The Chemistry of Cooling Systems, Larry Carley, Underhood Service, October 1996

Ever wonder why thermostat housings, aluminum radiators, water pumps and cylinder heads become badly corroded? Why radiator hoses eat through from the inside out, or why radiators sometimes clog up with gunk? You'll find the answer in cooling system chemistry.


You don't need a Ph.D. in chemistry to appreciate the fact that the coolant that needs attention. In many cases, the stuff is loaded with rust and sediment and looks more like hot chocolate than coolant. But even when the coolant is still green and looks good as new, the corrosion-fighting additives are often found to be depleted when the coolant is tested. So don't judge the condition of the coolant by appearances alone - looks can be deceiving. in many vehicles today is badly neglected.
If you test even further, you'll also find the concentration of antifreeze in the coolant varies wildly from the 50/50 mix recommended by most vehicle manufacturers. Out of every 12 vehicles, four on average either have too much water and not enough antifreeze, or too much antifreeze and not enough water. Another three or four may also be low on coolant.
One reason why the problem of coolant neglect is so rampant today is partly due to the fact that the average age of the U.S. vehicle fleet is well over nine years. Many vehicles have had only one or two coolant changes since they were new. And some are still pumping the same old tired coolant through their engines they had when they left the factory!
Coolant neglect also can be blamed on gas-and-go driving, extended service intervals and the elimination of many maintenance items that were once necessary to keep a vehicle in good running condition. Engines today no longer need annual tune-ups and few motorists see the need to winterize their vehicles. Consequently, many pay little or no attention to checking or changing the coolant until a problem occurs.
By the time you see these vehicles, the long-term effects of coolant.
neglect are usually self-evident - corroded and clogged radiators, eroded aluminum cylinder heads, water pumps and thermostat housings, rotten radiator hoses, leaky heater cores and freeze plugs - even blown head gaskets and chronic overheating. The root cause in most cases can be traced back to the poor condition of the coolant.

The Importance of the Mix


Though many people realize they need a certain amount of antifreeze in their cooling system to prevent the water from freezing during cold weather, many don't realize that antifreeze also helps prevent boilover during hot weather. Ethylene glycol, the main ingredient in conventional antifreeze, freezes at about 8 degrees F, boils at 330 degrees F and carries heat about 15 percent less efficiently than water.
Most vehicle manufacturers recommend a 50/50 mixture of antifreeze and water for optimum year-round protection. A 50/50 mix provides freezing protection down to minus 34 degrees F and boilover protection to 228 degrees F. Straight water obviously provides no freezing protection, no boilover protection beyond 212 degrees F, and no corrosion protection. Therefore, as you know, straight water should never be used in a cooling system. Straight ethylene glycol antifreeze also should never be used because it needs to be mixed with water (35 percent or more) to lower the solution's freezing point. Straight antifreeze also cools less efficiently than a mixture of antifreeze and water. Running straight antifreeze can raise metal operating temperatures inside an engine 70 to 80 degrees, which may be enough to cause galling problems in the exhaust valves. The maximum concentration of antifreeze should never exceed 65 percent.


Measuring the Mixture


The only way to know if the coolant is properly mixed is to measure the strength or concentration of antifreeze with a hydrometer, floating ball gauge, test strip or refractometer. Then, and only then, can you determine if more antifreeze or more water is needed. Simply opening the radiator cap and dumping in some extra antifreeze for added freezing protection may put too much antifreeze in the mixture. Exceeding an 80 percent concentration of antifreeze in the coolant can cause silicate gelling. The silicate corrosion inhibiting additives that are found in most "aluminum-safe" antifreezes can drop out of suspension, forming a gel or greenish goo that clogs the radiator and reduces heat transfer.
Even if the old coolant is drained out of the radiator and the system refilled with the proper 50/50 mix of antifreeze and water, the coolant may still not have the correct ratio. Up to a third of the old coolant can remain in the engine if only the radiator is drained. Unless this residual coolant is flushed out, it can dilute the fresh coolant as well as contaminate it with accumulated rust and sediment.
A thorough job of revitalizing the coolant, therefore, requires more than adding fresh antifreeze to the radiator or doing a simple drain and fill. It requires flushing the system to remove all traces of the old coolant, cleaning the cooling system if necessary to remove accumulated rust and scale, then refilling the cooling system with a properly balanced mixture of water and new or recycled antifreeze.


Chemical Breakdown


The "anti" part of antifreeze applies not only to freezing and boilover protection but also corrosion protection. A typical jug of antifreeze contains 96 percent ethylene glycol by weight, two percent corrosion inhibitors (silicates, phosphates and/or borates) and two percent water (a little water is necessary to help blend the inhibitors with the glycol).
Antifreeze formulated for aluminum engines typically contain a much higher concentration of silicates than that formulated for cast-iron engines. This can sometimes cause silica gelling problems if an antifreeze designed for passenger cars and light trucks ends up in a heavy-duty diesel truck engine. Heavy-duty trucks typically specify a low silicate formula antifreeze because they have mostly cast-iron engine parts and brass radiators. A supplemental additive that contains silicate is then added to the system to provide the required corrosion protection. But if there's already a high silicate antifreeze in the system, the coolant can become over saturated with silicate causing the silicate to react with itself and gel.
Silicate solubility also decreases as the ratio of antifreeze to water increases. In coolant that's accumulated a lot of miles, this isn't as much of a problem because much of the silicate will have been used up. But with fresh antifreeze and water, using too much antifreeze and not enough water may create conditions that are ripe for gelling.


Fighting Corrosion


The rate at which corrosion takes place inside a cooling system depends on a number of factors: the presence of minerals and other impurities in the coolant; the type of metals and alloys in the engine and radiator, and the acidity or alkalinity of the coolant itself.
Acidity and alkalinity is measured on a "pH" scale, where 7 is neutral, lower numbers represent increasing acidity and higher numbers increasing alkalinity (pH is chemist talk for the concentration of hydrogen ions in solution). Pure water is neutral with a pH of 7. Battery acid reads 2 or 3 on the pH scale, while baking soda might rate a 10 or 11.
Whether the coolant is acidic or alkaline makes a big difference. As long as it remains alkaline, corrosion is inhibited. But if it goes acidic, corrosion starts to eat away at the interior of the system. The corrosion-inhibiting additives in antifreeze are put there to keep the solution on the high side of the pH scale. The alkalinity of a typical antifreeze/water mixture will vary depending on the additives used and ratio of ingredients, but is usually somewhere between 8 and 11. The average for most antifreezes is around 10.5, but when diluted 50/50 with water and added to the cooling system the pH drops to the 8.5 to 9 range. Higher is not necessarily better, though, because some of the new long-life coolants have a pH of only 8.3. Staying power is what counts.
To ensure that the coolant remains alkaline for a reasonable length of time, there must be enough corrosion inhibitor to neutralize the acids formed from glycol degradation that occur over time. This neutralizing capability is called "reserve alkalinity," and it varies depending on the type and quantity of additives used in a particular brand of antifreeze.
Heat, dissolved oxygen, minerals in the water and corrosion inhibitor reactions at the metal surface gradually "use up" the corrosion inhibitors; once depleted, the coolant becomes acidic and corrosion accelerates. The trick to preventing internal corrosion, therefore, is to change the coolant before all the reserve alkalinity has been used up.
Under normal circumstances, conventional antifreeze usually contains enough reserve alkalinity and corrosion inhibitors to safely last two or three years. Replacing the antifreeze every two years or 30,000 miles for preventative maintenance, therefore, is usually recommended.
Periodic coolant changes are especially important with today's bi-metal engines and aluminum radiators because the different metals create a miniature battery cell that encourages electrolytic corrosion. Aluminum becomes the sacrificial anode, iron the cathode, with the coolant serving as the charge-carrying electrolyte. The higher the percentage of dissolved minerals and salts in the coolant, the better it conducts electricity and the faster the aluminum is eaten away. The corrosion inhibitors in quality antifreezes prevent this destructive electrolytic corrosion. But once the inhibitors are depleted, pinholes can form in radiators, cylinder heads can begin to weep coolant into the cylinders or crankcase, and water pumps and thermostat housings can start to leak.
Other problems can also accelerate the breakdown of the coolant. An exhaust leak into the cooling system through a cracked head or leaky gasket will quickly destroy reserve alkalinity because oxygen reacts with the additives in the antifreeze. If an engine has blown a head gasket, therefore, don't reuse the old antifreeze - replace it.
Using "hard" water can also shorten the life of the coolant. Tap water contains dissolved minerals that can react with and reduce the effectiveness of the corrosion inhibitors. Softened water contains fewer minerals but contains salts that can be just as bad. The best type of water to use, therefore, is pure distilled water. Distilled water is pH neutral, contains no acids, dissolved salts or minerals and will maximize the life of the coolant.


Antifreeze Analysis


There are a couple of ways to gauge the condition of the corrosion-inhibiting additives in the coolant. One is to use chemically treated test strips that change color to indicate the pH of the coolant. But all a litmus paper reading really tells you is whether or not the coolant is alkaline or acidic. It doesn't tell you how much reserve alkalinity or corrosion inhibitor the coolant has left. Nor does an alkaline pH reading always mean the coolant is still good. When aluminum starts to corrode, it can actually make the pH go back up!
Using pH to gauge the condition of the coolant is "iffy" at best because without knowing the specific additive package that was originally in the antifreeze, it's hard to tell what the pH reading actually means. So an alkaline reading may or may not mean the coolant is still good. A low pH reading (below 8), on the other hand, would generally indicate bad coolant and a need for a change. But be aware that Asian and the newer, long-life coolants can protect to lower pH (almost 7).
If it's been more than two or three years since your customer's coolantcoolant testers are available that measure the pH to determine its alkalinity. But test strips don't necessarily tell you how much life the coolant has left in it. But they can detect bad coolant. was last changed, however, chances are it may be nearing the end of its service life regardless of its pH reading. Electronic
You can also use an ordinary digital volt meter for the same purpose. With the engine off, touch the voltmeter positive test lead to the radiator or engine (making sure you get good metal-to-metal contact). Then open the radiator cap and insert the negative test lead into the coolant. A reading of up to 0.2 volts is considered acceptable and indicates the presence of reserve alkalinity in the coolant. If the coolant reads 0.3 to 0.6 volts, it is borderline and should be recycled or replaced. A reading of 0.7 volts or more would tell you the coolant is overdue for a change.
Internal corrosion in the cooling system can occur regardless of the condition of the coolant if voltage from various accessories (alternator, starter, ignition, etc.) flows through the coolant to ground rather than follows the intended ground path through the ground strap between the engine and chassis or the ground cable between the engine and battery. You can check for this condition by also using your DVOM. Use the same hookups as before to measure the voltage of the coolant, but this time while cranking the engine, then with the engine running and lights and heater on. If stray current is grounding through the coolant, you'll get a voltage reading. More than 0.15 volts can corrode aluminum, and 0.3 volts can be harmful to cast iron. Check, clean and tighten the ground straps and/or battery ground connection to eliminate the problem.


Coolant Additives


Can the corrosion inhibitors in the coolant be replenished or reconstituted by dumping a can of "rust inhibitor" or "coolant extender" in the radiator? Most vehicle manufacturers and antifreeze suppliers caution against the use of additives because it's a shotgun approach that runs a risk of upsetting the chemical balance in the system. Too much of a particular inhibitor can be just as bad as too little, causing sediment to precipitate out of solution and gum up the radiator. Short of running a laboratory chemical analysis on the coolant, there's no way to tell which inhibitors have been depleted, how much to add and whether or not the additives in the can will react adversely with those in the antifreeze.
The best advice is to replace or recycle the coolant according to the vehicle manufacturer's recommendations.

Import Additives


The additives used in European and Asian OEM antifreezes vary from those found in North American OEM and aftermarket antifreeze. European vehicle manufacturers specify an additive package that contains no phosphates and uses borates and low silicates. Their reason for doing so is because some areas of Europe have very hard water, which can react with phosphates to form calcium and magnesium sediments. The Japanese and Asian vehicle manufacturers, on the other hand, use phosphates but no borates and low or no silicates. They don't want borates in the system because they believe borates can corrode aluminum if the coolant is neglected for too long. Using an antifreeze that does not meet these requirements, therefore, may void the vehicle manufacturer's warranty.
Even so, topping off European or Asian cooling systems with a typical North American antifreeze (which contains silicates, phosphates and borates) should cause no problems. And once a vehicle is out of warranty, it should make no difference whatsoever what type of antifreeze is used in the cooling system, as long as it provides adequate corrosion protection.
There also are differences in the additive packages between various brands of aftermarket antifreeze. Some are better than others, so don't think for a moment that antifreeze is a "generic" product. There are differences. Some no-name, low-cost antifreeze products, for instance, can be straight ethylene glycol and contain no corrosion inhibitors or lubricants at all!


Coolant Recycling


Coolant recycling is growing in popularity because it eliminates most of the environmental concerns over coolant disposal, allows a valuable resource to be recovered and reused and combines flushing, recycling and refilling into one operation.
General Motors, Ford, Chrysler and most of the import manufacturers have endorsed coolant recycling provided the recycled coolant meets their quality specifications or the American Society of Testing Materials (ASTM) 3306 standard. General Motors, for example, says recycled coolant must meet their GM 1825-M standard. Most vehicle manufacturers list specific machines that have been tested and approved as meeting their requirements.
Coolant recycling machines use a variety of means to clean and replenish the coolant, including filtering out solids, chemically precipitating dissolved salts and other contaminants, de-ionization, reverse osmosis and distillation. Some processes are much more thorough than others, so it's important to make sure the equipment meets the required standards.
Once the coolant has been cleaned and decontaminated, corrosion inhibitors are added to it to replace the depleted additives. Some recycling equipment require separate additive packages for European, Asian and North American coolants, while others use the same basic additive package for all applications.
The economics of recycling coolant are debatable because the return on investment depends on the initial cost of the equipment, the cost of chemicals and other consumables (such as filters), periodic maintenance that may be required and the volume of work performed.


Long-Life Coolants


The problem with conventional antifreeze is that silicate and phosphate corrosion inhibitors are consumed in performing their job. After a couple years of service, the chemicals that keep the coolant alkaline are mostly used up, leaving little or no protection against electrolytic attack on the metals in the engine and radiator. Changing the coolant for preventative maintenance every two to three years or 30,000 miles can minimize such risks, but getting people to do so isn't easy. In response to this neglect, "long-life" formula coolants have been introduced.
The new long-life antifreezes still contain ethylene glycol as their main ingredient, but use different additives to extend coolant life to five years or 100,000 miles. These types of products are ideal for customers who tend to neglect preventative maintenance, yet want maximum protection for their vehicle's cooling system.
The vehicle manufacturers have taken note of the new long-life coolants, and some have started using them as factory-fill coolants to reduce maintenance. Saturn was one of the first domestic vehicle manufacturers to do so, going to a "European" long-life borate additive package that extended the service interval to three years or 36,000 miles.
The biggest improvement, however, came with the '96 model year when General Motors began factory filling all their cars and light trucks with "Dex-Cool" made by Texaco and marketed under the Havoline "Extended Life" brand name.
Dex-Cool has a service life of five years or 100,000 miles and is dyed orange so it can be easily distinguished from ordinary green antifreeze. It is a conventional ethylene glycol antifreeze but contains a unique corrosion inhibiting chemistry that uses carboxylate organic acids instead of the silicates, phosphates or borates.
GM says getting rid of traditional silicates helps to extend the life of the water pump. They believe that the microscopic particles of silicate in conventional antifreeze have an abrasive effect on water pump shaft seals that contribute to seal wear and leakage over time. The protective coating that's formed by Dex-Cool's organic acids on metal surfaces in the cooling system is also thinner than that formed by silicates, which initially improves heat transfer and cooling efficiency slightly.
GM says Dex-Cool should not be intermixed with ordinary antifreeze because doing so reduces the service life of the coolant. What's more, if ordinary antifreeze is used to top off a new vehicle with less than 3,000 miles on it, the additives in the conventional coolant can interfere with Dex-Cool's ability to form a protective layer on aluminum surfaces. This can allow the unprotected metal to corrode. So if the coolant has been contaminated, it should be completely drained or flushed and refilled with Dex-Cool. Unfortunately, it isn't easy to tell if Dex-Cool has been "contaminated" by ordinary antifreeze because the orange dye in Dex-Cool tends to mask the green dye in regular antifreeze. As much as 25 percent ordinary antifreeze can be added to the system without causing any noticeable change in the color of the coolant.
Once a vehicle has more than 3,000 miles on it, contamination with ordinary coolant should cause no short-term corrosion problems, but the long-term effectiveness of Dex-Cool will be reduced to that of ordinary antifreeze (two years or 30,000 miles). Bottom line: If a customer with a '96 or '97 GM product needs antifreeze, be sure to use Dex-Cool.
Note: To prevent intermixing of Dex-Cool with conventional antifreeze, it's important that all old antifreeze is completely drained from the cooling system. This requires flushing the system or using a flush and fill machine to exchange coolants.


The "Other" Antifreeze


Another type of antifreeze that's appeared in recent years is one containing propylene glycol (PG) rather than ethylene glycol (EG). Safe Brands/Arco Chemical markets a PG-based antifreeze under the "Sierra" brand name and Prestone under the "Low Tox" name as a safer, less toxic coolant. PG-based antifreeze is not safe to drink, but is significantly less toxic than EG. It also has an unpleasant taste, which reduces the risk of accidental poisoning of pets and small children.
In terms of freezing and boiling protection, PG performs about the same as EG. PG provides freezing protection down to minus 26 degrees in a 50/50 mixture, and boilover protection to 257 degrees (these are a few degrees less on both counts than EG, but can be offset by simply increasing the percentage of PG in the coolant). A 60/40 mixture of PG and water will protect down to minus 54 degrees F.
Though some auto makers initially said they did not approve of using PG antifreeze in their vehicles, GM said it does approve PG as an acceptable replacement coolant.
If you're adding PG to a customer's vehicle, you must first remove all ethylene glycol from the cooling system by flushing or completely draining all the old coolant. PG should not be mixed with EG because the freezing and boiling points are different, making it impossible to determine accurately how much freezing/boiling protection is in the coolant. An ordinary antifreeze tester cannot measure the concentration of PG in the coolant, so you need a second hydrometer, test strip or refractometer that's specially calibrated to read PG.

:banana:

Very informative, but my eyeballs gave out on me before I could fininsh this article.