Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/82—After-treatment
  • C23C22/83—Chemical after-treatment

Definitions

• This invention relates to aluminum and aluminum alloy articles having integral chemically-formed surface coatings that provide an improved combination of adherence and corrosion resistant properties to such products and to a process for making same. More particularly, the articles of this invention have on their aluminum or aluminum alloy surfaces an integral, chemically-formed coating that is strongly adherent and resistant to chipping or flaking at elevated temperatures and provides to the product a unique combination of corrosion properties including commercially satisfactory resistance to oxidation during use in gases at elevated temperatures such as encountered in the engine compartments of vehicle engines, resistance to corrosion from humidity, from organic solvents such as ethylene glycol, oils and gasoline, from acidic or alkaline solutions such as salt spray to the extent that is required of a base for paint or other protective organic or water-based protective coating on parts used within the engine compartments of vehicles.
• Chromate processes developed during the 1960's and 1970's have been asserted to provide improved paint bases relative to the chromate-phosphate coatings and are disclosed in a number of United States patents, including U. S. Patents, 3,009,482, 3,391,031, 3,404,043, 3,410,707, 3,447,972, 3,446,717, 3,982,951, 4,036,667, 4,146,410 and British Patent 1,409,413.
• the present invention provides aluminum and aluminum alloy articles that are coated with a new integral coating that satisfies specific commercial requirements for aluminum articles having a good corrosion resistance and resistance to dislodgment during use in vehicle environments.
• This invention also provides an economic, continuous process for producing the new aluminum coated articles of this invention, as will be described hereinafter.
• aluminum or aluminum alloy articles are coated with a new, thin colorless coating, which preserves the appearance of the uncoated aluminum or aluminum alloy articles.
• the coating contains as its essential component a chemical complex of alkali metal-chromium-­silicates as defined in the claims.
• the amount of that essential component in the coating is sufficient to provide the coated aluminum articles with an unexpectedly unique combination of properties of aluminum appearance, adherence, resistance to chipping and flaking, corrosion resistance to acidic and alkaline gases and aqueous solutions and oils, solvents and fuels sufficient to make it suitable as a base for paint and the equivalent of paint on parts within the engine compartment of vehicles.
• the preferred coatings are colorless and so thin as to be invisible to the naked eye.
• the coating thickness varies from about 50 angstroms, or 0.0005 micron, to about 2 microns.
• This invention also provides a process for the continuous, efficient production of the improved aluminum or aluminum alloy coated articles of this invention.
• the continuous process makes use of known production line dip or spray apparatus in which the aluminum articles or parts to be coated are mounted on racks or in rotating barrels supported on conveyor means capable of sequentially contacting the articles with aqueous solutions positioned in a plurality of in-line tanks, each tank containing an aqueous solution of selected coating-producing ingredients with intervening rinse solution-containing tanks, the in-line apparatus terminating in conventional means for drying the coated parts.
• the process of this invention has the advantages of using dilute aqueous solutions of inexpensive, commerically available chemicals that are maintained at low treatment bath temperatures ranging from ambient room temperatures up to about 160° F., or 71° C., and for short times of contact of the solution with the aluminum article being coated, for example, immersion contact in the range of about 20 - 180 seconds, preferably about 30 seconds, or spray contact for about 10 to 60 seconds and preferably 5 - 20 seconds.
• immersion contact in the range of about 20 - 180 seconds, preferably about 30 seconds, or spray contact for about 10 to 60 seconds and preferably 5 - 20 seconds.
• the process of this invention is useful to form coatings on aluminum and all of its alloys that are commercially available as sand castings, plate, sheet, forgings or extrusions. Particularly good results have been obtained by using the process for coating vehicle engine manifolds made from sand cast aluminum alloys as described in Example I.
• the new articles of this invention include articles fabricated from aluminum or an aluminum alloy which have on their surfaces a thin, adherent coating having a thickness up to about 2 microns comprising as its essential component a chemical complex of an alkali metal-chromium-silicate having proportions of each in the range, expressed as oxides in weight percent of: Na20 - 9.9% - 12.1% Cr203 - 4.1% - 4.3% Si02 - 76.8% - 91.2%
• the process for making the coated new articles of this invention comprises the following sequential steps, omitting intervening water rinsing steps:
• Cleaning solutions which have been found to be suitable for use in the first step of the process include a wide variety of commercially available inhibited acidic cleaners. Good results have been obtained by using an aqueous phosphoric acid solution containing phosphoric acid in an amount sufficient to give a pH in the range of about 5 to 6, and which may contain organic solvents such as tri- or diethylene glycol monobutyl ether in an amount of about 2% to 10% and may also contain any of a number of commercially available organic surfactants, for example, about 2 to 10% of a fluorocarbon surfactant such as FC 95 available under the tradename Fluorad from Minnesota Mining & Manufacturing Co.
• a fluorocarbon surfactant such as FC 95 available under the tradename Fluorad from Minnesota Mining & Manufacturing Co.
• the parts to be cleaned are immersed in such a cleaning solution at a temperature of about 130° to 180°F (54 to 82°C) for 2 to 5 mins, preferably about 3 mins, followed by rinsing in water at a temperature of about 120° to 140°F (49 to 60°C), preferably about 130°F (54°C), for 30 to 90 seconds.
• the cleaned articles from step 1 are then contacted with a stronger aqueous acidic solution capable of removing the aluminium and/or other oxides from the surfaces of the article.
• Good results have been obtained by using a chro­mic acid-based solution containing 70 to 80% chromic acid, 20 to 30% potassium dichromate and 2 to 4% ammonium silico­fluoride in a concentration of 3 to 6 oz/gal (45 g/l), preferably about 4 oz/gal (30 g/l) to form a solution hav­ing a pH in the range of about 0.5 to 1 and contacting the article with such solution for a time period in the range of 1 ⁇ 2 to about 3 mins.
• the oxide free cleaned articles are then water rinsed in one of three water tanks at ambient temperatures, for about 30 seconds in each rinse solution.
• the deoxidised, rinsed aluminium article is then subjected in step 3 to a coating forming step by contacting the article by dip or spray with an aqueous solution to form a silicon-chromate coating on the surface.
• a coating forming step by contacting the article by dip or spray with an aqueous solution to form a silicon-chromate coating on the surface.
• Good results have been obtained in forming such coatings by using an aqueous solution made up by adding to water, preferably deionised water, about 1 ⁇ 2 -2 oz/gal (3.7-15 g/l) of a composition containing in weight percent about 50 to 60% chromic acid, about 20 to 30% barium nitrate and about 15 to 20% sodium silicofluoride and preferably containing a catalyst in an amount of up to about 5% such as an alkali metal ferricyanide, i.e.
• compositions that are satisfactory for use may omit the barium nitrate component, and may include additional coating catalysts of the molybdic acid type in the event color is desired, such as the formulations disclosed in U. S. Patent 3,009,842 and in the other patents identified therein.
• Other useful, but less desirable compositions that are suitable for coated articles having less stringent requirements for salt spray resistance include those set forth in U. S. Patents 3,410,707 and 3,404,043. Compositions that are satisfactory are commercially available from a wide variety of suppliers in the United States and especially good results have been obtained by using the material commercially designated Iridit 14-2 which is available from Witco Chemical Company.
• the proportions of Cr203, barium nitrate and alkali metal silicofluoride in the preferred composition described above are not critical to the formation of the base chromium-silicate coating that is formed directly on the oxide free surface of the aluminum or aluminum alloy article being coated in accordance with this invention.
• Useful coated articles are formed when the formulation given above is varied to employ proportions within the ranges set forth in U. S. Patent 3,982,951.
• an immersion time of about 30 seconds is adequate when the temperature is maintained at less than 120° F., or 49° C.
• the article is sprayed at a similar temperature, about 5 to 20 seconds is adequate.
• step number three It is important to insure a thorough water rinsing of the silicon-chromate coating formed in step number three. This is best done using deionized water at ambient temperature, i.e., about 60° F. - 90° F. (15.6°C - 32°C), in 1 to 3 immersions, preferably three, for about 30 seconds each, or a single power spray for about 30 seconds.
• the fourth step is a final water rinse at a temperature that is higher than the ambient temperature employed in step 3.
• This higher temperature rinse serves to remove unwanted chromate colors, if present, and also to prepare the silicon-chromate coating to enhance its reactivity with the components in the strong alkaline solution to be next applied to form the alkali metal-chromium-silicate coating of this invention.
• Preferred conditions for step 4 include using deionized water at a temperature in the range of about 110° F. to 160° F., or about 43° C. to 71° C., and preferably about 130° F. or 54° - 55° C.
• the chromate-silicate coated article should be rinsed at the selected temperature for a time sufficient to raise the temperature of the article to about the elevated temperature of the rinse solution.
• the optimum time required varies for specific articles depending on the selected compositon used in step 3 and also depends on the size or bulk of the article.
• the optimum time may be affected by the particular alloy composition of the article being coated. For example, the time required may vary from about 30 seconds up to about 5 minutes, and the needed, or optimum, time is easily determinable by a few trials.
• the article may include pits or surface imperfections.
• the elevated temperature rinsed silicon-chromate coated article from step 4 is then subjected in step 5 to a second coating by step by contacting the coated article with a highly alkaline aqueous solution having a pH in the range of about 11 to 12 and containing disodium oxide and silicon dioxide components having a weight ratio of SiO2/Na2O in the range of about 2.4 to 3.25 and a range of densities between about 40 and 52° Baume' at 20°C.
• the silicate solutions may contain in weight percent, about 26.5% to about 33.2% SiO2 and about 8.6% to about 13.9% Na2O, at a similar range of densities.
• Preferred solutions are those which contain disodium oxide and silicon dioxide in a weight ratio of SiO2/Na2O of about 2.5 to 2.9 and a density in the range of about 42 to about 47° Baume' at 20°C.
• the best results have been obtained from a solution formulated by adding to water an amount of about 2 to 4% by volume of a sticky, heavy silicate having a weight ratio of SiO2/Na2O of 2.9 and a density of 7° Baume' at 20°C to thereby produce a coating solution having a pH of about 11.5.
• the articles from step 4 are then immersed for about 30 sec to 2 min in such a solution at a temperature of ambient to about 130°F (54°C), with the solution having a preferred pH between about 11.2 and 11.5.
• the thus coated articles are finally dried either in ambient air or in a low temperature furnace at 150° to 200°F (66 to 93°C) for 1 to 2 minutes.
• the dried, coated articles are the new articles of this invention.
• the articles have a thin, adherent coating that is substantially invisible to the naked eye but has been determined to have a thickness in the range of about 50 ⁇ to 20 000 ⁇ , or about 0.0005 ⁇ m to about 2 ⁇ m.
• the coated article has the same overall appearance as the uncoated article unless a yellowish chromate tint has been intentionally produced by varying the composition of step 5 or the temperature of step 4 as will be readily apparent to those skilled in the art of forming chromate coatings on aluminium.
• Tests conducted on the coated articles from step 5 have established that the coating is sufficiently adherent and hard to resist chipping or flaking when used at elevated temperatures up to about 400°F (204°C) such as may be attained in the engine compartments of automobiles and trucks.
• 204°C 400°F
• the articles from step 5 were vehicle intake manifolds and were tested for salt spray resistance under the conditions of ASTM B-117 test method no corrosion products were visible for 250 hours.
• Automobile intake manifolds were sand cast from a Ford Motor material designated 319 Aluminium having a specification of 5.5-6.5 Si, 0.4-0.6 Mn, 3.0-4.0 Cu, 0.1-­0,6 Mg, 0.7-1.0 Zn and 1.0 Max Fe.
• the articles were mounted on racks carried by a dip-type conveyor adapted to dip the racks into tanks to form coated manifold articles of this invention in the following sequence of steps:
• Coated articles from step 17 were analyzed using Electron Spectroscopy for Chemical Analysis (ESCA) to establish coating thickness and the elemental composition of the surface coating.
• the coating thickness of the dried articles from step 17 was greater than 50 angstroms and less than 2 microns.
• the coating composition in weight percent, expressed as oxides of the detected elements and taking into account the applicable accuracy level of the use conditions of the analyzing equipment, the coating contained: 9.9 - 12.1% Na20 4.1 - 4.3% Cr203 76.8 - 91.2% Si02
• the process was also used to coat other manifolds sand cast from the materials designated alloy 355.0 - T6, UNS Number A03550, and a die cast aluminum alloy designated BS 1490-LM20 having a specification of 13.0 Si, 1.0 Iron, 0.5 Mn, 0.4 Cu, 0.2 Mg, 0.2 Zn, 0.1 Ti, 0.1 Ni, 0.1 Pb and 0.1 Sn.
• Diode plates for automobile alternators that were stamped into the desired configuration using extruded aluminum alloy 6061-T6, AMS 4150G were coated using the process of this invention.
• the diode plates were approximately 5" (13 cm) long, 5/8" (1.6 cm) wide and 1/8" (3.2 mm) thick and in the shape of an arcuate segment of a circle having a radius of about 5" (13 cm), and provided with a plurality of openings for receiving and supporting diodes.
• a quantity of the stamped diode plates were positioned in rotatable barrels, as opposed to the racks described in Example I, and the barrels were sequentially processed through the same coating solutions used in Example I except that steps 4-6 were omitted and certain of the times of immersion in some of the other solutions were changed.
• step 1 the immersion was for 3 mins.
• step 7 the immersion was for 2-3 mins.
• step 11 the silicon-chromate coating forming tank, the immersion time was 12 mins and immersion time in the rinses in steps 12 - 15 was for a total of 5 mins.
• the coated diode plates retained the aluminium appearance of the stamped parts and were coated with an adherent, scratch and chip resistance coating having a thickness of approximately 2 ⁇ m.
• the coated diode plates from step 17 were tested for their ability to continue to pass current when assembled into an automobile alternator that was positioned in a salt spray cabinet using the slat spray test conditions of ASTM B-117.
• the diode plates were found to resist salt spray corrosion and to continue to pass the test current without failure for 1000 hours.

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• 1990
  • 1990-08-16 EP EP19900309006 patent/EP0419032A3/en not_active Withdrawn
  • 1990-08-29 CA CA 2024264 patent/CA2024264C/fr not_active Expired - Fee Related
  • 1990-09-17 JP JP24693890A patent/JPH03120379A/ja active Pending

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