By John Smith
It almost defies logic that a mighty diesel engine could be destroyed by something as fleeting and fragile as a bubble. But when that bubble is in coolant and it is formed in the cylinder liner, over time, the results can be devastating. It is called cavitation corrosion and it represents one of the reasons that today’s coolant formulas are so important.
The cavitation challenge can be traced to the combustion process, where the pounding motion of a piston will cause a cylinder to vibrate, generating bubbles in the coolant that flows around the cylinder liner. These bubbles can implode against the liner with enough force to pit the metal, eventually chewing their way into the combustion chamber.
Effective chemistry in today’s extended-life coolants (ELC) – including nitrites or nitrite-molybdates – can attack the specific location of any weakness in the cylinder liner’s protective coating.
“There still is a prevalent amount of conventional coolant out there,” according to Carmen Ulabarro, direct channel manager, Chevron Products. But the ELC products are approaching 50 percent of the marketplace, if they have not reached that point already, she says. That is due in no small part to the fact that the reservoirs in new trucks are filled with the advanced chemistry as soon as equipment rolls off the assembly line.
The trucking industry was introduced to these formulas in 1996, shortly after General Motors introduced the Dexcool extended-life coolants, which marked the beginning of the end for annual flush and fills in the automotive world. Caterpillar responded by releasing the standards for an ELC that could be used in a heavy-duty application.
That introduced a fundamental shift in the chemistry that could be poured into a reservoir.
“Most conventional coolants work by laying down a layer that forms a barrier between the metal surface and the coolant,” Ulabarro explains. Since water and glycol are extremely corrosive, additives are needed to provide extra protection.
But there is a downside.
“As the additives plate out to protect against corrosion, deposits are formed, the ability to transfer heat is decreased and the efficiency of the radiator decreases as well,” says Stede Granger, OEM technical service manager at Shell Lubricants. In contrast, an ELC will not make these deposits.
Still, the industry has yet to be able to produce one bottle that truly fits all, since each engine maker is looking for the chemical fixes that address specific needs.
An engine maker that speeds up a water pump is more likely to need cavitation protection, while the designers of a power unit with a larger cooling system won’t worry about this as much. Coolants also need to be compatible with any plastics, metals and seals that exist. Cummins, for example, needed to upgrade the gaskets on its ISX engine before an ELC formula could be used, Granger says.
Detroit Diesel and Mercedes-Benz engineers also have been balking at the use of nitrites since 1996 because nitrites can be aggressive to aluminum, and European regulators see the chemicals as carcinogens.
Recent service bulletins have eased the restrictions, allowing nitrated formulas in the Series 60 and DD15 engines.
“The only way nitrite can form a carcinogen is if there are amines in the coolant, and there are no amines in the coolant,” Ulabarro adds. (Ironically, nitrites are allowed in everything from sausages to wine.)
The users of test strips should be happy. One of the primary tests used to check the effectiveness of an ELC is a measurement of the nitrite or nitrite-molybdate. Without these tests, the strips would largely be limited to measuring the freeze point that reflects the ratio of glycol and water. Granted, some test strips still consider the presence of molybdates to be “contaminants” within the coolant, and still will recommend a flush if they appear.
The chemistry also has needed to evolve, addressing the changing emission requirements that introduced exhaust gas recirculation systems and the dramatic increases in temperatures that came with them. While coolant temperatures have increased by as little as 5° F to 10° F, hot spots still remain.
The high temperatures lead to a number of challenges. The glycol itself can oxidize and generate acidic and corrosive glycolates and acetates. Coupled with high flow rates and pressure drops in the cooling system, nitrites can even transform into ammonia that can corrode the soft metal in aluminum air coolers and radiators.
It hasn’t been the first time that the coolants have needed to change. The push against phosphates emerged because of the widespread availability of hard water, which can lead to the scale that affects the transfer of heat.
So far, ELCs have been up to the challenge. Some formulas now promise to last 600,000 miles or more, and in selected cases don’t even need the addition of “extenders” mid-way through their life.
Given the differences in chemistry, it is important to ensure that you are recommending the right coolants to your customers, and a quick look at the label on the container will identify the difference. While automotive coolants meet ASTM standard D-3306, the formulas for the trucking industry follow D-6210.
There also may be a reference to the truck-specific formula passing the standards outlined in the American Trucking Associations’ Technology & Maintenance Council Recommended Practice 329, although that standard is being updated.
Those who use conventional coolants also will need to monitor the supplemental coolant additives that meet specific needs.
The chemical balance is quite important. Sodium silicate may fight corrosion on aluminum surfaces, for example, but an excessive dose will form a “green goo” that can plug various passages in the cooling systems. A heavy dose of nitrites (above 3,000 ppm) will corrode solder. And a high level of dissolved solids will damage water pump seals.
“I have seen thermostats that are totally coated with an excess of inhibitors,” Ulabarro adds.
Does that mean service providers are maintaining the perfect chemical balance? Not always. Many shops will choose the middle ground of a four-unit or eight-unit inhibitor package, regardless of the readings on a test strip, she says.
Of course, when there is more than one formula on the market, there’s always the danger that conventional coolant will be used to top off a reservoir full of ELC. But there is no need to panic when this happens.
Traditional silicated coolants and their extended-life counterparts are chemically compatible. “When you mix the two, no chemical reaction takes place,” Granger says.
However, some of the added protection from corrosion will be sacrificed in the process. Shell even offers a conversion fluid that transforms a conventional coolant into an ELC, offering a more affordable solution to some fleet customers.
But Ulabarro also suggests there is little hope of one formula that meets every need. Talk of a “universal” coolant is simply a case of marketing managers glossing over technical issues, she says. “When an OEM prescribes an additive technology, they really are being specific to that particular engine.”
Each additive in a conventional coolant is designed to protect specific metals, adds Granger, referring to the way tolytriazols guard yellow metals like copper, silicates protect aluminum and nitrites protect iron.
There is a sign that glycol may give way to a more environmentally friendly option, however. Cummins Filtration recently announced the pre-diluted Fleetguard ES Compleat Glycerin, which uses a glycerin by-product of biodiesel instead of traditional ethylene glycol or propylene glycol. And it still meets the requirements of performance specifications such as the Cummins CES 14603.
“We take environmental responsibility very seriously at Cummins Filtration. Being able to support sustainability with a glycerin coolant that also offers excellent protection is very important to our mission,” says Mike Sarris, director of global coolants and chemicals for Cummins Filtration.
“We have long supported the importance of greener solutions through all our product development, and we are very pleased to introduce our new environmentally friendly glycerin coolant to the marketplace.”
Ulabarro admits that glycol has limits. “The higher the temperatures, the more the inhibitors get consumed,” she says. “At higher temperatures, your base fluid starts to break down. Glycol breaks down. The limiting factor in a coolant as far as temperature is concerned is the glycol.”
In contrast, a base fluid such as PDO tends not to oxidize.
The biggest challenge might include the addition of another fluid in the mix. Since it isn’t widely available, there is always a chance that someone will top off the radiator with a traditional formula.
“Everybody is on the green train until it hits your wallet,” Ulabarro adds.
