Have you ever paused at the sight of rusty steelwork and wondered what exactly is going on? What is rust, why does it form—and when should you worry about it?
At its core, rust is simply a chemical reaction: the interaction of steel with oxygen in the presence of water. It’s a basic metallurgical process, but one with serious implications. Despite being a natural transformation, rust can be extraordinarily costly to correct once it sets in.
Why does steel rust?
The irony is that the prevalence of rust in our built environment is due more to steel’s strengths than its weaknesses. Steel is the backbone of modern industrial economies: strong and incredibly versatile. But it also begins to corrode the moment it is exposed to the elements. In that respect, rust is not a failure of steel—it’s steel reverting to its original state: iron oxide.
That familiar reddish hue? That’s hydrated ferric oxide. But rust isn’t always that simple. It comes in many forms.
Rust Is Not Just Rust
The chemical formula often used to describe rust is:
4Fe + 2H₂O + 3O₂ → 2Fe₂O₃·H₂O
But this is a simplification. Rust can include a wide array of compounds depending on both the alloy of the steel and the environmental conditions in which the corrosion occurs. Air-formed rusts typically contain around 5% compounds other than Fe₂O₃·H₂O, including various oxides, hydroxides, and even salts. This diversity means that rust doesn’t always behave predictably—and it isn’t always easy to remove.
The Sulphur Dioxide Effect on Steel Rust
In industrial settings, sulphur dioxide (SO₂) can accelerate corrosion significantly. When released into the atmosphere, SO₂ reacts with moisture to form acidic compounds that attack the steel surface. These reactions form salts such as ferrous sulphate, which can settle in pits and crevices on the steel.
Unlike traditional rust, these salts are often white or light in colour and are notoriously difficult to remove—even with aggressive blast-cleaning. Worse still, these sulphates can regenerate sulphuric acid, creating a self-perpetuating cycle of corrosion that undermines the steel from the inside out. As the rust expands in volume compared to the original steel, it can also crack or lift any protective coatings applied after the fact.
What’s the difference between sulphate and chloride corrosion?
Marine Environments: The Chloride Challenge
A Chain Reaction You Don’t Want
How do you prevent steel from rusting?
Prevention Is the Only Real Cure
How long will galvanizing last in your area?
Is Galvanized Steel a solution for the Circular Economy?
