WO1997008364A1 - Passification of tin surfaces - Google Patents

Passification of tin surfaces Download PDF

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Publication number
WO1997008364A1
WO1997008364A1 PCT/US1996/013859 US9613859W WO9708364A1 WO 1997008364 A1 WO1997008364 A1 WO 1997008364A1 US 9613859 W US9613859 W US 9613859W WO 9708364 A1 WO9708364 A1 WO 9708364A1
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Prior art keywords
tin
chromium
trivalent chromium
article
solution
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PCT/US1996/013859
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French (fr)
Inventor
John W. Bibber
Original Assignee
Sanchem, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by Sanchem, Inc. filed Critical Sanchem, Inc.
Priority to AU68630/96A priority Critical patent/AU6863096A/en
Priority to US09/029,351 priority patent/US6099714A/en
Publication of WO1997008364A1 publication Critical patent/WO1997008364A1/en

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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
    • C25D11/38—Chromatising
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
    • C25D5/34—Pretreatment of metallic surfaces to be electroplated
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D9/00—Electrolytic coating other than with metals
    • C25D9/04—Electrolytic coating other than with metals with inorganic materials
    • C25D9/08—Electrolytic coating other than with metals with inorganic materials by cathodic processes

Definitions

  • the invention relates to a tin coating composition and a method of providing tin with a protective coating. More particularly, the present invention provides a tin coating composition having as its essential ingredient chromium in the trivalent state (chromium III) , and a method of coating the tin using a trivalent chromium composition having a pH of about 2 to about 3.
  • the thin natural oxide film which forms on tin surfaces provides a useful protective barrier and improves paint adhesion. If uncontrolled, however, the thin oxide film will form a thick yellow non-protective and non-adhesive layer. Therefore, a goal of tin producers has been to provide tin with an oxide film while preventing its rapid uncontrolled growth to a thick layer which prevents the effective adhesion of paint to the tin surface. Tin producers have passified tin surfaces in various ways to provide tin with a thin oxide film while preventing its rapid uncontrolled growth. In addition, proper passification of tin surfaces prevents or substantially reduces sulfide stains.
  • Patent 2,024,951 discloses a process which uses potassium permanganate to both stabilize the oxide film and reduce sulfide staining.
  • U.S. Patent #2,215,165 disclosed an electrolytic process which oxidized and then reduced the tin surface to thicken the oxide film and leave a passive tin surface. That same year Ken R., Protective Films on Tin Plate bv Chemical Treatment. J. Sol. Chem. Ind., 59, 259 C 1940, disclosed the results of an investigation which showed that an alkaline solution of chromates would passivate tin surfaces.
  • U.S. Patent 2,312,076, disclosed a process which used dichromates mixed with phosphates to passivate tin surfaces. Since that time new patents, i.e., Patent 2,606,866, have been issued, but have all centered around improvements in the basic dichromate/chromic acid processes mentioned.
  • chromium containing coating compositions A major drawback of the prior arts chromium containing coating compositions is their reliance on hexavalent chromium. Hexavalent chromium is extremely toxic and thus more costly to work with. For instance, hexavalent chromium requires special waste disposal.
  • It is an object of the present invention to provide a method for coating tin or tin plated articles with a non- hexavalent chromium protecting coating comprising coating said tin or tin plated article with a hexavalent chromium free trivalent chromium composition having a pH of about 2.0 to about 3.0.
  • one aspect of the invention is directed toward providing a method of coating an article of tin or tin plated material with a protective coating.
  • the process provides an electrolyte composition having an acid pH of about 2 to about 3 and as its essential ingredient, chromium in the trivalent state.
  • the process contacts the article with the electrolyte composition and deposits trivalent chromium on the article by electrolysis.
  • Another aspect of the invention is directed at a trivalent chromium coating composition for coating tin.
  • Still another aspect of the invention is directed at a manufacture of an article of tin or tin coated material having a trivalent chromium protective coating thereon.
  • the present invention can be further understood with reference to the following description and examples.
  • the present invention eliminates the need for hexavalent chromium compositions which, due to their extreme toxicity, are being forced out of the work place environment.
  • the invention provides substantially the same high level of corrosion resistance and sulfide stain resistance as that given by hexavalent chromium compositions while at the same time being considerably less toxic. Very little toxic waste is produced and thus very little needs to be disposed of.
  • this invention provides a protective coating for tin and tin plated surfaces which has as its essential ingredient trivalent chromium (Chromium (III) ) .
  • the process uses electrolysis of various trivalent chromium solutions to deposit a thin film of various reduced chromic oxides or hydroxides.
  • concentration of chromium (III) in solution and the amount of current applied determines the amounts of chromic oxide, Cr 2 0 3 , trivalent chromium hydroxide and chromium deposited and the time needed to deposit them.
  • a concentration of chromium (III) below 0.05 grams/liter is not economically feasible to deposit because the corrosion resistant film has insufficient thickness.
  • the upper concentration limit is the saturation point of the chromium
  • (III) salt in question To provide a strong thin coating, a pH range of about 2.0 to about 3.0 is used. A high pH produces a film with an undesirable gray color and reduced corrosion resistance. Below pH 2.0 the chromium oxides and/or hydroxides are dissolved off the metal surface as fast as they are formed. The most preferred pH is 2.25 to 2.40. The pH, when necessary, is adjusted and maintained adding an appropriate buffer. As seen in example 3, infra, a pH of 1.9 shows a marked increase in yellowing oxide formation which is not desirable. The prior art simply does not recognize the criticality of applicant's pH range. In fact Patent 4,875,983, indicates a preferable pH between 0.5 and 1.5.
  • the voltage should preferably be above the reduction potential of chromium (III). In water this is about 0.74 volts relative to the standard hydrogen electrode.
  • the preferred chromium (III) salts are the chloride and the sulfate.
  • Other chromium (III) salts have anions which inhibit production of a passive film on tin surfaces.
  • the sulfate is the most preferred because the chloride may release chlorine gas at the anode of the electrolytic cell and cause environmental problems. In addition, the sulfate is generally less expensive.
  • Potassium chromium sulfate is the most preferred sulfate as simple chromium sulfate is not as conductive and is less soluble.
  • Various other materials may be added to the bath to increase conductivity, prevent formation of hexavalent chromium and to clean or wet out the surface of the tin as long as these materials do not inhibit film formation or destroy film quality. Although not desirable from an environmental point of view the presence of hexavalent chromium will not be harmful to the bath.
  • My composition and process preferably does not contain any complexing agents or agents which interfere with the depositing of trivalent chromium oxide on tin.
  • Some examples of such agents are aminosulfuric acid, ammonium ions, formic acid, hypophosphite, glycine, gluconolactone, glycollic acid, glycollic acetate and glycollic formate.
  • the following examples 1-21 illustrate the invention in detail.
  • the tin plated steel surface or pure tin of the examples was cleaned of oils and/or loose soil with a non ⁇ ionic detergent. Then the cleaned tin plated steel or tin sheet is made the cathode of an electrolytic cell of 12 volts and 10 amps for 30 seconds in a dilute sodium carbonate solution (2.5 grams per liter) to obtain a clean reactive surface. The clean surface was then rinsed in D.I. water and treated as indicted immediately.
  • the tin plated steel and tin of the examples were 3 inches by 5 inch metal strips.
  • the applied voltage was 12 volts and the applied amperage was 10 or 2 amps. Exposure time was two to five seconds. Examples 1 through 3 show the unacceptable results of using a pH outside of the desired range.
  • Example 1 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2 12H 2 0) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 3.5. The light gray colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed slight sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed slight yellow stains due to tin oxide formation.
  • Example 2 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2 12H 2 0) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 3.25. The very light gray colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed slight sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed very light yellow stains due to tin oxide formation.
  • Example 3 A solution of 5.0 g/1 potassium chromium
  • a solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2. ⁇ 12H 2 0) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plated steel sheet was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3.
  • the tin plated steel was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes.
  • the exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
  • Example 5 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2 . 12H 2 0) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3. The tin was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
  • Example 6 A solution of 4.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2 . 12H 2 0) which contains about 0.42 grams of chromium (0.042%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3. The tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed slight sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed slight yellowing due to tin oxide formation.
  • Example 7 A saturated solution of potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2. 12H 2 0) was used as the electrolyte for an electrolytic cell in which tin plate as the cathode and stainless steel the anode. Twelve volts and two amps were applied to the solution for two seconds at 70°F and a pH of 2.3. The tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
  • Example 8 A saturated solution of potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2 12H 2 0) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and two amps were applied to the solution for two seconds at 200°F and a pH of 2.3.
  • the tin plate as rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1/0 g/1 non-ionic wetting agent for two minutes.
  • the exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
  • Example 9 A saturated solution of potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2, 12H 2 0) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and two amps were applied to the solution for two seconds at 30°F and a pH of 2.3. The tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
  • Example 10 A solution of 5.0 g/1 chromium (III) chloride hexahydrate was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for two seconds at 70°F and a pH of 2.3. The tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
  • Example 11 A solution of 5.0 g/1 chromium (III) sulfate hydrate was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for two seconds at 70°F and a pH of 2.3. The tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
  • Example 12 A solution of 5.0 g/1 chromium (III) sulfate hydrate was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless
  • a solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2 . 12H 2 0) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 3.0.
  • the silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes.
  • the exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellow stains due to tin oxide formation.
  • Example 13 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2 . 12H 2 0) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.5. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellow stains due to tin oxide formation.
  • Example 14 A solution of 5.0 g/1 potassium sulfate twelve hydrate (KCr(S0 4 ) 2 . 12H 2 0) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.25. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes.
  • the exposed tin surface showed a lack of sulfide staining even on the s de of the tin plate not facing the anode. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellow stains due to tin oxide formation.
  • Example 15 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2 12H 2 0) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.0. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed very slight sulfide stains. Baking the rest of the exposed tin surface at 420°F for one hour showed very light yellow stains due to tin oxide formation.
  • Example 16 A solution of 5.0 g/1 potassium chromium s
  • a solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2 . 12H 2 0) , which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.4. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes.
  • the exposed tin surface showed no sulfide staining and minor sulfide on the side not exposed to the anode. Baking the rest of the tin surface at 420°F for one hour showed no yellow stains, on either side of the tin plate, due to tin oxide formation.
  • Example 17 A solution of 5.0 g/1 chromium citrate was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3.
  • the silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes.
  • the exposed tin surface showed extensive sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed extensive yellow stains due to tin oxide formation.
  • Citric acid is known to form a protective film over tin plate.
  • Example 18 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2. 12H 2 0) , to which 100 mg/l of sulfite (SCV 2 ) was added to prevent the formation of hexavalent chromium, was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3. The silvery colored tin plate was rinsed in D.I.
  • the silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes.
  • the exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellow stains due to tin oxide formation.
  • Example 20 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2 12H 2 0) , to which 500 mg/l of a non-ionic wetting agent was added, was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed in tin surface at 420°F for one hour showed no yellow stains due to tin oxide formation.
  • Example 21 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S0 4 ) 2 12H 2 0) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3 maintained by use of a Potassium hydrogen phthalate sulfuric acid buffer system. The tin plate was rinsed in D.I.

Abstract

The present invention provides a method of coating a tin article with a trivalent chromium oxide and/or hydroxide in which the tin article is contacted with a trivalent chromium protective coating composition having an acid pH of about 2.0 to 3.0 and the trivalent chromium oxide and/or hydroxide is deposited on the article by electrolysis, the electroplating composition and an article of tin coated with a protective trivalent chromium oxide and/or hydroxide.

Description

PASSIFICATION OF TIN SURFACES
FIELD OF INVENTION
The invention relates to a tin coating composition and a method of providing tin with a protective coating. More particularly, the present invention provides a tin coating composition having as its essential ingredient chromium in the trivalent state (chromium III) , and a method of coating the tin using a trivalent chromium composition having a pH of about 2 to about 3.
BACKGROUND OF INVENTION The thin natural oxide film which forms on tin surfaces provides a useful protective barrier and improves paint adhesion. If uncontrolled, however, the thin oxide film will form a thick yellow non-protective and non-adhesive layer. Therefore, a goal of tin producers has been to provide tin with an oxide film while preventing its rapid uncontrolled growth to a thick layer which prevents the effective adhesion of paint to the tin surface. Tin producers have passified tin surfaces in various ways to provide tin with a thin oxide film while preventing its rapid uncontrolled growth. In addition, proper passification of tin surfaces prevents or substantially reduces sulfide stains.
Previous attempts at passification have mainly centered around thickening the natural oxide film with an oxidant while leaving a corrosion resistant film on the surface of the metal to retard further oxide growth and prevent sulfide stains. U.S. Patent 1,827,204, discloses an electrolytic process which uses chromates to both thicken the oxide film and leave a film of reduced chromic oxides to prevent further oxide growth and prevent sulfide stains. French Patent 777,314, Tichauer, discloses a process which used sodium molybdates as an oxidant and various heavy metals to provide a passive film on the surface of the metal. U.S. Patent 2,024,951 discloses a process which uses potassium permanganate to both stabilize the oxide film and reduce sulfide staining. In 1940, U.S. Patent #2,215,165, disclosed an electrolytic process which oxidized and then reduced the tin surface to thicken the oxide film and leave a passive tin surface. That same year Ken R., Protective Films on Tin Plate bv Chemical Treatment. J. Sol. Chem. Ind., 59, 259 C 1940, disclosed the results of an investigation which showed that an alkaline solution of chromates would passivate tin surfaces. In 1943, U.S. Patent 2,312,076, disclosed a process which used dichromates mixed with phosphates to passivate tin surfaces. Since that time new patents, i.e., Patent 2,606,866, have been issued, but have all centered around improvements in the basic dichromate/chromic acid processes mentioned.
A major drawback of the prior arts chromium containing coating compositions is their reliance on hexavalent chromium. Hexavalent chromium is extremely toxic and thus more costly to work with. For instance, hexavalent chromium requires special waste disposal. SUMMARY OF INVENTION
It is an object of the present invention to provide a method for coating tin or tin plated articles with a non- hexavalent chromium protecting coating comprising coating said tin or tin plated article with a hexavalent chromium free trivalent chromium composition having a pH of about 2.0 to about 3.0.
It is a further object of the invention to provide a passified tin or tin plated article having coated thereon a trivalent chromium protective coating.
Accordingly, one aspect of the invention is directed toward providing a method of coating an article of tin or tin plated material with a protective coating. The process provides an electrolyte composition having an acid pH of about 2 to about 3 and as its essential ingredient, chromium in the trivalent state. The process contacts the article with the electrolyte composition and deposits trivalent chromium on the article by electrolysis.
Another aspect of the invention is directed at a trivalent chromium coating composition for coating tin.
Still another aspect of the invention is directed at a manufacture of an article of tin or tin coated material having a trivalent chromium protective coating thereon.
DETAILED DESCRIPTION The present invention can be further understood with reference to the following description and examples. The present invention eliminates the need for hexavalent chromium compositions which, due to their extreme toxicity, are being forced out of the work place environment. In addition, the invention provides substantially the same high level of corrosion resistance and sulfide stain resistance as that given by hexavalent chromium compositions while at the same time being considerably less toxic. Very little toxic waste is produced and thus very little needs to be disposed of. Accordingly, this invention provides a protective coating for tin and tin plated surfaces which has as its essential ingredient trivalent chromium (Chromium (III) ) . The process uses electrolysis of various trivalent chromium solutions to deposit a thin film of various reduced chromic oxides or hydroxides. The concentration of chromium (III) in solution and the amount of current applied determines the amounts of chromic oxide, Cr203, trivalent chromium hydroxide and chromium deposited and the time needed to deposit them. A concentration of chromium (III) below 0.05 grams/liter is not economically feasible to deposit because the corrosion resistant film has insufficient thickness. The upper concentration limit is the saturation point of the chromium
(III) salt in question. To provide a strong thin coating, a pH range of about 2.0 to about 3.0 is used. A high pH produces a film with an undesirable gray color and reduced corrosion resistance. Below pH 2.0 the chromium oxides and/or hydroxides are dissolved off the metal surface as fast as they are formed. The most preferred pH is 2.25 to 2.40. The pH, when necessary, is adjusted and maintained adding an appropriate buffer. As seen in example 3, infra, a pH of 1.9 shows a marked increase in yellowing oxide formation which is not desirable. The prior art simply does not recognize the criticality of applicant's pH range. In fact Patent 4,875,983, indicates a preferable pH between 0.5 and 1.5.
The voltage should preferably be above the reduction potential of chromium (III). In water this is about 0.74 volts relative to the standard hydrogen electrode.
The preferred chromium (III) salts are the chloride and the sulfate. Other chromium (III) salts have anions which inhibit production of a passive film on tin surfaces. The sulfate is the most preferred because the chloride may release chlorine gas at the anode of the electrolytic cell and cause environmental problems. In addition, the sulfate is generally less expensive.
Potassium chromium sulfate is the most preferred sulfate as simple chromium sulfate is not as conductive and is less soluble. Various other materials may be added to the bath to increase conductivity, prevent formation of hexavalent chromium and to clean or wet out the surface of the tin as long as these materials do not inhibit film formation or destroy film quality. Although not desirable from an environmental point of view the presence of hexavalent chromium will not be harmful to the bath.
My composition and process preferably does not contain any complexing agents or agents which interfere with the depositing of trivalent chromium oxide on tin. Some examples of such agents are aminosulfuric acid, ammonium ions, formic acid, hypophosphite, glycine, gluconolactone, glycollic acid, glycollic acetate and glycollic formate.
The following examples 1-21 illustrate the invention in detail. The tin plated steel surface or pure tin of the examples was cleaned of oils and/or loose soil with a non¬ ionic detergent. Then the cleaned tin plated steel or tin sheet is made the cathode of an electrolytic cell of 12 volts and 10 amps for 30 seconds in a dilute sodium carbonate solution (2.5 grams per liter) to obtain a clean reactive surface. The clean surface was then rinsed in D.I. water and treated as indicted immediately. The tin plated steel and tin of the examples were 3 inches by 5 inch metal strips. The applied voltage was 12 volts and the applied amperage was 10 or 2 amps. Exposure time was two to five seconds. Examples 1 through 3 show the unacceptable results of using a pH outside of the desired range.
Example 1 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2 12H20) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 3.5. The light gray colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed slight sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed slight yellow stains due to tin oxide formation.
Example 2 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2 12H20) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 3.25. The very light gray colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed slight sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed very light yellow stains due to tin oxide formation. Example 3
A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2 12H20) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 1.9. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed yellow stains due to tin oxide formation. Example 4
A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2. β 12H20) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plated steel sheet was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3. The tin plated steel was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
Example 5 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2. 12H20) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3. The tin was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
Example 6 A solution of 4.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2. 12H20) which contains about 0.42 grams of chromium (0.042%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3. The tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed slight sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed slight yellowing due to tin oxide formation.
Example 7 A saturated solution of potassium chromium sulfate twelve hydrate (KCr(S04)2. 12H20) was used as the electrolyte for an electrolytic cell in which tin plate as the cathode and stainless steel the anode. Twelve volts and two amps were applied to the solution for two seconds at 70°F and a pH of 2.3. The tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
Example 8 A saturated solution of potassium chromium sulfate twelve hydrate (KCr(S04)2 12H20) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and two amps were applied to the solution for two seconds at 200°F and a pH of 2.3. The tin plate as rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1/0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
Example 9 A saturated solution of potassium chromium sulfate twelve hydrate (KCr(S04)2, 12H20) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and two amps were applied to the solution for two seconds at 30°F and a pH of 2.3. The tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
Example 10 A solution of 5.0 g/1 chromium (III) chloride hexahydrate was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for two seconds at 70°F and a pH of 2.3. The tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.
Example 11 A solution of 5.0 g/1 chromium (III) sulfate hydrate was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for two seconds at 70°F and a pH of 2.3. The tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation. Example 12
A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2. 12H20) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 3.0. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellow stains due to tin oxide formation.
Example 13 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2. 12H20) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.5. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellow stains due to tin oxide formation.
Example 14 A solution of 5.0 g/1 potassium sulfate twelve hydrate (KCr(S04)2. 12H20) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.25. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed a lack of sulfide staining even on the s de of the tin plate not facing the anode. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellow stains due to tin oxide formation.
Example 15 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2 12H20) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.0. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed very slight sulfide stains. Baking the rest of the exposed tin surface at 420°F for one hour showed very light yellow stains due to tin oxide formation. Example 16
A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2. 12H20) , which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.4. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining and minor sulfide on the side not exposed to the anode. Baking the rest of the tin surface at 420°F for one hour showed no yellow stains, on either side of the tin plate, due to tin oxide formation.
Example 17 A solution of 5.0 g/1 chromium citrate was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed extensive sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed extensive yellow stains due to tin oxide formation. Citric acid is known to form a protective film over tin plate.
Example 18 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2. 12H20) , to which 100 mg/l of sulfite (SCV2) was added to prevent the formation of hexavalent chromium, was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellow stains due to tin oxide formation. Example 19
A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2 12H20) , to which 500 mg/l of potassium dichromate was added, was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellow stains due to tin oxide formation.
Example 20 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2 12H20) , to which 500 mg/l of a non-ionic wetting agent was added, was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3. The silvery colored tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed in tin surface at 420°F for one hour showed no yellow stains due to tin oxide formation.
Example 21 A solution of 5.0 g/1 potassium chromium sulfate twelve hydrate (KCr(S04)2 12H20) which contains about 0.52 grams of chromium (0.052%) was used as the electrolyte for an electrolytic cell in which tin plate was the cathode and stainless steel the anode. Twelve volts and ten amps were applied to the solution for five seconds at 70°F and a pH of 2.3 maintained by use of a Potassium hydrogen phthalate sulfuric acid buffer system. The tin plate was rinsed in D.I. water, dried, and placed in a boiling solution of 6.67 g/1 sodium thiosulfate five hydrate, 1.67 g/1 sulfuric acid and 1.0 g/1 non-ionic wetting agent for two minutes. The exposed tin surface showed no sulfide staining. Baking the rest of the exposed tin surface at 420°F for one hour showed no yellowing due to tin oxide formation.

Claims

WHAT IS CLAIMED IS:
1. A method for coating a tin or tin plated article with a non-hexavalent trivalent chromium oxide and/or trivalent chromium hydroxide protective coating comprising: electrocoating said tin or tin plated article with a hexavalent chromium free trivalent chromium oxide and/or chromium hydroxide composition by exposing the tin or tin plated article to a trivalent chromium composition having a pH of about 2.0 to about 3.0 and depositing trivalent chromium oxide and/or hydroxide on said tin article.
2. The method of claim 1 wherein the composition is free of a complexing agent.
3. The method of claim 2 wherein the pH range is about 2.25 to about 2.40.
4. The method of claim 3 wherein said trivalent chromium composition comprises trivalent chromium compounds selected from the group consisting of potassium chromium sulfate, trivalent chromium chloride, trivalent chromium sulfate and the hydrates thereof.
5. The method of claim 3 wherein the article is exposed to the trivalent chromium composition for up to 30 seconds.
6. The method of claim 5 wherein the tin or tin plated article is exposed to the trivalent chromium composition for about 2 to 5 seconds.
7. The method of claim 4 wherein the tin or tin plated article is cleaned and then made a cathode of an electrolytic cell having a dilute sodium carbonate solution to provide a clean reactive surface tin or tin plated article rinsing the clean reactive surface tin or tin plated article, placing the rinsed clean reactive surface tin or tin plated article as a cathode in the electrolytic trivalent chromium electrolyte, applying electrical current to the cathode for about 2 to 10 seconds to provide a trivalent chromium oxide and/or hydroxide protective coated tin or tin plated article.
8. A process for coating a tin or tin plated article with a protective coating comprising:
providing an electrolyte solution having as an essential ingredient a trivalent chromium (III) component and an acid pH of about 2.0 to about 2.4;
contacting said article with said electrolyte solution for up to 30 seconds; depositing trivalent chromium oxide and/or hydroxide on said article by electrolysis, and
said electrolysis utilizing a voltage above the reduction potential of chromium (III) .
9. The process claimed in claim 8 wherein the concentration of chromium (III) is maintained above .05 grams per liter of solution and said solution is substantially free of complexing agents.
10. The process as claimed in claim 9 wherein said chromium
(III) is a chromium salt selected from the group consisting of potassium chromium sulfate, a chromium sulfate, a chromium chloride, and hydrates thereof.
11. A passified tin or tin plated article having coated thereon a trivalent chromium oxide and/or hydroxide protective coating.
12. A protectively coated tin or tin plated article comprising:
an article of tin or tin plated material having an electrolytic coating of chromium (III) oxide and/or hydroxide.
13. A protective trivalent chromium coating composition for electrocoating tin comprising an acid pH of between about 2.0 to about 2.4 and being free of a complexing agent and having as an essential ingredient a trivalent chromium compound selected from the group consisting of.potassium chromium sulfate, chromium sulfate, chromium chloride, and hydrates thereof.
14. The composition as claimed in claim 13 wherein the pH is between about 2.25 and 2.40.
15. The composition claimed in claim 14 wherein the concentration of chromium (III) is maintained above .05 grams per liter of solution.
16. The composition as claimed in claim 15 having sulfite or potassium dichromate added to said composition.
PCT/US1996/013859 1995-08-31 1996-08-30 Passification of tin surfaces WO1997008364A1 (en)

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