Whether it's used to melt ice on the road or comes in on the ocean breeze, every car owner knows that salt is the enemy. Salt is where rust comes from, and rust is a killer of cars. Road de-icing methods alone cause $3 billion in rust damage annually, as AAA notes. But why is salty slush so much worse than plain water? Blame electrolytes!
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Rust happens when metal oxidizes, turning your once-pristine car into a crumbling, hole-ridden mess. Oxidation involves—you guessed it—oxygen, a key molecular component of water.
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Shockingly (we can't have a chemistry explainer without a lame dad joke, can we?), oxidation is an electrochemical process. Iron atoms pass their electrons to oxygen atoms in the chemical reaction that creates rust, which the Growing With Science Blog notes as so:
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4Fe + 3O2 = 2Fe2O3

The atom to the right of that equal sign is responsible for the sadness, woe, disintegration, and fused-together hardware we hate so much on our snow-driven cars.

In the process of swapping these electrons around, the water becomes slightly acidic, which in turn allows the electrons to move even faster from the iron to the oxygen, notes Sciencing^[4].

That's why metal parts left in water will fare so much worse than ones that dry off: the oxidation process has a bit more help.

Winter roads, as we know, aren't just wet. Salt is used to lower the freezing point of water, per DMV.org, thus allowing icy roads to melt faster. Unfortunately, that's not the only reaction salt has with water.
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Salt also acts as a catalyst to speed up the oxidation process even further, as salt is a pretty potent electrolyte when dissolved in water. Electrolytes, for those of us who haven't set foot in a chemistry class since high school, aren't necessarily what plants crave^[6]. Rather, electrolytes conduct electricity when dissolved in a solvent like water.

In the case of salty water and ice your car comes in contact with on treated roads, the process of moving electrons over from iron to oxygen atoms happens even faster. That's the real reason why half the cars you'll see in upstate New York look like rotten Swiss cheese: electrolytes are not your car's friend.

Various methods such as painting, coating or galvanizing (dipping metal in molten zinc) are used to seal off iron from water it may come in contact with and thus, make an automobile less prone to rust. Most manufacturers do a fair amount of rust protection nowadays, but holes in these protective barriers can form over time.

If you're among the 150 million drivers in the United States who routinely encounter road salt, AAA recommends limiting your travel immediately before, during and after winter storms when salt is most likely to be freshly applied. Wash salt off as soon as you can after a storm with a car wash solution that won't strip the wax from your vehicle, and make sure to clean the undercarriage of your car well, as it's the most likely to have bare metal that rust loves to eat. Fill in paint chips quickly, and consider waxing your car beforehand to add an extra layer of protection.

Alternately, you could always move to a state like Texas where everything just shuts down if there's any threat of ice in the forecast. It's not worth keeping salt and other winter weather road maintenance tools on hand for one possible day of bad weather a year, and thus, the underbody bolts on my cars come off with only minimal persuasion.