What is corrosion?
To understand the nature of our device we need to know what it is to prevent. The most common and well-known corrosion is galvanic corrosion, which is the result of local galvanic cells formed on the surface of the metal. In any case, an important component of those cells is the electrolyte solution. In the case of atmospheric corrosion the electrolyte is condensed moisture and rain water always containing a certain amount of dissolved substances such as oxygen, carbon dioxide, sulfur dioxide, nitrogen oxides, and others. The corrosion process caused by mentioned corrosion cells is the anodic process – the formation of positive ions from the metal atoms. Electrochemical corrosion is the most common type of corrosion. It is one of the common atmospheric corrosion phenomena,which is caused by the action of moist impure air on metals.Electrochemical corrosion occurs when short-circuited cellstake action on the interface between a metal and an electrolyte. These cells are formed as a result of chemical heterogeneity (or physical) of a metal, e.g. at the interface between various metals, or as a result of heterogeneity in the crystalline structure of the metal. Corrosion of metals usually manifests itself in metal surface defects (stains and pitting), or the reduction of the metal strength.
Cathodic protection
Cathodic protection is an electrochemical corrosion protection. Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. The simplest method to apply CP is by connecting the metal to be protected with a piece of another more easily corroded "sacrificial metal" to act as the anode of the electrochemical cell. It is accomplished by direct contact (so-called anodic coating) or with a conductor. With the use of protectors large steel objects such as ship hulls, pipelines and underground tanks are protected against corrosion. Protectors are made of sheet metal or bars of active metals such as zinc, magnesium or aluminum, with the wires connected to a protected object. In the cell formed this way, the anode is the protector which corrodes. After being used, the protectors are exchanged for new ones. Anodic coatings (protectors) are made of metal
with a more negative electrode potential (less noble) than the metal being protected. Anodic coatings of metals provide the protected metal with cathodic protection as the coating made of less precious metal is made the anode and acts as a protector. Examples of anode coatings are zinc and cadmium. The most mportant practical application of anodic coating is galvanizing – the application of a zinc coating on steel surfaces. When cracks or fissures appear, a cell is formed in which the cathode is iron and the anode is zinc. In this situation, zinc ions go into solution instead of iron ions. Thus, in the case of anodic coating of metal surfaces, the coating doesn’t have to be perfectly sealed.
Here is a voltage range of metals from the least
noble one:
Na <Mg <Al <Zn <Fe <Cd <Co <Ni <Pb <H <Cu
<Hg <Ag <Au
The more negative the value of the normal
potential of the metal is, the greater tendency to pass into solution it has.
Shortly speaking, if we connected them all, they
would corrode in that order.
Active cathodic protection
We can achieve the same effect if we connect the protector with the
positive pole of the direct current, while the protected structure is connected
to the negative pole.