coolant system corrosion

jim beam · 2010-06-08T19:43:42+00:00

following from the recent thread on this topic, it seems there are some fundamental misunderstandings out there that need to be cleared up.the first thing to understand is where corrosion comes from in the first place. very simply, it comes down to electrolysis. remember when you made a battery in school with a strip of zinc, a strip of copper and a lemon? well, that works because of the difference in electrode potentials between the two metals, and the presence of an accommodating electrolyte.in car cooling systems, you have different metals used, iron, aluminum, copper, etc., and you have an electrolyte, the fluid in the cooling system. [there is more than that, but i'm simplifying for illustration]. so, that's going to lead to corrosion!!! how do you stop it?basic methods:1. remove the electrode potential difference as much as possible - use an aluminum radiator with an aluminum block for example.2. passivate the materials as much as possible - slows thing down.3. use a non-electrolytic coolant fluid.used together, all these work well and cooling systems can last many years with no obvious or at least, minimal degradation.but what are the practical realities?a. people tend to introduce electrolytes into their cooling system - the use of tap water being the prime example. not great, but it's life. and cars don't last forever.b. use of the above can in fact cause some passivation. for example, buildup of calcium carbonate in a cooling system can slow down corrosion rates since it interferes with electron flow. but it also interferes with heat transfer, a strongly negative and unwanted side-effect. again, not great, but it happens. indeed, as a passivation strategy, silicates were use as a corrosion inhibitor in cheap antifreeze for this reason - it passivated the system by coating it. [but it also coated and ruined pump seals]c. when, inevitably, a cooling system treated as above becomes too inefficient and fails and a radiator is replaced, the new...