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:
- 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.
- 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.
- 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.
You’ve probably been warned about building with dissimilar metals such as carbon steel and stainless steel; there’s good reason. This mistake has been the cause of major catastrophes, like the Santa Barbara oil spill.
Still, you don’t have to avoid this metallic combination altogether. We’re here to help you understand how to do it the right way.
Read on for a simple breakdown of galvanic corrosion and how to prevent corrosion between carbon and stainless steel.
What Is Galvanic Corrosion?
is the reason connecting carbon and stainless steel can lead to problems. Galvanic corrosion is when one metal causes another metal to corrode and break down.
Galvanic corrosion is the reason connecting carbon and stainless steel can lead to problems. Galvanic corrosion is when one metal causes another metal to corrode and break down.
For this corrosion to start, there need to be three things: an anode (one metal), a cathode (a second metal), and an electrolyte (water is a common one).
Some metals are more likely to give their electrons away, and others are more eager to pull in extra electrons. That means combining these different types of metals in electrolyte-heavy environments causes one metal to hand its electrons over to the other.
When a metal gives away electrons, galvanic corrosion starts, and the metal rusts.
How Does Galvanic Corrosion Happen?
When two dissimilar metals are connected to each other, the electrolyte helps move one metal’s electrons to the other metal. As the electron-generous metal loses electrons, it undergoes oxidation.
Oxidation causes the metal to rust, weaken, and disintegrate. The result is a corrupted metal piece and a weakened pipe, beam, or structure.
Carbon Steel vs. Stainless Steel: What’s the Difference?
Not all steel is the same. In fact, some steels don’t get along well together. This can be the case with carbon steel and stainless steel.
What’s the difference between these two common types of steel?
Carbon Steel
Carbon steel and stainless steel are both iron-based metals, but carbon steel has an especially high content of carbon. This makes carbon steel notably strong, heavy, and hard.
However, carbon steel is vulnerable to corrosion. That’s because it’s made of iron, and it’s easy for oxygen to corrupt iron. The result is iron oxide, or rust, which can completely eat away at carbon steel.
Stainless Steel
You might be thinking, “Wait, if both steels are iron-based, why is stainless steel resistant to corrosion?”
Stainless steel has a secret weapon: chromium.
Chromium can shrug off oxygen without corroding. The addition also pushes stainless steel higher up on the nobility chart.
So when stainless steel and carbon steel are connected, and an electrolyte such as moisture is introduced, stainless steel absorbs carbon steel’s electrons. Carbon steel can deteriorate rapidly, become weak, and come crashing down.
How to Prevent Corrosion Between Carbon and Stainless Steel
Despite not getting along, both carbon steel and stainless steel are strong and useful. Luckily, there are some ways to help them work in tandem without causing corrosion:
Use a Buffer
If the two types of steel can’t play nice, we might as well separate them. How? For piping, you can install things such as pipe shoes or wear pads. You can also add clamp liners or all kinds of insulators, like ProTek rods or flat plates.
These supports reinforce piping and keep metals from rubbing against one another.
They also help stabilize structures, which cuts down friction, makes cracks less frequent, and makes it harder for corruptive electrolytes to slip into metals.
Other buffers, such as nylon washers or fasteners, can add a protective layer between dissimilar metals in bolts or support beams, too.
Plate the Carbon Steel
Galvanizing is a way to protect carbon steel without completely changing its structure. When carbon steel is galvanized, a layer of zinc is spread over its surface.
Zinc is much lower on the galvanic scale than carbon steel, which means it’s more basic and more likely to give up its electrons than carbon steel.
After galvanizing, zinc sacrifices its electrons anytime a corrosive metal connects with the surface. Thus, the carbon steel structure can retain its sturdy form.
Reduce Exposure to Electrolytes
Remember, for galvanic corrosion to start, there need to be two metals and an electrolyte. So keeping electrolytes at bay can slow corrosion.
Some good options are to add aeration or smooth out surfaces whenever it’s possible. Waterproofing by adding water-resistant coatings can go a long way toward preserving metals, and you can use sealants to stop water or gunk from sliding between crevices.
It’s also a good idea to add drainage to avoid stagnant water. Pooling water erodes sections of metal and kick-starts the corrosion process. A good way to add drainage is through weep holes. This is where you drill holes in the bottom of hollow supports to give water an escape route.
More on Preventing Corrosion
Corrosion can be a scary hazard on a work site or in existing structures. However, knowing how it works will help you take the right actions to stop it. Download The Expert's Guide to Hot-Dip Galvanization to learn more.