Marsh Plating Corporation
 

Zinc Alloy Electroplating



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Electrochemically, alloys can be designed to produce different corrosion potentials than their alloying elements. It is possible, therefore, to maintain the sacrificial protection of zinc coating over steel, but at a different potential, closer to steel, by alloying it with another metal, preferably more noble than zinc. As a result, the alloy corrodes at a much slower rate than zinc alone, affording better corrosion protection. Some of these plating alloys have also been found to be excellent replacements for cadmium.

Zinc-Nickel Alloy

Some Advantages
  •   Excellent corrosion resistance, generally 5X better than pure Zinc
  •   Excellent corrosion resistance when exposed to high temperature
  •   Highest abrasion resistance of all zinc alloys
  •   Excellent covering power for deep recesses
  •   Replacement for the toxic metal, Cadmium
There are two types of zinc-nickel plating systems available commercially:
  • Alkaline-type (non-cyanide) bath
  • Acid-type bath
The nickel content in this alloy ranges from 5-15% by weight of the deposit; the balance is zinc. Corrosion resistance studies have shown a peak performance after chromating in alloys containing 10-15% nickel. At nickel levels higher than 25-30%, the deposit ceases to be sacrificial to steel.

The alkaline-type bath plates at 20-40% efficiency, and has the advantage of producing uniform thickness and nickel distribution in the deposit across low- and high- current density areas. The deposit has good ductility and has been successfully used on parts requiring post plate forming. The inherent alkalinity of the bath reduces the post plating corrosion tendency of unplated surfaces such as internal areas of tubular parts. The acid type bath plates faster at twice the efficiency and has been used to produce 10-15% nickel alloys. However, due to its higher efficiency, plated material distribution varies across the current density range, and nickel content in the deposit may be higher in the low- current density areas. Proper care must be exercised with zinc-nickel alloy baths to not substantially exceed 15% nickel in the deposit in order to maintain optimum corrosion resistance. As the nickel content increases beyond 15%, chromate Passivation becomes exceedingly difficult, and eventually impossible to obtain resulting in reduced corrosion resistance. Pilot lab and field tests have shown the zinc-nickel process to be superior to all other zinc alloys in terms of corrosion resistance, and range of applications, including cadmium replacement.

Zinc-Iron Alloy

Some Advantages
  • Economical Way to Get Extended Corrosion Protection Over Zinc
  • Easy to Make Black – Does Not Need Silver Based Black Chromate
This process produces alloy deposits containing 15-25 % iron. The deposit has inhance corrosion protection, good weldability and ductility that are needed in subsequent manufacturing steps. This alloy can be adjusted to improve adhesion of electropainting of formed steel components. When chromating, black is the most suitable for this type of alloy. Although zinc-iron offers good corrosion resistance as plated and chromated, exposure to heat deteriorates this resistance rapidly.

Zinc-Cobalt Alloy

  • Easy to Apply Black Chromate
  • Excellent Corrosion Protection
  • Bright Finish, Good Adhesion and Ductility
Zinc-cobalt alloy plating has become more popular because of its relatively lower cost of operation compared to zinc-nickel. It offers lower corrosion resistance; however, the level is still adequate for certain applications and an improvement over plain zinc of the same thickness. The coating can be applied in an acidic or caustic bath. The deposit will contain up to 1% cobalt. The acid-type bath has a higher cathode efficiency, and reduced hydrogen embrittlement, but its plating thickness distribution varies substantially between low- and high-current-density areas.

Tin-Zinc Alloy

  • Excellent Ductility
  • Excellent Solderability
  • Excellent Conductivity
  • Extremely high corrosion resistance to salt water and sulfur dioxide
  • Maintains high corrosion resistance even after hard crimping and bending
  • Can be safely applied to sensitive electronic components and glass to metal seals, with no attack to the glass
  • Replacement for the toxic metal, Cadmium
  • Provides good protection on steel when in contact with Aluminum
This alloy contains 70-90% tin with the balance being zinc. The deposit is ductile and maintains good solderability even after aging. Corrosion resistance equals or exceeds that of zinc-nickel alloys. Chromating is usually limited to clear or yellow.

Passivation and Post Plate Treatment

There are several hexavalent chromate Passivation treatments for the zinc alloys, which are necessary to produce the enhanced corrosion resistance of these alloys. Recent developments have introduced trivalent chromate formulations as well as chromate free passivates in various finishes ranging from clear to iridescent and black. Inorganic and organic topcoats are also available to further enhance overall corrosion resistance and provide other properties such as lubricity and torque and tension.

Contact me for information on the corrosion resistance of the various zinc alloy deposits.