The world loves steel. It's everywhere. Almost every nut and bolt, ships, buildings and bridges, and, of course, all cars and trucks. Steel is almost perfect. It's strong, cheap, ductile, easy to work with and weld, but it has one major flaw: it rusts. This video is a deep dive on rust and metal corrosion. We'll cover how galvanizing protects metals, why people think aluminum doesn't corrode and why they're wrong. I'll try to avoid the chemistry until I've lulled you into a false sense of security and then boom, I'll throw some electrons at you. Steel is an alloy of iron, that is, it's a mixture of iron and carbon, about 98% iron and a little sprinkle of carbon. And that small amount of carbon massively improves the usability and strength of steel, but the iron still rusts. Iron is a metal, and like all metals, it corrodes. But why do metals corrode? Well, all metals are chemically unstable in their pure form. Well, actually, that's not quite true, because gold is chemically stable, although it's soft and expensive. But aside from gold, all pure metals want to become chemically stable by combining with the oxygen to become oxides. And these oxides are the natural state of metals. We find these metal oxides in the ground where we call them ores. And then we extract them and put a huge amount of energy into smelting out the pure metal. But given the chance, these pure metals will recombine with oxygen and revert to that oxide forms. The iron oxide that we dig out of the ground is called iron ore. And the iron oxide that we fight with on bodywork is called rust, but they're both basically the same thing. Rust is the iconic corrosion, partly because there's so much steel and iron around us and also because it's bright orange. In order to rust, iron needs to be in contact with oxygen. But with oxygen alone, the process is so slow that we can ignore it. It's only when we add water and oxygen that the process becomes fast enough to actually matter. Technically, it's not just water that we need. It's a liquid containing salts or acids, an electrolyte. And all naturally occurring water on earth contains salts and minerals. The more salts in the liquid, the more quickly the iron will corrode. This explains why metals on the coast corrode more quickly than those inland, and why salt on the roads causes cars to corrode more quickly in the winter. Because of this need for water, we can really slow down corrosion by storing metals in a dry atmosphere. That's why you'll find aircraft stored for years in Nevada. There's exactly the same amount of oxygen there as anywhere else in the world, but it's dry. So, in the presence of water, iron atoms come off the surface of steel and combined with oxygen atoms to form iron oxide, which we call rust. Now, here's something important. That process is one way. The only way we can convert iron oxide back to iron is by throwing this in a furnace and smelting it back down again. All the rust on the surface of a part is made up of loss metal. And we can send off that rust, but the material has been permanently lost. But there's some good news, because rust takes up a bigger volume than the original metal. So you can send away on the surface like this, end up with a pile of rust, but still get back to a good sound surface. But hot on the heels of that good news is more bad news. Because rust takes up a greater volume than the original metal, it can cause fixings and bolts to seize together. The rust inside the threads expands to fill the gap and makes it much harder to undo. Okay, so we've accepted that rust is inevitable. But apart from the fact that it's orange, what's the problem? I mean, it still contains iron, right? If you've ever poked at rust, you'll know that is flaky, brittle, barely bonded together at all. In short, it lacks all the attributes that makes steel a great material. Just because rust contains iron, doesn't mean that it behaves like iron, in the same way that water contains hydrogen, but hydrogen is one of the most flammable elements on the planet, and we use water to put out fires. So, a compound can share no attributes whatsoever with its component's elements. With corrosion, the metal is converted from a lovely strong, ductile material into a week and brittle oxide, and the corroding object is weakened. And this corrosion only happens at the surface where the metal is in contact with oxygen. And here's a critical difference between rust and other oxides. Rust doesn't form a protective barrier. It's permeable and weakly bonded to the original metal, and that allows oxygen into contact with the surface below, so the process continues until all the metal is oxidized, and we have nothing but a pile of rusty flakes on the ground. This permeability of rust is steel's greatest weakness, and it's what makes rust different from the other oxides, which tend to form strong barriers. Copper is known for developing this beautiful green color and that color comes from a layer of copper oxide, a strong barrier that protects the underlying copper for hundreds of years. Now, we don't tend to build cars from copper, so let's talk about aluminum which behave similarly. Here's an aluminum bumper reinforcement. Just like iron, aluminum oxidizes in the presence of water and oxygen. In fact, it oxidizes even faster. Aluminum oxide is more subtle than iron. Instead of a rusty orange color, it has this white or gray powder. The key difference between this stuff and iron oxide is that aluminum oxide forms an impermeable barrier on the surface of the aluminum, preventing the metal from coming into further contact with the atmosphere. If we sand off some of this aluminum oxide, we'll reveal fresh aluminum underneath. But that fresh, pure aluminum will begin to oxidize within a few hours and form a new barrier preventing any further corrosion. This is how the aluminum panels on the '50s Airstream trailers continue to look so beautiful, because they're covered in a thin, invisible, tough layer of oxidation which won't progress any further unless the surface is scratched, and even then, it's somewhat self-healing. So, that's something to bear in mind. When it comes down to corrosion, the differences between the various metals, between steels and irons and aluminums, is not due to the metal themselves, but rather due to the characteristics of their oxides. This flywheel is untreated steel. I pulled this off a vehicle a few weeks ago. And this is what happens after just a couple of weeks exposed to the elements. Now, the only way to prevent ferrous metals like iron and steel from rusting is by keeping the metal away from the atmosphere. And to do this, we need some sort of barrier. Paint, oil, grease, wax, lacquer, all of these are barrier coatings. And so, maintain the paintwork on your car is not just about aesthetics. It's about stopping the atmosphere from reaching that underlying metal. Now, on a car, the internal mechanical parts tend not to be protected because they're expected to be coated in oil all the time. But for this reason, if you dismantle an engine or a gearbox and you plan on leaving the parts for any period of time before you put back, then you need to protect them from the atmosphere. So, for these camshafts, a good coating of oil and then wrapping them in clingfilm would be a good idea, particularly if you're dealing with precision components, camshaft's a great example, also valve gears, things like that. Now, apart from painting, there are two other treatments that are often used to protect metal. We've got galvanizing and chrome plating. In galvanizing, a steel surface is coated with another more reactive metal, usually zinc. This coating is sacrificial. It will oxidize before the underlying metal and it makes a protective barrier of its own. So, galvanized coatings are quite self-healing. The scratches can heal themselves because that zinc coating is still more reactive with the atmosphere than the iron, so oxidizes really easily and then spreads out to fill small breaches in the surface. But because this surface is sacrificial, it doesn't last forever. It wears away. Whereas with chrome plating, we have chromium, which is a very hard metal, corrodes much less easily than iron. And it's used for tough coatings which also have this metallic, shiny appearance. It forms a barrier against the atmosphere, but if the chrome plating is breached, then the underlying metal will rust in preference to the chromium itself. So, that's why chrome plating tends to bubble away from the surface, a bit like paintwork. So, chrome plating and galvanizing are kind of opposites. Galvanizing uses a more easily corroded metal as a sacrificial layer and chrome plating uses a tougher metal as a barrier. So, let's just recap. Steels and other materials containing iron will oxidize to form iron oxide, AKA rust. And the problem with rust is that it's permeable which means layer upon layer of rust is formed. Other metals like aluminum and copper form tougher oxides, which create their own protective barriers. So, for ferrous materials, and that is iron and steel, we have to create our own protective barrier, paint or any of the other coatings that we talked about. In another episode, we're going to look at what to do when you're faced with rusty corroded parts, how to remove and treat rust and prevent it continuing to gobble up your pride and joy.
