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The Newbies Arena Are you new to knife making? Here is all the help you will need. |
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Copper, Corrosion & the battery effect....
Someone asked a question about copper and corrosion on www.britishblades.com - I did a little research and thought I would share my findings here. If nothing else, it should provoke some interesting oppinions.
The question was something like, "when copper is in contact with steel, does it cause corrosion?" After researching a bit on google, the answer turns out to be quite complex, but very interesting. There is a phenomenon called galvanic corrosion, or the "battery effect" that occurs when 2 different metals come into contact with each other in and electrolyte solution. I'll try and explain what I've learned in simple language. Galvanic Corrosion Galvanic corrosion (also called ' dissimilar metal corrosion' or wrongly 'electrolysis') refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone. Either (or both) metal in the couple may or may not corrode by itself (themselves) in seawater. When contact with a dissimilar metal is made, however, the self corrosion rates will change:
In a bimetallic couple, the less noble material will become the anode of this corrosion cell and tend to corrode at an accelerated rate, compared with the uncoupled condition. The more noble material will act as the cathode in the corrosion cell. Galvanic corrosion can be one of the most common forms of corrosion as well as one of the most destructive. With this knowledge, we can look at the periodic table, partcicularly the nobility of elements and determine the likelyhood of galvanic corrosion occuring. The closer they are in nobility (gold & silver for example), the less likely they will undergo galvanic corrosion in even the harshest of conditions, while the further appart (copper & aluminium for example), the more likely they will corrode when in contact, even under controlled conditions. Galvanic series relationships are useful as a guide for selecting metals to be joined, it can help the selection of metals having minimal tendency to interact galvanically, or will indicate the need or degree of protection to be applied to lessen the expected potential interactions. In general, the further apart the materials are in the galvanic series, the higher the risk of galvanic corrosion, which should be prevented by design. Conversely, the farther one metal is from another, the greater the corrosion will be. Galvanic compatability... From the nobility of element, we can construct a table or index of anodic activity. At the top of the table is the most noble element - gold, with a value of 0.00 and so on. Using this table, we can make some recommendations... Anodic Index The following are recommendations for engineers...
According to the above, the difference between carbon steel (0.85v) and copper (0.35v), is 0.50v, which just makes it suitable for humidity controlled environments. Looks like anything more severe than a display case and mixing copper with carbon steel should be avoided. However, the above is based on engineering tolerances, for metals in direct contact, exposed to an electrolyte (water, blood, sweat - all of these will do) and not maintained. It is possible to protect the metals from this effect by insulating them from oneanother. A layer of epoxy for example. Also, regular care, a coating of oil etc, will reduce the likelyhood of this effect occuring. All knives need to be cared for, some more than others. I guess the use of copper would just put the knife into the "high maintainance" bracket. So, here are some tips...
Galvanic corrosion Anodic Index To answer the origional question... "when copper is in contact with steel, does it cause corrosion?" An answer would be, if the metals were in direct contact or contact was made by the presence of an electrolyte (blood, sweat, water etc), the less noble - steel, would corrode at an accelerated rate. Hope you found this interesting, opinions? Last edited by Martyn; 11-27-2002 at 01:12 PM. |
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This topic came up on an armour forum a while back. A lot of us use copper and brass rivets as well as brass ornamentation on steel armour. The person was concerned about brass/copper rivets corroding out due to sweat. I took several samples of metal combinations (brass/copper/steel rivets on brass/copper/steel sheet) and put some in mildly acidic solutions, salt water, plain water and air. They weren't really soaked in a solution, but were misted heavily once per day and stored between two layers of leather. The result was that brass/copper/steel will corrode if wet or in the presence of acidic compounds regardless of the metal it is in contact with. No differences were seen between any combinations that were definitely due to galvanic corrosion. Likewise, I have several knives with copper bolsters/shoulders and pommel caps where the cap does not contact steel...or has very limited contact, and it tarnishes at the same rate as the bolsters.
If you had two pieces that are constantly immersed, or are load bearing and immersed, I bet there's a huge effect! But for knives, I would guess that with proper care of a blade, you should never have to worry about this. |
#3
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Spot on Martyn, especially the part about coating, I think of it as a hole in a bucket of water, where the water is the less noble, the bucket the coating & the floor, the more noble. Coating the more noble (placing the bucket in another bucket) lessens the effect
__________________ colin@britishblades.com
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#4
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Still, when you see century+ knives with copper rivets & little (or certainly little evidence of) corrosion, it makes you wonder if it's really important:evil
__________________ colin@britishblades.com
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Well, the effect is there, however it's important to note that the values in the galvanic table, are volts measured while the element is immersed in salt saturated water. Like Don points out, if the knife were unprotected and immersed in seawater, then the effect would surely occur, but for a regularly oiled and polished blade, with layers epoxied together, I doubt it'd be of much significance.
If I were designing a diving knife or a sea fishing knife though, I'd probably choose something other than a copper/carbon combination. It would just seem good sense to do so. |
#6
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Quote:
Here a couple of examples of bimetallic corrosion, the tell tale sign: corrosion of the anode will accelerate corrosion of the cathode will decelerate or even stop. Which means one metal part ehibiting a lot of rust, with the other showing little or no rust. |
#7
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I can think of lots of places where it makes a BIG difference! Boat motors and other marine applications come to mind as well as heat exchangers and other large metal parts associated with the nuclear industry. I completely agree the effect is real, I just don't know that I would worry about it too much. I'm usually more concerned with "will the mokume pattern match the damascus pattern", rather than "will it accelerate the corrosion of the steel". The test we did was pretty crude and was done over a six month period to allieviate this person's fears that the brass rivits in a steel/leather/canvas coat of plates armour section were not going to corrode and fail in his lifetime. There wasn't enough control to attribute the corrosion to any one thing. I was suprised at how little therer was though. Once it thoroughly got a "patina"( couple weeks), it pretty much stayed the same with little buildup.
I use quite a bit of copper and high copper alloys with a lot of the migration era knives I make with complete disregard to galvanic effects...I've never noticed any effects, but I've never really looked for them either. Skin oils seem to be the biggest contributor to tarnishing. I tend to buff things down with silver polish every couple of months. It certainly makes sense to take a proactive approach when easily applicable, though. I know some people that have Viking style axes that use them for everything from hacking wood to cutting limes for magaritas...I'll have to look close at the axe next time I'm around them! On the pics, what materials are they, what type of environment, etc? |
#8
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Quote:
You're probably right Don, in that it aint really worth worrying about, but I also agree, if building in protection is just a matter of making sure you get good epoxy coverage, then it's worth the consideration at the time of making. It cant hurt, though I certainly wouldn't let it put me off using certain combinations of materials. It's may also worth thinking about who the knife is going to. I mean some people let carbon blades rust just out of improper care, but that's another subject |
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blade, fishing knife, knife, knives |
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