Factors Affecting General Corrosion Rate

Like most other chemical reactions, corrosion rates increase as temperature increases.

Temperature and pressure of the medium govern the solubilities of the corrosive species in the fluid, such as oxygen (O2), carbon dioxide (CO2), chlorides, and hydroxides.

A rule of thumb that the reaction rate doubles with a 20°F to 50°F temperature rise.

This linear increase with temperature does not continue indefinitely due, in part, to a change in the oxide film.

When water velocity is extremely high, the impact of the water tends to remove the protective oxide layer and some of the metal under it (erosion), thus, exposing more metal to corrosion.

Water velocities of 30 to 40 ft per second are usually considered to cause erosion. The presence of oxygen in water to which iron is exposed increases the corrosion rate.

The reason for this increase is the rapid reaction between oxygen and the polarizing layer of atomic hydrogen absorbed on the oxide layer.

The following reaction rapidly removes the polarizing layer.

(1)    O₂ +   4H⁺   --->  2H₂O

The overall reaction can be obtained by combining Equations (1).

and

H₃O⁺   +  e-   ---->  H   +  H₂O

with the following overall:

O2   +   4H₂O  +  4e-   <---->   6H₂O

The controlling step is believed to be diffusion of O2 to the metal surface where it can react directly with iron or with FeO.

(2)     2Fe   +   O₂   --->   2FeO

(3)     4FeO  +  O₂   --->   2Fe₂O₃

Oxygen, therefore, has two effects:

-it removes the polarizing layer of atomic hydrogen,

-it can react directly with the metal or metal oxide;

Thus, the corrosion rate increases.

Substances, such as O2 in this case, that remove the absorbed atomic hydrogen are called depolarizers.

The effect on Corrosion of the pH of water to which iron or steel is exposed is influenced by temperature in the following manner.

The potential of hydrogen or symbol (pH) is defined as the negative logarithm of the hydrogen concentration, represented as [H+] in moles/liter.

pH = -log [H+]

The pH value is used to represent the acidity of a solution.

this article is taken from: DOE FUNDAMENTALS HANDBOOK CHEMISTRY Volume 1 of 2 - DOE-HDBK-1015/1-93 JANUARY 1993