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More On "Antifreeze: Red or Green?"

Q. Dear Mr. Ciulla;

I read your Antifreeze: Red or Green? article today and found it interesting. I would agree with most of your comments. However, I would bring to your attention a comment on the use of phosphate in coolant. Phosphate does provide a number of benefits as you have noted. And, I agree with you that the amount of precipitate that is produced due to hard water is very small.

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Typically, you would not find it amid all the other junk in a cooling system. However, once you get past this argument with the German technical professionals, you will find that their real objection is that phosphate is what would be considered a "dangerous" inhibitor.

In a well formulated US green coolant, phosphate works to provide the benefits you describe. However, if the silicate is not well stabilized and the silicate protection is lost for an aluminum head, a phenomenon know as either "hot surface corrosion" or "aluminum transport deposition", depending on what car company you are talking to, can occur.

In this form of corrosion, the aluminum is corroded from the hot spots in the head, usually near the exhaust ports, and then reacts with the phosphate in solution to form aluminum phosphate. The aluminum phosphate is then deposited on a cool surface in a heat exchanger leading to over heat problems.

It has been reported as either a gelatinous precipitate that blocks the tubes or as a thin ceramic deposit on the radiator (original finding of the problem when Chrysler was testing VW engines to use in Omni/ Horizon in the late 70's) in the shape of the fan. If you write the chemical reactions that occur when the aluminum phosphate is precipitated, you will find that caustic is generated. The corrosion process then increases exponentially.

On the other hand, in the absence of phosphate, dissolved aluminum will precipitate as aluminum hydroxide. This reaction is pH neutral and the corrosion rate therefore does not increase rapidly.

This can be demonstrated in the lab very simply by electrochemically forcing corrosion of an aluminum specimen in plain 50% glycol solution with and with out phosphate present. In the system with only glycol, the pH will not increase. In the system containing the phosphate, the pH will rise.

It can also be demonstrated in a dynamometer. We wrote an ASTM paper on this in the early to mid 80's. Loren Beard, who is now at Chrysler, presented the paper at the ASTM symposium. What you see in the dynamometer test is the silicate first depletes, then you see aluminum in solution, then you see the phosphate deplete, and the pH will rise to 11 or more.

The corrosion coupons at first are well protected and then we saw a coupon with a small amount of damage followed by one that had a lot of damage. Interestingly, the coupon that had been in the cooling system from the start was still protected when the new coupons were attacked in this particular system. This does not happen in some other coolants.

I think the difference was that we were using phosphate to provide the entire buffer capacity and thus had no borate in the system. Borate is a surface cleaner and when it is present in the system during this corrosion process it must clean the silicate off the original coupon and therefore allow corrosion of that coupon.

Sorry to have been so long winded but this is one of my favorite topics. If you have any questions about my comments, please feel free to contact me.

John J. Conville

John Conville was manager of Antifreeze R&D for BASF in the US for about 15 years.

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