Corrosion Control
| When facing a corrosion problem, we must consider four variables: |
| a) Process variables |
| b) Design variables |
| c) Protection |
| d) Materials selection |
a) Among the process variables which affect corrosion, we might mention: concentration of aggressive ions, impurities, temperature, ph, airing, flow density, mechanical stress, and inhibitors. En each case we need to study the influence of each factor separately, paying special attention to the values they may reach in a non-stagnant environment.
It is interesting to briefly mention inhibitors. These are chemicals that are added to controlled environments to reduce the corrosivity of these environments. They may be classified as cathodic, anodic and mixed.
Cathodic inhibitors act by polarizing the cathodic reaction by restricting oxygen access, or increasing the hydrogen potential on the treated surface.
Among these inhibitors, we may find magnesium, manganese, zinc and nickel salts.
These inhibitors form a protective coating on the treated surface and, although they are not as effective as anodic inhibitors, they do not present the latter’s problems, which we will now discuss.
Anodic inhibitors retard anodic reaction. They may be oxidising substances (chromates, nitrates, ferric salts) which cause passivation or form a layer of corrosion product precipitates in the anodic area (alkali, phosphates, silicates). When these are found in insufficient concentrations they may be dangerous, because any breach in the layer would result in the formation of a small anodic area in which current density flow, and hence intensity of the attack, would be considerable.
b) A good design may avoid the following types of corrosion problems: crevice corrosion, galvanic corrosion, erosion corrosion and stress corrosion.
As a general rule, we must study the flow and is conditions of turbulence, stagnancy, etc.
c) Anodic and cathodic protection methods are widely used for corrosion control throughout the industry.
Cathodic protection makes the whole surface act as a cathode, by means of an impressed current system or by using sacrificial (galvanic) anodes.
In the first case a current is applied, which is opposite to the one generated by the metal distribution, and of a higher value.
The utilization of a sacrificial anode (manganese, zinc, aluminium) in based on the principles of galvanic corrosion.