passivation

=Passivation =

Some metals, such as stainless steel, undergo a process known as passivation. Passivation inhibits corrosion by forming a non-reactive surface film on top of a metal surface. The reaction is spontaneous and creates a thin layer (typically an oxide or hydroxide) that is only a few molecules deep.

The following Pourbaix diagram shows a plot of overall cell potential, in volts, versus pH for a metal that is a component of stainless steel, such as iron. Three different regions exist on this graph, depending on the combined effect of pH and net electromotive force:

The metal exists in its cationic form; corrosion will occur until the metal is consumed.
 * 1) Corrosive Region**

Corrosion is thermodynamically impossible.
 * 2) Immune Region**

An insoluble protective layer (oxide or hydroxide) is formed; corrosion will only occur until this layer is formed.
 * 3)** **Passive Region**

It should be ensured that the steel has a high weight percentage of chromium. The presence of chromium decreases the rate of corrosion by increasing the area encompassed by the passivation region. However, the percentage of chromium cannot be too high as this will cause the material to be brittle. The following graph shows the decrease in the rate of corrosion with an increased weight percentage of chromium in the steel:



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**References**
[1] //Electrochemistry encyclopedia.// [Online]. Available: []. [Accessed: November 30, 2009].

[2] "Lecture 11: Stainless steel," //Ubonrayathanee University.// [Online]. Available: [|http://app.eng.ubu.ac.th/~edocs/f20061122Suriya92.pdf]. [Accessed: November 28, 2009].