Mixing Dissimilar Coolant Technologies

There are three families of technologies of antifreeze or coolants in the American marketplace today. Some claim to be compatible with one or both of the others. Without a definition of "compatibility", the buyer should be aware that there exist supportable arguments that these technologies perform less well when mixed than any of them perform if used as single technologies.

This is a subject of active debate. ASTM has not addressed a definition for "compatibility" of antifreeze or engine coolants.

Conventional (usually green color) coolants are engineered to work well at a pH of 8.5 to 10.5. For decades, this technology has been effective and has provided good protection to the system's metals.

The higher (alkaline) pH is important to these technologies because it improves the solubility of the conventional additives, and also maintains a generally less friendly environment to corrosion.

When the coolant wears, the ethylene glycol breaks down a little forming acids, the pH drops and some of the components can oxidize. The coolant becomes worn out, so the recommendation and practice was to change the coolant every 2-3 years. This practice can still be followed with very satisfactory system performance

Extended service coolants can be subdivided into two categories: carboxylate and hybrid

The carboxylate (usually orange but sometimes yellow or blue) coolants generally operate at a pH between 6.0 and 8.5. At this pH, conventional inhibitors can be hard to keep in solution. The carboxylate inhibitors stay in solution and perform well in this pH range

The hybrid coolants contain carboxylates, and may contain a variety of conventional style components. They operate a in a pH range of 7.5 to 10.5, interestingly positioned between the other two.
Nevertheless, the stability of some additives has been a challenge that has been overcome by the scientists that formulate these technologies. Mixing some of these technologies with low pH carboxylates would seem problematic

ATA's Technology Maintenance Council (TMC) has recommended that mixing coolants be limited to 10% of a coolant. That is to say, mixing coolants is not recommended, and if more than 10% of the fluid is replaced the wrong technology, there is a question as to the capability of the coolant to prevent damage to the system.

Amalgatech agrees; laboratory investigations strongly suggest that mixing coolants above 10% by volume with other coolant types seriously compromises corrosion protection in standard ASTM tests.