/l of phosphate ions, 5 to 15 g/l nitrate, 0.5 to 1.5 g/l fluorine compound, 0.5 to 1.5 g/l divalent nickel, 2 to 20 mg/l trivalent iron, and 1.5 to 2.5 g/l divalent zinc, and the weight ratio of zinc to phosphate is 0.08 to 0.21. Since the free acid is dependent on the concentration of zinc, the third zinc phosphate solution contains a nitrite accelerator at a concentration of greater than 0.2 g/l of nitrite ions, and the process is conducted at a temperature of 15 to 39° C., or at 20 to 30 or 35° C. The dip treatment is generally conducted for between 0.5 to 3 minutes.
The aluminum may be precleaned with compositions and a preclean process, as described in, U.S. patent application Ser. No. 09/631,163 fled Nov. 4, 2003 entitled COMPOSITIONS USEFUL FOR DEGREASING METAL SURFACES, the disclosure of which is herein incorporated by reference. If the aluminum being treated by the process and compositions of the present invention is difficult to clean and the precleaning compositions and process of M6651 is incapable of cleaning the aluminum, then the first step of the process of the present invention may also be used to etch (oxidize) the aluminum to prepare it for the second step of the process and to remove any remaining grease, soil, oxides, etc. on the aluminum surface.
B. The Second Process Step
The second step (II) of the pretreatment process of the present invention is preferably the same as the second step (II) (which is a combination of steps) set forth in International Patent Application WO 99/12661, the disclosure of which is herein incorporated by reference. The second step (II) imparts corrosion protection to the aluminum while not reducing, and preferably improving, the corrosion prevention properties of the zinc-phosphated steel or zinc.
In accordance with WO 99/12661 and the present invention, in step (II), solutions according to the prior art that produce a conversion layer on aluminum may be used. These solutions must not, however, excessively dissolve the crystalline zinc phosphate layer formed in step (I). The pH of these solutions should therefore lie in the range from 2.5 to 10, preferably from 3.3 to 10. Advantageously, in step (II), solutions are chosen containing components that additionally passivate the crystalline zinc phosphate layers. Such solutions are mentioned hereinafter by way of example. In step (II), the metal structures are generally brought into contact with the treatment solutions by spraying or by dipping. The temperature of the treatment solution for step (II) is preferably chosen in the range from 20 to 70° C.
By way of example, in step (II) a treatment solution may be used that has a pH in the range from about 5 to about 5.5 and that contains overall about 0.3 to about 1.5 g/l of hexafluorotitanate and/or hexafluorozirconate ions. It may be advantageous for the corrosion protection of the crystalline zinc phosphate layer produced in step (I) if this treatment solution additionally contains about 0.01 to 0.1 g/l of copper ions for step (II).
Moreover, a treatment solution may be used in step (II) that has a pH in the range from 3.5 to 5.8 and that contains 10 to 500 mg/l of organic polymers chosen from poly-4-vinylphenol compounds of the immediately following general formula (I):
wherein n is an integer between 5 and 100, each of X and Y independently of each other denotes hydrogen or a CRR1OH moiety in which each of R and R1 independently is hydrogen or an aliphatic or aromatic moiety with I to 12 carbon atoms.
For step (II) in particular those treatment solutions are preferred that contain polyvinylphenol derivatives according to the teaching of European Patent No. 0 319 016 B1. This document also discloses the preparation of such polyvinylphenol derivatives. Accordingly, in step (II) a treatment solution is preferably used that has a pH in the range from 3.3 to 5.8 and contains 10 to 5000 mg/l of organic polymers selected from homopolymer or copolymer compounds containing amino groups, comprising at least one polymer selected from the group consisting of materials (α) and (β), wherein: (α) consists of polymer molecules each of which has at least one unit conforming to the immediately following general formula (II):
wherein:
The phrase “polymer molecule” in the above definitions of materials (α) and (β) including any electrically neutral molecule with a molecular weight of at least 300 daltons.
Ordinarily, primarily for reasons of economy, it is preferred to utilize as materials 30 (α) and/or (β) predominantly molecules which consist entirely, except for relatively short end groups, of units conforming to one of the general formulas (I) and (II) as described above. Again primarily for reasons of economy, such materials are generally prepared by reacting homopolymers of p-vinyl phenol, for material (α), or phenol-aldehyde condensation products, for material (β), with formaldehyde and secondary amines to graft moieties Z on some of the activated benzene rings in the materials thus reacted.
However, in some particular instances, it may be more useful to utilize more chemically complex types of materials (α) and/or (β). For example, molecules formed by reacting a condensable form of a molecule belonging to component (α) or (β) as defined above, except that the molecule reacted need not initially satisfy the requirement for component (α) or (β) that each molecule contain at least one moiety Z, with at least one other distinct type of molecule which is selected from the group consisting of phenols, tannins, novolak resins, lignin compounds, aldehydes, ketones, and mixtures thereof, in order to prepare a condensation reaction product, which optionally if needed is then further reacted with (1) an aldehyde or ketone and (2) a secondary amine to introduce at least one moiety Z as above defined to each molecule, so that the molecule can qualify as material (α) or (β).
Another example of more complex materials that can be utilized as material (α) is a material in which the polymer chains are at least predominantly copolymers of simple or substituted 4-vinyl phenol with another vinyl monomer such as acrylonitrile, metha-crylonitrile, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl methyl ketone, isopropenyl methyl ketone, acrylic acid, methacrylic acid, acrylamide, methacrylamide namyl methacrylate, styrene, n-bromostyrene, p-bromostyrene, pyridine, diallyldimethylammonium salts, 1,3-butadiene, n-butyl acrylate, t-butylamino-ethyl methacrylate, n-butyl methacrylate, t-butyl methacrylate n-butyl vinyl ether, t-butyl vinyl ether, m-chlorostyrene, p-chlorostyrene, p-chlorostyrene, n-decyl methacrylate, N,N-diallylmelamine, N,N-di-n-butylacrylamide, di-n-butyl itaconate, di-n-butyl maleate, diethylaminoethyl methacrylate, diethylene glycol monovinyl ether, diethyl fumarate, diethyl itaconate, diethylvinyl phosphate, vinylphosphonic acid, diisobutyl maleate, diisopropyl itaconate, diisopropyl maleate, dimethyl fumarate, dimethyl itaconate, dimethyl maleate, di-n-nonyl fumarate, di-n-nonyl maleate, dioctyl fumarate, di-n-octyl itaconate di-n-propyl itaconate, N-dodecyl vinyl ether, acidic ethyl fumarate, acidic ethyl maleate, ethyl acrylate, ethyl cinnamate, N-ethyl methacrylamide, ethyl methacrylate, ethyl vinyl ether, 5-ethyl-2-vinylpyridine, 5-ethyl-2-vinylpyridine-1-oxide, glycidyl acrylate, glycidyl methacrylate, n-hexyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, isobutyl methacrylate, isobutyl vinyl ether, isoprene, isopropyl methacrylate, isopropyl vinyl ether, itaionic acid, lauryl methacrylate, methacrylamide, methacrylic acid, methacrylonitrile, N-methylolacrylamide, N-methylol-methacrylamide, N-isobutoxymethylacrylamide, N-isobutoxy-methylmethacrylamide, N-alkyloxymethylacrylamide, N-alkyl-oxymethylmethacrylamide, N-vinylcaprolactam, methyl acrylate, N-methylmethacrylamide, a-methylstyrene, n-methylstyrene, a-methyl styrene, p-methylstyrene, 2-methyl-5-vinylpyridine, n-propyl methacrylate, sodium p-styrenesulfonate, stearyl methacrylate, styrene, p-styrenesulfonic acid, p-styrenesulfonamide, vinyl bromide, 9-vinyl carbazole, vinyl chloride, vinylidene chloride, 1-vinylnaphthalene, 2-vinylnaphthalene, 2-vinylpyridine, 4-vinylpyridine, 2-vinylpyridine N-oxide, 4-vinylpyrimidine, and N-vinylpyrrolidone.
The following preferences, primarily for reasons of economy, improve corrosion resistance, and/or increase water solubility, apply, independently for each preference, to the molecules of materials (α) and (β):
Poly(5-vinyl-2-hydroxy-N-benzyl)-N-methylglucamine is a specific polymer of the most preferred type, which, in the acidic pH range which is to be established, is present at least in part as an ammonium salt.
Solutions may be used that do not contain any further active constituents, apart from the polyvinyl phenol derivative and an acid for adjusting the pH, preferably phosphoric acid. Additions of further active constituents, in particular hexafluorotitanate or hexafluorozirconate ions, may however improve the layer formation on aluminum. For example, a solution may be used whose pH lies preferably in the range from about 3.3 to about 5.8 and which contains as organic polymer about 100 to about 5000 mg/l of an organic polymer in the form of a methylethanolamine derivative or N-methylglucamine derivative of polyvinyl phenol and in addition 10 to 2000 mg/l of phosphate ions, 10 to 2500 mg/l of hexafluorotitanate or hexafluorozirconate ions, and 10 to 1000 mg/l of manganese ions.
Instead of the polyvinyl phenol derivatives, whose preparation involves a certain expense, there may be used in step (II) solutions or dispersions of organic polymers selected from homopolymers and/or copolymers of acrylic acid and methacrylic acid as well as their esters. Preferably these solutions or dispersions have pH values in the range from about 3.3 to about 4.8 and contain about 250 to about 1500 mg/l of organic polymers. According to the teaching of European Patent 0 08 942 B1 these polymer solutions or dispersions may additionally contain hexafluorotitanates, hexafluorozirconates and/or hexafluorosilicates.
The two-step process of the present invention provides a chemical pretreatment, before an organic coating, of composite metal structures that contain aluminum, zinc, and steel in any ratio. The low phosphate ion concentration (less than 5 g/l), and high nitrite concentration (greater than 0.2 g/l) reduces the need for monitoring the process, and reduces the sludge produced during the first step of the process, reducing the environmental impact of the process. The low temperature operation (less than 40° C.) of the first step of the process reduces the energy consumed by the process, providing cost savings. Furthermore, the pretreatment process of the present invention forms a conversion layer on all exposed zinc, steel, and aluminum surfaces that is suitable as a corrosion-preventing paint substrate, especially before a cathodic electro-dipcoating.
A process sequence according to the invention was tested on sample metal sheets (four inch by four inch panels) of cold rolled steel (hereinafter abbreviated as “CRS”), aluminum 6111 (“Al 6111”), eletrolytically galvanized steel (hereinafter abbreviated as “EG steel”), and hot dip galvanized steel (hereinafter abbreviated as “HDG steel”). As is conventional in the automobile manufacturing sector, the metal sheets were first cleaned with alkali and activated with an activating solution containing titanium phosphate, and then rinsed with deionized water. The metal sheets were then dipped in a phosphating bath for 120 seconds. The general process cycle for the samples is shown below as Table 1. The composition of the phosphate solution used for the samples is shown below in Table 2, and a theoretical composition of the phosphate solution is shown below as Table 3. Table 4 shows examples of the substrate metal loss and phosphate coating weight for samples exposed to the phosphating process of the present invention. Table 5 shows examples of the weight loss on phosphated samples due to post-treatments.
*Polyvinylphenol derivative
Where in Table 5, the panels were treated for 30 seconds with the following post-treatments:
It will be apparent to those skilled in the art that various modifications and variations can be made in the method of the present invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
