June 13, 2008

Metallic Coating

Choice of Metallic Coating


In choosing a metallic coating for the protection of a metallic article it is not sufficient to consider the behavior of the coating metal alone. It's necessary also to have regard to the effect of the mutual exposure of coating metal and bases metal at pores, cuts and other initial or subsequent breaks ion the thin film of coating.


This is a consequence of the predominantly electrochemical nature of the corrosion of metals by the whether, by water and by aqueous solutions, which must now be briefly considered. All metal atoms in the metallic state are characterized by having one or more rather loosely attached electrons, the mobility of which accounts for the high electrical and thermal conductivity of metals. Metal atom in the combined state has lost these electrons and is bound to other atoms by their resultant positive charge.

Corrosion of a metal is therefore essentially the loss of one or more electrons from its atoms. But since both the metals and both the metals and the aqueous solutions which corrode them are conductors of electricity, the absorption of electrons essential to corrosion may occur elsewhere than at the site of the attack; that is to say, corrosion is electrochemical in nature.


The various metals differ in the relative ease with which they part from their outer electrons, and a table, known as the electrochemical series, can be arranged to show this. For this purpose hydrogen gas can also be considered to be a metal, because its atoms each have one loosely attached electron. In the combined state hydrogen loses this electron and becomes the hydrogen ion, which is present in all aqueous solutions to a greater of a lesser extent, because it is present both in water and in acids.


Metal with the properties more negative potential than hydrogen ion will have a greater willingness to part with their outer electrons than that of hydrogen. Some of the metallic potential properties show in the table below:


Table of Electrochemical series of Metal

Gold

+ 1.42 V

Nickel

- 0.23

Rhodium

+ 1.40 V

Cadmium

- 0.40

Silver

+ 0.80 V

Iron

- 0.44

Copper

+ 0.52 V

Chromium

- 0.56

Hydrogen

0 V

Zinc

- 0.76

Lead

- 0.13 V

Aluminum

- 1.67

Tin

- 0.14 V

Sodium

- 2.71


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