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
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/31—Coating with metals
• • 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
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/54—Contact plating, i.e. electroless electrochemical plating
• • 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

Definitions

• This invention relates to the electroless surface treatment, by immersion, of magnesium, magnesium base alloys, magnesium-dissimilar metal assemblies, and metals other than magnesium.
• the surface treatment involves the plating of tin or the coating of a combination of tin and MgSn(OH) on the metal treated.
• magnesium will be used to refer also to magnesium base alloys.
• the surface to be treated be cleaned. This may be done either mechanically, or chemically, or both, demnding on the nature of the foreign material on the surface. Another factor in determining what method of cleaning to use is the desired surface texture. If a very "Ice smooth texture is desired, a rough mechanical means would not be appropriate. Any conventional metal degreasing and cleaning method may be used.
• the surface of the metal may be activated by a rinse in an activator pickle, such as the phosphoric acid-ammonium acid fluoride treatment used to prepare magnesium for zinc immersion coating. It is possible to eliminate this activating pickle step by adding a relatively small amount of a water-soluble acetate e.g. 0.25 to 3 percent sodium acetate to the coating bath.
• an activator pickle such as the phosphoric acid-ammonium acid fluoride treatment used to prepare magnesium for zinc immersion coating. It is possible to eliminate this activating pickle step by adding a relatively small amount of a water-soluble acetate e.g. 0.25 to 3 percent sodium acetate to the coating bath.
• This invention involves the composition of an immersion coating bath suitable for coating magnesium surfaces and for plating other metals, such as iron, steel, brass, or copper, alone or in assemblies of magnesium and dissimilar metals.
• This invention also includes methods of use of the immersion coating bath referred to above.
• the bath may be prepared by first dissolving the watersoluble stannate e.g. potassium stannate (K SnO -3H O) in water at room temperature, to provide, for example, 37 to 60 grams per liter, usually followed by the addition of a caustic alkali e.g. sodium hydroxide (NaOH). The solution is then heated to 60-85 C. and the watersoluble pyrophosphate e.g. tetrasodium pyrophosphate (Na P O is added to provide 30 to grams per liter. The solution is stirred until the pyrophosphate is dissolved and the bath becomes clear. An acetate such as sodium acetate (NaC I-I O -3H O) may then also be added in amount to provide 7 to 30 grams per liter.
• K SnO -3H O watersoluble stannate
• Na P O sodium hydroxide
• the caustic alkali is added to raise the pH of the solution to the desired level (about 13.2). This higher pH results in a more uniform, gray tin-containing coating on the metal being plated. Where no caustic alkali is added, the coating is black or dark gray. Samples run at the lower pH are useful, but they showed a streaked, uneven coating. Addition of caustic alkali sufficient to raise the pH much above the recommended value of 13.2 was found to retard the reaction, with a resultant lack of coating on the magnesium surface.
• the pH of the bath with no caustic alkali added is about 12, and with the caustic added in the desired amount, the pH is raised to about 13.2.
• the pH of the bath should be adjusted periodically during use by the addition of caustic alkali, because the coating operation results in a lowering of the pH. Typically, the pH will decline from 13.2 to 12.8 during plating of 1720 square centimeters of magnesium surface per liter of coating bath.
• the acetate is added to the bath in order to complex the precipitates formed in the reaction, mainly MgSn(OH) This helps to keep the solution clear and gives a longer bath life. Another reward from this addition is a more uniform coating, with a resultant increase in corrosion resistance. The better coating is a result of the complexing of the reaction products. This prevents the MgSn(OH) from being precipitated on the magnesium surface by a sedimentation process which would leave a loose coat having poor adherence and appearance.
• the sodium acetate also acts as a surface activating agent eliminating the need for the activating pickle mentioned above.
• the bath may be operated without the acetate and without the caustic alkali, but for best results in coating magnesium surfaces, the solution composition should be as recommended below in Table I.
• sodium stannate is a useful equivalent to potassium stannate
• potassium hydroxide may replace sodium hydroxide
• sodium acetate may be replaced by either ammonium acetate or potassium acetate.
• the ranges of concentrations for these substitute compounds are the same as the recommended ranges in the preferred bath composition given above in Table I.
• concentrations of the stannate and pyrophosphate salts are less than 45 grams per liter, a useful but nonuniform, gray to dark gray, streaked coating will form on magnesium surfaces. Concentrations greater than 55 grams per liter produce a lighter, uneven but still useful coating.
• the bath is operable at temperatures ranging from room temperature to the boiling point of the solution. In general operation, however, the bath temperature is maintained at about 8085 C. This speeds the reaction so that an immersion time of about minutes is sufficient. At room temperature, a period of from 8 to 16 hours is required.
• a major advantage of thisinvention is that not only magnesium but magnesium-dissimilar metal assemblies may be coated in one operation. I-Ieretofore, this has been unheard of without a great loss of corrosion resistance.
• Both of the types of solutions described in this invention will plate a combination of about 50 percent Sn and 50 percent BgSn(OH) on a magnesium surface.
• the other metal surface receives a true tin plate.
• the anodic material may be magnesium or any other suitable metal electropositive to the metal to be coated, but below sodium in the series. Immersion time would be dependent on the electromotive potential which may limit the choice of anodic metal. It is to be noted that if a metal other than magnesium is to be plated, it must V sive electrical anodizing apparatus.
• the surface should be neutralized if it is to receive further treatment with organic materials at this stage (e.g. painting). This is due to the high alkalinity of the coating.
• This neutralization may be done by immersion in a 5 percent ammonium acid fluoride for 2 minutes, or in a 5 percent sodium bifluoride for 1 minute, or in some comparable solution for an appropriate time. Either neutralization suggested here may be carried out at room temperature.
• a major advantage of this invention over the Dow #17 anodize process is that it is not necessary to use expen- Further, using the process and bath described in this invention, magnesium, non-compatible metal assemblies maybe treated after fabrication, if desired.
• the Dow #17 treatment is good for general corrosion, but has little effect on galvanic corrosion.
• EXAMPLE 1 Two sets of 4 x 5 x 0.25" panels of A2318 magnesium alloy having a 1 X 5 x 0.125" mild steel strap coupled to the center of the magnesium panel with three A" mild steel round head bolts were treated, respectively, with: the Dow #17 anodize, and the immersion process and bath described as preferred in Table I of this application. After treatment, both sets of panels were coated with a clear primer and subjected to corrosion tests, the results of which appear below in Table II.
• the immersion process and solution described in this invention is superior over long exposure periods.
• the tests were run on magnesium-dissimilar metal assemblies, since these would show a more aggravated corrosion in a shorter time than would plain magneisum test panels.
• EMMPLE 2 LEI-142 magnesium alloy panels having the nominal composition of 14 percent Li, 2 percent Al, and the balance Mg were used in a stagnant immersion test with 3 percent NaCl as described in AST M B43T. The panels were 3 x 6 x 0.100" with four x flat head 7 mild steel fasteners attached. These samples were not painted after treatment. Table III.
• a process for providing a protective coating comprising tin on metal articles of the class of magnesium, magnesium base alloys, and assemblies thereof with metals of the class of iron, steel, copper, and brass which comprises contacting the clean metal article with an aqueous bath comprising a water-soluble stannate in a concentration of from 37 to 60 grams per liter, a water-soluble pyrophosphate in a concentration of from 30 to 100 grams per liter, and a water-soluble caustic alkali in a concentration of from 7.5 to 12.5 grams per liter.
• a process as in claim 1 wherein the water-soluble stannate is K SnO -3H O and the water-soluble pyrophosphate is Na P O 3.
• a process as in claim 1 wherein the water-soluble stannate is Na SnO 3H O.
• a process as in claim 7 wherein the water-soluble acetate is KC H O 11.
• stannate is K SnO -3H O
• the pyrophosphate is Na P O
• the caustic alkali is NaOH
• the acetate is NaC H O -3H O.
• An aqueous immersion coating bath composition comprising from 37 to grams per liter of a water-soluble stannate, from 30 to grams per liter of a watersoluble pyrophosphate, and from 7.5 to 12.5 grams per. liter of a water-soluble caustic alkali.
• An aqueous immersion coating bath composition consisting essentially of a water-soluble stannate in a concentration of from 45 to 60 grams per liter and a Watersoluble pyrophosphate in a concentration of from 45 to 100 grams per liter.
• An aqueous immersion coating bath composition as in claim 20 wherein the acetate is NH C H O 23.

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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)
• Electrochemistry (AREA)
• Chemical Treatment Of Metals (AREA)
• Chemically Coating (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

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• 0
  • BE BE621154D patent/BE621154A/xx unknown
• 1961
  • 1961-08-07 US US129505A patent/US3231396A/en not_active Expired - Lifetime
• 1962
  • 1962-08-02 GB GB29706/62A patent/GB955775A/en not_active Expired
  • 1962-08-07 DE DED39576A patent/DE1237871B/de active Pending

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