A general definition of corrosion is the deterioration of a substance, or its properties, because of a reaction with its environment. Most materials corrode when they exposed to certain environments. There are basically eight forms of corrosion listed below:
1. Uniform or general corrosion
2. Localized corrosion.
3. Galvanic corrosion.
4. Intergranular corrosion.
5. De-alloying attack.
6. Cracking phenomena.
7. High temperature corrosion.
8. Velocity effects.
Regardless of what form it takes, corrosion is inevitable. However, it can be slowed or temporary stopped. There are basically five forms of corrosion control listed below:
1. Change of materials.
2. Change of environment.
3. Barrier films.
4. Improve design.
5. Electrochemical techniques.
Many of the forms of corrosion can be controlled with treating chemicals. However , the use of treating chemicals is normally limited to internal systems. Paint and coating systems are the most widely used methods to control and minimize corrosion. Coatings protect materials in condition that range from atmospheric exposure to immersion in highly corrosive solutions.
Using the electrochemical technique called cathodic protection to control external corrosion of underground and marine structures. Cathodic protection is used to control internal corrosion of storage tanks, production , and process vessels that contain water. In many cases, coating is used with cathodic protection systems to reduce cathodic protection current requirements.
Cathodic protection (CP) is a proven method of corrosion control. However, CP must be economically justified because it can be expensive to install and maintain. If the life of the structure is not long enough to justify the expense of CP, it may be necessary to replace the structure. However, hazardous situation may make CP a necessary.
In many cases, CP is very advantageous. Its, economic justification is determined by comparing leak repair costs with the cost to install and operate a CP system.
Cathodic protection has decreased the number of leaks on different structures. After applying the CP program, the number of leaks will be decreased significantly.
There are different types of CP systems. Therefore, a thorough economic analysis may be needed to consider the cost of more than one CP system. The cost of the CP systems evaluated should include the following costs:
1. The initial cost to design and install the system.
2. The cost of power ( if required).
3. The cost to maintain the system ( inspection, repair and component replacement).
Obviously, this economic analysis requires good records. Accurate data ( cost per leak, power costs and system installation costs) is not easy to obtain. However, the validity of the analysis depend on the accuracy of cost figures.
02-11-2006, 10:23 PM
المواضيع المتشابهة 
