Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/04—Electroplating: Baths therefor from solutions of chromium
  • C25D3/06—Electroplating: Baths therefor from solutions of chromium from solutions of trivalent chromium
• • 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
  • C25D5/48—After-treatment of electroplated surfaces
• • 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
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12771—Transition metal-base component
  • Y10T428/12806—Refractory [Group IVB, VB, or VIB] metal-base component
  • Y10T428/12826—Group VIB metal-base component
  • Y10T428/12847—Cr-base component
  • Y10T428/12854—Next to Co-, Fe-, or Ni-base component

Definitions

• the present invention relates generally to a method of imparting improved corrosion protection to chromium plated substrates, which have been plated with chromium from a Cr ⁇ 3 plating bath.
• compositions and processes have been used or suggested for use in order to impart improved corrosion resistance to chromium plated substrates to prevent the formation of rust spots when exposed to a corrosive environment.
• the use of nickel/chromium electrodeposits on a metal or plastic substrate to provide a decorative and corrosion resistant finish is also well known,
• the nickel underlayer is deposited electrolyticaiiy from an electrolyte based on nickel sulfate or nickel chloride, and boric acid.
• This electrolyte also typically contains organic additives to make the deposit brighter and harder and also to confer leveling (i.e., scratch hiding) properties.
• the organic additives also control the electrochemical activity of the deposit and often duplex nickel deposits are applied where the layer closest to the substrate is more noble than the bright nickel deposited on top of it, This improves the overall corrosion performance as it delays the time required for penetration to the substrate by the corrosive environment.
• the total thickness of the nickel electrodeposited layer is between about 5 and about 30 micrometers in thickness.
• a thin deposit of chromium (typically about 300 nm in thickness) is applied from a solution of chromic acid containing various catalytic anions such as sulfate, fluoride, and methane disulfonate.
• the chromium metal deposited by this method is very hard and wear resistant and is electroehemlcaliy very passive due to the formation of an oxide layer on the surface. Because the chromium deposit is very thin, it tends to have discontinuities through which the underlying nickel is exposed. This leads to the formation of an electrochemical cell in which the chromium deposit is the cathode and the underlying nickel layer is the anode and thus corrodes.
• a further advantage of using chromic acid based electrolytes is that exposed substrate metal which is not covered by chromium in the plating process (such as steel on the inside of tubes and exposed steel through pores in the nickel deposit or even exposed nickel pores under the discontinuous chromium layer) is passivated by the strongly oxidizing nature of the chromic acid. This further reduces the rate of corrosion.
• chromic acid is extremely corrosive and toxic, it is also a carcinogen, a mutagen and is classified as reprotoxic. Because of this, the use of chromic acid is becoming more and more problematic, Tightening legislation is making it very difficult to justify the use of chromic acid in a commercial environment.
• Chromium plating processes based on the use of trivaleni chromium salts have been available since the mid-1970s and these processes have been refined over the years so that they are reliable and produce decorative chromium deposits.
• these chromium deposits do not behave the same in terms of their electrochemical properties as those deposited from a chromic acid solution.
• the chromium deposited from a trivaleni electrolyte is less pure than that deposited from a chromic acid solution and so is effectively an al loy of chromium.
• co-deposited materials may include carbon, nitrogen, iron and sulfur. These co-deposited materials have the effect of depolarizing the cathode reaction, thus increasing the rate of the electrochemical corrosion reaction and reducing the corrosion resistance of the coating.
• the trivaleni chromium electrolytes are not as strongly oxidizing in nature as hexavaleni chromium solutions, they do not passivate any exposed substrate material, having a further deleterious effect on the corrosion performance.
• there remains a need in the art for a method of passivating exposed substrates that is also able to decrease the rate of the cathodic reaction during galvanic corrosion of the nickel chromium deposit.
• ⁇ is another object of the present invention to improve the corrosion resistance of a chrorniurn(Ilf) plated article having an underlying nickel layer.
• the present invention relates generally to a method of treating a substrate, wherein the substrate comprises a plated layer deposited from a trivalent chromium electrolyte, the method comprising the steps of:
• Figure 1 depicts a Nyquist plot obtained from the results of Comparative Example 1.
• Figure 2 depicts a Bode plot obtained from the results of Comparative Example 1 .
• Figure 3 depicts a Nyquist plot obtained from the results of Example 1 .
• Figure 4 depicts a Bode plot obtained from the results of Example 1 .
• Figure 5 depicts a comparison of the corrosion of an unpassivated panel, a panel passivated with hexavalent chromium and a panel passivated with the trivalent chromium electrolyte of this invention
• the present invention relates generally to a method of providing improved corrosion protection to trivalent chromium plated substrates.
• the present invention is used to improve the corrosion resistance of trivalent chromium plated articles having a nickel plating layer underlying the chromium plated layer.
• the present invention may be used to improve the corrosion resistance of nickel plated substrates having a chromium layer deposited from a trivalent chromium electrolyte thereon.
• the inventors of the present invention have discovered a remarkable and unexpected synergy between cliromium alloy coatings produced from irivalent electrolytes and the coatings produced by treating such chromium alloy plated items cathodkally in a solution containing trivalent chromium salts and a suitable complexant.
• the present invention comprises a method of processing components plated with a chromium alloy deposit in a solution comprising a trivalent chromium salt and a complexant.
• the present invention relates generally to a method of treating a substrate, wherein the substrate comprises a plated layer deposited from a trivalent chromium electrolyte, the method comprising the steps of:
• the substrate is first plated with a nickel plating layer and the plated layer is deposited using a trivalent chromium electrolyte, over the nickel plated layer.
• the electrolyte solution typically comprises between about 0,01 and about 0.5 M, more preferably between about 0,02 and about 0.2M of the chromium(iil) salt.
• the trivalent cliromium salt is preferably selected from the group consisting of chromium sulfate, basic cliromium sulfate (cl rometan), and chromium chloride, although other similar chromium salts may also be used in the practice of the invention.
• the complexant is preferably a hydroxy organic acid, including, for example, malic acid, citric acid, tartaric acid, glycolic acid, lactic acid, gluconic acid, and salts of any of the foregoing. More preferably, the hydroxy organic acid is selected from the group consisting of malic acid, tartaric acid, lactic acid and gluconic acid and salts thereof.
• the chromium sail and the complexant are preferably present in the solution at a molar ratio of between about 0.3 :1 to about 0.7: 1.
• the solution may also optionally include conductivity salts, including, for example, sodium chloride, potassium chloride, sodium sulfate and potassium sulfate, by way of example and not limitation.
• the substrates to be processed are immersed in the passivate solution preferably at a temperature of between about 10 and about 40°C and a pH of between about 2 and about 5 and most preferably at about 3.5.
• the substrates are made cathodic at a current density of between about 0.1 and about 2 A/dm 2 for a period of time between about 20 seconds and about 5 minutes, more preferably for about 40 to about 240 seconds. Following this, the components are rinsed and dried. This treatment produces a remarkable improvement in the corrosion performance of the plated components.
• the process described herein works by depositing a thin layer of hydrated chromium compounds on the surface of the components. Making the components cathodic in an electrolyte of moderate pH liberates hydrogen ions at the surface which rapidly leads to a local increase in pH. This in turn leads to the precipitation of basic chromium compounds at the surface.
• the present invention relates generally to a substrate comprising a plated layer deposited from a trivalent chromium electrolyte passivated according to the process described herein, wherein the passivated chromium(IIi) plated layer exhibits a polarization resistance of at. least about 4.0 x 10 s ⁇ /cm 2 , more preferably a polarization resistance of at least about 8,0 x 10 s ⁇ /cm 2 , and most preferably a polarization resistance of at least about 9.0 x 10 s O/cmA
• chrormum(III) ions can form polymeric species at high pH (by a process known as "olation") and it is likely that, it is these compounds that fonn the passivate layer because chromiurn(III) hydroxide forms a flocculent precipitate that is adherent to surfaces.
• the inventors have found that the best results are obtained using chrometan as a source of chromium ions and sodium gluconate as the complexant.
• the inventors have also found that above a concentration of about 0,5 M, the coating produced is dark in color and detracts from tthhee vviissuuaall aappppeeaarraannccee ooff tthhee ccoommppoonneenntt..
• HHoowweevveerr HHoowweevveerr, tteemmppeerraattuurreess aabboovvee aabboouutt 4400°°CC rreeqquuiirree aa mmuucchh hhiigghheerr ccuurrrreenntt ddeennssiittyy iinn oorrddeerr t too pprroodduuccee aa ccooaattiinngg..
• the coating process was carried out at a temperature of 25°C and an average current density of 0.5 A dm2 for 120 seconds, The panels were then rinsed and dried.
• the corrosion performance of the panels was evaluated in a 5% sodium chloride solution by electrochemical impedance spectroscopy (EIS) using an EG&G model 263A potentiostat and a Solartron frequency response analyzer (FRA), This technique can he used to measure the polarization resistance of the test panel which is in turn related to the overall rate of corrosion of the surface, the higher the polarization resistance, the more corrosion resistant the coating.
• EIS electrochemical impedance spectroscopy
• FSA Solartron frequency response analyzer
• a frequency scan was carried out from 60,000 Hz to 0.01 Hz at the corrosion potential +/- 10 mV.
• the polarization resistance was determined by plotting the real impedance versus the imaginary impedance at every point on the frequency scan. This is called a Nyquist plot and for a normal charge transfer process yields a semicircular plot from which the polarization resistance can be calculated. Plots of frequency versus impedance and frequency versus phase angle were also plotted (these are called Bode plots and can generate more detailed information about the nature of the corrosion process).
• Figures 1 and 2 show the Nyquist and Bode plots obtained from an average of 5 results from each of the panels.
• Test panels were prepared in the same manner as in Comparative Example 1 except thai the chromium coating was applied from a trivaleni electrolyte (Trimac ⁇ , available from MacDermicL Inc.). This produces a chromium coating containing up to 2% sulfur and also having up to 0.5% carbon eodeposiied with the chromium, effectively making it an alloy. Again, two panels were left unpassivaied and two were passivated using the same process as described in Comparative Example 1. Again, EIS was used to examine the panels to determine the polarization resistance.
• Trimac ⁇ available from MacDermicL Inc.
• Test panels were prepared in the same maimer as in Comparative Example 1 except that the chromium coating was applied from a trivaleni electrolyte (Trimac III, available from MaeDerrnid, Inc.). One of the panels was left unpassivaied, one was cathodica!ly passivated in a solution of potassium dichromate and one was passivated using the process solution as described in Comparative Example 1.
• Trimac III available from MaeDerrnid, Inc.

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• Chemical & Material Sciences (AREA)
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• Inorganic Chemistry (AREA)
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• Chemical Treatment Of Metals (AREA)
• Electroplating And Plating Baths Therefor (AREA)
• Automation & Control Theory (AREA)
• Other Surface Treatments For Metallic Materials (AREA)

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