Here are the top misconceptions about strength and corrosion for steel and aluminum!
1. Steel is stronger than aluminum* Well.. sometimes.
Here’s a comparison of steel and aluminum yield strengths for various common alloys of both.
Stainless steel is over here around 200 MPa, along with steel studs. But, high strength
bolts (and other more brittle alloys with high nickel content) are over here towards 1000 MPa.
Typical structural steel members and rebar are in the middle here
at 350 to 400 MPa. A few other common steel alloys are shown too.
How about aluminum? High strength aluminum alloys such as the
7000 series used in aircraft components are up here around 700 MPa. Pure aluminum
is down here at 25 to 50 MPa for 1000 series alloys. Typical 6000 series extruded aluminum
for building members is here between 170 and 240 MPa, but can be as low as 70 MPa when welded!
As this plot shows, there is quite a lot of overlap in the
strengths of steel and aluminum. 2. Ok but how about if we consider
strength per weight, aluminum must actually be better then, right?
Well.. not necessarily. We can divide by unit weight to normalize
this plot by the relative weights of steel and aluminum, and see how this changes the overlap.
Still many of the steel and aluminum alloys are overlapping. However, this does not mean that
steel and aluminum are the same, and it doesn’t negate the fact that aluminum has a unit weight
approximately 3 times lighter than steel, and also deflects significantly more than steel.
Let’s say you are looking for a support beam for a structure. In this first graph
considering only strength, a typical steel beam would be at 350 MPa, whereas a typical
aluminum beam may be around 200 MPa. But, if weight is a concern, maybe you look at this
second graph normalized by weight instead. Here you would instead find that you’re getting more
strength (per weight) from the aluminum beam. Some prevalent examples of aluminum’s advantageous
strength to weight ratio are transmission lines, trailers, and aircraft components.
For cables spanning between transmission towers, cable self-weight is the primary structural design
load, so strength to weight is very important (and of course it helps that aluminum is
conductive since the whole point is to transport electricity). For transportation vehicles,
strength to weight is also a critical factor for consuming less energy and being more maneuverable.
Cars and planes tend to use higher strength aluminum alloys for this reason.
For planes, while aluminum is used for certain components, there are materials with better
strength to weight than aluminum for hulls or fuselages, such as carbon fiber (although older
planes did use aluminum for this). 3. Aluminum won’t corrode, will it?
Metal corrosion occurs when a surface layer of molecules reacts chemically to the
ambient conditions. In this reaction, valence electrons move from the metal’s surface layer
to the surrounding environment. For aluminum, the chemical composition of its surface layer, when
interfaced by air, is typically Aluminum Oxide. Aluminum Oxide has no free valence electrons
to give up, so it can continually form when aluminum is oxidized, since there’s always more
aluminum in the material to feed the reaction. So aluminum is referred to as corrosion resistant.
However, this surface layer of Aluminum Oxide is only a few nanometers thick. Highly humid
environments, abrasive actions, or high salinity conditions can continually wear out this surface
layer and cause corrosion of aluminum. To counter this, aluminum is often anodized to
increase the thickness of this Aluminum Oxide surface layer from nanometers to microns.
This is why aluminum is said to be corrosion resistant, but it is not corrosion-proof.
4. In contrast, steel definitely corrodes.. right?
Not always! There are some types of steel designed to perform as corrosion resistant,
similar to aluminum. Among these are stainless steel and weathering steel. For stainless steel,
the iron is alloyed with chromium which, similar to aluminum, continually forms a
thin protective layer of chromium oxide on the surface. Re-calling our strength chart, we can
see that this added corrosion resistance does come at a significant strength cost. Typical structural
steel has a yield strength of 350 MPa, whereas typical stainless steel yields around 200 MPa.
Weathering steel, on the other hand, manages to avoid this strength reduction while still
providing corrosion resistance. This type of steel, often seen for bridge girders
or outdoor art displays, is alloyed so that it maintains strength similar to typical structural
steel, but also so that it forms a special type of stable rust designed to live on the surface
with low porosity to protect the structural steel underneath. This plot shows how typical
rust forms, falls off, and allows another layer to grow, iteratively. In contrast, weathering
steel is designed to stop the cycle after the first iteration of rust layer formation.
Another very common method for corrosion protection is galvanizing steel. This method
employs a layer of zinc coating the steel. Since zinc gives up its valence electrons more easily
than steel, the zinc acts as a sacrificial coating to stop the steel from rusting.
Over time, zinc galvanizing can wear and require re-coating or other maintenance.
Its lifetime depends on the thickness applied and the environment it’s exposed to.
If you like these steel and aluminum facts, let us know and we’d be happy to share more.
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