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Underbelly protection from salt

This is my single largest worry.

Living in the Northeast, we get outrageous amounts of salt dumped on our roads to go with the snow and ice. I don't want to watch the battery pack under the floor of the Model S corroded by salt water and destroyed. Does anyone know for a fact what Tesla is doing to protect the battery pack from the masses of ice, snow, water, gravel, and salt which will be thrown up against the bottom of the car in typical driving? I've been assuming that there will be some sort of solid, hard protection plate, but does anyone know for sure?

@neroden
I believe that is a valid concern. However, I also believe Tesla has considered this (and other issues) because the battery pack sitting under the floor of the vehicle is the defining engineering benefit of the Model S. Watch the third video on the vehicle engineering blog (watch all three if you have a chance):

https://www.teslamotors.com/blog/tesla-vehicle-engineering

This is a big concern for me too. As a Canadain, I'll be driving my Model S Signature 12mths of the year. Salt is still the primary source of melting snow on our roads, especially in the Toronto area. This will be the ost expensive car I've purchased, so I don't want to see it eaten away.

You can imagine my distain if I have to go to ziebart and get the car sprayer with OIL to protect it from rust/corrosion.

Thing you have to remember is that the bottom of the model s is not open, it's basically a solid sheet of carbon fibre, which isn't metal and won't rust. The battery is enclosed above that carbofibre sheet, so unless you rip the underside of your car off, your battery will never taste salt.

For those wondering how tough carbon fibre is, keep in mind that they made a tank out of carbon fibre as an experiment in weight reduction. It's not quite as tough as an abrahms tank, but it's not too far off that either.

Wasn't Models S frame based on aluminum, and not carbon fibre? If you have source for this info about car bottom being coated by carbon fibre, could you tell us, it would be great to see another engineering stuff about that car.

What I remember is that battery pack is swappable from bottom, connected to frame by few dozen screws, so no cover AFAIK. However material which that battery pack is made of is unknown to me.

Aluminum is very corrode-resistant, way better than steel because of natural oxide sheet that forms in surface of it. Salt water doesn't do squat to that (they make boats out of aluminum). However aluminum may corrode if it is touching or close to other metals by galvanic reaction. I just hope Tesla have taken that in consideration when designing the car (I see no reason why they haven't, but it is a new company which is boldly going where no man has gone before).

It should be more resistant to all forms of corrosion than other cars due to all aluminum except steel pillars and boron steel bumpers in the frame. There is a rumor that the battery pack may have additional protection from the road but no word as to what that would be. Though the battery pack composition has not been specified-- it felt and looked like metal. Thus I would assume it is also aluminum.

Given that the battery needs to be encased in a non-metallic material to isolate ground, I don't see why it would react with salt. Only area of concern would be the connection points, similar to a normal car battery.

Good question. Lots of snow and salty roads here.

The frame is alum, and I thought the body panels were gonna be cf same as Roadster. Looking back on the old manufacturing blog, it's not cf at all, so my bad on that one.

I do know that the battery IS protected by a solid sheet of SOMETHING that it sits on, whatever the material is. It's that 'base plate' that allows for a rapid battery change.

The battery pack felt and looked like metal/aluminum at Detroit. Likely there is an insulator inside. A Tesla rep Indicated that the engineers were evaluating something to protect the bottom of the pack.

In the alpha prototype picture series, you can see that the battery pack has a welded metal casing. Manually welding aluminum is a hard job, so the alpha packs probably are made from steel.

Steel or aluminum, a metal casing needs some form of protection from galvanic corrosion, salt, and gravel.

Carbon fiber as encasing is not practical for mass production and as a shield is oversized (shield must not contribute to mechanical stiffness).

But we haven't heard any official statement from Tesla on that.

Aluminum most definitely corrodes in salty environments... especially high strength aluminum. Only freshwater boats use aluminum hulls.

The Al2O3 passivation layer on aluminum holds up great to freshwater (unlike steel!) but the Cl- ions in salty water easily dissolve that layer. Aluminum actually corrodes faster than steel in those environments.

I worry more about the load-bearing components like suspension linkages than the battery. The alpha car did not seem to have any coatings on those aluminum extrusions. Looks like the bottom of the battery at least had something.

A hard anodize with polymer sealant would do an excellent job of protecting underbody aluminum.... hopefully they implement somthing like that in the final production model!

Is (structural) aluminum prone to cracking?

No concern about cracking of Aluminum. The Battery cases was definitly different color than plain aluminum. This might have represented anodized aluminum. Tesla has Welded aluminum structural frames in the alpha models- why not welded aluminum on the battery frame? I would say that looking at the pack in person, I saw nothing that looked like a welded seam. The structure had plenty of welded seams. I agree that my biggest concern for corrosion is the suspension-- no different than any other car.

Audi is using aluminum for the A8 for some time now and I never heard the aluminum caused any problems.

Aluminum is way better than steel when it comes to corrosion. If you don't believe me, go look underneath a rust belt car. The body looks like a piece of cheese, while the trans/motor that are usually cast unpainted aluminum are affected very little.

Aluminium is most definately used as a hull material in saltwater boats, there is plenty of aluminium boats here in Norway and they are kept in salt water at marinas the whole season.

For example, take a look at the Buster line, very popular.
http://www.buster.fi/en/mainpage

"I do know that the battery IS protected by a solid sheet of SOMETHING that it sits on, whatever the material is."

Well, that answers my #1 question. I hope it's a really corrosion-resistant material, but even if it isn't, I can examine the solid sheet for damage and get it repaired or patched (and indeed hose it off) well before any damage to the battery pack itself actually occurs.

"A Tesla rep Indicated that the engineers were evaluating something to protect the bottom of the pack."

...and this statement is what worries me. "Evaluating" something? They need something, something which is cheaper than a battery pack. If the one-piece surface of the battery pack gets damaged, I don't see any way to repair it, so it will just get progressively worse until it finally does damage the battery. If a separate sheet gets damaged, no need to replace the battery.

should have specified - *bare* aluminum is not used in saltwater boat hulls.

Here is a decent article showing pictures of what happens to uncoated aluminum when exposed to de-icing salt: http://www.imoa.info/moly_uses/moly_grade_stainless_steels/architecture/...

"For example, there is a common misconception among architects that aluminum’s corrosion resistance is similar to that of stainless steel. This is incorrect. In locations that are exposed to chlorides (coastal or deicing salt), the corrosion rate of aluminum is typically 10 to 100 times that of stainless steel. The white to grayish white color of aluminum corrosion product may not bring attention to the problem (as the rusting red of steel does), until permanent aesthetic or structural damage has been done. This lack of knowledge makes aluminum a common replacement for stainless steel when construction costs must be reduced. Figures 4 and 5 show severe aluminum corrosion in a deicing salt laden environment and provide a significant contrast to Figure 2."

The point is that aluminum corrodes much less than steel. I don't see any stainless steel cars, do you?

Cars have a life expectancy, and are engineered to last a certaint amount of time. Tesla will be no different.

Also "steel" is not any element, it is a mixture of many materials, many of the aluminum alloys are also not pure aluminum, there are other things in them. There are differences between aluminum and aluminum :-P

I think this is a valid concern. It's not the only concern, of placing the battery pack under the floor, here are some more.
1. All sort of Corrosion
2. Very high chances of being hit by gravel, stone and some hard and high speed bump.
3. flooding during raining season of some country
4. Bumpy roads in some countries

So I think Tesla really need to address waterproofing & impact protection for their motors and battery packs.

I too would really appreciate an update regarding galvanic corrosion. Is there isolation between the steel frame areas and the aluminum? Furthermore is there any anodization or paint on the aluminum exposed to the elements?

This car needs to last me at least 10 years or my wife will cut something near and dear to me off. (For the record, the Model S was her idea...)

I can't speak about galvanic oxidation or isolation between steel and aluminum.

However, I have this tidbit about aluminum oxide from aluminumsulfate.net:

"Metallic aluminum is reactive with oxygen, which could cause corrosion to build up. However, when aluminum bonds with oxygen to form aluminum oxide, it creates a thin coating that protects it from oxidation. This keeps the aluminum from corroding and losing strength."

I never had a problem with the aluminium body and steel frame on the Land-Rover Series three (and I kept it for almost twenty years). I don't expect any problems with the Model S.

These kind of design issues don't bother me. Tesla studies, designs, test, changes, and ultimately passes all the crash worthiness, fire safety, etc. standards with flying colors. I know from my previous engineering work on aerospace electronics there are many established standards that must also be met (FCC, UL, SAE, CE) that specifically include salt/corrosion, electromagnetic interference, shock/fire, etc.

I do not see any evidence that Tesla would skimp on these sorts of fundamental design issues and testing. Just because we would all like to see more blogs from the company describing what they've done so we can ohh and agh, yet its not forthcoming, is not reason to be concerned imo.

ICE requires corrosion warranties because the exhaust system, for example, lives its entire life with corrosive hot gasses and acidic fluid in contact. Not to mention ICE also has to deal with issues of dissimilar metals.

It IS something to talk about while we wait......

pilot +1

Structural Aluminum definatly fails from salt corrosion. Check with anyone in the trucking industry. Lighter aluminum is desirable in the manufacture of semi-trailors.(inreases allowance of cargo weight) However, every year there are press storys of semi-trailor frames failing down under the weight of un-rigid loads such as loose cranberrys, watermellons, kernel-corn. Three to Five years are a long life for Aluminum frames under semis. Silicone and wax (Ziebart) sealing can extend the life of aluminum frames by several times.

I don't know if you have taken a look at the bottom plate of the battery pack. It is about 1/4" thick!
It will take a long time for that to corrode through even if it were regular steel and aluminum is better in those situations. I don't know if it is anodized though. If it is, then there is almost no chance that there will be any issues. The bigger problem is as always the welds.

The battery is enclosed in steel.


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