Exact questioned answered 4/16/02 by Tom, F&L GoR:
***********************************************************************
Sacrificial anode caps have a place in salt water applications, but there are some issues that everyone might want to consider in closed coolant systems.
For the aftermarket radiator caps with sacrificial anodes to work, the fluid must have some measurable "electrical potential." This potential is measurable, but that does not mean it is automatically corrosive, since the coolant should be protecting the metals in the cooling system. Then again, a sacrificial anode is designed to corrode, so it must be made to be reactive enough.
The most vulnerable part of the cooling system is the aluminum in high heat rejecting areas (heads.) High temperatures accelerate the reactivity of the corrosion process and even small potential differences may force a reaction to occur- but again the corrosion inhibitor package determines whether or not there will be corrosion.
That does not mean that the potential for corrosion exists in a normal cooling system since the metal reactivity of the sacrificial cap is so different.
The sacrificial anode works by corroding itself, which has a couple of concerns. One is the location of the anode in the radiator; there will be no benefit to other parts of the cooling system. Then the corrosion products will likely form deposits somewhere else in the cooling system. Deposits like to stick to hot surfaces and deposits on surfaces will enhance corrosion. For the radiator this may only result in plugging. For the heat rejecting surfaces, corrosion under the deposits formed will be likely. For extended life coolants, there is another possible disadvantage: since the inhibitors are free-floating in the coolant, they will deplete much more rapidly (and lose the 5-yr protection.) They will be brought to the highly reactive anode by the flow of the coolant and continually try to protect it instead of the cooling system metals!
Sacrificial anodes are used successfully in salt water applications where there are highly corrosive salts (Cl) which will react (corrode) with the more reactive "anode", typically zinc (perhaps also a Mg alloy?) to protect the cast iron block or outboard. One could make everything out of stainless or other nonreactive metal (and they do) in which case a "zinc" would not be needed. However, in cases where the propeller is made out of nonreactive material (brass, SS), and the rest is made out of cheaper cast iron, then a sacrificial anode completes the corrosion circuit/pathway that would normally be completed by the cast iron pieces and the noble propeller.
*********************************************
I have one on my GTS & it performs as stated. Sean knows his stuff!!
