Usually, the metals are Ti, Cr, Fe, Ni; and their alloys. They do form protective films in moderate corrosive environment, and their films dissolve away in very strong corrosive environment. But why not all metal behave so? What causes their protective films to be retained?
The preservation and formation of protective oxide films on metals depends on environmental conditions. For example, in humid air, metals such as chromium, aluminum, and titanium form dense, impermeable films of the corresponding oxides (Cr2O3, Al2O3 and TiO2), and iron forms a loose mixture of oxides with hydroxides, which poorly protects the base metal from corrosion.
However, there are environments in which oxide films cannot form and cannot exist; in this case, usually corrosion-resistant metals are quickly destroyed.
For example, if you put aluminum in an alkaline solution, then aluminum will quickly corrode. This is because in an alkaline medium, a Al2O3 film on aluminum is thermodynamically unstable and loses its protective properties.
For titanium and chromium, such a "deadly" medium is, for example, hydrochloric acid.
please refer to the electrochemical series of elements and their potentials
Dear Sumit,
This is due to close electron affinity and potential value orderly arranged in electrochemical series. Very few elements are closed in their potential values. Values of Co and Ni are having -0.28 and -0.25.
Ashish
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