ETCHANT FOR ALUMINIUM ALLOYS

Abstract: A tin immersion composition of matter is disclosed comprising a compound comprising divalent tin ion, a compound comprising fluoride ion, and a compound comprising acid hydrogen ion. A process for treating an aluminum-copper or an aluminum-silicon alloy to improve the adhesion of metal layers to the alloy is also disclosed comprising contacting the alloy with an acidic tin immersion composition to produce a tin immersion coating on the alloy, and contacting the tin immersion coating with an etchant to substantially remove the tin immersion coating and produce an etched alloy surface.

Description: ETCHANT FOR ALUMINUM ALLOYS BACKGROUND OF THE INVENTION Field of the Invention The field of the invention is a composition of matter and a process for metal plating an aluminum surface. Description of Related Art Harrison et al . in an entitled article "Plated Aluminum Wheel Characterization, " Metal Finishing. December 1994, pp. 11-16, notes that metal plating aluminum is one of the growing areas of decorative plating, especially aluminum automobile wheels. Although in the past plated aluminum wheels were a small after-market specialty item, this has become an original equipment manufacturer option and a special addition feature. The major concerns in production of metal plated aluminum are the reliability of the plating process, appearance and cost. In a typical sequence for applying metal coatings to aluminum, the substrate is polished and soak cleaned. The soak cleaner employed in the pretreatment of the aluminum surface removes finishing oils, grease and difficult-to- remove buffing compounds left on the surface of the aluminum from polishing. After the soak clean, the aluminum is immersed in a mild caustic or alkaline etch solution operated at elevated temperatures since etch rate is more dependent on temperature than caustic concentration. The mild alkaline etch removes the Beilby layer and roughens the surface. Employing aluminum-silicon alloys results in etching aluminum preferentially over the silicon, leaving coarse silicon crystals exposed on the surface. An examination of the surface of the aluminum-silicon alloy shows large areas of exposed silicon interspersed within the aluminum matrix. The silicon particles vary in size, do not appear to be uniformly distributed throughout the casting, and are not uniformly distributed on the surface of the aluminum, but rather in discrete areas. Silicon crystals protrude from the surface, most of which are oriented perpendicular to the surface. After the etch treatment, the substrate is then subjected to a desmut composition. Smaller, loosely adherent silicon particles (where a silicon containing alloy is employed) , as well as intermetallic compounds, are most likely removed during the desmut step. The substrate is then rinsed, zincated, stripped with nitric acid, zincated again, and followed by a nickel strike coating. This in turn is followed by a bright copper plating, optional copper buffing, nickel plating and an optional high sulfur nickel to improve corrosion resistance. After these preparatory steps, a decorative chromium plate is applied. As noted by Harrison et al. , a film is left on the aluminum after the mild caustic etch that is removed by the desmut step, and is one of the most crucial steps in processing the aluminum substrate to ensure adequate adhesion of the subsequently applied metal coatings. The tenacity of this film varies with the composition of the aluminum, especially where an aluminum alloy is employed. The desmut solution contains strong mineral acids, and when aluminum-silicon alloys are treated, fluoride ions. Both are selected to uniformly attack the aluminum surface, or the proportions varied to preferentially dissolve the silicon (e.g., high fluoride concentration) and/or the aluminum. The aluminum and exposed silicon particles are thereby rendered more active. Various combinations of additives, nitric, sulfuric, and phosphoric acids in combination with fluoride salts such as ammonium bifluoride or fluoroboric acid allow for adequate pretreatment of the aluminum to obtain good adhesion of subsequently applied metal coatings. Aluminum wheels employed by the automotive industry are generally A-356 aluminum alloy castings. The A-356 alloy is generally chosen for aluminum wheel applications because of its ease of use in casting, high resistance to hot cracking, high fluidity, low shrinkage tendency and moderate ease of machinability. The A-356 alloy is a hypoeutectic alloy consisting mainly of a two-phase microstructure. Iron is present to minimize sticking between the molds and casting. Magnesium and copper are added to impart strength to the alloy. Manganese is believed to improve the high temperature properties of the casting. The silicon in the alloy appears as very hard particles and imparts wear resistance. Most of the hypoeutectic aluminum-silicon alloy consists of a soft and ductile aluminum phase. The nominal composition of A-356 aluminum alloys is as follows: Element % by weight Al 91.9 Si 7.0 Cu 0.2 Mg 0.3 Mn 0.1 Zn 0.1 Fe 0.2 Ti 0.2 Treating aluminum alloys such as A-356 alloy in the foregoing manner leaves a heavy film on the aluminum after the mild caustic etch. This film or smut is a mixture of both aluminum oxides and alloying element oxides as well as exposed silicon in those alloys which contain silicon as an element. The zincating materials generally consisted of CN zinc compositions that optionally contained nickel, and because of environmental reasons and the state-of-the-art cyanide treatment technology, manufacturers sought cyanide free systems. Several cyanide free zincate compositions have been developed containing zinc and optionally nickel, copper, or iron and mixtures thereof; however, it was found in some instances that specific aluminum alloys, such as A-356 alloy, could not be pretreated satisfactorily in that a heterogeneous composition was formed on the surface of this alloy during the initial etch, sometimes referred to as segregation. This segregation in turn has an adverse affect on the adhesion of subsequently applied metal layers. It was further found that aluminum-copper alloys such as 2024 alloy could not be etched uniformly either by alkaline or acid etchants due to its low solution potential. It was also found that this compromised the adhesion of subsequently applied metal coatings. Oilman ' s Encyclopedia of Industrial Chemistry, Vol. A-l, p. 520 (1985), notes that the nature of the aluminum oxide surface and reactivity of aluminum after oxide removal makes electroplating more complicated. Additional factors include reactions between aluminum and the electroplating solutions, the galvanic reactions between aluminum and the plated metal, and the metallurgical structure of aluminum alloys that consists of solid solutions, constituents, dispersoids, and precipitates, each having a different reactivity. As is apparent from the foregoing, metal plating of aluminum surfaces is a highly complex field. It would therefore be an advantage to provide a process or composition for avoiding or minimizing the difficulties of smut formation, segregation, nonuniform etching and poor adhesion in the electrocoating of aluminum substrates in a process that utilizes cyanide-free zinc compositions. Accordingly, the present invention is directed to a composition of matter and a process that substantially obviates one or more of these and other problems due to limitations and disadvantages of the related art. SUMMARY OF THE INVENTION These and other advantages are obtained according to the present invention. Additional features and advantages of the invention are set forth in the description that follows, and in part are apparent from the description, or learned by practice of the invention. The objectives and other advantages of the invention are realized and obtained by the composition of matter and process particularly pointed out in the written description and claims hereof. To achieve these and other advantages, and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a novel acidic tin immersion composition of matter comprising a divalent tin ion-containing compound, a fluoride ion- containing compound, and an acid hydrogen ion-containing compound. The invention also comprises a process for treating an aluminum-copper or an aluminum-silicon alloy to improve the adhesion of metal layers to the alloy comprising: (a) contacting the alloy with an acidic tin immersion composition to produce a tin immersion coating on the alloy; (b) contacting the tin immersion coating with an etchant to substantially remove the tin immersion coating to produce an etched alloy surface. The substrate, and especially an aluminum-copper substrate is simultaneously etched and coated with the acidic tin immersion coating, and especially the novel acidic tin immersion coating of the invention. This is preferably followed by a separate etching step to substantially remove the tin immersion coating and provide a microporous structure on the surface of the aluminum alloy. In another embodiment, an aluminum silicon containing substrate is coated with the acidic tin immersion coating, especially the novel acidic tin immersion coating of the invention, again to simultaneously etch and deposit a tin immersion coating on the aluminum alloy. A fluoride etchant applied to the tin coating on the aluminum substrate substantially removes the tin immersion coating and produces a microporous structure on the aluminum surface. A cyanide-free zinc immersion coating is then applied to the aluminum substrate followed by electrodeposition of a metal layer such as nickel. The microporous structure produced by the acidic tin immersion coating/etching process provides improved adhesion for subsequently applied metal layers. DETAILED DESCRIPTION OF THE INVENTION The invention comprises a novel acidic tin immersion coating composition having a fluoride ion-containing compound, and is employed in a process that provides a microporous structure on an aluminum substrate. Employing an acidic tin immersion coating, and especially the novel acidic tin immersion composition of the invention having a fluoride ion-containing compound allows for the subsequent uniform etching of the aluminum substrate to substantially remove the tin to obtain an etched aluminum substrate, and especially a microporous structure on the substrate that promotes improved adhesion of subsequently applied metal coatings. The process of the invention is especially applicable to aluminum-copper alloys and aluminum-silicon alloys. Where the latter is employed, etching the tin immersion coating is preferably carried out using a fluoride ion containing etchant. After substantial removal of the tin immersion coating from the aluminum surface by etching, the aluminum surface may be coated with a metal by an immersion or electrolytic process. Alternatively the etched or metal coated aluminum substrate may be coated with a metal using other methods known in the art, such as non-immersion methods and non- electrolytic methods, including cathode sputtering, chemical vapor deposition (CVD) , ion beam coating and the like. Any metal may be coated in this regard such as zinc, chromium, copper, nickel, or combinations thereof, whether alloy combinations, or multiple layers of the same or different metals or alloys. The preferred process of the invention comprises coating an aluminum-copper or aluminum-silicon alloy substrate with the novel acidic tin immersion composition having a fluoride ion-containing compound. This is followed by substantially removing the tin coating on the aluminum by etching, and optionally coating with a metal. An ethcant having a fluoride ion-containing compound is preferred for etching the aluminum-silicon alloy coated with the tin immersion coating. The novel acidic tin immersion coating of the invention provides a highly porous surface on the aluminum, and it is believed this surface results from the immersion or substitution reaction between tin and aluminum. As noted before, other metal ions did not produce the same degree of microporosity in the substitution or immersion reaction. The novel acidic tin immersion coating of the invention acts in some respects as an acid etching solution, as well as an immersion coating to provide a tin layer on the aluminum alloy substrates. In the process of the present invention, this tin coating is subsequently etched, and the tin stripped away in part or completely, described herein as the substantial removal of tin. The stripping step leaves an exposed aluminum alloy surface that has a unique microporous surface that is further coated with a metal as described herein, and especially with cyanide free zinc immersion coatings. The application of the acidic tin immersion coating is undertaken, not to produce a lasting tin coating, but to create a microporous structure on the surface of the aluminum. The acidic tin immersion tin composition ' having a fluoride ion-containing compound comprises a divalent tin salt such as tin sulfate or any other equivalent salt of tin. These salts are the reaction product of tin compounds with an acid such as a mineral acid including the acids based on oxides of sulfur, phosphorus or nitrogen as well as organic acids, or halogen acids such as acids based on fluorine, chlorine, bromine and iodine. In addition to sulfuric acid, the mineral acids include sulfurous acid, nitric acid, nitrous acid, phosphoric acid, phosphonic acid, phosphinic acid and the halogen acids such as hydrochloric, hydrofluoric, hydrobromic and hydroiodic acids, all of which are known in the art . The organic acids in this regard comprise any monocarboxylic or polycarboxylic acids such as the dicarboxylic, tricarboxylic or tetracarboxylic acids known in the art. Examples include the aliphatic acids, cycloaliphatic acids and aromatic acids where the aliphatic acids contain from 1 to about 5 carbon atoms and the cycloaliphatic and aromatic acids contain from 6 to about 10 carbon atoms, and include acids such as acetic, hydroxyacetic acid, maleic acid, malic acid, phthalic acid, mellitic acid, trimellitic acid, benzoic acid and the like. Mixtures of acids can be used, including the two, three, or four component mixtures. The preferred acid comprises a mineral acid, and especially sulfuric acid. Preferred tin salts comprise tin sulfates. The acidic tin immersion coating of the invention has a fluoride ion-containing compound where the source of the fluoride ion can be hydrogen fluoride or any fluoride salt such as ammonium bifluoride, aluminum trifluoride, sodium fluoride, sodium bifluoride, potassium bifluoride, ammonium fluoride, fluoroboric acid or hydrofluoric acid. Ammonium bifluoride or ammonium fluoride are not ordinarily employed where ammonia fumes are a potential irritant. The alkali metal fluorides and hydrofluoric acid are especially suitable in this regard. Mixtures of the various compounds that will provide fluoride ion can be employed, especially the two, three, or four component mixtures. The acid hydrogen ion-containing compound is based on an acid as described herein, and especially the mineral acids. Preferred acids are those having ^ the same anion as the tin salt. The ratios of divalent tin ion, fluoride ion and acid hydrogen ion of the novel acidic tin immersion composition are selected to provide both etching and a tin immersion coating on the surface of the aluminum that will produce the desired microporous structure of the invention. The divalent tin ion generally is present in an amount from about 0.05 to about 0.15 mols and especially from about 0.075 to about 0.125 mols. The compound containing the fluoride ion is employed in the immersion composition so that the fluoride ion is present in an amount from about 0.25 to about 0.75 mols and especially from about 0.375 to about 0.625 mols. Lastly, the acid is selected so that the acidic hydrogen ion in the composition is anywhere from about 0.25 to about 0.75 mols and especially from about 0.375 to about 0.625 mols. These molar amounts of the ionic species are also the gram atoms of the ionic species employed , and take into account that some of the compounds that are used to provide these ionic species contain more than one gram atom of the particular ionic species per mol . For example, sulfuric acid contains two gram atoms of hydrogen per mol, whereas hydrochloric acid contains one gram atom of hydrogen per mol. Consequently, the quantities of the various components have not been expressed as molar amounts of the compounds employed, but rather the molar amounts of the ionic species contributed by the compounds. Although the foregoing molar amounts are used to define the ratios of the various divalent tin ion, fluoride ion and acid hydrogen ion, they also indicate the concentration of an aqueous tin immersion composition in that these molar amounts comprise the quantity of the components of the immersion composition that can be employed in one litre of water to make up the immersion composition. The etchant employed for removing the acidic tin immersion coating from the aluminum-silicon alloy comprises a composition having a fluoride ion-containing compound, the latter comprising any of the fluoride ion-containing compounds described herein. This etchant also includes these fluoride ion-containing compound used in combination with an acid such as a mineral acid as defined herein. The following examples are illustrative. A novel acidic tin immersion coating of the invention (also referred to as Microporous Etch) is prepared as follows: SnSO 21.5g/iiter HF (47%) 20 ml/liter H 2 S0 4 (95.0%) 5.2 ml/liter Optionally 2.0g/liter of gelatin can be added. Two standard process sequences were used to plate various aluminum substrates as follows. In these processes, the zincate was a standard cyanide-free zincate solution containing zinc ions and optionally nickel, copper and iron ions. Cyanide free zinc immersion coatings for aluminum are described by Stareck, U.S. Patent No. 2,511,952; and Ihrie et al, U.S. Patent No. 2709,847. Zelley, U.S. Patent No. 2,650,886 describes a zinc-iron cyanide-free zinc immersion coating for aluminum. Any of the foregoing zinc immersion coatings can be employed and modified to include in addition to iron, nickel, copper or any combination of nickel, copper and iron with zinc in the zinc immersion coatings. This modification is easily made by a person of ordinary skill in the art . Unless otherwise indicated, the desmut process was carried out with a composition comprising a mineral acid desmut composition as described herein comprising a fluoride ion-containing compound such as hydrogen fluoride or any fluoride salt as described herein. This desmut process, when applied to the acidic tin immersion coating, has also been referred to herein as an ethching step. The nickel plating comprised the application of a nickel coating by means of a Watts bath well-known in the art. Example 1. Process A 1. Soak Clean (Alkleen™ A-ll) 2. Alkaline etch (Alkleen™ A-77) 3. Desmut (Cleaner #30; Acidic product HNO3 and H2SO4 for silicon containing Al-alloy) 4. Zincate 5. Zincate-removal (HN0 3, 50% Vol) 6. Zincate 7. Ni Plating Example 2. Process B 1. Soak Clean (Alkleen™ A-ll) 2. Desmut 3. Zincate 4. Zincate removal (HN0 3/ 50% Vol) 5. Zincate 6. Nickel Plating Adhesion Results: Process A Process B ** Pure Aluminum 1100 Good **Cu-Rich Aluminum 2024 Poor **Mg-Rich Aluminum 5052 Good *A1-Si Alloy A-356 Poor Poor *Al-Cu-Si-Alloy A-380 Poor * Cast material ** Extruded material As can be seen from the foregoing, poor results were obtained for aluminum alloys containing copper or silicon. Accordingly, processes A and B were modified to include the novel acidic tin immersion composition of the present invention containing fluoride. This composition is described as a Microporous Etch in the following examples: Example 3 Process A Modified. 1. Soak Clean (Alkleen™ A-ll) 2. Alkaline Etch (Alkleen™ A-77) 3. Desmut (Cleaner No. 30) 4. Microporous Etch 5. HN0 3 (50% vol) 6. Zincating 7. Ni Plating Example 4 Process B Modified. 1. Soak Clean (Alkleen™ A-ll) 2. Microporous Etch 3. Desmut 4. Zincating 5. Zincate-removal (HNO 3 50% vol) 6. Zincating 7. Ni Plating Process A Process B Modified Modified Pure Aluminum 1100 Poor Cu-Rich Aluminum Good 2024 Mg-Rich Aluminum Poor 5052 Al-Si Alloy A-356 Poor Good Al-Cu-Si Alloy A-380 Good The desmut, zincating and nickel plating steps employed were the same as in Examples 1 and 2. As can be seen from the results employing modified processes A and B, good adhesion to copper and silicon containing aluminum alloys were obtained by employing the composition and processes of present invention. Where tin deposition is too fast, thereby impairing the formation of an acceptable microporous structure, an organic compound, such as gelatin, can be added to the novel acidic tin immersion composition containing fluoride. It will be apparent to those skilled in the art that modifications and variations can be made in the novel composition of matter and process for a coating in aluminum substrate of the present invention without departing from the spirit or scope of the invention. It is intended that these modifications and variations and their equivalents are to be included as part of this invention, provided they come within the scope of the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A tin immersion composition of matter comprising a divalent tin ion-containing compound, a fluoride ion- containing compound, and an acid hydrogen ion-containing compound. 2. The composition of Claim 1 wherein said divalent tin ion-containing compound comprises a tin salt, said fluoride ion-containing compound comprises hydrofluoric acid or a fluoride salt and said acid hydrogen ion- containing compound comprises a mineral acid. 3. The composition of Claim 2 wherein said divalent tin ion is present in an amount from about 0.05 to about 0.15 mols, said fluoride ion is present in an amount from about 0.25 to about 0.75 mols, and said acid hydrogen ion is present in an amount from about 0.25 to about 0.75 mols. 4. A process for treating an aluminum-copper or an aluminum-silicon alloy to improve adhesion of metal layers to said alloy comprising: (a) contacting said alloy with an acidic tin immersion composition to produce a tin immersion coating on said alloy; (b) contacting said tin immersion coating with an etchant to substantially remove said tin immersion coating to produce an etched alloy surface. 5. The process of Claim 4 for producing a microporous structure on said aluminum alloy wherein said acidic tin immersion composition is a tin immersion composition of matter comprising a compound a divalent tin ion-containing compound, a fluoride ion-containing compound, and an acid hydrogen ion-containing compound. 6. The process of Claim 5 where said divalent tin ion-containing compound comprises a tin salt, said fluoride ion-containing compound comprises hydrofluoric acid or a fluoride salt, and said acid hydrogen ion-containing compound comprises a mineral acid. 7. The process of Claim 6 wherein said divalent tin ion is present in an amount from about 0.05 to about 0.15 mols, said fluoride ion is present in an amount from about 0.25 to about 0.75 mols, and said acid hydrogen ion is present in an amount from about 0.25 to about 0.75 mols. 8. The process of Claim 4 comprising further coating said etched alloy with a metal. 9. The process of Claim 5 comprising further coating said etched alloy with a metal . 10. The process of Claim 4 comprising further coating said etched alloy with a metal by an immersion coating process . 11. The process of Claim 5 comprising further coating said etched alloy with a metal by an immersion coating process. 12. The process of Claim 4 comprising further coating said etched alloy with a metal by a zinc immersion metal coating process to yield a zinc coated aluminum substrate. 13. The process of Claim 5 further comprising coating said etched alloy with a metal by a zinc immersion metal coating process to obtain a zinc coated aluminum substrate. 14. The process of Claim 12 comprising electrolytically coating said zinc coated aluminum substrate with a metal . 15. The process of Claim 13 comprising electrolytically coating said zinc coated aluminum substrate with a metal. 16. The process of Claim 14 comprising electrolytically coating said zinc coated aluminum substrate with nickel . 17. The process of Claim 15 comprising electrolytically coating said zinc coated aluminum substrate with nickel. 18. A product produced by the process of Claim 4. 19. A product produced by the process of Claim 5. 20. A product produced by the process of Claim 8. 21. A product produced by the process of Claim 9. 22. A product produced by the process of Claim 10. 23. A product produced by the process of Claim 11. 24. A product produced by the process of Claim 12. 25. A product produced by the process of Claim 13. 26. A product produced by the process of Claim 14. 27. A product produced by the process of Claim 15.

Inventors: MURANUSHI YOSHIHISA, MURANUSHI, YOSHIHISA
Applicants: ATOTECH USA INC, ATOTECH USA INC.
Application year: 1996
Main classification: C25D5
IPC codes: C23C14, C23C16, C23C18, C23F1, C25D5