Antifreeze & additives

Coolant is not only freeze protection

Freezing point is the least interesting property of a modern coolant, because any reasonable concentration takes care of it. What actually matters is the inhibitor package: it keeps cast iron, aluminium, brass and radiator solder safe from electrochemical corrosion in a circuit where a conductive liquid runs at 90 °C for years. The second job is raising the boiling point under pressure, which is what prevents vapour forming against the combustion chamber walls. The third, on wet-liner diesels, is cavitation protection — bubbles that implode against the outside of the liner and perforate it from the outside in, a failure mode nothing about the fluid's appearance announces.

Technologies and specifications, not colours

Coolant colour is a manufacturer's commercial code, not a technical classification — green products exist on entirely different technologies. The real classification is by inhibitor: conventional on silicates and phosphates, with a short life; OAT on organic acids, long-life and silicate-free; hybrid HOAT, with a small silicate content to protect aluminium. The references that count at purchase are ASTM D3306 and D6210 on the American side, and the European approvals: VW TL 774-D/F/G (G12, G12+, G13), MB 325.5 and 326.x, MAN 324 SNF/NF, Volvo 128 6083. On heavy-duty engines and wet-liner designs, cavitation protection is required explicitly through ASTM D6210 — it is not assumed.

Simple rules that avoid most of the trouble

A cooling circuit forgives little and warns late:

Do not mix technologies. An OAT on top of a conventional silicate coolant forms gel and blocks the heat exchanger — identical colour is no guarantee of compatibility.
Respect the concentration. Below 33 % corrosion protection drops; above 60 % heat transfer drops, so the engine runs hotter on more coolant.
Use demineralised water to dilute concentrate. Tap water brings calcium, which deposits on exactly the heat-transfer surfaces.
On wet-liner engines, check explicitly for cavitation protection; not every "heavy-duty" product has it.
Change at the interval even when the fluid looks fine. The inhibitors are consumed before the appearance changes.