Does salt really rust a car?

Here me out, as whilst I’m sure my argument will be wrong, I struggle with this:

Let us focus on undercarriages: Yes, salt is extremely corrosive for cars. All those grit bits sounding like stones under the car as you drive on freshly laid stuff.
And yes, I can imagine that ‘next day’ look, when the roads are socked with wet, saline solution as the salt has done its stuff on road ice is horrendous for car’s undercarriage.

My struggle comes next: In the south West, the pattern typically is a week’s salt is laid night after night (and early afternoon sometimes with the school ‘run’ times) in a cold snap, and cars get plastered, all the horrible stuff. ‘It gets everywhere’ etc is often said.
But…the cold snap generally breaks , the temps go up, the salt stops being laid and it hammers down for days on end. Granted, it takes a bit of rain to swoosh most of the saline salt away, but it does.
My confusion is thus: surely this ‘pretty much no salty rain on roads anymore’, just normal wet roads, is going to wash away salt lying on the undercarriage? If the argument is ‘salt gets everywhere’, I’m going to take convincing that 5pm, hammering down duel carriage or rush hour traffic rain, weeks after a cold snap finished, don’t also ‘get everywhere’ too i.e the same places the salt did, and hence neutralises it?
Q: Does the prior salt get washed away from multiple ‘normal’ rain in non-salt road trips?

And is it a case that, yes, salt is hardly a car’s friend, but is rust on an undercarriage also due to it just getting wet, irrespective of salt, in the same way a house gate rusts even though a house gate don’t ‘travel’ on the road where there’s salt? i.e, the house gate just gets wet with normal rain.
Educate me.

If you sprayed your finger with hydrochloric acid (dont) and immediately sprayed it with clean water, would it stop burning do you think? I think it wouldn’t, because all you have done is diluted it a bit. How many times tdo you have to spray it to get it all off? Lots I suspect.

That’s.my thought process anyway for what it is worth.

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Good point. But in the UK, how many times do you travel in hammering down rain when there’s no salt around? Loads. So using the analogy, it gets sprayed a lot?

Going on the spray patterns left on my car salt is detectable on the roads for at least several weeks after the ice snap and subsequent rains are over and you’d have to wash the all salt off extraordinarily thoroughly to wash off the corrosive effect, plus the corrosive effect will be accelerated by the heat generated from the engine and exhaust.

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Sounds reasonable. I didn’t think of the heat effect, either.
Mind you, if salt is still present many rains and weeks after a cold snap, surly taking a car out in even May wouldn’t be good, as salt can still be layed in April? Yet pretty much everyone got their pride and joys out by May.

I would also question whether a layer of ash, from a log burner, would corrode the top of your car.

Sorry, couldn’t resist it🤣………as you were………

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Well, water and iron don’t mix , and when they do, an electrochemical process is set up. The addition of salt increases the conductivity significantly. :frowning:

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Things like sills tend to rust from the inside out from condensation in any case, so salt is probably irrelevant there.

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I think the focus should be, does salt rust Mazdas, yes would be the answer? If in standard form as it came from the factory, driven on salty roads most definitely the rusty stuff will appear.

Daily drivers I’ve had from new and driven throughout winter on salty roads include a SEAT, a VW and now a Skoda. They’ve had a fair few years on salty roads between them, nothing to note of rust on any, they are/ were well protected.

Btw, I love my Mazda, it just doesn’t love salty roads unprotected.

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This is more of an fyi than directly about the ops question.

I have some maritime experience of corrosion issues.

On a salt water boat a sacrificial anode is always connected below the water line. This is because when 2 different metals are connected in salt water a natural eclectic current occurs, which corrodes the connecting metals.Galvanic Corrosion. Ie, you would never connect an anchor chain to its mooring shackle using different metals. The job of the anode is to corroded itself rather than important metal parts of the boat.

Cars are made up of many differing metal parts so all salt solution on those parts may hasten corrosive actions.

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I’ve seen those anodes (I think) on the hulls of canal boats when lifted out for blacking.

I agree with this.
I didn’t want to add ‘but other cars I’ve got/had haven’t had MX-5llitis’, as I thought it would meander from the thread question, but yes, I agree with you ref not all of them suffer the same.

Rust tends to propagate along crystal boundaries in the steel; this is how it can burrow deep into a sheet with not much appearance on the surface until it bubbles and flakes away.

Both air and water are required; remember water can hold some air, so it is usually there. Salt merely helps speed up mobility, it is not essential!

Different steels have different alloys and crystal structures, usually for strength or hardness or springy or ductile, etc; all affect how, or if, it rusts.

Welding changes the crystal structure and different heating-cooling methods can produce different temper yet again, often resulting in a more rust-prone area. Hence all the many welding techniques in use, some better, some quicker, some cheaper, some more convenient.
It’s a complex science; I helped proof read a thesis on this years ago, mind utterly boggled, and it has made great strides forward since. I know just enough about it to know how little I actually do know; SFA.

In practical terms on our cars; the simple preventative is to wash away any build-up of muddy or salty material from crevices and traps under the car.

This cloggage holds moisture and tends to accumulate in flow eddies, it is not washed away by hours of travel on a pouring rain wet road.

The in-fold lip on a wheel arch is a classic flow eddy, combing the grit out of the water, as also is the join where the rear facing vertical edge of a cross member supports a floor pan.

Removing these hard-dried-on accumulations usually require a generous pre-soak, followed by a mild pressure washer aided by an old non-scratch bristle brush, maybe 1/2" to 1" for a wheel arch lip.

My cars last well, and don’t seem to rust much, but then I do wash them underneath and pay attention to any eddy points I can spot.

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That was part of the arguments about galvanising a car and should it be positive earth or negative earth electrics.

I forget the electro-chemistry now, but it might be a useful line to follow.

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Great replies. ND12 is getting educated, even though I doubt any poster thought I needed educating :sweat_smile:

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Not for one second! :rofl:

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We have a water softener. It’s in the cupboard under the kitchen sink. It takes salt blocks. These are enclosed under a cover. The cupboard door hinges closest to the water softener rust very quickly - and there is not direct contact. I always used to think the saying about ‘don’t buy a car that’s lived on the cost’ was a bit dramatic, but it’s scary stuff.

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Mine too, but slightly dusty tablets instead of a block!

I thought it was because there is some dust spray and slight splash, and the generic hinges feel very much like they are made down to a price.

But they still work after fourteen years in an unheated, well ventilated laundry, so it’s only a mild surface rusting.

I took a picture for a better look and notice its mostly on the top surfaces, eg salt dust and washing powder falling, etc. The grey plastic tub is the water softener, I need to slide it out from that corner to top it up.

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Parchiepig is getting very confused! :rofl:

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Coming from a motorcycling perspective, I can confirm that road salt is a killer as far as corrosion is concerned. Not only on steel frame/chassis parts but also aluminium engine casings which get furry and damaged after just one journey on salted roads. It is hideous stuff. It isn’t used on airport runways and neither is it applied on the Humber Bridge because of the potential for catastrophic damage. Just take a look at at the state of a council gritting wagon to get an idea of the damage that salt causes.

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