Date: Sun, 17 Jan 1999 10:52:27 -0800
Reply-To: YauMan Chan <YAUMAN@CCHEM.BERKELEY.EDU>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: YauMan Chan <YAUMAN@CCHEM.BERKELEY.EDU>
Subject: Re: corrosion & anodes (long)
Content-Type: text/plain; charset=US-ASCII
>>> Austin <austins@IX.NETCOM.COM> 01/16 10:36 PM >>>
Thus the question for all the electron cognizenti out there - does an anode
have to be electrically attached to a vehicle ground, or is what I'm doing
now eventually going to work? i.e. for this to work, do I have to fish a
wire out of the tank to connect the anode to a convenient ground???
>>>
Here's a 2 minute lesson in electrochemistry:
A battery is created whenever you have two "dis-similar" metals in contact with each other or through a third medium call the electrolyte. Thus, you can stick a copper pin and a zinc pin into a lemon or potato and light up a light bulb - you have created a battery. We can look up the different metals "Potential volts" in any chemistry handbook under "Electrochemical Series." Thus under Copper is listed .0158v and under Zinc is -0.763v, and so the Cu/Zn lemon battery gets 0.158-(-0.763) or 0.92 Volts. While it is acting as a battery, the metal from the Zn side will migrate to the Cu side as ions - thus the Zn side will be "eroded" while the Cu side will get coated with Cu+lemon juice compounds. The kicker here is that the higher the potential difference between the metals, the more reactive this process is.. I.e. faster it will happen. The key, however, is that there has to be an electrolyte for this to happen. Acids (lemon juice) make good electrolyte and most base/alkalies do not. Salt solutions also make good electrolytes.
This is what is suppose to happen when the coolant acts as an electrolyte between the engine and the radiator. The radiator is copper Cu and the engine is something else - what? If the engine is steel, with is mostly Iron (Fe) or cast ion, the potential differences is only (Cu= 0.158 and Fe=-0.036 ) 0.194volts). If the engine is Aluminum (Al) the potential difference is (Cu=0.158 and Al=-1.706) 1.864 volts. As you can see, the reaction rate is substantially higher in the case of an aluminum engine.
Between the engine and the radiator, for most cars, there only common current path is the coolant which acts as electrolyte. Thus we have created a gigantic battery and when the reaction is taking place, the engine gets "eroded" while the copper radiator gets clotted up (Al migrates and deposits to the Cu.) From the above paragraph, you can see that the cast iron engine is a lot less "reactive" than the Aluminum engine, ie you can make a much battery with Al and Cu than with Fe and Cu. The engine is the anode and the copper radiator is the cathode.
To stop, this nonsense is easy. Do not immerse these metals in an electrolyte. Make the coolant alkyl and not acidic. Make sure the is no "salt" in the coolant. "Salt" used in the chemical sense is not just table salt. Phosphate compound is a good salt. This is what all the fuss is about the blue phosphate-free coolants.
Now, in the case of the boat it is not so easy. The hull, if metal, or any metal parts in contact with water is the anode, sea water is the electrolyte and mother earth is the cathode. So you have a HUGE battery and the reaction is very fast and furious. Sea water is a very very good electrolyte, in fact as good as sulfuric acid used in batteries! Here's a case where a sacrificial anode is effective. A metal, more active on the electrochemical series than the metal of the boat is immersed in water, but in electrical contact with the boat, "moves" the electrolysis action from the boat to the sacrificial anode.
So, while a sacrificial anode works wonders in marine applications where the anode is Fe compound like steel, it is not very useful in the Vanagon engine. For it to be effective, first, it must be metal more negative than Al on the electrochemical series and there are very few, Al being one of the most negative of common metals. You are left the likes of Magnesium (-2.3v) or Potassium (-2.94v) or some Al compound made for that purpose. Second, it must be immersed in the cooling jacket of the engine itself and be in electrical contact with the aluminum and immersed in the coolant.
Yau-Man Chan
87 GL
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