Galvanic Corrosion

[fishbulb]

This came up in amother post http://www.bladeforums.com/forums/showthread.php?s=&threadid=177315 when Joe Talmadge mentioned what he called the "battery effect" between a Ti handle and a steel blade. I responded with the following, but I decided to start a new thread for it, as I would really appreciate others' opinions or knowledge of this process:

"Battery Effect"
I believe that what Joe was talking about is "galvanic corrosion", which occurs when two dissimilar metals are in close contact for long periods. Although I'm not an authority on this, I just finished a Industrial Chem. course a few weeks ago and various kinds of corrosion were about half the class. What happens is that the two conductive metals exchange electrons between each other, resulting in corrosion because each reverts into its most stable form, which for steel is rust. This occurs for most metals, however I seem to remember that Ti is chemically inert so I am fairly sure that galvanic corrosion would not be an issue in a steel + Ti folder because the electrons could not be exchanged. Not entirely sure though.

This same principle is why bead-blast finishes rust so readily, each of the little "pockets" in the metal fill with atmospheric contaminants and create a mini galvanic cell, causing the nearby metal to corrode.

However, galvanic tendancies can also be used to prevent corrosion too, by the use of a "sacrificial anode". In this setup a piece of a more electrochemicly reactive metal (usually magnesium) is placed in contact with a steel or iron structure. Because of electron exchange, the steel can't corrode until the other metal is oxidized. It is used frequently to prevent rust on underground storage tanks. This principle could easily be applied to a folder or fixed blade by making a removable, replacable magnesium insert for the handle/blade, thereby producing a very corrosion-resistant knife.

I'm farily sure anout all this, but anybody who is/knows a chemistry expert might want to ask them to make sure.

 

[Clint Simpson]

I'm not a chemistry expert but here goes...

Originally posted by fishbulb
however I seem to remember that Ti is chemically inert so I am fairly sure that galvanic corrosion would not be an issue in a steel + Ti folder because the electrons could not be exchanged.

Electrons are exchanged, but Ti is in its most stable form. Steel however... Well, you wrote about "sacrificial anode". Guess what, steel can work as well as magnesium. BTW Ti may change color over time but that is the extent of its corrodibility.

This principle could easily be applied to a folder or fixed blade by making a removable, replaceable magnesium insert for the handle/blade, thereby producing a very corrosion-resistant knife.

Naysayer alert. I doubt easy is the right word, the removable, replaceable aspect adds weight and production problems. KISS (not the knife, but more on that later). If you use MG, it is going to stay in the knife, maybe as a non-spring liner? Of course, how long will the anode last? Furthermore, only the steel parts in contact with MG benefit. Great for the tang, useless for the edge (so what’s the point). The best application is give the blade a Mg coating, but it ain’t exactly Teflon you know. BTW MG is flammable, freaks me out; a metal that burns! And before someone posts that it only burns in power form, with normal wear that is what the insert will eventually become. MG blades don't exist for a reason: neither toughness nor wear resistance. Of course, flare up is a minor concern due to improbability (but not impossibility). Not trying to put you down, just sharing my thoughts.

This same principle is why bead-blast finishes rust so readily, each of the little "pockets" in the metal fill with atmospheric contaminants and create a mini galvanic cell, causing the nearby metal to corrode.

D@MN! I like(d) bead-blast finishes. No Glare and full profiling, doesn't rub off! What more could you ask for in military knife? How about maintainability? And it sounded to good to be true.

My CRKT KISS has a bead-blast finish AND a copper washer on the pivot pin. That area of the blade rust out faster than the rest. My point? Tuf-cloth fixed it right up!

 

[fishbulb]

Have to disagree with this a bit.

Electrons are exchanged, but Ti is in its most stable form. Steel however... Well, you wrote about "sacrificial anode"

Since the Ti is in its most stable form, individual electrons may be exchanged between the two metals but the total electron concentration would remain the same in the steel, because the Ti cann't absorb any more electrons. This is why gold plating is used over copper circuits on high-class electronics. Galvanic corrosion cannot take place because stable gold is unable to accept any more electrons.

I doubt easy is the right word, the removable, replaceable aspect adds weight and production problems. KISS (not the knife, but more on that later). If you use MG, it is going to stay in the knife, maybe as a non-spring liner? Of course, how long will the anode last? Furthermore, only the steel parts in contact with MG benefit.

I agree that "easily" might have been the wrong word, there would likely be formidable technical problems. However, I don't see any reason it should not be possible. Any good machine shop should be able to drill a hole in a handle and whip up a sacrificial pin or screw to put through it. Also, all parts of the item that are connected to the sacrificial anode by a conductive material would be protected. Storage tanks are generallly attached by a long wire to their anode. Regarding Flammablity, there is no reason to use another metal, I just put magnesium because it is the most common. Any conductive material that is more electrochemically reactive than the material to be protected would work, I have to check my books but I am fairly sure that nickel, tin, and copper would all work.

Either way, glad to hear some feedback.

 

[Overgaar]

I'm exactly an expert either. However, I do know that Zinc anodes are used on steel ships and on the iron keel og yachts.

Zink is however not exactly nice to look at, so I guess that you would have to conceal the tiny Zinc anode somewhere inside the handle.

Just my .02

Overgaar

 

[Ed Fowler]

I have seen it in some very high ticket antique knives. I thought at the time that it was a area that we should pay more attention to, but gave the matter no more thought since then. I believe that I will spend some time on the subject. Thanks for a thoughtful thread.

 

[Jeff Clark]

Titanium is neither chemically inert nor in its most stable form when it is in an unoxidized metalic form. A clue to this is that you never run across a nugget of titanium while out prospecting (contrast this with gold or copper which do occur in a metalic form in nature). Titanium oxide is the most stable form. The key with titanium is that it quickly forms a dense nonconductive protective oxide layer that chokes off corrosion. In effect it self anodizes. Aluminum does the same thing to a lesser degree. Iron forms a more conductive and porous oxide (rust) that does not provide protection. You might have corrosion problems in high friction areas where the oxide layer frequently gets rubbed off.

When you are dealing with ships and water tanks you are dealing with a saturated environment where the electrons can move around rather freely. The sacrificial anodes work pretty well in that setup. I suspect that you would have trouble making sacrificial anodes to be effective in your pocket.

 

[Cobalt]

Here is a link on a similar subject. I posted some several firms that if I remember correctly have a lot of information on the net about the subject. You can research it there and get some more answers:

http://www.bladeforums.com/forums/showthread.php?threadid=82472&highlight=anode+or+anodic


What I will add on the subject is that in order to have the anode/cathode exchange you must have the liguid medium or enough moisture in the air to cause this process. There were many heated debates on corrosion at the old knifeforums, but I am not sure if those where ever saved or archived.

I remember reading about an F-4U corsair from WWII that was found in very good condition under water. The reason was determined to be a great amount of zinc used (sacrificial metal) which corroded and protected the remainder of the aircraft's steel parts.

hope this helps

PS, hello to all the old friends on here, haven't been around for a long time. Those that know me will know why. In any case Hello to all

 

[HM]

That seems to be the right thread to raise my long-burning question. I could not confirm it quickly but according to my best recollection steel and brass have similar standard potential (??E0) or affinity to give/receive electrons. Therefore, when these two metals form a microcell (on your knife in wet conditions) they not corrode each other. Now, is that correct and does it have something to do with the fact that brass rivets and part are often used on knives?

HM