SOLVED Dissimilar Metals???

[Kryten]

I have just read a post metioning about using dissimilar metals namely copper and alluminium, I am familiar with lead and steel can cause electrolisis, but I see that using copper and alluminium can cause a battery effect.
can anyone enlighten me on how this takes place and does it matter where in the system you use the components, because I was considering making a resovior (about 3 or 4 gallons) out of ally and getting a copper water blocks and radiator.
can anybody help the chemically challenged?

 

[Hoot]

All the transitional metals have an anodic / cathodic relationship to one another. Here is a link to a table listing them in order of most reactive to least.

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If two of these metals have a conductive path between them (IE, water with impurities) electrons will migrate from the anodic (+) to the cathodic (-) metal. That's a technical way of saying electrical current will flow between them. Depending upon how great the dissimularity between the two metals and how conductive the path, the reation can vary from barely detectible to raucous corrosion. There's a pretty good difference between aluminum (AL) and copper (CU), so the potential exists for deterioration of the surfaces between them. If you were using real pure distilled water only, it would take a while for it to contaminate with impurities enough to provide a sufficiently conductive path to cause severe deterioration. additives change that picture. Salt would be a good example of a bad choice of additives since it increases the conductivity considerable. As far as Ethylene Glycol, Water Wetter, that Purple stuff, etc, I don't know how they contribute to the conductivity of distilled water. Alcohols should not be a big deal though. There is a lot more that I don't know, than I do, about this subject, starting with no "hands on" experience with water cooling. Perhaps some more experienced water cooling overclockers can "wade in" ;D on this.

Hoot

 

[Badger]

In order to get dissimilar metal corrosion you need to make a circuit, i.e. as well as having an electrolyte the metals need to be in electrical contact with each other.
Think of a car battery, until you connect the + and - terminals together there is no current flow.
What I'm getting around to is providing there is no electrical path between the aluminium components in the system and the copper ones no circuit can be created and no corrosion will occur.
Assuming your using plastic (non conductive) tubing the only connection could be through the PC case itself so make sure you electrically isolate your aluminium reservoir from the case and you should have no trouble.
Trust me on this one, I engineer piping systems in offshore oil platforms for a living and it is an issue there too.

 

[doubled]

Along those lines (offshore rigs), couldn't one use a sacrificial anode the way that outboard engine manufacturers use?

Just a thought.

Dave

 

[Badger]

doubled (May 11, 2001 03:16 p.m.):
Along those lines (offshore rigs), couldn't one use a sacrificial anode the way that outboard engine manufacturers use?

Just a thought.

Dave

Thats a good idea! a sacrificial zink anode placed in the reservoir would be nice insurance.

 

[William]

i think the zinc anode has been mentioned, but I had forgotten about that. If this electrolyses only happens between transition metals, then why is aluminum a problem, it is not in the transition period.

 

[Badger]

dunno260 (May 12, 2001 12:45 a.m.):
i think the zinc anode has been mentioned, but I had forgotten about that. If this electrolyses only happens between transition metals, then why is aluminum a problem, it is not in the transition period.

Given the right conditions dissimilar metal corrosion will occur between any dissimilar metals (carbon does it too) Not sure what your refering to by 'transition metals'

 

[Kryten]

Thanks for the info guys, I was intending to mount the radiator attached to a custom built resovior (on the outside) but I guess I could place rubber or something between the ally and the copper radiator to insulate it.
Also I would have copper water blocks inside the case with tubes running to my external cooling.

 

[Ridenow]

Hoot appears to read the same stuff I do, but is better at summarizing.
I was under the impression that over time the metals released ions into the water and these ions were enough to create the circuit. It would take a while for the corrosion reaction, but it would still happen.

 

[Badger]

Ridenow (May 12, 2001 11:25 p.m.):
Hoot appears to read the same stuff I do, but is better at summarizing.
I was under the impression that over time the metals released ions into the water and these ions were enough to create the circuit. It would take a while for the corrosion reaction, but it would still happen.

I know some of the stuff out on the net might lead you to believe this, but it's not the case. To comlete a curcuit you need a closed loop (thats what a curcuit means) so a well as the electrolite to transfer the cherged ions the metals need to be in electrical contact so the current can flow (like the car battery).
Quite often this closing of the loop is inadvertantly accomplished by earthing the components to the case.

 

[Ridenow]

Badger (May 13, 2001 12:43 a.m.):

Ridenow (May 12, 2001 11:25 p.m.):
Hoot appears to read the same stuff I do, but is better at summarizing.
I was under the impression that over time the metals released ions into the water and these ions were enough to create the circuit. It would take a while for the corrosion reaction, but it would still happen.

I know some of the stuff out on the net might lead you to believe this, but it's not the case. To comlete a curcuit you need a closed loop (thats what a curcuit means) so a well as the electrolite to transfer the cherged ions the metals need to be in electrical contact so the current can flow (like the car battery).
Quite often this closing of the loop is inadvertantly accomplished by earthing the components to the case.

Ok, the guy I saw have this problem had most of the corrosion in the waterblock. It was mounted to the plastic socket lugs and had plastic tubes. I can not imagine any electrical conductivity through the processor. This block was electrically isolated, except with the fluid. I still think the cooling fluid had enough ions to close the circuit. Remember, this fluid is circulating from the block to the radiatior to the reservoir and back.

 

[Badger]

Ridenow,
I'm not too concerned about what some guy reported, the basic science on dissimilar matal (Galvanic) corrosion is the metals must be in electrical contact as well as contact through the electrolite. dont believe me ? check this site out: http://www.alliedcorrosion.com/techinfo.html

Here is a quote from the site:

Basic Theory of Corrosion
Corrosion is basically an electrochemical process in which four conditions must be present:
1. There must be a positive or anodic area, referred to as the "anode."
2. There must be a negative or cathodic area, referred to as the "cathode."
3. There must be a path for ionic current flow, or "electrolyte."
4. There must be a path for electronic current flow, which is normally a "metallic path."
[End Quote]

Of course is you would rather believe what 'some guy' says rather than established scientific fact it's up to you.

 

[Hoot]

The following is an anecdotal experience. I don't know why it is occurring, but it definitely is occurring. With my MC-462 heatsink, which has a copper baseplate, using AS as a thermal compound, I can measure a potential from the copper baseplate to chassis ground, sufficient to produce a current somewhere in the region of 2-3 ma. This is with the PC running and occurs with or without the HS fan running. I don't know if this potential is created as a result of some minor leakage from the core to the baseplate, or if it is enhanced by the presence of silver in the junction. It is a DC current, not an AC current, so I doubt it is a function of capacitive coupling. Now, whether that small potential can contribute to the galvanic process would probably hinge on there being a DC path from the radiator core and/or metal reservoir to chassis ground. I have only observed this on my system. Now, here is where it gets real kinky. If I leave my HS on the CPU for more than a week or two (rare from experimenting), the AS seems to recombine and leave small flecks of silver on both the copper base and the core. These will not scrub off in the normal degreasing process and must be shaved off with a razor blade. They occur most often where the printing is etched onto the core. Just some fuel to the flames.

Hoot