How is corrosion produced?
Corrosion may be classified according to:
Based on the study of these charts, we will be able to understand how corrosion is produced.
Corrosion Mechanisms
The electrochemical corrosion mechanism is produced in presence of a liquid solution (electrolyte) in what we will call corrosion cell.
These cells are produced when two dissimilar metals are put in contact with each other, in the presence of a corrosive environment. The metals then act as electrodes, and an electric current, called corrosion current, flows between them. However, it is also possible that these cells form on the surface of one metal.
Small differences in chemical composition are enough to trigger a reaction. In each case, whether corrosion occurs between two different metals or on different areas on the surface of the same metal, one area will be more “noble” than the other, i.e. one will present a higher electrical potential than the other, and this difference will generate a current flow. The bigger the difference, the stronger the flow and the greater the corrosion.
| Normally, the electrodes in this cell are known as: | |
| ANODE: | This is the area where oxidation (metal corrosion) occurs, and the chemical reaction is produced by an electron loss. |
| CATHODE: | This is the area where a reduction takes place, and the chemical reaction involves receiving electrons. |
Anodic reaction always involves metal corrosion. Cathodic reaction always varies, depending on the type of electrolyte.
| The speed of metal corrosion depends on the following factors: |
| 1- Presence of an oxidising agent. |
| 2- Difference of potential between anode and cathode. |
| 3- Lower anode than cathode area. |
| 4- Temperature, which accelerates the reaction. |
In laboratory studies, by generating a certain potential (polarizing) on a certain metal which acts as an anode, before a platinum cathode, in a certain environment, we may answer the following questions:
What is stainless steel?
What makes stainless steels resistant to corrosion in a certain environment?
The figure shows the anodic polarization curve of different types of steel, with increasing content of CHROMIUM.
This element shows a dramatic effect on the curve. Within certain potential ranges, increasing contents of chromium show low density flow, which implies lower corrosion.
Within this range, steel is passivated, which means metal corrosion is inhibited. In this area, the metal’s surface is covered by a thin but dense layer of chromium oxide, iron and oxygen, which protects the material beneath.
Only with a minimum 12% content of chromium can steel be passivated lo the lowest potential, and as from there we may properly call these alloys STAINLESS STEELS.
The polarization curve of a stainless steel immersed in an acidic solution presents the following conditions, and we may distinguish three particular areas:
| 1. Active Area: | In this area, the metal’s surface is partially covered by the protective layer, with a low-solubility oxide. Current density flow, and therefore corrosion, is high. In consequence, stainless steel in these conditions should not be used. |
| 2. Passivation Area: | In this area, the surface is completely covered by the oxide layer, so current density flow, and therefore corrosion, is low. Imax values measure the passivation potential of steel |
| 3. Transpassivity Area: | In this area, the chromium in the protective layer is again oxidized. The layer is quickly dissolved, and corrosion rates rise. |
Logically, we tend to broaden the range of the passive area, and move the curve towards the lower I values.
The chart shows the influence of the different alloy elements.
Chromium evidently has the most favourable effect in general terms. Other elements which show an appreciable influence are molybdenum and nickel.
As steels present unique corrosion potentials in different defined solutions it is possible, by combining alloy elements, to develop steels which present good resistance to certain environments.