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
  • C23C22/34—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 containing fluorides or complex fluorides
• • 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

Definitions

• Such Cr VI - free, Cr III - based systems are an improvement on Cr VI - based systems especially with regard to effluent treatment, but they are not a complete answer to the problem because Cr III is itself somewhat toxic, although much less so than Cr , and the effluent concentration maximum limits are fairly stringent (ca 2 ppm).
• the conversion coating contains hydrated aluminium oxide and will usually include cationic species derived from the substrate metal, probably also as hydrated oxides, and may include other species occluded from the solution.
• the low acidity of the solution causes the substrate dissolution reaction, which is a necessary prerequisite for the formation of a conversion coating, to be choked preventing the formation of satisfactory coherent conversion coatings.
• an alternative reaction path is provided which does not require high acidity to promote the dissolution reaction. The nitrate reacts according to the following equation: This reaction effectively absorbs H thus causing the pH to rise.
• the solutions of Al III used in this invention present a very small polution hazard especially as compared with Cr and even more so as compared with Cr IV . Indeed, it is well known that aluminium is effectively non toxic in dilute solution and aluminium salts are added to waste waters, especially those containing heavy metal pollutants, in the detoxification of such wastes. Further, the other components used in the solutions do not give rise to severe pollution problems. In some cases the solutions contain fluoride, but the concentration of fluoride used in low and fluoride effluent treatment techniques are established and we do not expect any problems in the handling of this fluoride.
• the solutions used in the invention may include one or more surfactants to enhance wetting of the substrate by the solution.
• surfactants we have not found it essential to use surfactants, but under less carefully controlled conditions, such as on an industrial production line, their inclusion may be beneficial particularly in preventing air bubbles adhereing to and obscurring the substrate surface or in removing small particles of dirt or smears of oil or grease on the substrate from handling prior to conversion coating.
• the surfactants used are "compatible surfactants" by which we mean that they do not form insoluble products with other components of the solution. For example the aluminium salts of stearic and other long chain carboxylic acids are insoluble and, therefore, simple salts of such acids are not compatible surfactants.
• the amount of compatible surfactant used will depend upon the particular circumstances but will typically be from 5 to 250 ppm and more usually from 10 to 100 ppm, by weight.
• boric acid or a borate salt to a solution containing fluoride will generate fluoborate in situ.
• concentration of fluoride used in solutions for providing conversion coatings on aluminium or its alloys will usually be from 1 to 20 gl -1 , optimally 3 to 8 gl , (expressed as NaF) when added as a simple salt and from 3 to 15 gl -1 (as fluorosilicate) when added as a complex salt.
• aluminium(including aluminium alloys) as substrates in this invention is important because aluminium is finding increasingly wider applications in which the surface of the metal is to be subsequently painted, lacquered or laminated to plastics. In such applications it has been found that the oxide film on the aluminium surface generally does not allow paint or lacquer to adhere strongly. Contrary to this the aluminium oxide conversion coatings produced according to the present invention do enhance the adherion of paint or lacquer films and laminated layers. Furthermore, coating aluminium with a conversion coating is a generally quicker and cheaper process than the usual anodizing process.
• the absolute value of pH is not the only factor and we have found that the most marked beneficial results are obtained when the pH is close to that at which hydrated aluminium oxide spontaneously precipitates from the bulk of the solution.
• pH the “pH of precipitation” in any particular case.
• the absolute value of the pH of precipitation is a property of the aluminium containing species in the solution, rather than simply of aluminium itself, and the species present depend on the ligands in the solution. Additionally, the solution may contain non- equilibrium species, typically hydrated aluminium-anion complex ions which are not thermodynamically stable but which persist for times comparable with the practical useful life of any particular sample of solution.
• phosphate can cause precipitation of the metal from solution either by premature precipitation in the film of solution adjacent to the substrate or in extreme cases by precipitation from the bulk of-the solution. Additionally, we have found that the presence of phosphate in the conversion coating has a deleterious effect on the properties of the coating. As well as phosphate other anions, especially the highly oxidizing, high oxidation state metal oxyanions interfere with the deposition of the desired conversion coatings. Examples of such anions and manganate., permanganate chromate and dichromate. These materials are not deliberately added to the solutions used in this invention and care will generally be taken to ensure their absence.
• contact of the substrate with the reaction solution for a period of only a few seconds will generally be sufficient to give a coherent conversion coating on the substrate.
• the period of time of contact will be selected depending on the thickness of the conversion coating desired which, in turn, will depend on how the coated substrate will be used. Typically, the period of time for which the substrate is in contact with the solution (the “contact time”) will be from 5 seconds to 2 minutes.
• the contact time will be chosen to suit the intended use of the treated substrate. For “key” coatings (further described below) the contact time will typically be from 5 to 15 seconds. For corrosion resistant coatings the contact time will typically be from 10 seconds to 2 minutes. The contact time will usually be longer than time for which the substrate is immersed in the solution (the immersion time").
• Freshly deposited coatings are soft and can be removed from the substrate by mild abrasion.
• the coatings can, however, be hardened and made more resistant to mechanical abrasion by air drying, e.g. at ambient temperature preferably for not less than 24 hours.
• air drying e.g. at ambient temperature preferably for not less than 24 hours.
• the beneficial effects of drying can be accelerated by placing the coated substrates in an oven, preferably at temperatures in the range 60 to 120 0 C for periods of from 1 to 2 hours.
• conversion coatings may serve as primer coatings for subsequent coatings of paint or lacquer.
• the conversion coating enables enhanced adhesion of the paint or lacquer coating to be achieved and provides additional protection against corrosion by suppressing under-film (i.e. under paint or lacquer) corrosion of substrate metal.
• the conversion coatings may also be used to "key" layers of plastics materials to the metal substrates in the productions of laminates, to improve the strength of the metal-plastic bond.
• Example 1 was repeated varying the concentrations of the solution constituents, solution pH and dipping time within the limits set out below Panels treated at ambient temperature (20 to 25°C) throughout these ranges had salt spray resistances of up to 1000 hours.
• a steel panel was electroplated with 5 ⁇ m tin, dried and immersed for 30 seconds in a solution of the following composition After drying a pale blue film was visible on the surface.
• the solution was modified by adding 10 gl -1 of sodium borate and the pH was adjusted(increased)until a slight precipitate of hydrated aluminium oxide was present.
• a second similar tin plate panel was immersed for 30 seconds in the solution and after drying a blue-yellow film was visible on the surface.
• These two panels together with a third similar but untreated tin plate panel were exposed to neutral salt spray testing. After 24 hours of testing the untreated panel had tin corrosion products on its surface but no corrosion of either of the coated panels had occurred.

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• Chemical & Material Sciences (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemical Treatment Of Metals (AREA)
• Separation By Low-Temperature Treatments (AREA)
• Fertilizers (AREA)
• Nonmetallic Welding Materials (AREA)

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Publications (1)

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Cited By (2)

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Citations (8)

• 1981
  • 1981-06-30 ES ES503554A patent/ES8203984A1/es not_active Expired
  • 1981-07-01 AU AU73201/81A patent/AU7320181A/en not_active Abandoned
  • 1981-07-01 EP EP81302990A patent/EP0044171A1/en not_active Withdrawn
  • 1981-07-01 JP JP56502227A patent/JPS57501532A/ja active Pending
  • 1981-07-01 WO PCT/GB1981/000119 patent/WO1982000160A1/en unknown
• 1982
  • 1982-02-26 DK DK85182A patent/DK85182A/da not_active Application Discontinuation
  • 1982-03-01 NO NO820627A patent/NO820627L/no unknown

Patent Citations (10)

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