Electroplating and Electroforming

December 6, 2002

• Electroplating involves the reduction of a metal ion to the metal
  • the basic reaction is M⁺ⁿ + n e^- ---> M
  • the reaction occurs at the negative electrode of an electrolysis cell
  • an electrode that reduces material is called a cathode
• By definition electroplating is the deposition of a metal coating on a metal surface

• Why would you electroplate a metal surfaces?
  • to provide a more decorative finish (say a gold or silver plating on a less luxurious metal)
  • to save money (such as gold plating a less expensive metal)
  • to make the object out of a stronger material (but not having that material show)
    • pure gold, for example, is quite soft
  • to provide a corrosion resistant surface (chromium plating is often used to protect exposed iron)
  • to provide improved electrical contact (electronic contacts are often gold plated)
  • to increase the thickness of an object (it's one way to repair wear on a metal part.)
• Metals that are frequently applied as electroplate
  • silver
  • gold
  • copper (actually relatively easy to do but rarely done)
  • chromium
  • nickel
  • zinc
  • cadmium
  • brass (a zinc + copper alloy)
    • Many reactive metals cannot be plated from aqueous solutions
    • we get H₂ produced instead
    • this would include Mg, Al, Ca, Na, K, Li

• What is the reaction at the anode?
  • frequently the anode is made of the same metal
  • the reaction is simply oxidation of the metal
  • the net chemical reaction is moving metal from anode to cathode
  • the metal ion concentration remains constant in the solution
    • (in gold and chromium plating the anode is often graphite and the reaction is the oxidation of water to O₂ and H⁺. These electroplating baths will need fresh additions of the metal ion)

• What's the actual composition of the electroplating bath?
  • We could simply use an aqueous solution of the ion; this works reasonably well with copper
  • for most other metals this often produces a relatively coarse metal deposit
  • Finer deposits generally are possible only when the metal ion concentration is extremely low
    • this often involves typing up most of the metal ion in an inorganic complex
    • For over a century the electroplating industry relied on baths that contained high concentrations of cyanide
      • such plating baths are quite toxic for the workers
      • such plating baths are a serious environmental risk if spilled or improperly disposed of
      • Silver is the only common electroplating process that still routinely relies on cyanide plating baths.

• Electroplating is subject to Faraday's Law
  • typical electroplating currents are around 100 amps per square foot
    • or 1 amp per square inch
    • or 0.2 amp per cm²
  • we need n electrons (n=1,2 3 typically)
  • Let's use copper as an example...
    • 0.25 Amp for a period of 30 minutes
    • #Faraday = 0.25 x 30 x 60 (amp-sec) /96500 =4.7x10^-3
    • # moles of Cu deposited = 1/2 that value or 2.3x 10^-3 moles
    • mass of copper = 63.5 x 2.3 x10^-3 = 0.15 grams
    • volume of copper = 0.15g / 8.92 (g/cc) = .0166 cm³
    • if this is deposited on a 1 cm² surface (0.25 Amp/cm²)
      • the metal deposited is 0.017 cm or 0.17 mm thick
      • this is about 6.5 mil (1 mil = 0.001 " thick)
    • this is a typical value for a rugged plating
      • (a purely decorative finish might be 0.1 mil in thickness)

• There is another way to deposit a metal coating by a simple oxidation/reduction process
  • a more reactive metal (like iron) will reduce the ions of a less reactive metal like silver or copper)
    • this deposited metal generally does not adhere well, partly because the metal underneath is being continuously eaten away
    • in fact, this can be a complicating factor in electroplating if such a reaction occurs
    • with the low free metal ion concentration in most plating baths this is not a problem

Experimental:

• We can copper plate a metal object using a dilution solution of copper sulfate / sulfuric acid
  • the anode is a copper wire or a strip of copper
    • connect this to the positive terminal of a battery
  • the sample is an iron nail or a graphite rod
    • connect this to the negative terminal
    • an ammeter can be used to measure the current
• We may also provide a zinc plating bath
  • you can then produce a silvery zinc finish on a copper object

Electroforming

• We started this during the photography experiment.
• We used a photoresist to mask a stainless steel surface
  • we then copper plated the exposed areas
• Unfortunately, the power supply was limited to about 0.050 Amps
  • our samples were about 1-2 cm² in area (10 cm² if we didn't cver the back)
  • the calculation above suggests we can expect less than a 0.002" thick coating in 30 minutes
• An electroform that can be peeled off and remain intact probably needs to be about .005"
  • we did get suitable samples several hours later
  • we will repeat with higher currents and a bit more patience.
• Again-- the deposited metal will not adhere to the stainless steel

Electroforming over nonconductive forms

• The image is a maple leaf that has been immortalized in metal. This came from a Web site catalog for bridal gifts
  • The leaf is first dried
  • Then the surface is made electrically conducting
    • often with electroless silver plating (see notes on electroless plating)
  • Then a thin layer of copper is plated as a base for further plating
  • Additional copper may be applied, but most artisans switch to a stronger metal like nickel
  • For appearance, the leaf is usually finished with a final plating of gold or silver
    • (the original dry leaf remains sealed inside the metal shell)

• Industrial uses of electroforms
  • CDs and Vinyl Phonograph records are manufactured from electroforms
    • the original recording is mechanically cut into a plastic disk

    • the disk is made conductive

    • the surface is then plated with a hard metal like nickel or chromium

    • the electroform is the removed and used to press the product
      • additional electroforms can be made to provide a large number of dies

  • the treasury department prints bills from electroformed printing plates
    • an artisan called an engraver prepares the original metal engraving

    • this is then used as the base for an electroform

    • the electroforms become the actual printing plates

  • Treasury Department Web site How coin dies are prepared (includes galvano or electroform copy)

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