General corrosion is well understood in the engineering industry, and most design codes offer guidance on how to reduce the problem. However, a lesser understood form of corrosion is bimetallic corrosion, which can be catastrophic if allowed to develop. Check it out below.
What is Bimetallic Corrosion?
Bimetallic (or galvanic) corrosion occurs when two different parent metals come into contact with one another. For corrosion to occur, there must also be water present. However, this doesn't mean that both metals must be submerged. Rather, the moisture content of air is usually enough to permit corrosion of both metals.
The process of bimetallic corrosion causes one of the metals to deteriorate significantly and occurs in environments that may not usually be considered corrosive. In the metal coupling, the corrosive material is referred to as the anode, while the non-corrosive material is referred to as the cathode.
During corrosion, positive current flows from anode to cathode through the moisture present. This current causes the anode to heavily degrade, whilst the cathode remains untouched.
What Factors Affect Bimetallic Corrosion?
As alluded to above, there are specific criteria that must be met for bimetallic corrosion to occur. However, this doesn't mean that all metal pairings will corrode at the same rate. Rather, there are a number of factors that dictate the extent of bimetallic corrosion.
The most important factor in determining the corrosion rate is the galvanic ranking of both metals. These are deduced from the galvanic series, which ranks metals in terms of their "nobility". The noblest materials are the ones that have the highest resistant to bimetallic corrosion, whereas the least noble materials are the ones that suffer the greatest degradation. As such, the corrosion rate is highly dependent on the gap between the metals in question on the galvanic series.
The next factor that controls corrosion damage is the exposed surface areas present in the metal coupling. Again, it is the difference between both metals that dictates the rate of corrosion. An anode with a small surface area will corrode heavily when paired with a cathode of large surface area. The reverse is also true – if the anode has a relatively large surface area compared to the cathode, corrosion rates will be kept minimal.
Finally, the strength of the electrical connection is important in determining corrosion rates. Solid connections with low resistance will transfer a high rate of current flow, whilst "weak point" contacts will transfer a very small amount of current. As the process of corrosion relies on the current transferring material from one metal to another, it depends entirely on how much current is flowing between the metals.
How Can Bimetallic Corrosion Be Prevented?
The most obvious solution to controlling bimetallic corrosion is to avoid bringing two dissimilar metals into close contact. With that said, sometimes this is unavoidable and measures must be taken to control the extent of corrosion damage on the anodic material.
One of the best ways of reducing galvanic corrosion where dissimilar metals must be in contact is to ensure that the cathode area does not dominate. This means keeping the cathode area relatively small when compared to the anode. By doing this, your metal coupling will still experience corrosion; however, the flow of electrons will not be as concentrated and your metal will corrode at a much slower rate. During the design stage, you should size your anode and cathode areas to ensure that the corrosion is kept at a reasonable rate throughout the material's design life.
If sizing the cathode is proving difficult due to the structure's layout, you should consider treating the metal prior to application. Typically, there are two treatments used throughout the engineering industry:
- Sulphuric anodising – an electrolytic treatment specifically for aluminium that creates a protective oxide layer. This layer is highly resistant to corrosion and can be made in a number of colors to suit your application.
- Paint coatings – a protective treatment that does not create an oxide layer; however, the protection offered by the paint is usually enough to reduce the problem of galvanic corrosion. Care must be taken to ensure that scratched paint does not reveal small anode areas, as this will lead to high corrosion.