Definition: A Chemical reaction between a metal and its environment to form metal compound.
A Chemical or Electrochemical process through which a metal is attacked by natural agents such as air and water resulting in partial or complete dissolution, deterioration or weakening of any solid substance.
A common Example of corrosion is “Rusting of Iron” Iron combines with Oxygen in air and water to form Hydrated Oxide of Iron.
Corrosion rate increases with time, and the process of corrosion will be accelerated in the following conditions:
- Metal Surface under stress
- With inter-granular impurities in the metal
- Situations where the corrosion products do not completely cover the substrate metal.
Types of Corrosion:
- Non Aqueous corrosion (or) Chemical (or) Dry Corrosion
- Aqueous Corrosion (or) Electrolytic (or) Wet Corrosion
1. Non-Aqueous Corrosion (or) Chemical (or) Dry Corrosion:
As the name depicts Corrosion occurs in the absence of Water or any Fluid Electrolytes hence called as Dry Corrosion. The process of Chemical Corrosion goes as follows:
Most of the metals except Gold and few other Noble Metals will form Oxide layers when they react with Oxygen in Air.
- Metal reacts with Oxygen from Air.
- Forms Surface Oxide Layers.
- The metals gain weight due to formation of Oxide layers
Weight Gain is seen in the following ways:
- Parabolic Gain: The Oxide layer is very stable, the metal ions takes very long to diffuse through the thick oxide surface layer.
- Linear Gain: The Oxide layer is not very stable, they tend to crack or separate partially from the surface.
- Linear Loss: Here the Oxide layer Evaporates as soon as it formed due to the high temperatures. This leads to Weight Loss.
2. Aqueous Corrosion (or) Electrolytic (or) Wet Corrosion:
This type of Corrosion occurs in the Oral Environment. The presence of moisture, temperature fluctuations and the changing pH caused by diet and decomposition of foodstuffs contributes to this type of corrosion. Aqueous Corrosion requires presence of Water or other fluid electrolytes so it termed as wet corrosion. this occurs by Electrochemical reactions as a pathway to the transport of electrons is required.
Types of Electrochemical Corrosion:
Galvanic Corrosion (or) Dissimilar Metal Corrosion:
Galvanism is the production of Electrical current that the patient can feel, which is produced when 2 dissimilar metals are in direct physical contact. Aluminum alloys used in Temporary crowns have a Electrode potential of +1.66 volts and Gold – 1.49 volts. In presence of Saliva or Oral fluids function as Electrolytes and act similar to that of an Electrical cell. This produces Galvanic Pain and metallic taste. Most patients feel pain between 20 to 50 Uamp.
Conditions where Galvanic Corrosion Occurs:
- In presence of Adjacent Dissimilar restorations, with Galvanic Action material goes into solution and roughness and pitting occurs.
- A Single Metallic Restoration canalso show Corrosion, between an External surface exposed to Sailva and internal surface exposed to Dentinal Fluid.
- When an external metal like, tin foil, silver fork,, Spoon or any other metal comes in contact with the metal restoration.
- Can be seen in Similar metals also as their surface composition differs
Stress Corrosion: Degradaion by the combined effect of mechanical Stress and Corrosive Environment, usually in the form of crackcing.
A metal which has been stressed by cold-working bending and burnishing, produces the localized stress in some part of the structure. If stressed and under-stressed metals are in contact in an electrolyte, the stressed area will become the anode of a galvanic cell and will corrode.”Hence Excessive Burnishing of metal restoration should be avoided”
In orthopedic surgery, stainless steel plates and screws are frequently embedded in tissues. Any differences in the extent of cold working between screws and plate must be avoided. Similarly, different portions of the same piece of steel may be stressed to different extends, thus results in corrosion.
Concentrataion Cell Corrosion (or) Cervice Corrosion: An Elecrtochemical corrosion cell, in which the potential difference is associated with the difference in the concentration of the solutions causing corrosion at different parts of the metal surface.
A homogenous metal or alloy can undergo electrolytic corrosion where there is a difference in electrolyte concentration across the specimen examples:
- Accumulation of food debris over a metallic restoration or interproximal areas. One type of Electrolyte is formed under food debris and one type in Saliva which causes the Electrochemical corrosion under the layer of food debris.
- Where there is difference in the oxygen concentration in an electrolyte, an oxidation-type concentration cell is produced. Corrosion is greater at portions where oxygen concentration is lower. This type of corrosion occurs in an unpolished metallic restorations like pits in the restoration.There is less oxygen content at the bottom of the pit which acts as an Anode, Pitting occurs at the bottom of the Pit. This is called Pitting Corrosion. To avoid this type of corrosion the metallic corrosion should be well polished.