This article is the second in a three-part series addressing metal corrosion in buildings and other structures. The previous installment addressed pitting corrosion, and the next will address crevice corrosion.
By Michael Harkin
Galvanic corrosion refers to the damage that occurs to an asset when two different kinds of metal are joined in a corrosive environment. When the environmental conditions are right and the metals are in electrical contact with one another, one of the metals will corrode more quickly than if it were on its own while the other will corrode much more slowly.
While galvanic corrosion can cause serious failures, preventing it is as simple as understanding the factors and chemistry behind it, designing against it and planning ahead for it if assets must be exposed to the elements.
Sometimes exposing assets made of different kinds of metals to the elements is unavoidable. With that comes the risk of a degradation of one metal due to its proximity to another coupled with the right environmental conditions.
Metals that are prone to this corrosion are known as anodes; metals resistant to it are known as cathodes. The risk of galvanic corrosion depends on the chemical properties of the metals of an asset. As the Galvanic Series illustrates, metals like gold, platinum and titanium are more cathodic (also known as more noble) and are therefore more resistant to corrosion. Metals like magnesium, zinc, beryllium and aluminum alloys are more anodic (less noble) and are more susceptible to corrosion. The greater the difference between metals on the Series, the higher the risk of galvanic corrosion if the two are paired in a corrosive environment.
Single pieces made of high-alloy metal also are at risk if the different metals present are far apart on the Galvanic Series and if the piece is exposed to a corrosive environment.
Preventing Galvanic Corrosion
The most effective way to prevent galvanic corrosion is to understand the Galvanic Series and design assets using metal combinations that won’t support the genesis of the corrosion. For example, joining metals that are farther apart on the Galvanic Series (such as titanium and aluminum) increases the risk of galvanic corrosion. Similarly, joining metals that are closer together on the scale (such as tin and copper) reduces the risk.
Time, resources, and even lives can be saved far in advance when asset designers know the risks associated with galvanic corrosion and eliminate those risks before they ever become a problem. Sometimes, though, asset specifications require joining certain metals. There are two prevention options when the risk of damage due to galvanic corrosion is inevitable:
Control the environment. If assets don’t need to be in corrosive environments, see that they aren’t introduced to or stored within them to reduce the risk of corrosion.
Coat the asset. The proper protective coating can prevent corrosion if the asset will encounter corrosive environments.
Galvanic Corrosion: Extend Asset Life?
In a number of industries, galvanic corrosion can be used to actually lengthen the life of an asset. Attaching a piece of metal to a surface that’s higher on the Galvanic Series will spur corrosion of the anodic metal while protecting the meaningful, more cathodic parts of an asset that otherwise would have been at risk. This technique is called “cathodic protection” and is a popular — although at times expensive — option for hard-to-reach surfaces like the hull of a ship or a buried pipeline.
There are some tradeoffs with cathodic protection. Developing a cathodic protection system for an asset is more expensive than just using a traditional “barrier” methods like coating the surface. Additionally, maintenance managers must keep a constant watch on sacrificial anodes, replacing them frequently to ensure maximum corrosion protection. Also, avoid putting any type of protective coating over sacrificial anodes which will negate their effectiveness.
Inspect And Plan Accordingly
Galvanic corrosion is a significant and costly issue that often leaves asset owners and operators overwhelmed and frustrated. But there are ways to fight the good fight and achieve true peace of mind about the state of your asset. Finding a trusted partner to conduct surveys and develop corrosion control and prevention plans goes a long way to ensuring long, safe, and useful lives for assets.
Harkin is a NACE and SSPC coating inspector and current President of FeO, a QP5-certified coating inspection and consulting company located in Virginia Beach, VA. Prior to FeO, Harkin served as an Army soldier and a Marine Corps officer.