Strong, malleable, and versatile, iron is a hugely common metal, found in countless products all around the world. It’s been in use for millennia, and with good reason – for a quick history lesson, check out our history of the Iron Age and how it transformed the world.
But as iron ages and the environment around it takes its toll, the metal eventually corrodes and begins to form what we know as rust. So, what causes this rusting of iron? And how can we prevent it and better protect our valuable iron products?
The reddish-brown flaky coating that we call rust is actually a substance called iron oxide, and it is a type of corrosion. It’s specific to iron and iron alloys, and can come in other colours too, depending on its chemical composition. Other metals have different names for similar corrosive processes – copper develops a patina, while silver becomes tarnished.
As well discolouring and altering the appearance of the metal, rust also causes structural weaknesses. These get progressively worse and potentially dangerous the longer the rust is left unchecked. Some bridge collapses, including the Silver Bridge in 1967 and the Morandi Bridge in 2018, were at least partially attributed to corrosion of steel and iron components.
If iron is used in moving parts, rust can hinder those movements, as smooth surfaces give way to rough and corroded metal instead. Rusted iron is also a potential breeding ground for tetanus bacteria, while rusted water pipes can discolour water and alter the taste.
That said, iron oxide also has some positive uses too. It’s used as a method of polishing metallic jewellery, and the powdered form has found use as a pigment in some cosmetics and as part of calamine lotion. It was also used in magnetic storage and recording media, before cobalt alloy replaced it.
Now we know what it is, let’s look at what causes it.
Rust is a form of corrosion, a gradual degradation that occurs when metals react with their environment, usually through air and water. The compounds that result from these reactions are known as oxides – in this case, iron oxide.
Depending on how much exposure the iron receives, the process can take anywhere from a few weeks to months to even years to really make an impact, but rusting remains an almost inevitable part of iron’s life cycle.
At its simplest, the process looks like this:
Iron + Oxygen + Water = Hydrated Iron (III) Oxide, better known as rust.
The rusting of iron is a redox reaction, meaning it contains both oxidation and reduction processes. These processes refer to the loss and gain of electrons by elements involved in the reaction. Oxidation doesn’t necessarily have to involve oxygen – it’s actually just a name derived from oxygen being the first oxidising agent to be identified.
In this case, the iron loses electrons (oxidation), while the oxygen gains electrons (reduction).
When exposed to oxygen, an oxidising agent, iron will readily give up its electrons. This is because iron is a reducing agent. The formula for this oxidation reaction, also known as an anodic reaction, is:
2Fe → 2Fe2+ + 4e-
The oxygen and water then go through a cathodic reduction, resulting in a hydroxide ion:
O2 + 2H2O + 4e- → 4OH-
It’s worth noting that if the water contains any dissolved electrolytes – such as in saltwater – this process becomes much faster. For example, rust occurs more quickly in saltwater than in freshwater.
The iron and hydroxide ions then react to form iron hydroxide:
2Fe2+ + 4OH → 2Fe(OH)2
These iron hydroxides are then dehydrated – this is what gives us the iron oxides that make up rust. There are many processes involved in this step, eventually giving us that immediately recognisable red rust, which has the formula Fe2O3H2O.
It’s important to remember that our air isn’t just composed of oxygen. Carbon Dioxide (CO2) is also very prevalent, and when combined with water forms a weak carbonic acid. This is an even better electrolyte than pure water and can help move the rusting process along. As CO2 emissions continue to rise, an increase in rusting of iron may follow.
Only exposed surfaces will rust, but because iron oxide flakes and breaks away, this means new areas are exposed over and over again, allowing the process to continue. This is why non-ferrous (non-iron) metals like aluminium or iron alloys like stainless steel don’t corrode in the same way. They’re able to form protective films that not only shield the layers below, but also prevent any cracks or gaps for environmental factors like water to get in and start the process again.
So now we know how rust occurs, but how can you protect your iron products from it?
Dirt, dust, and sand are abrasive particles, and can scratch the surface of iron. This wears away protective coatings and makes rust more likely to occur.
Get some warm water, a mild cleaning product, and something non-abrasive to clean with, such as a sponge, and give your iron products a good clean. Be sure to dry them thoroughly afterwards. For some expert iron cleaning tips, check out our guide to cleaning wrought iron.
While you can’t do much to protect any iron gates from the elements, there’s still plenty you can do to look after smaller items in the home. If you live in a particularly humid area, a dehumidifier could be a good way to protect any iron around you. This will remove moisture from the air, making those redox reactions less likely to occur. You can also place silica gel packs in smaller areas, like tool boxes or drawers, to dry out the air and protect any iron-based contents. And, of course, if any of them come into contact with water, dry them quickly.
As mentioned earlier, there are some metals and alloys that don’t suffer from the same issues with corrosion as iron does. Stainless steel, for example, contains chromium, which oxidises quickly and forms a protective layer around the metal underneath, one which won’t flake away the same way iron oxide does. You’ll still need to take care of the products, of course – so keep that steel wool away from those stainless steel pots and pans!
Beat the rust before it even appears by purchasing and applying a protective product. There’s no shortage of options on the market, across a range of price points.
The only thing you’ll need to do is make sure you give the product enough time to dry – if there’s rain on the radar, you’ll have to wait.
Check for rust spots regularly and deal with them before they get any worse. Smaller spots can be sanded away using steel wool or sandpaper, before applying a protective product to prevent it happening again.
For larger projects or rust that’s simply got out of hand, you might want to bring in professionals for all your restoration and repair needs. Whether that’s some wrought iron gates that need a bit of love, or a sweeping staircase that needs resurrecting, reaching out to an expert will see you on the right path!
For a detailed dive into protecting your property, take a look at our guide to protecting wrought iron from rust.
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