Region 7 - Corrosion Protection Homepage
Basically corrosion is an electrochemical process based on four conditions that must be present:
- There must be a positive or anodic area, referred to as the "anode."
- There must be a negative or cathodic area, referred to as the "cathode."
- There must be a path for ionic current flow, or "electrolyte."
- There must be a path for electronic current flow, which is normally a "metallic path."
The electrical potential between the anode and cathode causes the corrosion current to flow. The anode is the area that suffers metal loss and corrosion. The amount of metal that will be removed is directly proportional to the amount of current flow. As an example, one ampere of direct current discharging into a soil electrolyte can remove approximately 20 pounds of steel in one year. This rate would vary from metal to metal, and the corrosion current normally encountered is recorded as thousandths of ampere, or milliamperes.
If one of the four conditions mentioned above can be eliminated, the corrosion process and metal loss can be prevented. Cathodic Protection, one of the most common methods of corrosion mitigation in the pipeline and storage tank industry, mitigates corrosion by eliminating all anodic areas on an underground metallic structure.
Cathodic Protection is usually accomplished through one of two methods: the first method would be to connect a sacrificial metal (with a higher natural electromotive force) through a metallic conductor or wire to the structure intended to be protected. Magnesium is a common sacrificial or galvanic anode. This type of galvanic cathodic protection relies on the natural electrical potential between the two metals to cause the cathodic protection current to flow. Since the driving voltage is limited to the very small potential difference existing between the metals, and the current output is relatively low, this type of cathodic protection is normally associated with very small or very well-coated structures.
The second common type of cathodic protection is referred to as impressed current cathodic protection. This type of cathodic protection relies on an external direct current source such as a rectifier or battery. An anode material is placed in the electrolyte with the structure intended to be protected, and is made more positive than the structure by connecting both the anode and the structure to the direct current supply. Any conductive materials can be utilized as an impressed current anode, but since corrosion takes place at the anode, materials with very low consumption rates are most desirable.
The four conditions required for corrosion to occur can involve two separate structures or can be present on one structure. The following are four common types of corrosion cells:
1) Dissimilar Electrolyte Corrosion Cells
This type of corrosion occurs when a structure passes through an electrolyte of varying properties. Normally, the electrolyte varies in chemical composition or electrical resistivity. When variations of resistivity occur along the same structure, normally the area of the structure in contact with the lower resistive electrolyte will be the anodic area. The natural electrical potential of a metal in an electrolyte can vary significantly with differences in electrolyte compositions. The development of a potential difference, even between two points on the same structure, can provide the criteria necessary for corrosion to occur.
2) Dissimilar Metal Corrosion Cells
This type of corrosion is more commonly referred to as galvanic corrosion. It occurs when two metals of different compositions are metallically contacting each other in a common electrolyte. The magnitude of potential difference between the two metals, and which of the metals has the more negative potential, will determine which metal will be the cathode, which will be the anode, and the rate at which corrosion will occur at the anode.
3) Existing Structure/New Structure Corrosion Cell
This type of corrosion is very similar to dissimilar metal corrosion in that you have an electrical potential between two metals in a common electrolyte. New sections of the same type of metal are commonly used when making repairs or additions to a structure. The unfortunate thing about this type of corrosion is that the newer structure will normally become the anode.
4) Differential Aeration Corrosion Cells
Another important source of corrosion activity on a structure is differential aeration of the electrolyte (commonly soil). In a situation where part of a structure is in soil having a free supply of oxygen (well aerated), and an adjacent area is in oxygen-starved (poorly aerated) soil, the part of the structure in the well-aerated soil will be the cathode and the part of the structure in the poorly aerated soil will be the anode.
We would like to make it known that there are many other types of corrosion cells, and the corrosion mechanism is far more involved than stated here; however, the above is provided as basic information and reference for structure owners.