A comment from Mick and Maggie of The Rose of Arden yesterday focussed my thoughts on sheet steel piling, and its susceptibility to rust. I had posted a photo of cows drinking from a section of algae-covered Montgomery Canal between Four Crosses and Vyrnwy Aqueduct which showed what appeared to be rusty piling.
Here's another photo, showing the piling slightly closer up.
As I understand it, rust - ferrous oxide - needs oxygen in order to form. It needs water too, but water without oxygen won't cause rusting. Galvanising steel - coating it with a layer of zinc - protects against rusting, but fixing holes and scratches can expose the steel underneath, which can then rust.
Ordinarily, steel under water low in oxygen shouldn't rust quickly, certainly not as quickly as steel in highly oxygenated water. Perhaps the algae in the photo indicates a high oxygen content, hence the rusting of the piling.
Mick and Maggie wondered whether it might actually have been brown algae, but the way the green algae has sloshed onto the brown makes it look as if it really is rust.
I haven't attempted to go into the electrolytic aspects, but the zinc coating in the galvanising process also acts as a sacrificial anode, in exactly the same way as the lump of magnesium on steel narrowboats' hulls.
In a comment on yesterday's post Martin suggests that the rust is indicative of varying water levels. Fluctuating levels, intermittently wetting the steel and then exposing it to the air, would certainly provide the right conditions for rusting to occur. Add a bit of heat from the sun, and, like putting your wet car into a warm garage, the tin worm is bound to strike.
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