Galvanic corrosion is relatively common, and it’s an issue that can be prevented by proper fastener selection. Galvanic corrosion occurs when two different metals contact each other in an environment that contains oxygen or moisture. When this happens, a small electrical current travels between the two metals, causing one of the metals to corrode more quickly than it would on its own.
Electrical current?
That’s right. Dissimilar metals and alloys have different electrode potentials, and when two or more come into contact in an electrolyte, one metal (more reactive) acts as an anode and the other (less reactive) as a cathode. When there is a major difference in the chemical composition of the two metals, galvanic corrosion will likely occur.
You want to avoid galvanic corrosion because this process accelerates the breakdown of your fasteners. When the typical corrosion is amplified through this process, you can experience degradation, crevice corrosion, pitting, and other common forms of material breakdown leading to the fastener not performing to its optimal use.
Galvanic Corrosion Environments
As mentioned, the corrosion of two dissimilar metals in contact with each occurs when there is a difference in electrical potential between the two metals. The metal with higher electrical potential will corrode more quickly than the metal with lower electrical potential, which means that it can rust at different rates and attract dirt and debris from the environment.
If you submerge an aluminum door with carbon steel fasteners into seawater, for example, the aluminum will corrode faster due to the electrical potential being different from the carbon steel. This is why you sometimes see rust marks along rivets on a boat or exterior facing of a metal door when the bolts or fasteners used in welding have a different electrical potential than the metal wall/door they are securing.
Consider these:
– The difference in reactivity between the two metals: The greater the difference in reactivity, the greater the corrosion rate.
– The surface area of each metal: The larger the surface area, the greater the corrosion rate.
– The level of electrolyte: The higher the electrolyte, the greater the corrosion rate.
How to Prevent Galvanic Corrosion?
Several methods ensure the metal materials you are working with do not experience accelerated galvanic corrosion. These include:
Similar Corrosion Potentials
Use materials that closely match each other on the galvanic series chart, which shows the relative corrosion rates of different metals in relation to each other. A material with a lower corrosion potential will not corrode as quickly when placed in contact with a higher-corrosion-potential material.
Insulating the Two Metals from Each Other
The more common practice is to insulate the two metals from each other, using a non-conductive material and ensuring that they are isolated from each other so that no galvanic current can flow between them. This can be accomplished using grease, oil, coatings like paint or varnish, or thicker materials like PVC, plastic, and wood.
Using Corrosion Inhibitors
You can also use an inhibitor applied before and after the project has been completed. If you choose to apply them both, ensure enough time between applications for the product to dry thoroughly.
Select High-Quality Fasteners to Avoid Galvanic Corrosion
To avoid galvanic corrosion, select high-quality fasteners made from stainless steel or galvanized treated fasteners that include some form of alloy coating. This ensures a drastically reduced electrical consideration so you can complete your project with as little risk of future accelerated corrosion as possible.
The goal is to have an installation with as little rust, corrosion, or degradation effect as possible, so the structural integrity is unaffected for years to come.
A well-documented example of galvanic corrosion occurred in the Statue of Liberty when regular maintenance checks in the 1980s revealed that corrosion had occurred between the outer copper skin and the wrought iron support structure. Although the problem was anticipated in the 1880s, the shellac insulating the two metals failed faster than projected and resulted in the iron supports rusting. An extensive renovation replaced the original insulation with PTFE.