Rick's Free Auto Repair Advice

Coolant types — What’s the difference between coolants?

Coolant types — What’s the difference between IAT, OAT, HOAT

Automotive coolants are divided into two main categories: the older “Green” Inorganic Acid Technology (IAT) coolants and the newer organic acid technology coolants (OAT, HOAT, PHOAT, and NHOAT).

What does coolant do?

• Removes combustion generated heat from the engine
• Prevents freezing
• Prevents corrosion due to oxidation and galvanic corrosion
• Prevents/reduces cavitation
• Lubricate the water pump.

What’s in coolant?

Approximately 95% of coolant is mono-ethylene, polyethylene, or polypropylene glycol. The other 5% are the lubricants, anti-foaming agents, buffers, anti-oxidants and anti-corrosion additives. In other words; the additive package

Does coolant wear out?

Yes. The additive package definately wears out over time and mileage. However, the base fluid does not, except in certain circumstances of overheating. If you’ve overheated your engine, you’ve also damaged the base fluid and should get a full engine flush.

What additives are used in IAT coolant?

Borate coolant additive

Borate is used in IAT coolant to reduce acidity. It’s generally not used in OAT or HOAT coolants because they maintain a different pH and borate would throw the pH out of whack.

Nitrite coolant additive

Nitrite is used in cast iron engines to prevent corrosion and cavitation. However, Nitrite don’t work well with newer aluminum engines. If it is used in a coolant for aluminum engines, the coolant must contain other additives to protect the aluminum.

Phosphate coolant additive

Phosphate lays down a coating film on cooling system metals to prevent corrosion. If you’re mixing conenctrated coolant that contains phosphate, it’s critical to use distilled water as tap water (hard water) can cause the phosphate coating to flake off and form layers of scale that can clog the radiator and heater core.

Phosphate is used because it quickly lays down a “passivating” layer on metal surfaces, so it protects right away. However, phosphate coatings don’t last very long.

Silicate coolant additives

Silicate in IAT coolants is often used as a substite for phosphate. It’s considered another fast-acting acid that lays doesn a film barrier to metals from corrosion. Silicate works fast to protect but has a short life like phosphates. However, some Asian carmakers believe that silicates are abrasive and wear out water pump seals, so you generally won’t find silicates in Aisian coolants.

Silicates have one other drawback; they reduce heat transfer.

How does IAT coolant work?

The Silicate or phosphate additives deposit on the walls of all the metals in the cooling system to form an Anodic layer of protection. Once the anodic coating layer is higher than any cathodic current in the solution, the system is considered “passivated,” preventing electrolysis. In other words, the anodic coating prevents the transfer of electrons from one metal to another, something we call corrosion.

Downsides to IAT coolant technology:

• Short life — The silicates and phosphates precipitate out of the solution, so they have a short life of less than 30,000 miles or about 2-years.
• Borate as an anodic corrosion inhibitors works best with cast iron engine parts, but is corrosive to aluminum and aluminum alloys
• Silicates and phosphates are abrasive, so they cause wear to water pump seals
• IAT green coolant reacts poorly with hard water, forming scale in the cooling system that can plug the radiator and heater core.
• Nitrates have a short life and don’t work well to protect aluminum or aluminum alloys.
• Pitting —As the passivating film deteriorates, the metals develop pitting and the corrosion accelerates.

Silicates and phosphates are designed to protect copper, brass, cast iron and aluminum. It is NOT recommended for use on newer cars that were filled with OAT or HOAT coolants.

Since most modern engines are built with aluminum engines, radiators, water pumps, and plastics, IAT coolant is no longer the best choice.

What is Organic Acid Technology (OAT)

OAT coolant is still built on a 95% glycol base fluid. But OAT doesn’t use silicates, phosphates or  borates. Instead, they use sebacate and carboxylic acids, azoles, 2-ethylhexanoic acid (2-EHA), aliphatic mono- and dicarboxylic acids and tolyltriazole as the additives.

OAT additives work differently than IAT additives. Instead of laying down a film coating, OAT coolant works on a molecular level, bonding the additive molecules to the metals. While OAT additives last much longer than IAT additives 5-years or 150,000 miles compared to 2-years or 24,000 miles), they also take much longer to start working; several thousand miles of driving before they take full effect.

Advantages of OAT coolant

• Longer life
• Better able to protect aluminum than silicates and phosphates
• Non-abrasive so it doesn’t wear out water pump seals.

Disadvantages of OAT coolant

OAT coolant take longer to provide protection. In some cases they don’t reach maximum protection until 3,000 to 5,000 miles.

The bad rap OAT gets isn’t deserved

When GM came out with OAT coolant, their vehicles experienced upper intake manifold leaks and sludge buildup. Owners formed a class action lawsuit to recover damages from GM DexCool OAT coolant. Owners blamed the OAT coolant.

The cause of the upper intake gasket was the choice of plastic used in the gasket

But the fault wasn’t with the coolant. The gasket leaks were caused by poor design. GM engineers designed the gaskets from plastic with a silicone sealing area. Unfortunately, they never considered the plastic softening effect of the 2-EHA additive. Over time, the 2-EHA additive that was so effective at preventing corrosion, wound up damaging the gasket. Once GM changed the plastic gasket carrier material to a type not affected by 2-EHA, the gasket leak problem disappeared.

The sludge problem with DexCool was caused by low coolant levels and a poorly designed radiator cap

When DexCool is used in a cast iron engine and the engine is operated with low coolant, the exposed cast iron rusts. The rust powder accumulates over time to cause sludge formation in the tiny passages of the heater core. Some experts believe that a poorly designed radiator cap was the cause of the low coolant levels. But operator error is also a huge contributing factor.

When using DexCool in case iron engine, maintaining properly coolant level is critical.

Hybrid Organic Acid Technology (HOAT) coolant type

HOAT coolant combines the best of both worlds. It’s main protection is provided by organic acid, supplemented by either a low dose of silicate or phosphate, depending on whether it’s an Asian or European vehicle.

Asian carmakers use phosphate in their HOAT coolants to provide immediate corrosion protection until the organic acids take effect. They also like phosphate because it doesn’t have the abrasive effect of silicate additives.

European carmakers, on the other hand, contain silicates but no phosphates.

What is G-05 yellow HOAT coolant?

G-05 is used by many shops to avoid stocking multiple types of coolants. It is a low silicate, no phosphate formula, so it’s considered an HOAT coolant. However, since it constains silicates, it should not be used on Asian vehicles.

What about all-makes, all models coolant or universal coolant?

Is there really a coolant that works in all makes, all models?

Not really. What you’re really getting with a universal coolant is a coolant that doesn’t do anything very well. The coolant manufacturers have to throw in small amounts of all the additives to protect all possible combinations of metals, plastics, rubber and gasket materials. As a result, it does nothing very well.

Universal coolant is a mix of IAT and OAT additives with nitrites added to protect cast iron. You really don’t want nitrites in a modern aluminum engine.

Universal coolant isn’t any cheaper than the recommended coolant and it doesn’t work as well as the recommended coolant. Why risk cooling system failures that could cost thousands of dollars just to save $10 on coolant?

Coolant colors and which carmakers use which coolant?

Green IAT coolant. Light-medium green with inhibitors that include
silicates and phosphates. Recommended service life is 2-years/24,000 miles.

Who uses HOAT coolants?

VW/Audi pink. Contains some silicates and an organic acid, but is phosphate free

Ford/Mercury yellow. Contains low amounts of silicate and no phosphate
Honda blue. Contains a special coolant with just one organic acid

European/Korean blue. Contains low silicates and no phosphates

Asian red. Contains no silicates but has some phosphate

Yellow Gold. HOAT coolant with silicates and phosphates.
Orange DexCool. An OAT coolant with sebacate and 2-EHA as the primary inhibitors. Texaco Havoline, Prestone, Extended Life and Zerex Extended Life make DexCool

Toyota Red. A conventional Japanese coolant with phosphates but, again, no silicates.
Toyota Pink and Honda Deep Green Extended Life. These new extended-life Japanese coolants use phosphates for fast protection (no silicates) and a single organic acid (sebacate) and other inhibitors.

VW/Audi Pink. The version used in models sold in the U.S. is an organic
acid type, primarily sebacate. However, some European models reportedly
use combinations of sebacate and 2-EHA, so it seems to be a matter of
who’s the supplier rather than an objection to 2-EHA.

European/Ford Yellow. More of a yellow than Prestone American Yellow
Gold. Its inhibitors include a relatively low dose of silicates (no phosphates),
plus benzoate. It’s often called a HOAT (hybrid OAT), because
benzoate is an OAT, though benzoate is not used in “true” OAT formulas.
Euro yellow is sold in the U.S. aftermarket as type G-05.

Chrysler Group Orange. The dye is orange, but the formula is G-05, the
same as European/Ford Yellow.

Korean Blue. You also may have seen blue dye in some European cars
(BMW and Volvo, among others). It’s similar to European/Ford yellow.
Brown. Usually a combination of two coolants mixed together by a topup—
for example, orange coolant repeatedly topped up with green, gold
or yellow. It even may be an orange coolant discolored by heavy doses of
sealer installed to avoid replacing a leaking heater or radiator.

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