I have seen other questions on this forum where preventive measures against salt water on bike chain are discussed. I have seen them in context to rusting.
What I want to ask is that, rusting will be caused by water, then what is so special about salt water ? What effect does salt specifically have on bike chains ?
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.
Answer: Salt accelerates rust of iron and steel.
Any exposed metal will rust faster and sooner when exposed to salt. Even a pinhole in your paint will start turning into a rust spot where in a non-salty location it would probably be fine.
With respect to chains, there's probably a bit more oil and lube on the chain than on the rest of your bike. This may have a minor protective function until it washes off.
Chains are made of plain steel or less commonly stainless steel. Sometimes the plain steel chains are powder coated, but this doesn't offer any protection to the rolling contact surfaces of the chain. Stainless steel will last longer generally, but its not immune to corrosion either.
Doesn't matter if its road salt that some places spray around in winter, or if its sea salt that has come from seawater or sea spray, I don't think there's a significant difference.
Defence against the dark salts:
Your best bet is to avoid the salt. Don't be riding through the ocean at any time.
Anytime your bike has been exposed to salt, wash it off after your ride. Not uncommon for people to shower after swimming in the ocean, why not your bike? Avoid power washers, you just want to rain on it for a while, and ideally not over a lawn. Do this as soon as possible after your salty ride. Let it air dry and then relube the chain before storing the bike away, that way you can get on and ride next time with minimum prep.
Your regular bearing maintenance needs to be stepped up in frequency if you're regularly exposing the bike to salt.
There are antirust coatings, but they're more intended for inside frames etc rather than on chains.
Finally remember chains are consumables and should be changed every 2000 miles. Perhaps your chain change interval drops a little to 1500 miles. They're not overly-expensive items.
Rust is not specific to a chain. Any iron will rust. Steel is mainly iron.
Salt accelerates rust because it is an electrolyte. Rust is oxidation and salt increase the number of oxidizing agents.
From wikipedia
The rusting of iron is an electrochemical process that begins with the transfer of electrons from iron to oxygen. The iron is the reducing agent (gives up electrons) while the oxygen is the oxidizing agent (gains electrons). The rate of corrosion is affected by water and accelerated by electrolytes, as illustrated by the effects of road salt on the corrosion of automobiles. The key reaction is the reduction of oxygen:
O2 + 4 e− + 2 H2O → 4 OH− Providing the electrons for the above reaction is the oxidation of iron that may be described as follows:
Fe → Fe2+ + 2 e− The following redox reaction also occurs in the presence of water and is crucial to the formation of rust:
4 Fe2+ + O2 → 4 Fe3+ + 2 O2− In addition, the following multistep acid-base reactions affect the course of rust formation:
Fe2+ + 2 H2O ⇌ Fe(OH)2 + 2 H+ Fe3+ + 3 H2O ⇌ Fe(OH)3 + 3 H+ as do the following dehydration equilibria:
Fe(OH)2 ⇌ FeO + H2O Fe(OH)3 ⇌ FeO(OH) + H2O 2 FeO(OH) ⇌ Fe2O3 + H2O From the above equations, it is also seen that the corrosion products are dictated by the availability of water and oxygen. With limited dissolved oxygen, iron(II)-containing materials are favored, including FeO and black lodestone or magnetite (Fe3O4). High oxygen concentrations favor ferric materials with the nominal formula Fe(OH)3-xOx/2. The nature of rust changes with time, reflecting the slow rates of the reactions of solids.
This question would probably be a better fit on chemistry.stackexchange.com
