Effect of salt on bike chain
what is so special about salt water


Pure water when it dries is gone, there's no residue.

With salt water when it dries you get salt crystals, which are hydroscopic (they suck up water from the air). This means that even if the air is dry where you are now, the salt will attract water as soon as you put the bike somewhere more humid. And the water will form rust in contact with steel, since steel reacts more vigorously with water than salt does. In other words, the salt sucks water out of the air, then exposed steel sucks water out of the salt.

There's a second problem that salt water gets sticky as it dries, and that attracts dust. It's like an oily chain lube in that regard, fine as long as you clean it off frequently. But that grit grinds away any protective coating on the steel, and any protective layer of rust as it forms. Then the salty water makes more rust.

Chains are especially vulnerable to this as water wicks more easily than chain lube (especially in cold weather). So it will be drawn into the inner workings of the chain. Then when the chain dries a bit the salt is already in the middle of the chain and ready to soak up more water and keep the wear surfaces of the chain links wet. So they'll rust faster.

Solid salt spread on icy roads will work the same way as salt water, but probably more so since the lumps of salt will be saltier than actual salt water.