CT Corrosion 4

[bimr]

Ed,

Have you seen the numerous reports of stainless steel from the tesla cyber truck owners. Here is one today:

https://www.torquenews.com/11826/tesla-cybertruck-becomes-extensively-corroded-after-exposed-magnet

This doesn't appear to me to be a good application for stainless steel for numerous reasons.

 

[EdStainless]

304L with moderate cold reduction (20-25%) still has elongation over 25% and is profoundly magnetic.
To the point that to do Eddy Current Testing you have to run saturation coils.
It doesn't happen with 305, 309, 310 and some of the 200 (Nitronic) alloys.
It is a matter of the chemistry, and the Ni is so low in most grades that they are only meta-stable austenite.

 

[dik]

You could likely run some form of catalytic protection using the electric features of the vehicle.

 

[TugboatEng]

Cathodic? That only works if the vehicle is submerged in an electrolyte.

 

[MintJulep]

Rail cars made of 200 SS have been in service for decades, so it isn't the material per se.

301L and 304L have been used for rail cars since the 1950's.
201LN is more recent.

 

[dik]

Cathodic? That only works if the vehicle is submerged in an electrolyte.

I figured there had to be some sort of moisture to initiate most of the corrosion.

 

[TugboatEng]

Corrosion and cathodic protection are the same processes with the polarity reversed. They require the same conditions to exist. In the case of galvanizing where the, yes the metal is protected in moist environments. However, when there are discrete anodes, an very strong electrical connection is required which means it only works with full immersion.

 

[WKTaylor]

I got myself thinkun'...

One element/rule of certain SStl alloys in aerospace is 'do not disturb the natural passive finish... let it do it's job'.

What 'IF' these owners decided to clean/polish/brighten the natural SStl 'passivation patina'... using chemicals/abrasives that would strip-off the natural passivation... and embed 'tramp' particles and/or leave active acid/basic compounds in microscopic levels on the surface...? Hmmmmm.

 

[EdStainless]

embedded material and residual acids or soaps (chlorides) are bad news for SS.
If the surface is clean then it will naturally repassivate.

 

[mrfailure]

I wonder if real world environmental conditions are simply too harsh to use unpainted stainless steel as on Cybertruck. It easily shows up dings and imperfections as compared with painted/coated surfaces, and as this thread notes corrosion can occur if conditions are not perfect. (Personally, I also think Cybertrucks are much more appealing when painted...)

 

[TugboatEng]

The problem is that stainless steel requires a different maintenance routine than paint. People do not like changing habits.

Problem two is the misled expectation that stainless steel is actually corrosion proof. It's only corrosion resistant.

Tesla is probably going to have to develop their own brand of car washes. A sodium hydroxide wash to remove organic materials, nitric acid with a touch of hydrofluoric acid to clean and passivate, and a DI water wash to give it that new car shine.

 

[3DDave]

That's the sort of cleaning that will really bring out the dings and scratches.

 

[EdStainless]

A simple Cl free soap will be good enough to keep SS vehicles clean and corrosion free.
My daughter commutes on a SS bicycle.
She rides daily for seven years now.
Once a week she hoses it off.
She only scrubs it to remove mud or bird dirt.
It still shines.
An occasional rinse with mild citric acid would be plenty good for maintaining a nice clean surface.