Aluminum vs. Steel

Steel is harder than aluminum, but hardness doesn’t fully capture the meaning of strength in relation to metals. 

Steel outperforms aluminum by far in terms of sheer tensile and compressive strength. The carbon content in steel makes it inherently superior and able to withstand extreme strength without deforming, cracking or breaking. Steel is also more durable and resilient under extreme conditions except in corrosive environments.

The question of what is stronger between aluminum and steel becomes more complex with the addition of alloying elements, which can significantly increase the strength of each material, further blurring the lines of strength.

Aluminum is inherently corrosion-resistant due to an oxide layer that becomes passive when the metal is exposed to air. The protective layer prevents the metal from corroding in various environmental conditions within the 4–9 pH range. Since aluminum can resist corrosion without alloying elements or extra treatment, it’s an effective and economical solution for applications where corrosion resistance is needed.

Unlike aluminum, carbon steel corrodes when exposed to natural conditions involving oxygen and moisture. The iron in steel reacts with oxygen to form rust, a layer of iron oxide. In its pure form, aluminum resists corrosion, while steel rapidly breaks down.

Metals are better at maintaining and transferring heat than nonmetals. However, that characteristic varies widely. While steel can withstand extreme temperatures compared to aluminum, it’s one of the poorest heat conductors. Aluminum’s thermal conductivity is higher at 237 W/(mK), while carbon steel’s conductivity is about 45 W/(mK).

Compared to stainless steel — which has an even lower thermal conductivity at about 15 W/(mK) — aluminum’s heat dissipation is 15 times faster. Due to aluminum’s good thermal conductivity, it’s used to make heat sinks.

Considering thermal and electrical conductivities are proportional at a given temperature, aluminum has a higher electrical conductivity than steel. As a superconductor, the metal equally demonstrates low electrical resistance with 61% IACS, meaning its conductivity is about 61% that of copper. The metal’s conductivity makes it the best alternative to copper for electrical transmission. Carbon steel’s electrical conductivity is about 10% that of copper and even lower for stainless steel. 

While steel is much harder and more resilient, it can’t achieve the dimensional limits possible with aluminum and will crack under pressure to form. As such, steel’s strength and toughness make it a more robust metal but a less malleable one. Aluminum offers a high level of malleability that allows it to mold into intricate designs.

You can still shape steel, but not to the degree of aluminum. The process will also be labor-intensive and costly because the metal’s high strength makes it more challenging to work with. In applications where formability is a priority, you would need aluminum’s impressive malleability.

Aluminum’s density is 2.70 g/cm3, about three times lower than that of steel. While steel’s density contributes to its hardness and is desirable for load-bearing structures, less weight is a more attractive quality for metals because it makes transportation, machining and molding easier and more cost-efficient.

The density of aluminum compared to steel is critical in industrial applications. Aluminum’s lightweight quality makes it effective in industries where low weight is a requirement. Where strength is also a priority, aluminum still provides the best strength-to-weight ratio compared to steel, which makes the aluminum vs. steel weight trade-off a factor with significant implications.

Steel and aluminum have various properties where one excels over the other. Besides these characteristics, the cost of each material emerges as a decisive factor. While bauxite is among the most abundant ores on the planet, it requires a lot of energy to process into aluminum, which increases its overall cost and makes it more expensive than iron. However, other factors come into play that make the cost of the two metals less straightforward:

• Production cost: Aluminum is much easier to work with than steel. Production is more cost-effective since aluminum takes less energy and time to cast and machine.
• Movement cost: Due to aluminum’s lightweight, it costs less to transport from one point to another.
• Life cycle cost: Aluminum’s resistance to corrosion makes it easier to maintain over time. Whereas steel rusts unless treated or alloyed with chromium to form stainless steel. In addition, aluminum is 100% recyclable and more affordable to recycle than steel.

So, is aluminum cheaper than steel? Generally, no. However, when you consider these accompanying factors like processing, transportation and maintenance, aluminum can be a more cost-effective alternative overall.