What Does Local Corrosion Cell Mean?
A local corrosion cell occurs when two different metals (in terms of microstructures and compositions) come into contact in an environment filled with electrolytes.
The corrosion produced in this type of cell may vary. It will depend on the direct reaction that occurs between the metals, as well as the type of solution that comes in contact with the two. In some cases, corrosion may be differentiated into cathodic and anodic parts that may occur at separated areas by fixed distances. If the corrosion occurs with no separation, it can be classified as direct chemical corrosion. If the two parts in the local corrosion cell are separated, it is known as electrolytic corrosion. Regardless of the type, the reaction course still conforms to thermodynamics and electrochemistry laws.
Corrosionpedia Explains Local Corrosion Cell
Local corrosion cells come in many forms. Some of the most common are:
- Dissimilar metals: This corrosion cell is composed of two metals in a single phase that are in contact. Good examples are zinc, nickel, iron and gold. The metal that is higher in the electrochemical series will be classified as the cathode. The other will be the anode and will suffer corrosion. Dissimilar metals that are in contact, bathed in an appropriate electrolyte, is the principle behind batteries. In this type of reaction, controlled corrosion can be observed.
- Multi-phase alloy: This is formed through metal alloys created out of several phases like cast iron, stainless steel and aluminum alloy. Each phase possesses dissimilar electrode potentials, which results in one phase producing an anodic reaction and later, corrosion.
- Stress cells: This can be described as one piece of metal wherein a part of its microstructure contains more saved strain energy compared to the rest of the area. Metal atoms tend to have very low strain energy when placed in a crystal array.
- Concentration cells: These occur when one concentration in the series differs with the electrolytes surrounding the cells. It can be an electrolyte concentration where the anodic portion corrodes faster to enhance the local concentration of ions.
- Oxidation concentration: This is the most typical corrosion cell that affects engineering structures through oxygen gas. For this to occur, oxygen needs to have access to metal in an environment favorable to corrosion.