Three ways to prevent Galvanic Corrosion

A frequently asked question in the context of metal connections is how dissimilar metals behave and if galvanic corrosion can take place for example between carbon steel and stainless steel, or aluminum and carbon steel. The short answer to this question is: usually galvanic corrosion is not a problem.

The long answer is: for galvanic corrosion to take place, the following 3 conditions need to be met simultaneously:

• different types of metals,
• presence of an electrolyte (e.g. water),
• electrical continuity between the two metals (source)

The potential for galvanic corrosion between metals is dictated by how far they are apart on the galvanic series of metals (source). The higher the potential difference, the higher the risk for corrosion to take place.

The most common structural materials in timber buildings are stainless steel, carbon steel, aluminum, and zinc (as a coating for carbon steel). The fact that zinc is commonly used in close contact with steel as a coating provides us with a couple of interesting insights:

• in principle it's not a problem to combine dissimilar materials. Zinc doesn't start to deteriorate automatically as soon as it comes in touch with steel
• zinc is less noble than steel (i.e., further to the left on the above chart) and therefore acts like a sacrificial layer to prevent corrosion of the steel, because noble materials corrode less noble materials

One of the 3 conditions for galvanic corrosion to take place is the presence of a electrolyte. The electrolyte enables the movement of ions and as a consequence starts to corrode the less noble material. Water is one of the most common electrolytes and if you keep the water out of the connection, you prevent galvanic corrosion to take place.

But, doesn't wood contain moisture that could potentially act as an electrolyte and trigger galvanic corrosion? Even after years in a closed environment timber only reaches an equilibrium moisture content, but it will never reach 0% moisture content. Here it's important to distinguish between free water and bound water. Free water could potentially act as a electrolyte but even then the risk associated with it would be very very low because the electrolyte needs to touch both dissimilar materials, and free water is not exactly pouring in streams out of the wood cells. Bound water in turn can't act as a because it's bound within the cells of the timber. It's safe to say that there's no free water left in timber with a moisture content below 20% (source) and given that the moisture equilibrium of timber is closer to 10%, the timber that surrounds the connection can actually protect the connection from galvanic corrosion by absorbing excess moisture and prevent a buildup of water.

Sometimes we cannot avoid using dissimilar metals. Or by using a mix of different metals we yield efficiencies for the project and we don't want to give up on those. And we don't need to give up on them if we adhere to a few principles:

1. KEEP THAT WATER OUT. No water, no electrolyte, no galvanic corrosion. Water is not only a problem for galvanic corrosion, but also for conventional corrosion, swelling and deformation of the timber, rot, stains and funghi. Actually, with water in the structure, galvanic corrosion can be the least of your concerns.
2. Insulate dissimilar materials, i.e., break the electrical continuity between the materials. For example the LOCK EVO is made of extruded aluminum and is used with stainless steel screw fasteners. The aluminum plates are painted to avoid galvanic corrosion.
3. Make sure fasteners are the more noble material in the connection. For example, our concealed hangers plates in aluminum use carbon steel screws and dowels (where applicable). Carbon steel and aluminum don't have a too big potential difference on the galvanic series to start with, corrosion is therefore unlikely and even if it takes place it's very slow. Further, as carbon steel is the more noble material, it would not be the aluminum connector cutting through the small diameter fasteners and immediately jeopardizing the structural integrity of the connection, but it would be the small diameter fasteners corroding the less noble aluminum, therefore visibly displacing the connection before it comes to catastrophic failure.

The conclusion is, that the most commonly used materials in timber construction do not show a significant risk for galvanic corrosion if we follow the basic principles, first of all to keep water out of the structure. The theoretical considerations outlined in this article are backed up with rothoblaas' decades-long experience in testing and in production of structural solutions for timber construction.