The phenomenon of rusting

In studying the corrosion behavior of some metals, an interesting phenomenon is observed, the results of which will be useful for protecting materials. The aforementioned behavior is such that the combinational tendency of these metals in a corrosive environment initially increases normally with an increase in the concentration of corrosive agents (the oxidizing power of the solution), and the corrosion of metals is intensified, but at a certain concentration of corrosive agents, the corrosion rate suddenly decreases significantly and the metal loses its combinational tendency and becomes inactive. The reason for the reduction in corrosion at a certain concentration of oxidizing components is the formation of a protective or inactive layer on the metal surface. The protective layer can be oxide, hydroxide or other compounds. This phenomenon is called rusting.

The behavior of metals that exhibit the property of passivation can be divided into three zones: active, inactive and transition to the rusting state. In the active zone, the behavior of the metal is similar to the behavior of a normal metal. A slight increase in the oxidizing power of the solution causes a rapid increase in the corrosion rate. If the increase in the oxidizing agent continues, the corrosion rate suddenly decreases. This state corresponds to the beginning of the inactive zone (rust). Further increase in the oxidizing agent will have very little effect on the corrosion rate of the metal in question, and finally, at very high concentrations of the oxidizing agent or in the presence of very strong oxidizing agents, the corrosion rate increases again and this region is called the transition zone.

Rusting mechanism

The exact cause of the active-inactive-rusting state transition is not completely clear. It is a special case of polarization of activity that results from the formation of a protective surface layer or shell.

In explaining the rusting mechanism, studies show that several reactions occur on the metal surface that usually cause the formation of a protective layer on its surface. This protective layer is stable over a wide range of the oxidizing power of the solution, but if the oxidizing power increases too much, this protective layer is destroyed and corrosion in the metal begins again.

In short, it can be said that the formation of oxide layers on the surface of the metal, which are actually corrosion products and have good protective properties, causes the metal to rust.

The phenomenon of deactivation (rusting) is seen in metals such as iron, silicon, nickel, chromium, titanium and their alloys, and is also observed to a small extent in zinc, cadmium, tin, uranium and thorium.

Example: Iron does not corrode at all in concentrated nitric acid, even if the acid is gradually diluted, corrosion is not observed, but if the metal surface is scratched, a layer of gas bubbles covers the metal surface and the significant production of hydrogen and nitrogen oxides indicates an increase in the corrosion rate of iron.