Rust is not corrosion! Print E-mail
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Yesterday morning, they botched it once again on Radio 2 - a Flemish broadcasting station - by associating a certain word with an incorrect meaning. It's true that the Van Dale - official dictionary of the Dutch language - turns a blind eye to this meaning, but from a technical point of view, it's not at all correct.

The occasion? The 'Boerentoren' (Farmers' Tower) – the pride of Antwerp - has been put in scaffolding for a while, in order to do repairs and to apply a system to it that will better protect the underlying steel structure against 'corrosion'. That's right, 'corrosion' is the correct term here, and not 'rust', as was implied for the umpteenth time. Of course, rust is the consequence of corrosion, but rust is not the same as corrosion.

Allow me to clarify: corrosion is the solution of a metal, for example in water. This mechanism could more or less be compared to the dissolution of table salt in water. When you pour table salt into a pot of water in quantities that stay under the so-called 'solubility limit' of salt in water, the salt will dissolve into its building blocks, sodium and chlorine ions. If the concentration of salt remains below the solubility limit of salt in water, you won't be able to see that there's salt in the water. What happens to table salt in water can also happen with metals: they dissolve. For example, in steel the iron atoms 'dissolve', which means that iron ions are released into the water. That is what we call 'corrosion'. However, as long as the solubility limit of those iron ions in water hasn't been reached, you won't be able to see what's happening. In other words, you can't see any rust yet, but that doesn't mean there's no corrosion, no loss of metal, no loss of strength, etc. Therefore, it is perfectly possible for corrosion to occur, even though you can't even see a speck of rust yet.

So, what's this thing called 'rust' then? Rust is what you get when you exceed the solubility limit of the iron ions in water. Let's go back to our table salt comparison. If you keep adding salt to the pot of water, at some point you will exceed the solubility limit of salt in water. From that moment on, any salt you add will be 'too much' for the solution, and the excess salt will no longer dissolve, but instead visibly precipitate on the bottom of the pot. A different analogy: when you have a pot of water in which the solubility limit of the salt has not yet been reached, but you start to boil the water, at a certain point the solubility limit will be exceeded: the loss of water will increase the concentration of salt. When the solubility limit is exceeded, the sodium and chlorine ions will be bound together again and result in the precipitation of sodium crystals. For steel, this means that when the solubility limit of iron ions in water is exceeded, the excess iron ions will precipitate by binding themselves to hydroxide ions or oxygen. This chemical compound will manifest itself onto the nearby zones that are already corroding. Rust is the chemical bond between iron ions and hydroxide ions or oxygen.

If we summarize the previous two paragraphs, we come to this conclusion: rust is not corrosion, but rather the consequence of corrosion.

But the thing that also grinds my gears, is that language should be used to clearly and correctly inform people. The manner in which dictionaries and the media use 'rust' and 'to rust', causes much confusion from a technical standpoint. Unfortunately, even many people in the industry are confused. Too many people in the industry still wait 'until they see rust' or 'until it starts to rust', before taking action. But in many cases, by then it's already too late. The process of corrosion - loss of metal - has already been doing damage for a long time.

 

Frans Vos

General Manager Materials Consult bvba

Visiting professor KU Leuven

 
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