Auto Service World
Feature   August 1, 2011   by Mike Farrell, Curriculum Developer CARS

The Invisible Enemy

Summer is here and people are traveling to vacation destinations. With the hotter temperatures, trailer towing and air conditioning use, the vehicle’s cooling system becomes taxed. This could ultimately lead to overheating or a cooling...

Summer is here and people are traveling to vacation destinations. With the hotter temperatures, trailer towing and air conditioning use, the vehicle’s cooling system becomes taxed. This could ultimately lead to overheating or a cooling system component failure.
The introduction of long-life coolants, extended oil and fluid change intervals means that vehicles are not in for service as often. The extended life coolants and the extensive use of aluminum in the radiators, heater cores and engine components can lead to internal corrosion. This condition is more evident as the coolant ages and becomes acidic. The acidic environment creates salts from the metals and minerals in the water. This increases the coolant’s voltage conductivity, allowing it to develop an electric charge—one cause of corrosion.
There are two different types of corrosion in the cooling system, natural and electrochemical. An example of natural corrosion would be the rotting of the radiator fins. Cooling system electrochemical corrosion has two forms, galvanic and electrolysis. Galvanic corrosion is caused by the chemical reaction between dissimilar metals and does not require an outside voltage source. Electrolysis, on the other hand, requires an outside potential voltage source. The higher the conductivity of the coolant the greater the amount of electrochemical corrosion will be evident. If the vehicle’s negative ground connections are loose, corroded or missing, the electrolysis will be more severe.
The electrical charge in the cooling system results from several different sources. Static electricity is created by the friction of the coolant and water solution against two dissimilar metal surfaces (cast iron and aluminum). The engine block on many of today’s vehicles is used as the electrical negative circuit for many vehicle systems. This causes electrical current to flow through it. The water in the coolant solution captures the voltage charge. The engine management system with all the sensors, computers and fuel injectors also contributes to the cooling systems becoming electrically charged.
As the corrosion inhibitors in the coolant wear out they can no longer protect the cooling system components. As the coolant changes from an alkaline solution to an acidic solution, electrolysis begins to corrode engine and cooling system components. Long Life Organic Acid Technology (OAT) and Hybrid Organic Acid Technology (HOAT) coolants should be changed at the recommended manufacturer’s intervals, the general rule of thumb being five years or 160,000 km. (100,000 miles) whichever comes first.
If the customer is unsure of the coolant’s age, begin by testing the pH level of the coolant. Test strips are readily available. The pH scale ranges from 0 to 14 with any number below 7 being acidic and any number above 7 alkaline. Regular ethylene glycol coolant has a pH range of between 9.0 and 10.5 and the long life coolants (OAT and HOAT) have a range of between 7.5 and 8.5. If the pH level for OAT and HOAT coolants drops below 7.0, synthetic materials often used for intake manifolds, thermostat housings and radiator tanks will begin to deteriorate. If the pH reading is above 9.0, the solder and aluminum parts can be affected.
To test for galvanic corrosion insert the positive end of a digital multimeter (DDM) into the coolant at the radiator cap and then attach the negative lead to the battery negative post. The initial voltage reading should be ignored because the coolant solution will begin to react to the test lead. Wait several minutes before taking the reading. If the voltage reading is under 200 mV galvanic activities are not severe.
When testing for electrolysis, leave the positive lead in the coolant but move the negative lead to the engine ground or the alternator housing. Start the engine and allow the idle to stabilize. Take an initial reading with no accessories on. If the reading stays below 300 mV begin to turn on components that will cause a draw on the electrical system such as the headlights or air conditioning system. So long as the voltage reading stays below 300 mV there is no problem with electrolysis. A normal reading for both galvanic and electrolysis corrosion is 50-70 mV.
The correction for electrochemical corrosion is coolant replacement. Once the electrical charge is present in the water it cannot be removed. The entire cooling system must be flushed, not just drained and refilled. After completing the cooling system flush recheck for electrochemical corrosion. If the readings are still above the 50-70 mV mark flush the cooling system again but use distilled water to mix with the coolant. Distilled water does not contain minerals and salts that assist the water to conduct and carry an electrical charge.
By checking and verifying the pH and electrical charge in the cooling system after a repair, the potential for a comeback can be reduced.
For more information on automotive technology visit CARS OnDemand training at:
If you have completed any of the Advanced Technology CARS OnDemand courses, CARS would appreciate you taking the time to complete a short 3 minute survey. To access the survey please go to the CARS website, log-in and under “Account Details” choose “Participants Surveys” — “Advanced Technologies Student Survey.” Your feedback is important to us.

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