Definition Corrosion

Definition Corrosion

While we want to know about why the material will be degraded or corroded, first we must know the definition corrosion in order to know it.

1. Definition:

Many ways talks about definition corrosion, but the usual interpretation of the term is “an attack on a metallic material by reaction with its environment”. The concept of corrosion can also be used in a broader sense, where this includes attack on nonmetallic materials, but such attacks are outside the scope of this writing. 

There are four requirement in presenting corrosion as you can see in the picture below namely:
1. Anode
2. Cathode
3. Electrolyte
4. Electrolyte Media

2. Main Group of Corrosion

Corrosion of metallic materials, based on electrolyte media and temperature, can be divided into three main groups:

1. Wet corrosion, where the corrosive environment is water with dissolved species. The liquid is an electrolyte and the process is typically electrochemical and best – known as low temperature corrosion.

2. Corrosion in other fluids such as fused salts and molten metals.

3. Dry corrosion, where the corrosive environment is a dry gas. Dry corrosion is also frequently called chemical corrosion and the best-known example is high temperature corrosion.

Based on the attacking location, corrosion divided into:

a. Uniform corrosion

Uniform corrosion, as the name suggests, occurs over the majority of the surface of a metal at a steady and often predictable rate. Although it is unsightly its predictability facilitates easy control, the most basic method being to make the material thick enough to function for the lifetime of the component. Uniform corrosion can be slowed or stopped by using the five basic facts;

(1) Slow down or stop the movement of electrons

(a) Coat the surface with a non-conducting medium such as paint, lacquer or oil

(b) Reduce the conductivity of the solution in contact with the metal an extreme case being to keep it dry. Wash away conductive pollutants regularly.

(c) Apply a current to the material (see cathodic protection).

(2) Slow down or stop oxygen from reaching the surface. Difficult to do completely but coatings can help.

(3) Prevent the metal from giving up electrons by using a more corrosion resistant metal higher in the electrochemical series.

a. Use a sacrificial coating which gives up its electrons more easily than the metal being protected. 

b. Apply cathodic protection.

c. Use inhibitors.

(4) Select a metal that forms an oxide that is protective and stops the reaction. Control and consideration of environmental and thermal factors is also essential.

b. Localized Corrosion

The consequences of localized corrosion can be a great deal more severe than uniform corrosion generally because the failure occurs without warning and after a surprisingly short period of use or exposure. Application of the five basic facts needs greater thought and insight.