Corrosion resistance in Cybertruck may not be perfect

David R Kirkpatrick

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What forms of corrosion can occur in stainless steels?
The most common forms of corrosion in stainless steel are:
  1. Pitting corrosion - The passive layer on stainless steel can be attacked by certain chemical species. The chloride ion Cl- is the most common of these and is found in everyday materials such as salt and bleach. Pitting corrosion is avoided by making sure that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel which is more resistant to attack. The pitting corrosion resistance can be assessed using the Pitting Resistance Equivalent Number calculated from the alloy content.
  2. Crevice corrosion - Stainless steel requires a supply of oxygen to make sure that the passive layer can form on the surface. In very tight crevices, it is not always possible for the oxygen to gain access to the stainless steel surface thereby causing it to be vulnerable to attack. Crevice corrosion is avoided by sealing crevices with a flexible sealant or by using a more corrosion resistant grade.
  3. General corrosion - Normally, stainless steel does not corrode uniformly as do ordinary carbon and alloy steels. However, with some chemicals, notably acids, the passive layer may be attacked uniformly depending on concentration and temperature and the metal loss is distributed over the entire surface of the steel. Hydrochloric acid and sulphuric acid at some concentrations are particular aggressive towards stainless steel.
  4. Stress corrosion cracking (SCC) - This is a relatively rare form of corrosion which requires a very specific combination of tensile stress, temperature and corrosive species, often the chloride ion, for it to occur. Typical applications where SCC can occur are hot water tanks and swimming pools. Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production.
  5. Intergranular corrosion - This is now quite a rare form of corrosion. If the Carbon level in the steel is too high, Chromium can combine with Carbon to form Chromium Carbide. This occurs at temperatures between about 450-850 deg C. This process is also called sensitisation and typically occurs during welding. The Chromium available to form the passive layer is effectively reduced and corrosion can occur. It is avoided by choosing a low carbon grade the so-called 'L' grades or by using a steel with Titanium or Niobium which preferentially combines with Carbon.
  6. Galvanic corrosion - If two dissimilar metals are in contact with each other and with an electrolyte e.g. water or other solution, it is possible for a galvanic cell to be set up. This is rather like a battery and can accelerate corrosion of the less 'noble' metal. It can avoided by separating the metals with a non-metallic insulator such as rubber.
 

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I have a background in corrosion as well. as a Coast Guard Structural Aircraft Mechanic. I have seen many types of corrosion on aircraft and I can attest to one thing is that Corrosion Never Sleeps. Thanks for the brief, definitely something to guard against. Fluid Film and Corrosion X are my friends in the battle against the enemy.
 

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Pitting corrosion is something we will definitely see, especially where road salt is used. But it is mostly cosmetic damage, and it will take decades for the pits to be larger than pin-pricks.
In salt mines they use all kinds of corrosion inhibitors on their trucks and still they never last two years, usually only one year. I imagine those guys are looking at the cybertruck and it's smooth underside very, very closely.

Galvanic corrosion is the one where... If we see that, Tesla really screwed up. It will be in areas where separate parts join together, meaning loss of strength in that area will matter, a lot. It's effects can accrue quite quickly. In extreme cases the non-stainless part will seem to disappear. I imagine they'll use lots of rubber grommets and modern adhesives in the areas where stainless joins to non-stainless.
A few of the earliest aluminum F-150's had spots of galvanic corrosion, but ford fixed it pretty quickly. It's something that shade tree mechanics need to be aware of, and is easily overlooked.
You know those little box mail trucks you've seen all around the USA since the mid 70's? The bodies of those are made from riveted aluminum sheets. Most of those bodies are on their third or fourth frame. If they can do that with aluminum, I'm not worried about 3mm stainless at all.

The other types of corrosion mentioned above will be rare to almost nonexistent, unless of course Tesla screwed it up.
 

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Tesla has more electrochemists on its staff than the average automaker. They ought to be able to figure it out. It's not, AFAIK, a problem with any of the current production which have 1)Aluminum and 2)other (dissimilar) metals. Any place where roads are salted in the winter chloride ion abounds. I'll not be losing sleep over corrosion worries.
 
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David R Kirkpatrick

David R Kirkpatrick

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Wow, thanks for comments. I guess that the ubiquitous salt on upstate NY roads will always be a problem. I’ll try the sealants.
I still have my doubts about restricting oxygen to passive layer with wrap or paint. What do you think?
 

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Corrosion, at least on the stainless steel portions, will be a non-issue.

Have you seem this? Tesla says they are using 3mm of cold-rolled stainless.
3mm is slightly thicker than 12 gauge. It's almost an eighth inch thick.

12-Gauge-14-Gauge1-600x333.jpg
 

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The material comes from the mill passivated. If the manufacturer cuts it or abrades it or damages the passivation layer it needs to be repassivated but exposure to atmospheric oxygen will generally do that. If not (don't know what the value of X is) the manufacturer can re passsivate chemically. If the passivation layer is not damaged there is no need to repassivate it that I am aware of but I'm not a metallurgist.

But I think you are wondering whether this layer needs to be protected against chloride. I know that brewers, who use lots of 307, tend to avoid bleach (a mixture of NaOCl and NaOH which releases chloride ion when it does what it is supposed to do) out of concern over this. Bleach will attack the passivation layer. Don't ask me how as it is an oxidizing agent.

Again I don't now what the X is in 30x. It may be an alloy less likely to be attacked by chloride. As to what action we should take I'd say - none. I think it safe to assume that Tesla is aware that there is lots of Cl- out there in the world and that their vehicles had better be resistant to it. They have two years to test this alloy, both in the lab and on the road in winter and near the seashore. They know they won't be able to sell the truck if it is corrosion prone.
 

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I'm looking for information about wrapping stainless steel, not finding much. In the meantime, I'll make some guesses.
I do not claim to be any kind of expert.

If you put a sticker on it, and then five years later remove the sticker, the area under the sticker will be LESS weathered than the rest of the panel. You don't have to worry about the passivation layer unless you use an acidic glue or something else weird.

I heard that someone had asked Elon in a tweet about wrapping the CT, but then another employee answered the question. Has anyone seen this tweet? Anyway, the answer was something like... "it can be difficult to get wraps to cling well to stainless.", To me, this means that the surface will not be highly polished. The more rough the surface is, the harder it is to get things to stick to it.
A brushed finish is very common, looks decent, hides flaws from the factory, can be repaired easily with the right tools, and would be very rough when trying to get a wrap to stay attached. Will they use a brushed finish? If they do, they have to get all the brush lines across the entire vehicle to line up, or it looks stupid. Robots can do that.
I'm pretty sure that any stainless grade can be polished to a mirror-like finish, Steels with higher chromium content will be easier to polish, and turn out much nicer in the end. Some stainless screws that I buy at the hardware store are pretty shiny without being polished. Somebody somewhere will definitely polish their cybertruck. How hard it is for them to do that will depend on the grade of steel and finish that Tesla ends up using.
It's really hard to tell on video exactly what the surface finish is, and when Elon says things like "we could make the CT an inch narrower and six inches shorter", you can know that what we see today is definitely not what will roll out of the factory in a couple years. We will have to wait and see.

Tesla is not divulging the exact grade of stainless they intend to use. The exact phrase they use is "Ultra-Hard 30X Cold-Rolled stainless-steel structural skin" That does not mean it was run through the rollers thirty times. It would be better to say "30?" grade stainless steel. It's probably an alloy above 304, hopefully closer to 309.

Here I have shamelessly stolen some text from the website of McMaster-Carr. McMaster.com
it illustrates some grades and why they are used, in layman terms. Maybe it helps some of you.

Weldable 321 Stainless Steel
The titanium content of 321 stainless steel preserves corrosion resistance around weld points.

High-Temperature 309/310 Stainless Steel
309/310 stainless steel has high levels of chromium and nickel to provide good corrosion resistance at elevated temperatures. It'soften used in heat exchangers and furnaces.

Polished Multipurpose 304 Stainless Steel
Polished to either a brushed or mirror-like finish, this material is often used to fabricate decorative enclosures and industrial workstations.

Hardened Multipurpose 304 Stainless Steel
Harder than our other 304 stainless steel, these thin sheets offer better wear resistance for use as a wrap, liner, or cover.

Multipurpose 304 Stainless Steel
From cookware to chemical-processing equipment, 304 stainless steel is a good all-around choice for a wide range of applications.

Economical 430 Stainless Steel
Use 430 stainless steel in decorative and light structural applications,rather than outdoors or in extreme temperatures.

Fatigue-Resistant 301 Stainless Steel
Able to withstand repeated stress and wear, 301 stainless steel has the strength required for applications such as springs and fasteners.

Impact-Resistant 440A Stainless Steel
440A stainless steel resists damage from impact and abrasion. It's often used for cutlery and valve components.

Hardened Very-Wear-Resistant 420 Stainless Steel
This material is hardened for increased wear resistance over standard 420 stainless steel.

Hardened Corrosion-Resistant
316 Stainless Steel

Because these thin sheets are harder than our other 316 stainless steel, they offer better wear resistance.Use them as a wrap, liner,or cover.

High-Strength 17-4 PH Stainless Steel
With a higher chromium content than 15-5 PH stainless steel, this high-strength 17-4 PH offers better corrosion resistance.It is also known as 630 stainless steel.
 

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It was mentioned that a portion of the stainless from Starship would be utilized in Cybertruck production.

"The body of the Cybertruck will be cold-rolled stainless steel, just like that of Starship"

https://www.google.com/amp/s/www.space.com/amp/tesla-cybertruck-stainless-steel-spacex-starship.html

I took a look around and found that they were using 301 stainless. The following is the text and webpage where I found that information.

"Musk says SpaceX will initially use 301 stainless — that’s similar to the metal used in pots and pans."

https://www.extremetech.com/extreme/284346-elon-musk-explains-why-the-starship-will-be-stainless-steel
 

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I found a tweet from Musk that indicates it might actually be 310S stainless. Would be better than 301 for resistance and pitting from what I've read.

 

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I wonder why main line auto makers never used this material?
 

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I'm looking for information about wrapping stainless steel, not finding much. In the meantime, I'll make some guesses.
I do not claim to be any kind of expert.

If you put a sticker on it, and then five years later remove the sticker, the area under the sticker will be LESS weathered than the rest of the panel. You don't have to worry about the passivation layer unless you use an acidic glue or something else weird.

I heard that someone had asked Elon in a tweet about wrapping the CT, but then another employee answered the question. Has anyone seen this tweet? Anyway, the answer was something like... "it can be difficult to get wraps to cling well to stainless.", To me, this means that the surface will not be highly polished. The more rough the surface is, the harder it is to get things to stick to it.
A brushed finish is very common, looks decent, hides flaws from the factory, can be repaired easily with the right tools, and would be very rough when trying to get a wrap to stay attached. Will they use a brushed finish? If they do, they have to get all the brush lines across the entire vehicle to line up, or it looks stupid. Robots can do that.
I'm pretty sure that any stainless grade can be polished to a mirror-like finish, Steels with higher chromium content will be easier to polish, and turn out much nicer in the end. Some stainless screws that I buy at the hardware store are pretty shiny without being polished. Somebody somewhere will definitely polish their cybertruck. How hard it is for them to do that will depend on the grade of steel and finish that Tesla ends up using.
It's really hard to tell on video exactly what the surface finish is, and when Elon says things like "we could make the CT an inch narrower and six inches shorter", you can know that what we see today is definitely not what will roll out of the factory in a couple years. We will have to wait and see.

Tesla is not divulging the exact grade of stainless they intend to use. The exact phrase they use is "Ultra-Hard 30X Cold-Rolled stainless-steel structural skin" That does not mean it was run through the rollers thirty times. It would be better to say "30?" grade stainless steel. It's probably an alloy above 304, hopefully closer to 309.

Here I have shamelessly stolen some text from the website of McMaster-Carr. McMaster.com
it illustrates some grades and why they are used, in layman terms. Maybe it helps some of you.

Weldable 321 Stainless Steel
The titanium content of 321 stainless steel preserves corrosion resistance around weld points.

High-Temperature 309/310 Stainless Steel
309/310 stainless steel has high levels of chromium and nickel to provide good corrosion resistance at elevated temperatures. It'soften used in heat exchangers and furnaces.

Polished Multipurpose 304 Stainless Steel
Polished to either a brushed or mirror-like finish, this material is often used to fabricate decorative enclosures and industrial workstations.

Hardened Multipurpose 304 Stainless Steel
Harder than our other 304 stainless steel, these thin sheets offer better wear resistance for use as a wrap, liner, or cover.

Multipurpose 304 Stainless Steel
From cookware to chemical-processing equipment, 304 stainless steel is a good all-around choice for a wide range of applications.

Economical 430 Stainless Steel
Use 430 stainless steel in decorative and light structural applications,rather than outdoors or in extreme temperatures.

Fatigue-Resistant 301 Stainless Steel
Able to withstand repeated stress and wear, 301 stainless steel has the strength required for applications such as springs and fasteners.

Impact-Resistant 440A Stainless Steel
440A stainless steel resists damage from impact and abrasion. It's often used for cutlery and valve components.

Hardened Very-Wear-Resistant 420 Stainless Steel
This material is hardened for increased wear resistance over standard 420 stainless steel.

Hardened Corrosion-Resistant
316 Stainless Steel

Because these thin sheets are harder than our other 316 stainless steel, they offer better wear resistance.Use them as a wrap, liner,or cover.

High-Strength 17-4 PH Stainless Steel
With a higher chromium content than 15-5 PH stainless steel, this high-strength 17-4 PH offers better corrosion resistance.It is also known as 630 stainless steel.
Thanks for the research time... informative
 

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Have been looking for more pics showing the steel edge-on, to hopefully get a better idea of how thick...
Here's a couple of screen caps. Both are from the unveiling event.

CT_unveil_bed.png

This is inside the bed, with the tailgate down, and you can see the latch the tailgate hooks to.
Look just above the red rectangle. THICK!
Under his arm is the track that the vault door uses. It looks like there's 3mm steel both at the top and bottom of that track.
Being held open is the door to the charging port for the CyberQuad. That door is not 3mm thick.



ct_unveil_doorjam.png

And here is the side of the door. THICK SKIN!
But something is missing at the top fold, it's too thin.
 
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