Galvanic corrosion can cause significant damage to fasteners and structures without being noticed. When two different metals come into contact with each other in a damp environment, the less noble metal can corrode at an accelerated rate. This happens, for example, with stainless steel screws in aluminum or galvanized steel, or when copper and steel are combined.
For roof and facade structures, which are exposed to rain, condensation, and pollution on a daily basis, this is an important consideration.
In this article, you’ll learn how galvanic corrosion occurs, which metal combinations pose an increased risk, and how you can significantly extend the lifespan of your fasteners by choosing the right materials and proper sealing.
What is galvanic corrosion?
Galvanic corrosion, also known as contact corrosion or bimetallic corrosion, is a form of corrosion that occurs when two different metals are electrically connected in a moist or conductive environment. Examples include rainwater, condensation, or salt spray between a stainless steel screw and an aluminum plate.
This creates a small electric current. The less noble metal becomes the anode and can dissolve more rapidly, while the more noble metal is protected as the cathode.
This principle is also explained in the technical information sheet on contact corrosion from ZinkInfo Benelux, which specifically addresses galvanized steel in combination with other metals.
How does galvanic corrosion work?
Galvanic corrosion occurs only when three conditions are present simultaneously:
- Two different metals in contact with each other.
- An electrolyte, such as moisture, condensation, rainwater, or salt water.
- A potential difference between the two metals.
A greater difference in nobility increases the driving force for galvanic corrosion, but does not in itself determine how quickly the process proceeds. The surface area ratio, the conductivity of the electrolyte, and the duration of wetting also play an important role.
| Position | Metal | Behavior |
|---|---|---|
| Least noble (anode) | Magnesium | Corrodes the fastest |
| ↓ | Aluminum | Vulnerable when coupled with a more noble metal |
| ↓ | Zinc / galvanized steel | Acts as the sacrificial metal |
| ↓ | Steel / iron | Corrodes when coupled with stainless steel or copper |
| ↓ | Copper / brass | Relatively noble; can accelerate corrosion of steel, zinc, and aluminum |
| Most noble (cathode) | A2 / A4 stainless steel (passive) | The most noble metal in this selection in many environments |
(This order is indicative of operating conditions in which stainless steel remains passive. The exact position depends on the electrolyte, the alloy, the temperature, and the surface condition; stainless steel may move lower in the series if the protective passive layer is compromised.)
Metals that are close to each other in the galvanic series generally pose less risk than combinations with a large potential difference. However, no combination is entirely risk-free. The ultimate degree of corrosion is also influenced by the environment, the construction, and the duration of exposure to moisture.
When the passive layer is intact, stainless steel is among the more noble metals in many environments. As a result, it can accelerate the corrosion of less noble metals, such as aluminum, zinc, and carbon steel.
The aspect ratio is more important than many people realize
In practice, people often focus solely on the type of material, while the ratio between the two surfaces is at least as important.
The most unfavorable situation arises when a small surface area of a base metal is in contact with a large surface area of a noble metal. In that case, the corrosion of the base metal is greatly accelerated.
Therefore, a small stainless steel fastener in a large aluminum plate is generally a more favorable combination than a small galvanized fastener in a large stainless steel surface.
Although the former situation is considerably safer, a thorough assessment of the application is always necessary.
Which metal combinations require extra attention?
These combinations are common in roof and facade structures.
Stainless Steel and Aluminum
Stainless steel and aluminum can perform well, but require extra attention in humid environments or when exposed to chlorides, such as in coastal areas. The risk increases especially when the aluminum surface area is small relative to the stainless steel surface area.
Stainless Steel and Galvanized Steel
When stainless steel and galvanized steel are combined, the zinc coating may corrode more rapidly. How quickly this happens depends on the surface area ratio, the amount of moisture, and the presence of salt.
A small galvanized fastener embedded in a large stainless steel surface is one of the most unfavorable situations.
Copper or Brass on Steel
Copper and brass are more noble than steel. When these two metals come into contact with each other in a humid environment, the steel can corrode more rapidly.
Galvanized Steel and Copper
Zinc and copper also form an unfavorable combination due to the large potential difference between the two materials.
In dry indoor spaces, the risk is usually limited. After all, galvanic corrosion cannot occur without moisture. Outdoors, where rain, condensation, and salt are regularly present, the risk increases significantly. Joints that dry out slowly deserve special attention.
Case Study: The Ring That Nobody Put on the Bill of Materials
A common mistake does not occur with the screw, but with the washer.
A buyer carefully orders stainless steel screws for a facade or roof structure. During installation, however, it turns out that the washers are not listed on the bill of materials. As a result, a galvanized washer is quickly grabbed from the stock on the construction site.
At first glance, this doesn’t seem like a problem. In reality, however, this results in an undesirable material combination. With prolonged exposure to moisture and chlorides, the galvanized washers can corrode more quickly, leading to rust, leaks, and costly repairs.
Therefore, always specify the screw and washer as a single, complete, and compatible fastening system.
How do you prevent galvanic corrosion?
Fortunately, galvanic corrosion can often be effectively prevented. By carefully selecting the right materials, implementing electrical isolation, and keeping moisture out of the connection, you can significantly reduce the risk.
1. Choose the Same Type of Material
The safest solution is to use the same alloy for all components that are electrically connected to one another.
This prevents galvanic corrosion, although other forms of corrosion, such as pitting corrosion or crevice corrosion, may still occur.
2. Isolate the Metals from One Another
If a combination of different metals is unavoidable, ensure electrical isolation.
This can be achieved using non-conductive rings, plastic sleeves, or insulating spacers. A stainless steel sealing ring with vulcanized EPDM not only helps keep moisture out of the joint but also limits direct metal-to-metal contact beneath the ring.
For complete electrical insulation, it is important that contact is interrupted not only beneath the ring, but also along the screw shaft and around the drill hole.
3. Prevent moisture in the joint
Without moisture, galvanic corrosion cannot occur.
Therefore, ensure adequate drainage, ventilation, and a design that prevents water from pooling.
Be aware, however, that damaged sealant joints or poorly applied seals can actually trap moisture. Instead of providing protection, they create an environment where corrosion occurs more quickly.
A2 or A4 Stainless Steel: Which Grade Should You Choose?
When choosing stainless steel fasteners, the next question is often whether A2 or A4 is the best choice.
The main difference in terms of corrosion resistance is the addition of molybdenum to A4. However, A2 and A4 are material groups according to ISO 3506 and may include several specific stainless steel alloys.
| A2 Stainless Steel (often 304) | A4 Stainless Steel (often 316) | |
|---|---|---|
| Typical composition | Approx. 17.5–19.5% chromium, 8–10.5% nickel | Approx. 16.5–18.5% chromium, 10–13% nickel, 2–2.5% molybdenum |
| Corrosion resistance | Good; suitable for general atmospheric exposure | Higher; also suitable for chloride-containing environments |
| Application | Indoor and outdoor use with low chloride exposure and low corrosion loads | Coastal, chloride-containing, and aggressive industrial environments |
| Strength class | Specify separately, e.g., A2-70 or A2-80 | Specify separately, e.g., A4-70 or A4-80 |
For many indoor environments and standard outdoor applications, A2 is an excellent choice.
A4 offers greater resistance to chlorides and is therefore more commonly used in coastal areas, industrial environments, and locations where road salt is a factor.
Keep in mind that the final choice of material also depends on factors such as distance from the sea, the degree of shelter, the surface finish, the cleaning frequency, and the construction details.
In cases of heavy salt exposure, seawater spray, or environments with high chloride levels, such as certain areas in indoor swimming pools, a higher-alloyed stainless steel grade may even be necessary.
In a nutshell
- Galvanic corrosion occurs when different metals come into electrical contact in the presence of moisture and a potential difference.
- The surface area ratio plays an important role. A small fastener made of a base metal embedded in a large surface of a noble metal represents the most unfavorable combination.
- Common combinations that warrant attention include stainless steel with aluminum, galvanized steel, or copper.
- Prevent galvanic corrosion by using the same alloy, electrically isolating the metals from each other, and keeping moisture out of the joint.
- For outdoor applications exposed to chloride, preferentially choose A4 stainless steel. In highly aggressive environments, a higher-alloyed stainless steel grade may be necessary.
Are you unsure about the right combination of fasteners or the appropriate stainless steel grade for your project? Our technical advisors are happy to work with you to find a durable and corrosion-resistant solution.