Galvanic corrosion is an electrochemical process, resulting when two very small electrical currents from two dissimilar metals come together.
The result is that the more anodic of the two metals will corrode.
For example, if you have carbon steel screws securing a stainless steel window frame, it is likely that the screws will corrode at an accelerated rate.
But by using the right kind of powder coating you can prevent galvanic corrosion.
What Conditions Cause Galvanic Corrosion?
The conditions necessary for galvanic corrosion to occur involve:
- Two different metals, each with a different potential for corrosion
- Direct contact between them
- A conductive electrolyte solution, typically water.
Galvanic corrosion will only occur if all three of these conditions are present.
Anything that blocks a direct path between the two metals will prevent galvanic corrosion occurring.
Galvanic corrosion is also known as bimetallic corrosion.
How Does Galvanic Corrosion Occur?
Metals vary in how noble they are. Noble metals are resistant to corrosion and oxidation in moist air.
The less noble a metal is, the less resistant it is.
In the electrochemical process of galvanic corrosion, the noble metal is the cathode and the less noble metal the anode. The potential difference in voltage between these two electrodes becomes a driving force for corrosion to attack the less noble metal.
For example, galvanic corrosion occurs when different metals that are connected are submerged in seawater.
Galvanic corrosion does not occur in dry surroundings, but in surroundings where there is chloride present in the atmosphere, on the coast for example, then it can happen.
The same applies if the metals concerned are on structures exposed for prolonged periods to the elements.
Consequently, in some external metal architectural features, you can see galvanic corrosion happening.
Why is Surface Area a Factor?
Surface area can play an important part in galvanic corrosion.
Basically, the larger the cathode metal is compared to the anode, the more of a reaction can occur.
For example, if you have steel rivets in a copper bar submerged over a long period, eventually the corrosion will eat away at the rivets.
But in a reverse situation, where copper rivets are applied to an iron bar, the iron only suffers a slight acceleration in corrosion.
Another example would be using carbon screws in a steel window frame. Over time, the bolts would corrode much faster than the frame.
What Examples are there of Galvanic Corrosion?
Perhaps the most famous example of galvanic corrosion occurring is on the Statue of Liberty in New York.
The statue combines an iron armature with a copper skin. The original design of the statue had included a shellac-soaked asbestos cloth insulation to prevent galvanic corrosion happening when these two metals made contact.
But this was only a temporary solution, and eventually the electrolytes breached this barrier, causing corrosion of the iron armature on the statue.
As part of a restoration project, the armature was eventually replaced with ferralium 225, a high-performance, stainless steel alloy.
On a much smaller scale, you can observe the process of galvanic corrosion if you cover something salty in a steel pan with aluminium foil in the fridge. Where the food has contact with the foil, it will form holes quickly, and there will be corrosion appearing in spots on the food.
The salty food acts as the electrolyte, and the aluminium foil is the anode. The steel pan is the cathode.
Why is Galvanic Corrosion a Risk?
Corrosion raises various issues in fabrication, architecture and manufacturing.
As we have also seen, surface area can be a factor too, with galvanic corrosion degrading bolts that fix things together.
Galvanic corrosion will seldom occur between two stainless steels, but materials such as graphic gaskets will increase the risk and problems from galvanic corrosion.
There are, however, ways to prevent galvanic corrosion occurring, including applying powder coating to parts and surfaces.
How Do You Protect Against Galvanic Corrosion?
One method of corrosion protection is to select metals that are as close together in the galvanic series.
The galvanic series determines the nobility of metals, which is an indicator of how they will react if the conditions for galvanic corrosion are present.
Another is to avoid the surface area effect of combining a small anode and a large cathode.
Insulating dissimilar metals will help, if this is practical.
One versatile and adaptable alternative, though, is to apply a powder coating, either to the cathode or, preferably, both metals.
Can Powder Coating Prevent Corrosion?
Powder coating helps metal surfaces resist various forms of corrosion.
It provides a durable finish, which you can apply evenly. Once you bake it to cure it, it is resistant to abrasion, peeling, chipping and cracking.
Also, because you apply it in powder form, it can reach difficult areas and cover sharp edges and corners, giving a proper, protective seal to the surfaces and objects you coat.
Powder coating is also UV and weather-resistant, making it ideal for outdoor applications.
To protect against galvanic corrosion, powder coating insulates metal surfaces from galvanic currents.
The best type of powder coating for this purpose is epoxy-based.
Powder coating is an effective solution, but you will still need to maintain your powder coated surfaces, to ensure they continue to provide the right level of corrosion prevention.
This means checking the coverage is even and has adhered properly to the substrate, and then that you keep your coated surfaces clean and well maintained.
For more information about powder coating, or a quote, please contact us.