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  #1  
Old 10-22-2006, 11:52 AM
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B.Finnigan B.Finnigan is offline
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Dissimilar metals cause corrosion

In all the years I have been on knife making forums I have not read any info about dissimilar metals. The materials we use to make a knife can actually accelerate the corrosion of steel. Metals such as nickel, silver, copper, stainless, brass and bronze in a moist enviroment can react with iron to make a very weak battery. This translates to corrosion which carbon steel is very prone to in the best of enviroments.

"When two different metals are in contact in a corrosive environment, one of the metals experiences accelerated galvanic corrosion while the other metal remains galvanically protected.

Galvanic corrosion occurs when two different metals are in contact in a corrosive environment: one of the metals (The carbon steel blade) experiences an accelerated corrosion rate. The contacting metals form a bimetallic couple because of their different affinities (or attraction) for electrons. These different affinities create an electrical potential between the two metals, allowing current to flow. (ie; rust)"

-American Galvanizers Association.


Nickel is one of the "worst" ones since it is highly reactive. The two types of rechargable batteries we commonly use are nickel based, Nickel cadmium (NiCd) and nickel metal hydride (NiMH).

By being aware of this potential reaction we can insure our knives will be around long after we are gone. The possibly un-informed customer will not think about galvanic corrosion they will just chalk it up to the maker using inferior products and techniques.

Your knife might be able to chop through two 2x4's, a 1" rope and still shave arm hair and then be bent to a 45 degree angle but what good is that if it rusts a few years down the road? That nickel silver or stainless guard looks pretty on that new knife but think about it's future.

Here is a picture of a stainless(?) knife that was put in the drawer and contacting a carbon steel knife while they were both damp. The carbon steel knife had serious pitting but it also electroplated the stainless knife. This is after I went over it with steel wool. They were in the drawer for less then two weeks.


Last edited by B.Finnigan; 10-23-2006 at 06:44 PM.
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Old 10-22-2006, 12:55 PM
Tai Google Tai Google is offline
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Oh Boy!
Sounds like a juicy topic for another gut wrenching thread!

...Good point!


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  #3  
Old 10-22-2006, 12:59 PM
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Shakudo Shakudo is offline
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check the search function.i believe this subject was discussed at length more than a year ago.
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  #4  
Old 10-22-2006, 02:18 PM
EdStreet EdStreet is offline
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sounds ilke a job for mythbusters!

seriously tho what was the outcome of the treat a year+ ago?

Ed


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Old 10-22-2006, 02:50 PM
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B.Finnigan B.Finnigan is offline
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I searched "galvanic corrosion, corrosion and dissimilar metals" and found nothing. It would be great if we could have two different search functions, one for advertisers and one for raw info.

Many months ago a knifemaker posted a question in the Newbies forum why his knife had greenish blue staining on the blade. He used nickel silver and a copper spacer. Alot of people commented that it was copper corrosion but nobody went into any detail why it happened so quickly. I touched briefly on galvanic corrosion but no one else chimed in with any additional info. It was sort of a digital "deer in the headlights" reaction.

If the knife you make is going to be in a humid enviroment especially in a coastal area this really needs to be addressed. It will happen, the question is how fast and to what extent.
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  #6  
Old 10-22-2006, 05:05 PM
Tai Google Tai Google is offline
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I guess there might be something to be said about integral guards and wrapped integral socket handles...

No dissimilar metals here.




You get corrosion from dissimilar metals aggravated by a little hydrogen embrittlement from acid etching and all hell can break loose! The blade can literally rot off at the guard...


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Last edited by Tai Google; 10-22-2006 at 05:28 PM.
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  #7  
Old 10-22-2006, 05:24 PM
EdStreet EdStreet is offline
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so basicaly the first image that was posted had some type of chemical on it to start with.

Also anyone have a list of reactions each metal has?


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Old 10-22-2006, 07:14 PM
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B.Finnigan B.Finnigan is offline
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The first pic is a stainless steel blade that reacted with a plain carbon steel blade due to a small amount of dampness. The two blades were put in a drawer with the blade tips touching when niether of them were dried off completely. No chemicals were involved except H2O and a very weak current of electricity. Seven year olds don't always do the dishes correctly.

Last edited by B.Finnigan; 10-22-2006 at 07:19 PM.
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Old 10-22-2006, 08:09 PM
EdStreet EdStreet is offline
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that photo actually looks like something I would see on a dive boat after a weekend of diving. only difference would be the dive boat thing would have it wide spread.

I did run across some charts and some very fascinating info on this.



Quote:
In fact three conditions are required for galvanic corrosion to proceed; the two metals must be widely separated on the galvanic series (see Figure 2), they must be in electrical contact and their surfaces must be bridged by an electrically conducting fluid. Removal of any of these three conditions will prevent galvanic corrosion.
Quote:
Frequently this is not practical, but prevention can also be by removing the electrical contact - this can be achieved by the use of plastic or rubber washers or sleeves, or by ensuring the absence of the electrolyte such as by improvement to draining or by the use of protective hoods. This effect is also dependent upon the relative areas of the dissimilar metals. If the area of the less noble material (the anodic material, further towards the right in Figure 2) is large compared to that of the more noble (cathodic) the corrosive effect is greatly reduced, and may in fact become negligible. Conversely a large area of noble metal in contact with a small area of less noble will accelerate the galvanic corrosion rate. For example it is common practice to fasten aluminium sheets with stainless steel screws, but aluminium screws in a large area of stainless steel are likely to rapidly corrode.
Quote:
This combines elements of pitting, crevice and galvanic corrosion, and occurs where small particles of foreign matter, in particular carbon steel, are left on a stainless steel surface. The attack starts as a galvanic cell - the particle of foreign matter is anodic and hence likely to be quickly corroded away, but in severe cases a pit may also form in the stainless steel, and pitting corrosion can continue from this point. The most prevalent cause is debris from nearby grinding of carbon steel, or use of tools contaminated with carbon steel. For this reason some fabricators have dedicated stainless steel workshops where contact with carbon steel is totally avoided.

All workshops and warehouses handling or storing stainless steels must also be aware of this potential problem, and take precautions to prevent it. Protective plastic, wood or carpet strips can be used to prevent contact between stainless steel products and carbon steel storage racks. Other handling equipment to be protected includes fork lift tynes and crane lifting fixtures. Clean fabric slings have often been found to be a useful alternative.
also ran across this one.


Quote:
Electrolyte
The degree of bi-metallic corrosion is affected by the electrolyte pH and conductivity. The intensity of the corrosion can increase with the conductivity of the electrolyte. Typical values of conductivity of various fluids are listed below;
Distilled Water 0.5-2 μS/cm
Stored Distilled Water 2-4 μS/cm
Supply Water 50-1500 μS/cm
Sea Water 50,000 μS/cm
Sat. Sodium Chloride 250,000 μS/cm
Sulphuric Acid up to 800,000 μS/cm

Bi-metallic corrosion is seldom a problem when the metals are immersed in pure water.


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  #10  
Old 10-22-2006, 09:12 PM
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This page explains the anodic index and galvanic series pretty clearly: http://www.corrosion-doctors.org/Def...nic-series.htm


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Old 10-23-2006, 01:59 PM
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The electrolyte series is a little deceiving - distilled water is very corrosive as it has no buffering.


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Old 10-23-2006, 02:12 PM
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I'm a little confused here because I was under the impression that the nickel in 15N20 and L6 played a notable part in the corosion resistance of those metals. 15N20 in particular seems to be pretty much a plain-jane carbon steel with a little nickel added. It seems to rust much slower in our humid environment than 10xx series or O-1 steels seem to. Is this because they are in a "solution" of some sort and there is no surface for an electrolytic reaction or am I missing something here?


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Old 10-23-2006, 04:34 PM
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It could be that you keep them oiled adequately. I have had some pattern weld billets that I did not like the patten so I pitched them in the scrap pile and they rusted within days. That is with only 1.5% nickel. Alot of stainless has 8-10% Ni.
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Old 10-23-2006, 05:03 PM
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Raymond Richard Raymond Richard is offline
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One of the jobs I had years ago was doing all the flat stainless steel in the galleys of super tankers. The stainless steel fastend to steel furring. Before I attached the stainless I had to run a strip of plastic tape to the furring to stop the electrolisis.

As soon as I posted this I just happend to think that I used stainless steel screws to hold the pannels on. Nothing was done to stop the electrolisis for that application.


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Last edited by Raymond Richard; 10-23-2006 at 05:07 PM.
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  #15  
Old 10-23-2006, 05:15 PM
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B.Finnigan B.Finnigan is offline
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ooops!!!
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