Video Transcript
Vehicle components are sometimes made of aluminum instead of steel in order to reduce the risk of oxidation. Why is aluminum less susceptible than steel to oxidation? Aluminum is less reactive than steel. Aluminum binds water less strongly than steel. Aluminum is protected by an unreactive carbonate layer. Aluminum cracks less easily than steel. Or aluminum is protected by an unreactive oxide layer.
To begin with, let’s review what we know about the oxidation of metals. Let’s imagine that we make our vehicle component from a metal and leave it exposed to air and moisture. What’s gonna happen to the metal? Gradually, the surface which is exposed to air, which contains moisture, is going to start to oxidise. If our metal is iron, this is known as rusting and becomes this kind of red iron oxide colour. Gradually, over time, this process continues. And more and more of the metal becomes eroded. As more and more of the metal is oxidised, the component can become brittle and may fall apart.
This question is asking us to compare this oxidation process for aluminum and steel. We’re already told that aluminum is less susceptible than steel to this process of oxidation. But what we need to work out is why that is. Being less susceptible to oxidation means that vehicle components made of aluminum are going to last longer, which is obviously a good thing. We’re given five possible answers. It makes sense to go through each answer and first work out whether the reason given would actually make aluminum less susceptible to oxidation. Then, we can work out whether the statement is actually true for aluminum.
Let’s start with the first one. Aluminum is less reactive than steel. If this is true, aluminum being less reactive would indeed make it less susceptible to oxidation. So this could be a possible answer. Let’s have a look at the next one. Aluminum binds water less strongly than steel. For oxidation to occur, the metal needs to be exposed to both air and moisture over a long period of time.
Metals aren’t really known for their ability to bind water molecules. It’s the reaction of the metal atoms along the surface with both the moisture and the oxygen in the air which causes this oxidation. So water binding isn’t really an important part of oxidation. So if aluminum did bind water less strongly, it’s not really that useful when it comes to being more or less susceptible to oxidation. So this is not a correct answer.
Let’s move on to the next one. Aluminum is protected by an unreactive carbonate layer. If this was true, aluminum being protected by a layer of something unreactive would indeed protect it from oxidation, making it less susceptible to the process. So this would be useful. In fact, the last suggested answer is also about aluminum being protected by an unreactive layer, but this time an oxide layer. So again, if this were true, this would be really useful in protecting it from oxidation.
Going back to the fourth answer, aluminum cracks less easily than steel. We know that oxidation simply requires surface area of metal and being exposed to both moisture and air. So whether or not it cracks more or less doesn’t really make it any more or less susceptible to oxidation. So we can also rule out this answer.
Now we need to simply look at the three potential answers that we have left and work out which of them is true and which of them are not. Let’s begin with “aluminum is less reactive than steel.” If you look at a reactivity series, you’ll see that aluminum is actually higher in that series, meaning more reactive than iron, which is the main component of steel. It turns out that aluminum is actually really reactive. So this answer is simply not true.
Aluminum is so reactive in fact that it’s always coated in a protective layer, which brings us to our last two potential answers, one an unreactive oxide layer and the other an unreactive carbonate layer. If we think about this logically, we can work out which answer is correct. When a pure aluminum surface is exposed to air, it reacts and forms this unreactive layer. But think, what does it react with?
It’s most likely to react with the oxygen in the air, producing an unreactive oxide layer. This oxide layer turns out to be Al₂O₃, aluminum oxide. And this protects the underlying aluminum from any further oxidation. So the reason that aluminum is less susceptible to oxidation than steel is because it’s already protected by an unreactive aluminum oxide layer.
Which of the following is another advantage of using aluminum instead of steel in vehicle components? Aluminum is stronger than steel so it deforms less easily. Aluminum is less dense than steel. Aluminum is harder than steel so it scratches less easily. Aluminum has a higher melting point than steel. Or aluminum is less expensive than steel.
So let’s use the same approach as the last question. We’ll work out whether each answer is potentially an advantage of using aluminum and then look at which ones are true. The first potential answer is that aluminum is stronger than steel so it deforms less easily. When considering the use of a metal for vehicle components, it is important that that material is strong and doesn’t deform while the vehicle is being used. So this is a potentially useful suggestion.
The next one, aluminum is less dense than steel. If you make the same vehicle component from two different metals, whichever metal has the lowest density will weigh the least. So it will be the lighter component. If aluminum is less dense than steel, making a component from aluminum means that it will weigh less. It’ll be lighter. And that means that the vehicle potentially can go faster because it’s not carrying such a lot of weight. So this could be very useful.
The next answer, aluminum is harder than steel so it scratches less easily. Well, this is potentially not as useful as the first answer. It is slightly useful if aluminum were harder than steel. Scratching a vehicle components is not quite as important as, say, scratching the paint on a vehicle. We’ll give this answer a small tick in the useful column as it’s slightly useful if this were true.
Next, aluminum has a higher melting point than steel. Cars are unlikely to be subjected to very high temperatures. Well, the insides of, say, the engine of a car may reach reasonable temperatures. It’s unlikely to be high enough to melt a metal like aluminum or steel. So again, we’ll have a very small tick because this one is not massively useful. Finally, aluminum is less expensive than steel. If this were true, this is of course a huge advantage.
So now, let’s analyse each answer again and think about whether or not it could be true for aluminum. First, we’ll consider whether aluminum is stronger than steel. Of course, there’re many different types of steel, steel being made of mainly iron but a few other components added in. But generally speaking, all forms of steel are much stronger than aluminum. You can check this out by looking up the tensile strength of both aluminum and steel. So this first answer is not true. Equally, aluminum cannot be harder than steel, the third answer. So we can also rule this out.
Next, the density of aluminum versus steel. We could look up the values for the densities of both of these metals. And we can see that steel is significantly more dense than aluminum. So this makes this answer true. Aluminum is less dense than steel, which does make it good for making vehicle components because when the components are made, they are much lighter. So this is a good candidate for the right answer.
If we look at the melting points of both steel and aluminum, we can see that aluminum has a much lower melting point. So this statement isn’t true. And we come to the final possible answer. Aluminum is less expensive than steel. It turns out that aluminum is actually significantly more expensive than steel, up to even three times more expensive. So this is not the correct answer.
So another advantage of using aluminum instead of steel when making vehicle components is that aluminum is less dense than steel. So this is our final answer.
