antifreeze

Antifreeze is a liquid additive mixed with water in an engine cooling system to prevent the coolant from freezing at sub-zero temperatures and from boiling away at high engine temperatures. The base ingredient is almost always ethylene glycol, a viscous organic compound with a freezing point around minus 12 degrees Celsius when pure. When mixed with water at the right ratio, typically 50:50 by volume, the mixture freezes at roughly minus 37 degrees Celsius, well below the point where water alone would turn to ice and rupture engine block castings.

Beyond freeze protection, antifreeze performs several other critical functions. It raises the boiling point of the coolant by roughly 10 degrees Celsius, allowing the system to run hotter without cavitation or coolant loss. It contains corrosion inhibitors, usually silicates or organic acid technology compounds, that protect cast iron, aluminum, and copper components in the cooling circuit. Hard water minerals are buffered by alkaline compounds in the mixture so they do not form scale deposits inside passages. Red dyes or fluorescent green colors serve as visual indicators of the coolant and help spot leaks.

Two main antifreeze chemistries are used in automotive work. Inorganic additive technology (IAT) antifreeze relies on silicates and phosphates as corrosion inhibitors and was the standard in most vehicles until the early 2000s; it typically requires a drain and refill every 50,000 kilometers. Organic acid technology (OAT) antifreeze uses compounds like sebacate or 2-EHA instead and lasts up to 200,000 kilometers in many modern vehicles. Hybrid formulations attempt to bridge both approaches. Mixing different types in the same system is generally unsafe because their chemistries can react unpredictably.

Common failure modes and maintenance

Antifreeze degrades over time through oxidation and loss of alkalinity, a process accelerated by high temperature cycling and air exposure. Once the pH drops below 7.5 or so, corrosion inhibitors become ineffective and rust, pitting, and erosion of internal engine passages accelerate. Coolant hoses and gaskets can weaken if the mixture is not maintained, leading to leaks that progressively dilute the antifreeze and reduce its protective effect. Water can enter the system if seals fail, diluting the glycol and lowering the freeze protection threshold.

Disposal of spent antifreeze is a legal and environmental concern in most jurisdictions because ethylene glycol is toxic to mammals and aquatic life even in small concentrations. Used antifreeze should never be poured down drains or onto soil. Professional garages and service stations typically collect it for recycling or safe incineration. Propylene glycol is sometimes used in RV and marine applications as a less toxic alternative, though it offers similar freeze and boil protection.