Short answer
IAT, OAT, HOAT and P-HOAT are names for the corrosion inhibitor packages in glycol antifreeze. IAT uses inorganic inhibitors such as silicates and phosphates, OAT uses organic acids, and hybrid (HOAT) coolants combine organic acids with a smaller amount of an inorganic inhibitor such as silicate or phosphate. Engine makers choose a chemistry to suit their metals and seals, so the manual or the coolant’s stated specification matters more than its color.
IAT: inorganic additive technology
Traditional coolant, often dyed green, relies on inorganic inhibitors such as silicates, phosphates, borates and nitrites. Silicate forms a protective layer quickly on aluminum and other metals, but it is consumed in doing so, which is why IAT coolants have traditionally carried shorter change intervals.
In heavy-duty diesel engines, conventional low-silicate coolants are typically maintained with supplemental coolant additives (SCAs) that replenish nitrite or nitrite-molybdate, which protect wet cylinder liners against cavitation pitting.
OAT: organic acid technology
OAT coolants use carboxylate organic acids instead of silicates and phosphates. They protect metal by reacting at corrosion sites rather than coating every surface, and they deplete more slowly, which is the basis of extended-life coolant. GM’s DEX-COOL is a widely known OAT coolant. Heavy-duty OAT products often add nitrite for liner protection and are sometimes called NOAT.
Hybrids: HOAT, Si-OAT and P-HOAT
Hybrid coolants combine organic acids with a small dose of an inorganic inhibitor. A silicate-hybrid (HOAT or Si-OAT) has been common in North American and European applications; phosphate-hybrid (P-HOAT) coolants without silicate are associated with several Asian manufacturers. The differences reflect each maker’s choices about aluminum protection, water quality and seal materials.
Color is not a specification
Dye colors overlap across chemistries and brands: orange, pink, blue, yellow and green can each be found on more than one type. Identify coolant by the specification on the label and the vehicle or engine maker’s requirement, not by color.
Mixing different chemistries dilutes each inhibitor package and can shorten service life. When switching types, a complete drain and flush is the usual approach. Coolant test strips or a lab coolant analysis can check freeze point, pH and inhibitor levels before deciding.
Frequently asked questions
Is “universal” coolant a chemistry?
No. It is a marketing term for coolants intended to be compatible with several types. Check that the label lists the specification your manual names.
Why does heavy-duty coolant differ from car coolant?
Many diesel engines have wet cylinder liners that are prone to cavitation erosion, so heavy-duty coolants include inhibitors such as nitrite or molybdate and are covered by heavy-duty standards such as ASTM D6210.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].