Quote:
Originally posted by Althornin
If you could direct your content to showing me the flaw in my thinking rather than repeating yourself, maybe we could move on.
You seem to be neglecting the electrolyte in your considerations...
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I don't know how to show you the flaw in your thinking without repeating myself.
I am neglecting the electrolyte in my considerations, because it is irrelevant to the power dissipation involved in trying to prevent galvanic corrosion using an external power source.
The goal, in adding an external power source, is to stop any flow of current through the electrolyte/coolant. If there is no current flow through the electrolyte, then there is no electrical power dissipation in the coolant. Ergo, the electrical conductivity of the electrolyte is irrelevant for the purposes of this discussion.
I'm going to go through this all once again. Please tell me what you dont understand, or disagree with.
Figure 1 represents aluminum and copper in an electrolyte, with no electrical path between the metals, aside from the electrolyte. (Equivalent to a battery sitting unused.) After initial creation, this system will reach a state of equilibrium at which point the voltage between the two metals will be that defined by the difference in the metal's electrode potentials. The resistance of the electrical path through the electrolyte is represented by Rbat. No current flows through the electrolyte in this system because there is no conductor available to complete the loop, and Galvanic corrosion is nonexistant. (Though there may be other types of corrosion going on.)
Figure 2 represents the same system as Figure 1 with the addition of an electrical conduction path other than through the electrolyte. This is the typical situation with a copper and aluminum waterblock where the two metals are bolted together. (Equivalent to a battery with its terminals shorted.) The existance of the electrical path outside the electrolyte (Rs) allows current to flow through the coolant.
Now suppose the resistance through the coolant (Rbat) is 1,000,000 ohms and the resistance of the metal to metal connection is 0.001 ohms. (Probably reasonable numbers to within an order of magnitude or two.)
The current flow in the system will be:
2 Volts / (1,000,000 ohms + 0.001 ohms) = 2 microamps
The power dissipated in the system will be:
2 Volts * 2 microamps = 4 microwatts
Fairly small current and power dissipation, but enough to eventually allow the aluminum to get corroded through.
Are you with me so far?
Figure 3 represents a system that attempts to prevent the Galvanic corrosion using an external power source. By connecting an external power source so that the voltage differential across Rbat is zero, the Galvanic corrosion can be stopped.
BUT, the voltage applied by the external power source is applied across Rs. Therefore the current through Rs is:
2 Volts / 0.001 Ohms = 2000 Amps
And, the power dissipated in Rs is:
2 Volts * 2000 Amps = 4000 Watts
(The resistance Rbat is totally irrelevant to this calculation, because the voltage of the external power source has been selected specifically to provide for zero voltage differential across Rbat, and therefore zero current flow through Rbat.)