Why Salt Water in the Subway Is So Extremely Dangerous
by Patrick DiJusto
Hurricane Sandy’s storm surge, which bulged into New York Harbor on Monday night, inundated much of lower Manhattan, parts of Brooklyn, and the barrier islands like the Rockaways in Queens. The water got into some of the city’s subway, automobile, and electrical tunnels, and pumping the water out and bringing those tunnels back into working order is going to be a tall order—we’re talking many days or weeks to complete.
What’s the big deal about water getting into subway and electrical tunnels?
The ocean is salty because rains fall on land, and because rivers flow to the sea. Pure rainwater can be very slightly acidic, as the water absorbs some of the carbon dioxide in the air. With each drop that falls on a piece of rock, the slightly acidic rain dissolves away a tiny amount of salts and minerals. The rain then carries this dissolved salt to a nearby river, which ultimately flows to the ocean. On the way some of that salt gets redeposited on land, and some gets used by living creatures, but over the billions of years that is has been raining on Earth, the oceans have filled with 7.67 * 10^19 pounds of salt#. (The United States Geological Survey estimates that if you desalinated all the oceans and spread the surplus salt over the land surface, it would form a crystalline white layer some 500 feet thick.) This may seem like a lot of salt (and it is), but the oceans are so vast that they are, on average, only 3.5% saline.
in contact with electricity and/or bare metal,
In the 1890s, any city worth its, *ahem*, salt was developing a train-based mass transit system. The smoke-filled tunnels of London’s Underground provided these engineers with a guide for what not to do when designing a subterranean train route: New York’s Board of Rapid Transit Commissioners were adamant that the city’s subway would be powered by clean, non-smoky electricity. To do this they borrowed the European concept of the third rail: In addition to the two carbon steel rails of the train tracks, New York’s subway has a third rail, made of highly conductive steel and pumped with 625 volts of DC electricity, placed next to the other rails to power the trains. A “shoe” on the bottom of each subway carriage makes contact with the third rail, and energies the train’s motors.
Power for the third rail is distributed throughout the system by nearly 2,500 miles of power cable, which passes through more than 7,600 manholes throughout the city.
produces intense amounts of corrosion.
This type of rail system is safe to use in nearly any type of environment except being submerged in salt 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. And if there’s any external electricity in the system (such as an energized third rail), the whole process runs on fast-forward and becomes one big dangerous pool of electrified hell. It seems the city’s decision to turn off the subway was the right one.
1. Wolfram alpha says that there are 3.196×10^8 cubic miles of seawater on earth. The USGS says that 1 cubic mile of seawater contains 120 million tons of salt. That works out to 7.67* 10^19 pounds of salt in the ocean.