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#049 Metal Corrosion and Protection

Category : Corrosion - Corrosion Protection
July 9, 2010

Metal Corrosion and Corrosion and corrosion protection

 From this vol. forward, let us think about metal corrosions and protections.

It is well know that metals corrode in air, water, and in the ground causing gas/water leaks as well as playground structure collapses resulting in human injuries. Not all cases may result in serious accidents, but incidents such as corroded jewelry and railings are quite numerous.
It is quite logical to think that the metals, originally contained within the earth in forms of stable oxides and sulfides were mined and highly refined, will want to revert back to their original stable form if exposed to water rich in dissolved oxygen and chlorine ions, and air with 21% oxygen by volume. In order to prevent this and maintain the original functional purposes, alloying of metals, Plating for corrosion protection, and anti-corrosive painting processes are performed.

(1)Metal activation Series

Let us take a look at corrosiveness and non-corrosiveness of metals. In reactions of metals and acids, the metals with lower ionization energy than hydrogen will generate hydrogen. An order of metals based on reactiveness is called Metal activation Series, and is shown in Table 1.

[Table 1] Metal activation Series
Active MetalsReacts with water and acidsCs、Rb、K、Na、Li、Ba、Sr、Ca、Mg
Reacts with acidsAL、Zn、Fe、Pb、Sn
Hydrogen H
Non-active MetalsReacts with strong oxidantsCu、As、Sb、Bi、Hg、Ag
Precious MetalsNot reacts with acid and oxidants PtPt、Au、Os

The metals shown near the top are active (corrosion prone) metals, and they react with acids and generate hydrogen. The metals Cu and below only react when oxidants are present, and further Pt and below do not react even when oxidants are present. As can be seen above, metals have different levels of ionization energy, and with different levels of corrosion tendencies.

However, the actual corrosion phenomena are not so simple since the environmental variations affect the corrosion processes. For example, if the initial corrosion creates a an oxidized surface layer dense and stable sufficient in shielding the inner metal from the environment, no further oxidation would proceed. Since corrosion processes are largely affected by the surrounding environment, discussion the subject of corrosion without environmental concerns would be meaningless.