Coatings
Electroplating
Electroplating is a process that uses electrolysis to apply a thin layer of metal on top of another metal. Electroplated metallic coatings can serve decorative or protective purposes and can range widely in thickness. For example, an electrodeposited chromium layer can be as thin as 2.5 x 10-4 mils in a decorative coating or a thick as 1 mil in hard chromium plate. A benefit of the electroplating process is the control that it allows over the dimensions of the applied coating. Many different types of metals can be applied to surfaces using this method. Zinc, chromium, tin, and nickel are some of the metals most commonly electroplated on steel and other ferrous alloys. Ferrous and non-ferrous alloys are also frequently electroplated with aluminum, brass, bronze, cadmium, copper, gold, iron, lead, platinum, and silver.
Process
In electroplating, the structure to be plated is submerged in an electrolyte solution containing metal ions of the species to be deposited. The structure is then made the cathode in an electrolytic cell causing metal ions in solution to plate out on the cathode surface. Electroplated metal coatings of any metal M are applied by cathodic polarization of the reaction,
Mn+ + ne- M.
This reaction is the reverse of the anodic reaction for metal dissolution during corrosion. In the case of chrome plating, the metal to be plated is placed in a hot bath of sulfuric and chromic acid (H2CrO4) and cathodically polarized. As current passes through the cell, chromium reduces from the +6 oxidation state in chromic acid to metallic chromium and plates out on the cathode surface.
Controlling the parameters of the process, including potential, current, temperature, residence times, and the composition of bath solutions regulate electroplating. The electrolyte solution must fulfill certain criteria. The salts of the metal in the electrolyte must be soluble at a sufficient concentration and at a reasonable temperature. The electrolyte must be a relatively good electrical conductor in order to achieve an even distribution of the coating and avoid excessive heating of the bath. The pH-value of the bath must also be maintained so that reduction of the metal occurs before evolution of hydrogen.
In some cases using a water-based electrolyte for electroplating is not possible. For example, Ti and Al will deposit only from organic electrolytes; and metals such as Mg, Nb, Ta, and W will only plate from molten salt electrolytes.
Metals plateable from aqueous solutions (red background). Elements with yellow background are only plateable in combination with one of the others (alloy plating). Grey text indicates that the element can be deposited either by an auto-catalytic electroless process or by electroplating. [Courtesy of Technical University of Denmark.]
Applications
Zinc can be electroplated onto steel and other ferrous alloys to provide sacrificial protection. This process is known as electrogalvanizing. This method of zinc coating application is often preferable to hot dipping because of improved surface finish and finer control of dimensions. Electrogalvanized steel surfaces lack the spangle (surface crystals) found in hot-dipped coatings and can easily be prepared to receive decorative finishes. The pure zinc coating deposited is highly ductile and, because of excellent adhesion, electrogalvanized steel has good working properties. The zinc coating can remain intact even after severe deformation.
Electroplated tin (tinplate) is another common application of the electroplating process. Tinplate is used as a protective coating for food and beverage containers. Tin is active to steel in most food products and provides galvanic as well as barrier protection in sealed cans.
References
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