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The key feature of antifreeze--as the name implies--is that it doesn't freeze.

When I was a kid, my Dad used to use just plain water in the radiator of his 1967 Oldsmobile Cutlass during the summer. It worked ok. He'd only use antifreeze in winter.

Since salt significantly lowers the freezing temperature of water from 32 to -6F (0 to -21C) [1], can salt water be used as a cheap alternative to antifreeze in winter and below-freezing temperatures?

  1. https://van.physics.illinois.edu/qa/listing.php?id=18774&t=lower-freezing-point-to--10%B0f#:~:text=There's%20only%20a%20certain%20amount,%2C%20or%20%2D6%C2%B0F.
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    Routinely having to top up the radiator should not happen. Granted, if you are, ahem, well into your middle age (as I) and your father tended to get very well used cars (as mine) then possibly you have seen him on a pre 70s car which tended to steam off radiator water if the engine was hot. Nowadays it is a closed cycle, you don't put anything in your radiator - it is there already. – Stian Yttervik Feb 22 at 8:33
  • Why would your father change coolant twice a year? It's not as if you need to drain the radiator and replace it twice a year. Are you sure you're not mis-remembering and seeing him add water only when it overheated, you know: in the summer? – spuck Feb 22 at 17:37
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    Note that many modern water pumps use the coolant as a lubricant, so running plain water may eventually ruin them as well. Obviously it's better than letting the radiator run dry, and technically there are other lubricating additives you could substitute. But really you pretty much always need at least a little anti-freeze in a modern engine or it will be unhappy. If you can't get it, your best substitute is generally some kind of alcohol. Note also that anti-freeze also functions as anti-boil, which is hugely important for some engines that run hotter than 195. – Perkins Feb 22 at 17:48
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    @spuck on older cars it was common for them to be designed to shed excess heat under heavy load via evaporation. In the summer you'd run straight water and top-up as necessary. Then in the winter you'd add some amount of ethanol or isopropanol to prevent freezing. Come spring the higher temperatures would result in the alcohol boiling off leaving your radiator with just water in it again. Those who couldn't afford alcohol in winter would drain the radiator after shutting the engine down and then refill it before starting. – Perkins Feb 22 at 17:51
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    Even just normal "plain water" is a bad idea, because it can still have small amounts of salts and minerals in it. This is why they use distilled water as coolant (or to dilute the antifreeze). – vsz Feb 23 at 14:23
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Uh ... absolutely not. Salt water will cause corrosion with in the engine block. Salt water is an electrolyte, so will pass electricity which will cause electrolysis. Also, it might be able to handle a bit lower/higher temps than straight water, it doesn't work as well as antifreeze. Antifreeze is usually good to -40°F/-40°C, plus it protects your engine from electrolysis and corrosion. I'd use straight water before I'd use saltwater ... not something which is good for your engine.

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    Also there are several different metals involved in the cooling circuit: radiator is copper or ally alloy, block cast iron or am ally alloy, head cast iron or alloy and even gaskets can be different again... – Solar Mike Feb 21 at 5:07
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    Which would mean galvanic corrosion, which would be all sorts of 'fun' – Journeyman Geek Feb 21 at 15:00
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    @supercat Actually, in marine, bottom-coats are a very big deal. They use a special paint that is unlike topside paint. Subs also have all sorts of other crud on them, and by "crud" I mean state-of-the-art anti-echo coating. – Harper - Reinstate Monica Feb 21 at 19:22
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    @supercat - Would they work? They might ... however, I'm sure they'd cost money to not only buy the zinc anode, but you'd also need to fit them properly, which there's no place engineered into the cooling system to accept them. Both of these are costs which are just better if you'd use proper antifreeze in the first place. – Pᴀᴜʟsᴛᴇʀ2 Feb 21 at 19:32
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    @Pᴀᴜʟsᴛᴇʀ2: Of course I wasn't trying to suggest that it would be practical, but rather to suggest that using saltwater would fall in the realm of "possible but impractical". I wouldn't be surprised if some engines specialized for use in remote areas have included sacrificial anodes to allow emergency use of salt water, or perhaps even routine use of such (e.g. in a fuel-powered ocean-water pump). – supercat Feb 21 at 19:38
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Terrible idea.

The salt will precipitate and block the radiator fins

Of course you know you can dissolve things like salt into water. You probably also know there's a limit to how much it can hold - called saturation. Saturation is dependent on temperature.

If you saturate warm water and then let it get cold, the material will precipitate - in salt's case, crystallize on a solid surface. Where will it do this? Where it is the coldest, and since your use-case calls for temperatures below 0C/32F, that will be in the radiator, inside the fine passages.

This is a perfect recipe for clogging the radiator.

But more worrisome, with the salt not in the water, it is more vulnerable to freezing. The colder the weather, the poorer this "antifreeze" performs.

Also, it will corrode everything.

As discussed in Paulster2's answer.

You can protect the engine with zinc plates, and you already are - the radiator has a lot of zinc. The radiator will sacrifice itself to save the engine, so good news there anyway.

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    Nice answer, but your iced tea example is not really a good one. That's a matter of kinetics, not the equilibrium. Given enough time, sugar is plainly ridiculously soluble even in freezing water. See Figure 4 in this paper by van der Sman. The cyan line is the solubility line and note that it stays above 60% sucrose mass fraction (1.5 kg of sucrose in a litre of water) all the way down to the freezing point. – TooTea Feb 22 at 9:56
  • @TooTea Thanks. Yeah, that cyan line certainly explains Mountain Dew :) – Harper - Reinstate Monica Feb 22 at 20:43
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Salt-water (sodium chloride) solution was, and often still is, used for weight/ballast inside tractor and equipment tires, as that cheap alternative to antifreeze in winter.

Very nasty stuff and highly corrosive to metals not prepared for it. No way would I want that inside an engine block!

Interestingly current trends are towards using antifreeze and other less corrosive liquids.

Sources: personal experience, and any search for “fluid filled tires”.

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    Actually, calcium chloride solution. That's what I remember from growing up on a hobby farm, and the search you recommended confirms it. – Fred Larson Feb 22 at 23:09
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Most modern engines have aluminium heads (or other components) for reduced weight, and absolutely require antifreeze in the coolant for it's anti-corrosion properties. Aluminium heads at temperature are prone to corrosion and using tapwater, other than quite temporarily, will allow corrosion to start.

Use antifreeze if you don't want to pay for a new engine (or at least, the head gaskets). Choose a quality one, especially if you have aluminium heads -- corrosion can eventually occur with inferior ones. Coolant only needs to be changed occasionally; perhaps 2 years for green, 5 years for long-life antifreeze; so getting this right is a very minimal cost.

In at least a couple of instances, I have observed once tapwater was been used in an aluminium engine for a more than a month or two, issues tended to arise switching back to antifreeze -- eg. scale in the engine lifted and deposited in the radiator, tending to block it. So even temporary use may have costs.

Salt water, as other posters have said, will enable galvanic corrosion and greatly increase the rate of corrosion over tapwater. Don't even consider it.

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In addition to the corrosive nature of salt water antifreeze does another duty--it has a higher boiling point than water.

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    Salt solutions have higher boiling point as well. – fraxinus Feb 22 at 7:37
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I have not hear about using salt water to cool combustion engines.

But salt water IS used as coolant in commercial cooling equipment. You could find it in large supermarkets, or in office centers, as it is very convenient, that all equipment in supermarket connected to shared cooling network.

For example, they use large industrial chillers mounted on some place external of building (on roof or on some field near building), and feed salt water via ordinary water pipes to building, where industrial refrigerators and conditioner blocks connected to those pipes.

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Some marine engines are cooled directly by the overboard water and some of them tolerate salt water as well.

Other than that, @Harper answer sums it best.

Edit: If I was in a dire need to travel in a sub-freezing conditions (or any other conditions where only e.g. sea water is available, I would pretty much try. Few hours of salty water can't be as bad as 10 years of italian antifreeze.

If the car has any value above the scrap by weight - the cooling system can be flushed from the salt.

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    No, marine engines cooled by salt water use heat exchangers to keep the salt out of the engine. – longneck Feb 22 at 12:58
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    @longneck, you may be thinking large marine engines. All outboards use open loop salt water cooling. Many sterndrives do as well. – swordfish45 Feb 22 at 20:24
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    Plenty of inboard diesels on sailboats are raw-water cooled, too (directly through the block, no external heat exchanger). Not great for longevity and more popular on freshwater boats than salt-water boats, but it definitely is a thing. – nobody Feb 22 at 23:14
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I believe the antifreeze also acts as a lubricant to moving parts if I'm thinking right. Water not a lubricant plus adding salt might be bad for that part.

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    No, you're thinking of the engine oil. – Vikki - formerly Sean Feb 21 at 22:23
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    The only moving part the antifreeze is in contact with is the coolant pump. It is known to run on rusty water more or less the same. – fraxinus Feb 22 at 7:41
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The main thing that came to mind is rust. Salt water will cause this in many metals. So you would need to look into the material. Remember you usually need iron present (for most commonly used alloys) for rust (but not oxidisation, which is a different ball game. So any form of steel or iron and I wouldn't put salt anywhere near it. Probably safer with copper, alumnium, titanium etc.

Plain water is indeed fine in the summer. In the winter your best bet is just antifreeze unless like written above you are sure the metal won't rust. There are other alternatives to antifreeze but they are mostly just forms of alcohol (ehthylene gycol) which is what antifreeze is anyway.

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