Galvanic Corrosion, Understand and Control This Type of Corrosion
Galvanic corrosion is a form of corrosion that shall be properly addressed during the design phase because the consequence of this type of corrosion can be quite expensive. Usually when asked to define the mechanism of Galvanic Corrosion a Corrosionist, people involved in Corrosion Prevention and Corrosion Protection, uses something similar to the following: “Galvanic corrosion, also called Bimetallic Corrosion, takes place when two dissimilar metals are electrically connected in an electrically conducting fluid.”
In very simple terms this means that when two different metals are placed in contact in presence of water or another conductive fluid, one will corrode while the other will not corrode.
To make it easier to understand is better to break it into three basic concepts:
- You need to have two dissimilar metals
- The dissimilar metals shall be electrically connected
- The dissimilar metals shall be electrically connected and in contact with an electrically conducting fluid
Now let’s analyze each single point:
How much they have to be “dissimilar” to be considered as a potential for galvanic corrosion?
This is where, usually, most people take the wrong direction in assessing the likelihood of galvanic corrosion.
Examples of dissimilar metals can be steel and copper, aluminum and nickel, zinc and magnesium and basically every time that you have two “different” metals in contact you’ll have to assess if galvanic corrosion can be an issue.
Corrosionists developed what is called the “Galvanic Series of Metals” or “Galvanic Chart” that in simple terms is a list of all the most common metals arranged in an order that goes from the “most active” or “less noble” to the “less active” or “most noble”.
The meaning of this Galvanic Series or Galvanic Chart is that when you join two of the metals listed, the one that is lower, usually the Series or Chart is arranged with the “most active” or “less noble” on the bottom of the list, will be the metal that corrodes, while the “less active” or “most noble” one, located in the higher part of the list, will not corrode.
Electrically connected means that there must be an electrical connection between the two metals and this could be a weld, an electrical wire, a bolted connection…
The last point is very important because the two metals shall be immersed in a conducting fluid, e.g. water, seawater, in order to have galvanic corrosion and the corrosion attack will takes place in the immersed areas of the most active metal.
“Immersed area” can be somewhat misleading because someone can think that galvanic corrosion occurs only if there is a complete immersion, like a ship in seawater, while this can be a thin layer of condensed water from the atmosphere or rainwater.
So when you have to define the likelihood of galvanic corrosion, these are the basic steps that you’ve to follow:
- Define the type of materials
- Define which metals will potentially corrode; the “most active” or “less noble” in the galvanic series
- Define if there is a continuous contact between the two metals: e.g. weld, bolted connection…
- Define if there is, or there could be a conductive fluid, e.g. water, in the area of contact
These are the factors that allow you to define if Galvanic Corrosion shall be considered in your design, but in order to define how much corrosion you will have or the severity of attack, you will need to go deeper into the mechanism and evaluate other factors like: area ratio, type of electrolyte, time of exposure to wet conditions…
Once you’ve assessed if Galvanic Corrosion can be a problem and the severity of damage, you can evaluate how to stop or prevent it.