Salt water is still winning. Unfortunately.
Remember back during the Fukushima crisis, when you heard a lot of talk about why the people trying to save the plant didn't want to use sea water to cool the reactors? There were a number of reasons for that (check out this interview Scientific American's Larry Greeenemeier did with a nuclear engineer), but one factor was the fact that salt water corrodes the heck out of metal. Pump it into a metal reactor unit and that unit won't be usable again.
Now, the corrosive power of salt water is in the news again — and this time it's ripping through New York City's underground network of subways and utility infrastructure. I like the short piece that Gizmodo's Patrick DiJusto put together, explaining why salt water in your subway is even worse than plain, old regular water:
When two different types of metal (or metal with two different components) are placed in water, they become a battery: the metal that is more reactive corrodes first, losing electrons and forming positive ions, which then go into water, while the less reactive metal becomes a cathode, absorbing those ions. This process happens much more vigorously when the water is electrically conductive, and salt water contains enough sodium and chloride ions to be 40 times more conductive than fresh water. (The chloride ion also easily penetrates the surface films of most metals, speeding corrosion even further.) Other dissolved metals in sea water, like magnesium or potassium, can cause spots of concentrated local corrosion.
Via Tom Levenson