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Surface Finishing Tutorial

#073 Corrosion Protection Measures - Metal Coating - 1

Category : Corrosion - Corrosion Protection
March11, 2011

There are corrosion protection methods of covering the surfaces of metal objects with other metals with higher corrosion resistance properties called Plating, Thermal Spraying, Diffusion Coating, and Metal Cladding, and are used based on the applicable corrosion environment the metals are placed in. Various surface coating are used for home appliances, automobile, communication equipment, architecture, chemical plants, marine vessels, bridges, and ground embedded objects.

Most metal coatings are "plating". They are either "hot dip plating" where metal objects to be plated are dipped in hot melted coating metal, or "electrolytic plating" where electrical current is applied on the subject metal as an anode in a electrolyte solution containing plating metal ions.

In addition to the plating mentioned above, there are others where melted metal is sprayed onto metal surfaces as Thermal Spraying, and Cladding where different metals are welded, explosion welded, or rolled together.

Normally, industrially processed plating has pin holes and scratches occurring from usage, and the metal is exposed at these locations. The behaviors of corrosions occurring would differ whether the plating metal is either more noble than the base metal or not. Nickel, silver, copper and chromium are more noble than steel, and cadmium and zinc are less noble than steel. Aluminum and tin can be either depending on the environments.

With the pin holes on noble metal coat film as shown in [Fig.1], a battery is formed with the base steel being the negative pole, and corrosion will occur.

[Fig.1] Pin Hole Corrosion

With this type of plating, the pin holes cannot be completely eliminated although the thickest possible film is applied to reduce the porosity. Therefore the usage is limited to ornamental decorating purposes in relatively mild corrosive environments, and not for corrosion protection purposes. Chrome plating is often used as noble metal plating but is not at all effective in environments with water due to the cracks in its film. For office equipment and automotive applications, chrome plating over copper or nickel base plating is often used to improve wear resistance characteristics. An automotive bumper plating example is shown in [Fig.2].

[Fig.2] Automotive Bumper Plating Example