Can aluminum and steel touch?

You're working on a project joining aluminum and steel components, but a nagging doubt arises. Is it safe for them to touch? Ignoring this could lead to premature failure and costly repairs due to unexpected corrosion.

Yes, aluminum and steel can touch, but if an electrolyte (like moisture or saltwater) is present, galvanic corrosion can occur, where the more reactive aluminum will corrode preferentially to protect the less reactive steel.

As a manufacturer of aluminum alloy forged rings¹ and discs at SWA Forging since 2012, we've encountered many situations where our aluminum components interface with other materials. Understanding material compatibility is crucial for the longevity and performance of any assembly. This is especially true for our clients, including traders and machining companies, who rely on the integrity of the materials they use. Let's delve into what happens when these two common metals meet.

Can you put steel and aluminum together?

You're designing an assembly and need to connect steel and aluminum parts. Is this even feasible, or are you asking for trouble? Combining them incorrectly can lead to structural weakness over time.

Yes, you can put steel and aluminum together. It's done frequently in many industries. However, you must take precautions to prevent galvanic corrosion, especially if moisture will be present.

It's very common to see steel and aluminum used together in various applications, from automotive components and marine structures to building facades and industrial machinery. The engineering reasons for doing so are often compelling – perhaps you need the strength of steel in one area and the light weight of aluminum in another. So, the direct answer is yes, you absolutely can put them together.
The main challenge, as hinted before, is galvanic corrosion. This happens when two dissimilar metals are in electrical contact and are also bridged by an electrolyte (a conductive liquid, often just moisture with some dissolved salts or pollutants). In the aluminum-steel couple, aluminum is more "anodic" (more reactive) than most steels. This means the aluminum will corrode preferentially, acting like a sacrificial anode to protect the steel.
Imagine a tiny battery being formed at the interface. The aluminum gives up its electrons more readily, leading to its own degradation. Over time, this can lead to significant loss of aluminum material, weakening the joint or the aluminum component itself. This is a concern we always highlight when discussing applications where our forged aluminum parts might be joined to steel. We advise our clients, especially those in machining who prepare components for assembly, to be mindful of the joint design and environmental exposure.

Are aluminium and steel compatible?

You're considering a design that uses both aluminum and steel. Are they truly compatible, or will they fight each other? Choosing incompatible materials can compromise the entire project's lifespan.

Aluminium and steel are not inherently compatible in corrosive environments without protective measures. Direct contact in the presence of moisture will lead to galvanic corrosion of the aluminum.

When engineers talk about material "compatibility," they often mean several things: can they be joined effectively, do their mechanical properties work together, and will they react negatively with each other over time?
Mechanically, aluminum and steel can be joined. They have different strengths, stiffnesses, and thermal expansion rates, which designers must account for, but these are solvable engineering challenges.
The real compatibility issue arises from their electrochemical differences. Steel (especially carbon steel) and aluminum are quite far apart in the galvanic series. This series ranks metals based on their electrochemical potential in a given environment, typically seawater. The further apart two metals are, the greater the potential difference between them, and thus, the more severe the galvanic corrosion of the more anodic metal when they are connected in an electrolyte.
Aluminum is significantly more anodic than steel. So, if you bolt an aluminum plate directly to a steel frame and it gets wet, the aluminum will start to corrode at an accelerated rate, especially near the interface. The steel, being more cathodic (less reactive), will be protected at the expense of the aluminum.
For our clients at SWA Forging, who often machine our aluminum forged rings and discs for various assemblies, understanding this lack of electrochemical compatibility is vital. We stress the importance of isolating the two metals or using other protective strategies if the assembly will see moisture. This ensures the longevity of the aluminum components we supply.

What metals should not touch aluminum?

You're working with aluminum and want to avoid corrosion problems. What other metals are a bad idea to pair directly with aluminum? Making the wrong metallic pairing can quickly degrade your aluminum parts.

Metals significantly more noble (cathodic) than aluminum, like copper, brass, bronze, and many types of steel (especially stainless steel in some conditions), should not touch aluminum directly in a corrosive environment without isolation.

To prevent galvanic corrosion of aluminum, you need to be careful about what metals it comes into direct electrical contact with, particularly if an electrolyte might be present. The general rule is to avoid pairing aluminum with metals that are much more "noble" or cathodic.
Here's a breakdown:

• Copper and its alloys (Brass, Bronze): These are classic problem metals with aluminum. Copper is significantly more cathodic. Direct contact in a moist environment will cause aggressive corrosion of the aluminum. I've seen aluminum parts practically dissolve when in contact with copper water pipes that had a persistent leak.
• Steel (Carbon Steel, Alloy Steel): As discussed, steel is more cathodic than aluminum, leading to aluminum corrosion.
• Stainless Steel: This is a bit more complex. While stainless steel is generally more noble than aluminum, the severity of corrosion can depend on the specific grades of both aluminum and stainless steel, and the environment. In some marine environments, stainless steel fasteners can cause severe corrosion of aluminum.
• Graphite (Carbon Fibers): While not a metal, graphite is highly conductive and very noble. Contact between aluminum and carbon fiber reinforced polymers (CFRP) in the presence of moisture can cause severe galvanic corrosion of the aluminum.
  Metals that are closer to aluminum in the galvanic series, or even more anodic, pose less of a risk or might even sacrificially protect the aluminum. Examples include zinc (often used to galvanize steel or as sacrificial anodes for aluminum) and magnesium.
  For our customers receiving aluminum forgings from SWA Forging, this is key information. If their application involves bolting our aluminum rings to, say, a bronze fitting, we'd strongly advise on isolation methods.

What happens when aluminum comes in contact with steel?

You're about to join aluminum and steel. What exactly will occur at their interface, especially if conditions aren't perfect? Understanding the process helps in preventing unwanted degradation.

When aluminum contacts steel in the presence of an electrolyte (like moisture), galvanic corrosion occurs. The aluminum acts as an anode and corrodes, while the steel acts as a cathode and is protected.

When aluminum and steel are in direct electrical contact and an electrolyte (even just humidity condensing on the surface) bridges them, an electrochemical cell is formed.
Here's a simplified step-by-step of what happens:

1. Potential Difference: Aluminum has a more negative electrochemical potential than steel. This means aluminum atoms are more willing to give up electrons (oxidize) than iron atoms in the steel.
2. Anode and Cathode Formation: The aluminum surface becomes the anode (where corrosion occurs), and the steel surface becomes the cathode (where a reduction reaction occurs, typically involving oxygen and water).
3. Electron Flow: Electrons flow from the aluminum (anode) through the metallic connection to the steel (cathode).
4. Ion Formation (Corrosion): At the aluminum anode, aluminum atoms lose electrons and become aluminum ions (Al³⁺), which then react with the environment (e.g., with hydroxide ions from water) to form corrosion products like aluminum hydroxide. This is the physical loss of aluminum material.
5. Cathodic Reaction: At the steel cathode, the electrons are consumed by a reaction, often the reduction of oxygen and water to form hydroxide ions (OH⁻). This process itself doesn't typically damage the steel; in fact, the steel is "cathodically protected" by the sacrificing aluminum.
   The rate of this corrosion depends on several factors: the conductivity of the electrolyte, the surface area ratio of the cathode to the anode (a large steel cathode and small aluminum anode is a very bad combination for the aluminum), and the temperature. At SWA Forging, we ensure our clients are aware that simply providing a high-quality aluminum forging (ISO9001 certified) is only part of the solution; proper assembly practices are crucial for long-term performance.

Conclusion

Aluminum and steel can touch, but beware of moisture, as it triggers galvanic corrosion, degrading the aluminum. Proper isolation or coatings are essential for durable mixed-metal assemblies.