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
  • C23C22/05—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 using aqueous solutions
  • C23C22/06—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 using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/48—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 using aqueous solutions using aqueous acidic solutions with pH less than 6 not containing phosphates, hexavalent chromium compounds, fluorides or complex fluorides, molybdates, tungstates, vanadates or oxalates
  • C23C22/56—Treatment of aluminium or alloys based thereon
• • 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
  • C23C22/05—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 using aqueous solutions
  • C23C22/60—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 using aqueous solutions using alkaline aqueous solutions with pH greater than 8
  • C23C22/66—Treatment of aluminium or alloys based thereon
• • 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
  • C23C22/05—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 using aqueous solutions
  • C23C22/68—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 using aqueous solutions using aqueous solutions with pH between 6 and 8

Definitions

• This environmental-quality invention is in the field of chemical conversion coatings formed on metal substrates, for example, on aluminum substrates. More particularly, one aspect of the invention is a new type of oxide coating (which I refer to as a "cobalt conversion coating") which is chemically formed on metal substrates.
• the invention enhances the quality of the environment of mankind by contributing to the maintenance of air and water quality.
• chemical conversion coatings are formed chemically by causing the surface of the metal to be "converted" into a tightly adherent coating, all or part of which consists of an oxidized form of the substrate metal.
• Chemical conversion coatings can provide high corrosion resistance as well as strong bonding affinity for paint.
• the industrial application of paint (organic finishes) to metals generally requires the use of a chemical conversion coating, particularly when the performance demands are high.
• aluminum protects itself against corrosion by forming a natural oxide coating, the protection is not complete.
• aluminum alloys particularly of the high-copper 2000-series aluminum alloys, such as alloy 2024-T3, corrode much more rapidly than pure aluminum.
• the first is by anodic oxidation (anodization) in which the aluminum component is immersed in a chemical bath, such as a chromic or sulfuric acid bath, and an electric current is passed through the aluminum component and the chemical bath.
• a chemical bath such as a chromic or sulfuric acid bath
• an electric current is passed through the aluminum component and the chemical bath.
• the resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes.
• the second type of process is by chemically producing a conversion coating, which is commonly referred to as a chemical conversion coating, by subjecting the aluminum component to a chemical solution, such as a chromic acid solution, but without using an electric current in the process.
• a chemical solution such as a chromic acid solution
• the chemical solution may be applied by immersion application, by manual application, or by spray application.
• the present invention relates to this second type of process for producing chemical conversion coatings.
• the chemical solution may be applied by immersion application, by various types of manual application, or by spray application.
• chromic acid process for forming chemical conversion coatings on aluminum substrates is described in various embodiments in Ostrader et al. U.S. Patent 2,796,370 and Ostrader et al. U.S. Patent 2,796,371, in military process specification MIL-C-5541, and in Boeing Process Specification BAC 5719.
• These chromic acid chemical conversion baths contain hexavalent chromium, fluorides, and cyanides, all of which present significant environmental as well as health and safety problems.
• chromic acid conversion bath such as ALODINE 1200
• Cr ⁇ 3 - "chromic acid” hexavalent chromium
• NaF - sodium fluoride KBF4 - potassium tetrafluoroborate
• Chromic acid conversion films as formed on aluminum substrates, meet a 168 hours corrosion resistance criterion, but they primarily serve as a surface substrate for paint adhesion. Because of their relative thinness and low coating weights (40-150 milligrams/ft ⁇ ), chromic acid conversion coatings do not cause a fatigue life reduction in the aluminum structure.
• the invention is a process for forming a cobalt conversion coating on a metal substrate, thereby imparting corrosion resistance and paint adhesion properties.
• the invention was developed as a replacement for the prior art chromic acid process. The process includes the steps of: (a) providing a cobalt
• the substrate may be aluminum, aluminum alloy, as well as Cd plated, Zn plated, Zn-Ni plated, and steel.
• the cobalt-III hexavalent complex is preferably present in the form of Me m [Co(R)6] n , wherein Me is Na, Li, K, Ca, Zn, Mg, or Mn, and wherein ⁇ i is 2 or
• n 1 or 2
• R is a carboxylate having 1 to 6 C atoms.
• the invention is a chemical conversion coating solution for producing a cobalt conversion coating on a metal substrate, the solution including an aqueous solution containing a soluble cobalt-III hexacarboxylate complex, the concentration of the cobalt-III hexacarboxylate complex being from about 0.01 mole per liter of solution to the saturation limit of the cobalt-III hexavalent complex.
• the cobalt conversion solution may be prepared by a bath makeup sequence including the steps of: (a) dissolving a soluble cobalt-II salt, preferably cobalt acetate, and (b) dissolving a metal acetate salt such as sodium, magnesium or calcium acetate to form a conversion coating solution.
• wetting agents such as alkyl fluorides, fluorocarbons, and metal fluorides can be added to the conversion coating solutions. Addition of these wetting agents eliminate the need for a costly sealing step following formation of the conversion coating.
• FIGURES 1 through 4 are photomicrographs (scanning electron microscope operated at 20 KV) of alloy 2024-T3 test panels with cobalt conversion coatings made by the invention.
• FIGURES 1 through 4 show a cobalt conversion coatings 410 formed by a 15 minute immersion in a typical cobalt conversion coating solution at 140°F.
• FIGURE 1 is a photomicrograph at XI 0,000 magnification of a test panel showing a cobalt conversation coating 410 of the invention.
• the photomicrograph is a top view of the upper surface of oxide coating 410.
• the top of oxide coating 410 is porous and looks like a sponge. This test panel was immersed in the cobalt conversion coating solution for 15 minutes.
• the white bar is a length of 1 micron.
• FIGURE 2 is a photomicrograph at X70,000 magnification of the test panel of FIGURE 1.
• the photomicrograph is a top view of the upper surface of oxide coating 410.
• FIGURE 2 is a close-up, at higher magnification, of a small area of the test panel.
• the white bar is a length of 1 micron.
• FIGURE 3 is a photomicron at XI 0,000 magnification of another test panel showing a side view, from an elevated angle, of a fractured cross section of a cobalt conversion coating 420 of the invention.
• the fractured cross section of the aluminum substrate of the test panel is indicated by reference numeral 422. This test panel was immersed in the coating bath for ⁇ 5 minutes. To make the photomicrograph, the test panel was bent and broken off to expose a cross section of oxide coating 420.
• the white bar is a length of 1 micron.
• FIGURE 4 is a photomicrograph at X70.000 magnification of the test panel of
• FIGURE 3 showing a side view, from an elevated angle, of a fractured cross section of cobalt conversion coating 420 of the invention.
• FIGURE 4 is a close-up, at higher magnification, of a small area of the test panel.
• the aluminum substrate of the test panel is indicated by reference numeral 422.
• the white bar is a length of 1 micron.
• FIGURE 5 is a graph showing the tradeoff between paint adhesion and corrosion resistance as a function of immersion time.
• the present invention relates to a new cobalt conversion coating.
• the cobalt conversion coating can be made so resistant to corrosion that the conventional sealing step is no longer required. This result is achieved by adding metal fluorides and wetting agents such as alkyl fluorides and fluorocarbons to the conversion coating solution. It is believed that the combination of the wetting agents and the metal fluorides ⁇ impart a small etch effect on the aluminum substrate surface which is believed to aid in the coating formation.
• Cobalt ammine complexes were thus produced with a number of reactants, i.e., Co(NO 3 6H 2 O, CoCl 2 « 6H 2 O, NH4NO3, NH4CI and NH4OH.
• the resultant coatings formed on aluminum substrates were found to have substantially improved corrosion resistant over the simple salt immersion described earlier.
• a review of cobalt complexing chemistry yielded the following information:
• the ammonium nitrate is used to prevent initial precipitation of the reaction products.
• This chemistry is extensively reviewed in prior copending application Serial No. 07/525,800, filed May 17, 1990.
• An examination of reaction (4) revealed however, that while well defined iridescent coatings could be formed on aluminum substrates, an excess of ammonia, i.e., NH4OH was required to drive this reaction.
• NH4OH ammonia
• the amount of excess ammonia in the bath had a pronounced effect on the paint adhesion and corrosion resistance characteristics of coatings formed by this method. Paint adhesion and corrosion performance ranged from superb to complete failure, depending on the amount of ammonia in the bath.
• Coatings formed by this method are strongly colored, similar to prior chromated coatings in appearance. Coating weights in excess of 350 mg/ft ⁇ can be achieved. Nitrates such as NaNO3, Mg(NO3) 2 » 6H 2 O or Ca(NO3) 2 * 6H O were added to this formulation to aid in coating firmness at higher coating weight thus avoiding powdery loose deposits. As a result of this research, two bath formulations that resulted in a clear coating and a colored coating were selected for further testing. The following two equations below establish the chemistries for the clear and colored cobalt conversion coatings. Clear Coat:
• Equation (12) NH 4 (C 2 H3O 2 ) in Equation (12) would eliminate the above competing reactions described in conjunction with complexes (13) and (14), and result in the same strong colored coatings as the original ammonium acetate solutions.
• Sodium acetate is the most preferred metal carboxylate.
• Other metal carboxylates such as zinc, lithium, potassium, and manganese acetate will work but are not preferred.
• the typical reactions are
• Metal fluorides such as MgF 2 and CaF 2 and wetting agents such as water soluble alkyl-fluorides and fluorocarbons can also be added in very small quantities to these solutions (Tables III and IV) to improve corrosion protection and manufacturing ease.
• alkyl fluoride wetting agents such as MSP-ST alkyl fluoride by M&T Harshaw, Cleveland, Ohio, and fluorocarbons FC99 or FC95 wetting agents by 3M Company, St. Paul, Minnesota, have been successfully used.
• the presence of the fluorinated wetting agents, metal fluorides, or mixtures thereof raise the corrosion performance level of resultant coatings to such a degree that sealing of these conversion coatings in a secondary seal step is no longer required.
• any water soluble fluorinated wetting agent capable of lowering liquid surface tension into the range of 30 to 40 dynes per centimeter at 20 °C is usable.
• Solutions operated as detailed below yield coatings that pass a 168 hours salt spray corrosion resistance criteria in accordance with Boeing Process Specification BAC 5719, "Chromated Conversion Coatings.”
• the single step conversion coating has yielded in excess of 240 hours of salt spray resistance before showing any sign of corrosion pitting. It became apparent during experimentation with formulations (16), (17) and
• Cobalt acetate is the most preferred soluble cobalt-II salt.
• Other water soluble cobalt salts such as Co NO3) 2 , C0SO 4 , CoCl 2 , C0PO 4 , C0CO3, may be substituted for cobalt acetate, but are not preferred for the reasons illustrated in Table I.
• These cobalt salts are preferably reacted with soluble metal carboxylates having from 1 to 5 carbon atoms, although metal salts of acetic acid are most preferred.
• the carboxylate salts of Ca, Mg, and Na are preferred, with the Na carboxylate being most preferred. while Zn, Li, K, and Mn may also be used.
• the limitations on using carboxylates other than the acetates is water solubility.
• carboxylates that will work are for example sodium propionate.
• the minimum solubility needed to produce an effective coating is about 0.01 moles of cobalt-II salt per liter of water at 20°C (68°F)
• the salts may be used up to their solubility limits.
• fluorinated wetting agents may be added to the bath as discussed above. When these wetting agents are employed, a conversion coating is created that does not need to be subjected to a conventional sealing step in order to exhibit satisfactory corrosion resistance.
• the concentration of dissolved cobalt-II salt used may be from about 0.01 mole per liter of final solution up to the solubility limit of the cobalt-II salt employed at 20 °C (68°F)
• the concentration of dissolved cobalt-II salt used may be from about 0 04 mole per liter of final solution up to 0.15 mole per liter of final solution
• the concentration of the cobalt-III hexcarboxylate coordination complex may be from about 0.01 mole per liter of final solution up to the solubility limit of the cobalt-III hexcarboxylate coordination complex employed.
• the concentration of the cobalt-III hexcarboxylate coordination complex may be from about 0.04 mole per liter of final solution up to 0.15 mole per liter of final solution.
• the concentration of dissolved metal carboxylate may be from about 0.03 to 2.5 moles per liter of final solution.
• concentration of dissolved metal carboxylate used may be from about 0.05 mole per liter of final solution up to 0.2 mole per liter of final solution.
• the concentration of the fluorinated wetting agents is preferably sufficient to hold solution surface tension between 30 " to 40 dynes per centimeter at 20°C.
• the metal fluorides, MgF and CaF 2 may be present in a concentration from 0 to solubility limit. It is to be understood that the fluorinated wetting agents, metal fluorides, or mixtures thereof are not required, but are preferred. If the wetting agents and metal fluorides are not used, the conversion coating must be subjected to a sealing step to achieve high corrosion resistance. By using the wetting agents and fluorides, the sealing step can be eliminated, thus making the use of the present invention even more economical.
• the pH of the bath may be from about 5.0 to 9.0 with 6.0 to 7.5 being
• the temperature of the bath may be from about 68°F to 160°F. Above 160°F, gradual decomposition of the cobalt-III hexcarboxylate complex may occur. The optimum temperature is 140 ⁇ 5 °F.
• the immersion time may be from about 3 minutes to 60 minutes, more preferably from 5 to 30 minutes. When sodium acetate is employed, the immersion time can be reduced to 5 to 8 minutes. Use of these parameters will result in coating weights ranging for example from 20 to 240 mg/ft 2 .
• a " stainless steel tank is equipped with air agitation and temperature control equipment capable of controlling temperature within ⁇ 5°F. (The tank may be lined with an inert material capable of withstanding
• the tank is filled to 3/4-full with deionized water and heated to 120°F Air agitation is commenced to achieve a gentle boil.
• the cobalt conversion coating should be applied after all trimming and fabrication have been completed. Parts, where solution entrapment is possible, should not be subjected to immersion alkaline cleaning or immersion deoxidizing; manual cleaning and manual deoxidizing procedures should be used to obtain water break- free surfaces before applying cobalt conversion treatment.
• a water break-free surface is a surface which maintains a continuous water film for a period of at least 30 seconds after having been sprayed or immersion rinsed in clean water at a temperature below 100°F.
• Vapor degrease may be performed in accordance with Boeing Process Specification BAC 5408. Emulsion clean in accordance with Boeing Process Specification BAC 5763, or solvent clean in accordance with Boeing Process Specification BAC 5750 if parts are greasy or oily. Parts with open faying surfaces or spot-welded joints where solution entrapment is possible should be immersed in cold water (or in hot and cold water) for 2 minutes after precleaning.
• Alkaline clean and rinse may be performed in accordance with Boeing Process Specification BAC 5749, except for parts with open faying surfaces or spot welded joints, in which case, rinsing should be for at least 10 minutes using agitation with multiple immersions (a minimum of four times) followed by manual spray rinsing as required to prevent solution entrapment.
• Deoxidize Deoxidize and rinse may be performed in accordance with Boeing Process
• Castings may be deoxidized by either of the following methods: a. Deoxidize in accordance with Boeing Process Specification BAC 5765, Solution 37, 38 or 39. b. Dry abrasive blast castings in accordance with Boeing Process Specification BAC 5748, Type II, Class 1 and rinse.
• Optimum bath temperature from the standpoint of corrosion and adhesion was found to be 140°F ⁇ 5°F.
• FIGURE 5 depicts the general behavior of cobalt conversion coatings with respect to corrosion performance vs. paint adhesion.
• the intercept point of the corrosion and adhesion curve represents the bath parameters where the two divergent properties (corrosion and adhesion) are at optimum with respect to each other.
• the pH be maintained between pH 6.0 and 7.5, although coatings have been produced between pH 5.0 and 9.0. Adjustments to the pH may be required after the solutions have been used for extended periods. Corrosion Resistance
• ESCA electron spectroscopy for chemical analysis (also known as XPS or X-ray photoelectron spectroscopy).) These analyses show that the cobalt conversion coating consists of a mixture of oxides, namely, aluminum oxide Al 2 O 3 as the largest volume percent, and cobalt oxides CoO, Co 3 O 4 , and Co 2 O 3 .
• the term “largest volume percent” means that the volume of this oxide exceeds the volume of any other oxide which is present, but the term “largest volume percent” means that the volume of this oxide exceeds the volume of any other oxide which is present, but the term “largest volume present” does not necessarily imply that the volume of this oxide is more than 50 volume percent.
• the data further shows that in the lower portion of the oxide coating (that is, next to the aluminum substrate), the largest volume percent is Al 2 O 3 .
• the middle portion of the oxide coating is a mixture of CoO, Co 2 O 3 , Co 3 O4, and Al O 3 .
• the data shows that in the top portion of the oxide coating, the largest volume percent is a mixture of Co 2 O 3 and Co 3 O 4 .
• FIGS. 1 through 4 show a cobalt conversion coating 410 and 420 formed by a 15 minute immersion in a typical cobalt conversion coating solution.
• the top surface of the cobalt conversion coating as shown in FIGS. 1 through 4 bears a resemblance to a sponge, thus providing substantial surface area and porosity for good paint adhesion. Below the top surface, the coating becomes more dense and solid (non- porous).

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• Chemical & Material Sciences (AREA)
• Metallurgy (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• General Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Chemical Treatment Of Metals (AREA)
• Paints Or Removers (AREA)
• Chemically Coating (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
• Insulated Metal Substrates For Printed Circuits (AREA)

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Citations (4)

• 1993
  • 1993-01-18 CN CNB931017378A patent/CN1138873C/zh not_active Expired - Fee Related
  • 1993-06-22 MX MX9303745A patent/MX9303745A/es unknown
  • 1993-06-23 BR BR9306602A patent/BR9306602A/pt not_active IP Right Cessation
  • 1993-06-23 AU AU45008/93A patent/AU687740B2/en not_active Expired
  • 1993-06-23 NZ NZ253699A patent/NZ253699A/en not_active IP Right Cessation
  • 1993-06-23 JP JP50204994A patent/JP3345010B2/ja not_active Expired - Lifetime
  • 1993-06-23 SG SG1996004772A patent/SG43169A1/en unknown
  • 1993-06-23 RU RU94046218A patent/RU2135637C1/ru not_active IP Right Cessation
  • 1993-06-23 AT AT93914728T patent/ATE198631T1/de not_active IP Right Cessation
  • 1993-06-23 DE DE69329853T patent/DE69329853T2/de not_active Expired - Fee Related
  • 1993-06-23 ES ES93914728T patent/ES2152950T3/es not_active Expired - Lifetime
  • 1993-06-23 EP EP93914728A patent/EP0646187B1/en not_active Expired - Lifetime
  • 1993-06-23 DK DK93914728T patent/DK0646187T3/da active
  • 1993-06-23 WO PCT/EP1993/001630 patent/WO1994000619A1/en active IP Right Grant
  • 1993-06-23 CA CA002138790A patent/CA2138790C/en not_active Expired - Fee Related
  • 1993-06-24 ZA ZA934544A patent/ZA934544B/xx unknown
  • 1993-06-25 CN CNB931075831A patent/CN1195893C/zh not_active Expired - Fee Related
• 1994
  • 1994-12-23 NO NO19945026A patent/NO315522B1/no unknown
• 2001
  • 2001-03-09 GR GR20010400395T patent/GR3035554T3/el not_active IP Right Cessation

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