Steel and aluminum are the most commonly used metals in the world. Aluminum is the second-most abundant metallic element on Earth, while steel is by far the most utilized alloy worldwide. However, while they may look similar, there are key differences between steel and aluminum, and they each have unique properties and applications.

In this post, we’ll compare the cost, strength, weight, corrosion resistance, processing methods, and applications of steel vs. aluminum—factors that can help you determine which one is best for the job.

 

Cost

While steel and aluminum prices are constantly changing based on global market conditions, generally, steel is cheaper by weight than aluminum. However, both metals come in a variety of different types and alloys that can vary significantly in price. Mild and carbon steels tend to be the least expensive, while stainless steel prices can be close to those of aluminum.

 

Strength

There’s no question that stainless steel is stronger and harder than aluminum—that’s why the vast majority of automotive frames are made of steel. Steel is an alloy of iron and carbon, among other elements, and the carbon gives steel its tensile strength, hardness, and durability. It’s less likely than aluminum to deform from force or heat.

Because aluminum is softer than steel, it is much more prone to deformation. While aluminum will never be quite as strong as stainless steel, its strength increases as its temperature decreases, making it useful for low-temperature applications.

 

Weight and Density

As mentioned above, aluminum costs more than steel by weight, but stainless steel is about 2.5 times denser. That means for an equal volume of steel vs. aluminum, the aluminum is less expensive because it’s significantly lighter.

Aluminum’s low density gives it a very appealing strength-to-weight ratio. Even though steel is stronger, aluminum provides much more strength per unit mass, allowing aluminum structural components to weigh much less than stainless steel components while still having adequate tensile strength for many applications. With the right design, aluminum can provide similar durability to steel at half the weight.

 

Corrosion Resistance

One of aluminum’s most prized characteristics is its superior corrosion resistance. Unlike stainless steel, aluminum doesn’t rust. The same oxidation reaction that produces iron oxide (rust) causes aluminum to form an outer passivation layer of aluminum oxide—which, unlike rust, actually protects the metal from corrosion and decay and prevents further oxidation.

Stainless steels are corrosion-resistant, but they can still rust. Depending on the application and environment, carbon steel usually requires a surface coating or paint to protect it from rust and corrosion. Unlike aluminum’s passivation layer, this coating can scratch or wear off, exposing the steel and making it vulnerable to rust and corrosion.

 

Ease of Processing

The physical properties of stainless steel vs. aluminum make them very different to work with. Aluminum is much more malleable than steel, allowing it to be formed into more delicate geometries like very thin walls. Its relative softness makes it easier to cut than mild steel products, making aluminum machining faster than stainless steel machining. Because it is so light, it can be machined on smaller equipment.

While aluminum is easier to form and machine, steel is typically easier to weld. Aluminum’s passivation layer and high thermal conductivity can make welding challenging. A skilled welder can still weld aluminum, but it takes more precision and is less forgiving than welding stainless steel. Also, some aluminum alloys are easier to weld than others, like aluminum 5052.

Aluminum has a much lower melting point than steel—1,221°F vs. approximately 2,700°F—so it’s much easier to cast. It also cools more quickly and evenly, preventing deformation and providing excellent dimensional stability. Casting steel alloys requires special mold materials such as graphite and more complex cooling systems to ensure even cooling.

 

Applications

Stainless steel applications are distinct compared to those of aluminum thanks to their difference in physical properties. In general, stainless steel is used when aluminum’s strength is insufficient for the application or when cost is the most important factor. Aluminum is often used for applications that require very low weight or high corrosion resistance.

Steel’s strength and durability make it an ideal construction material. Buildings, railways, and other infrastructure almost always rely on a framework of steel, though aluminum is also useful for some construction applications because of its strength-to-weight ratio. Steel also makes up about half of the average automobile.

Aluminum is essential to the aerospace industry, where weight must be minimized, but strength and durability are critical to safety. Aircraft and spacecraft can consist of up to 90% aluminum alloys. While steel is still the material of choice for cars and trucks, the automotive industry is increasingly turning to aluminum to minimize weight and improve fuel efficiency in newer vehicles.

Aluminum’s low density, high conductivity, and formability also make it a perfect material for electrical lines. It’s cheaper than copper and an even better conductor, and it’s easily drawn into wire.

 

Steel vs. Aluminum: Summary

Property	Steel	Aluminum
Cost	Low	Moderate
Ultimate Tensile Strength	400 – 2000+ MPa	75-500 MPa
Density	0.28 lb/in3	0.1 lb/in3
Corrosion Resistance	Low (carbon steel) to moderate (stainless steel)	High
Malleability	Low	High
Machinability	Moderate	High
Weldability	High	Low
Castability	Low	High
Application Examples	Construction, infrastructure, automobiles, appliances, tools, fasteners	Aircraft, electrical wiring, electronics, utensils, foil

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