Surface Finishing Tutorial

#149 Characteristics of Corrosion Resistant Plate Coatings

Category : Electroplating

February 8, 2013

Corrosion resistant plate coatings have the following characteristics. Most corrosion resistant platings are of "Single layer".

Plate coating	Decorativeness	Corrosion resistance	Wear resistance	Other characteristics
Zinc Zn	○	◎		Low abrasion coefficient, Secondary machinability, Adhesiveness
Zinc-Nickel alloy		◎		High corrosion resistance
Zinc-Iron alloy		◎		High corrosion resistance
Tin-Zinc alloy		◎		Secondary machinability, Solderability, Sea water corrosion resistance
Cadmium		◎		Secondary machinability, Solderability, Sea water corrosion resistance
Chemical conversion coating		◎		Adhesiveness

※Secondary machinability: Ability to accept post-plating processes such as bending and deep drawing, etc.

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(Note 1) Principle of corrosion resistance
The anti-corrosiveness of the corrosion resistant plating is brought by a plate coating that is more chemically active (Zinc) than the plated base metal (Steel). This is called Sacrificial Corrosion Prevention where the electro-chemically inferior plate coatings deposited on the metal surfaces will dissolve first before the base metals in corrosive environments. In other words, the plate coatings become the sacrifices to protect the base metals from corrosions.
Traditionally, zinc has been used as the sacrificial metal for galvanized sheet steels, but lately zinc and nickel (5~10%) and steel (0.3~0.6%) alloy platings are used with 2~20 times of the corrosion resistance of the traditional means, and used as highly corrosion resistant yet thin and strong materials for automotive components.

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(Note 2) Ionization tendency
The [chemically active metals] mentioned in (Note 1) points to the metals with high ionization tendencies, generally. The following is the metals with high ionization tendencies in increasing order:
K>Na>Mg>AL>Zn>Cr>Fe>Cd>Co>Ni>Sn>Pb>H>Cu>Hg>Ag>Pt>Au As shown, zinc is more tendency to ionize than steel. However, this order is not necessarily absolute. The order may change as the corrosive environment changes, such as metal ion concentration, etc.

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