Odd Leatherman Kick failure

[Spoonrobot]

So here's the backstory.

A few days ago I see a post over at the Spyderco forums about soaking a ZDP-189 knife in Coca-Cola overnight to give it a golden brown patina. I figure, hey, that's a pretty hot color. So I get some Coke, fill up a small container with just enough to cover my Delica. Seeing as I had about a half can of Coke left I decided I might as well test the rust removal properties of the soda too. My modded Kick has been neglected for some time, developing a bit of rust on the file so I open all the tools a little bit and then drop it in it's own container and cover it with Coke. This is @10pm last night.

Today I wake up @8am, pull the Kick and Delica and wash them off. I am disappointed, the Delica looks exactly the same. 

However, all the rust spots on the Kick are gone. It's cleaned up amazingly; no scrubbing, just the dunk and rinse and all the corrosion I chronicled earlier has disappeared. So I guess the myth is that it is a myth that Coke doesn't remove corrosion from stainless steel. As I'm working the tools under the water I notice they are under no tension from the backspring, I stop, check for a set and push ever so gently on the spring. And it snaps off in my hand with an audible *tink*.

This is very odd; the way I had the tools partially open the deviation from rest for the spring is barely measurable, maybe a millimeter or half. I can't even get a picture of it to illustrate.

Here're the broken bits. 


I have half a mind to send it back just so they can take a look at it. Understand I don't want any sort of warranty claim or replacement at all, just was curious if they have seen this before and if there is anything to learn from the pieces. My understanding of springs is that something like this should not happen, but I have a fairly limited knowledge of flat springs.

No rust though, which is kind of cool.


The story ends well though, I had won an auction for a riveted PST earlier this month and I guess it came with a Kick too (  ) so I was able to swap the tools and be back to EDCing my favorite multi within the same day.

*Oddly enough the small bit driver now has rust on it. Seems like the Coke works well on stainless steels but not on the carbon steel of the bits. It is a sexy golden rust though.

[NeitherExtreme]

My understanding of springs is that something like this should not happen, but I have a fairly limited knowledge of flat springs.

Me too... Seems odd. Good luck that you had a back-up though.

I wonder if LM will have anything to say about that one.

[Mike, Lord of the Spammers!]

That is an odd failure!, flat springs are usually dead reliable , maybe they over tempered it

[NeitherExtreme]

Just another thought... was the tension on the spring lopsided? I mean was a tool pushing just one side or the other so that it had twisting kind of pressure? I'm not sure if that makes sense, or even if it does if that could explain anything, but just a thought.

[DaveK]

  That ain't good!

Just looking at a Kick, and the springs look pretty substantial and should take a bit more than gentle pressure!

It'll be really interesting to see what LM say Spoon.

[Anthony]

Wow that's weird...especially BOTH sides snapping at the same time.

[J-sews]

So am I understanding correctly that the backsprings worked fine before the Coke bath, but then became super brittle afterwards???!!!

[jzmtl]

If you want to remove rust from any more stuff, try this, not acidic. I've used it on some old stuff and it works.

http://www.wr6wr.com/newSite/articles/columns/wp0906/wp0906.html

[Spoonrobot]

Just another thought... was the tension on the spring lopsided? I mean was a tool pushing just one side or the other so that it had twisting kind of pressure? I'm not sure if that makes sense, or even if it does if that could explain anything, but just a thought.

It's possible this may be the case for one handle but the other side, with the phillips driver, couldn't have been lopsided because the phillips is so wide and is centered.

So am I understanding correctly that the backsprings worked fine before the Coke bath, but then became super brittle afterwards???!!!

Well, yes, but I not sure the Coke really had anything to do with it. It appears the problem was that keeping the backsprings under tension for long periods by having all the tools partially opened. The tang of the tools was engaging the backspring but not to a very high degree.

 I'm tempted to try leaving all the tools partially opened without the Coke and see what happens then, as well as leaving all the tools closed and doing another coke bath.

[Sea Monster]

I'm tempted to try leaving all the tools partially opened without the Coke and see what happens then, as well as leaving all the tools closed and doing another coke bath.

Is this in the spirit of thoroughness or do you just enjoy breaking tools?

[Spoonrobot]

Can it not be both?

[J-sews]

I wouldn't think that simply maintaining tension on those backsprings for long periods of time would lead to breakage. It had to have been the Coke bath did it somehow.

[NeitherExtreme]

I'm tempted to try leaving all the tools partially opened without the Coke and see what happens then, as well as leaving all the tools closed and doing another coke bath.

Is this in the spirit of thoroughness or do you just enjoy breaking tools?

I think he's make the sacrifice for the sake of science.

 

[joebw]

I’ve looked at this Leatherman Kick back spring failure while immersed in Coca Cola and believe that I can explain it.

Before getting into my theory on the failure, it is necessary to discuss hydrogen and its fairly unique behavior as an ion, an atom and a molecule. Hydrogen (H) is the first element in the periodic table.  It consists of one proton and one neutron which make up the nucleus of the hydrogen atom.  Orbiting around this nucleus is one electron.

Hydrogen is further interesting in that it exists as a diatomic (meaning two) molecule.  Thus, gaseous or liquid hydrogen is referred to as “H2”.  Both nitrogen (N2) and oxygen (O2) exhibit diatomic molecules in nature.

Of great significance in this failure of the Leatherman Kick is what happens when the H2 molecule forms from two hydrogen atoms.  You’ve heard that opposites repel each other such as two north poles on magnets.  The electrons orbiting around the hydrogen nuclei do exactly that.  Assume that the volume occupied by one hydrogen atom equals “one” (don’t worry about the units).  The repulsion created by the orbiting electrons causes the two hydrogen atoms which each had a volume of “1” for a total of “2” when added up to actually have a volume of “2.3” when the diatomic molecule forms.  More on the significance of this further on.

Coca Cola has been used as a rust remover for as long as I can remember.  The reason that it works as this is the presence of carbonic acid (H2CO3) in the Coca Cola.  Part of the proprietary formula of this beverage is to carbonate the drink.  This serves to give it fizz and to make the pH acidic as part of the drinks unique flavor.  In water (base for Coca Cola), the carbonic acid will disassociate as follows;
H2CO3 ⇌ HCO3− + H+

While rust removal was Spoonrobot’s objective, inadvertently, the generation of the hydrogen ion (H+) led to the cracking of the Kick back springs.

If you look at the back spring failure, you’ll find that it occurred where the back spring met the handles.  This is the region of the highest stress in the spring.  In fact, I would think that with enough openings and closings, this would be where a fatigue failure would occur as the bending stress may generate a tiny crack which in time would grow until the critical crack length is reached.  At that point, such a crack would propagate at the speed on sound resulting in a failure which would look quite like the one we see here.

Before continuing this explanation, a word or two about the electrons associated with metal atoms.  Unlike plastics or ceramics, the electrons in the outer orbits of a metal atom are not tied to a particular atom.  Rather they are referred to as “free electrons” are able to move throughout the metal.  It is this feature unique to metals that allows them to conduct electricity.  In the case of plastics and ceramics, their electron are tied to a particular atom thus making them insulators

 

Here is what I believe happened.  The carbonic acid disassociated as noted above.  A hydrogen ion entered a beginning submicroscopic crack at the base of the back spring.  This hydrogen ion “borrowed’ a free electron from the stainless steel and transformed from an ion to an atom.  This was repeated with another hydrogen ion.  When these two atoms met, it was a case of 1 + 1 = 2.3.  That volume expansion actually creates a gas pressure of over one million psi at the crack tip.  Since the resultant diatomic molecule is far greater in diameter than that of the ions, it is unable to escape through the microcrack.  Net result is that the pressure is relieved by causing the crack to length a a fraction of a millionths of a centimeter.  This process repeats until the critical crack length is achieved at which point the back springs break.  This is referred to as “hydrogen stress cracking” or hydrogen embrittlement.

[Spoonrobot]

Holy cow joebw! That's exactly the kind of explanation I was hoping someone could supply. Thanks a lot for taking the time to write that up.

[ducktapehero]

.  This is referred to as “hydrogen stress cracking” or hydrogen embrittlement.

That's what I figured too. 




What in the hell did he just say?

[Spoonrobot]

Here's the wiki on Hydrogen Embrittlement if anyone's interested.

[Sea Monster]

Holy Science Batman!

[hawkchucker]

Joebw has a big old brain now dont he!

[BIG-TARGET]

Can you get Adam Savage and Jamie Hyeman to back it up??

[DaveK]

I'm assuming that wasn't a lucky guess then Joe?

[joebw]

Hi Dave,

No, not a lucky guess.  I've been in the metallurgical field for some 44 years and have seen lots of cool stuff like this.

Joe

[hawkchucker]

Cool now the important ? Can you make me a titanium handle for a Hawk?

[J-sews]

Still and all, its amazing to think that metal - strong modern stainless steel in this case - can be altered and weakened by a lowly can of soda.

[Offroad]

Still and all, its amazing to think that metal - strong modern stainless steel in this case - can be altered and weakened by a lowly can of soda.

It sure is

[Benner]

Just think what it does to your insides. 

[J-sews]

Just think what it does to your insides. 

I'm a two-Coke-a-day addict, and can confirm that it generates gas as a by-product. Did not realize the gas was hydrogen though........

Perhaps my ____'s are worth money?!

[Benner]

I drink a lot of Coke as well and have had the same results. 

[ducktapehero]

Just think what it does to your insides. 

I used to be a Coca-Cola addict. I'd drink over a 12 pack a day. Since quitting I feel so much better. And I'm actually a much more reasonable person to deal with because I'm not totally strung out on caffeine. I still like to have a soft drink now and then but now it's usually just generic orange flavor or when I'm feeling spendy I'll get Dr Pepper.

I think a lot of America's obesity problem is due to soda pop and lack of exercise. When I was a kid I'd get maybe 1 can of Coke a day if I was lucky and was outside playing football all day. Now kids drink pop by the gallon and spend all day on their ass playing football on their computer.

[Anthony]

I can't remember the last time I had a Coke or Pepsi

It tastes good going down, but it leaves me with a funny taste in my mouth and I just feel sluggish afterwards.  I'm convinced there's a chemical in soda to make you want another after you've finished one..