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Antifreeze and corrosion inhibitors
Corrosion Inhibitors in Modern Automotive Antifreeze
(credits to MNScooter)

All automotive/motorcycle antifreezes are Ethylene and/or Propylene Glycol-based. Glycol serves the primary purpose of antifreeze: to depress the freezing temperature, and raise the boiling temperature, of the water in which it is dissolved - a necessary property given the range of temperatures and pressures in which engine coolants must function. A simple glycol/water solution alone cannot be used as a coolant, however, because the hot metallic internal surfaces of radiators and engine water jackets would become corroded by direct contact with water. To prevent metal corrosion, antifreezes are formulated with small percentages of additives called corrosion inhibitors. Antifreezes also contain small quantities of other additives as well, such as lubricants, antifoaming agents, bittering agents, and dyes.

There are two different types of corrosion inhibitors currently in use: inorganic oxides and carboxylates.

Inorganic oxides are the corrosion inhibitors used in conventional antifreezes. These inhibitors, including phosphates, silicates, borates, nitrates, nitrites, and amines (or mixtures of these), work by depositing a barrier film on the metallic internal surfaces of radiators and water jackets, thereby forming a protective layer that isolates the coolant from the metal and thus prevents corrosion. These inhibitors are gradually depleted in service, so inorganic oxide antifreezes must be changed at intervals, generally every two years, to maintain effectiveness.

Carboxylates, also called "Organic Acid Technology" (OAT), are another, more modern type of inhibitor. These include Sodium Ethyl Hexanoate and Sodium Neodecanoate. These inhibitors work by reacting with, and stabilizing corroded metal only at local corrosion sites. These inhibitors are depleted more slowly than inorganic oxides, but still must be changed, albeit at longer intervals; generally every five years. They provide better heat transfer than do inorganic oxides since they do not form a barrier between metal and coolant. They are also significantly more effective at arresting corrosion to aluminum.

Conventional American antifreezes contain inorganic oxide inhibitors. This type of antifreeze is being phased out of use in new vehicles, in favor of OAT antifreeze.

European antifreezes contain a mix of inorganic oxide (silicate) and carboxylate inhibitors. Water in Europe is generally quite hard, with a high calcium and magnesium mineral content. Phosphates react with these minerals to form calcium and magnesium phosphate, which deposit on hot metallic surfaces as scale. This causes corrosion and reduced heat transfer. European antifreezes are therefore phosphate-free.

Asian antifreezes contain a mix of inorganic oxide (phosphate) and carboxylate inhibitors. In Asia, water hardness is not high; but issues with toxicity and biodegradability led to a ban on silicate inhibitors.

The new, extended-life OAT antifreezes were formulated to be globally acceptable, and to provide improved performance over inorganic oxide antifreezes. Because they contain neither phosphates nor silicates, they meet both European and Asian requirements. Also, as noted above, they provide improved heat transfer, protect aluminum better, and require changing less frequently. OAT antifreezes are classified as G12 in America, and G30 or G34 in Europe. This is the type of antifreeze used in Piaggio's water-cooled engines.

Care must be exercised when mixing antifreezes with different inhibitor types. Claims of universal compatibility are often false. Silicate inhibitors jell when mixed with some OAT inhibitors, resulting in obstructed radiator passages, and possibly in serious engine damage from overheating. Other compatibility issues exist as well. It is generally best not to mix one type of antifreeze with another. Only distilled water should be used since tap water contains minerals that will deposit on metal surfaces, causing corrosion and reduced heat transfer.
Last Updated Sun Jan 03, 2010 8:21 am
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