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AUTO TECH: Corrosion: silent killer of vehicles

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While antifreeze coolants do protect the engine from freezing in cold weather, their ability to provide heat transfer and raise the boiling temperature in hot weather are just as important to engine durability.

With our busy lives, it's easy to overlook some vehicle maintenance items, such as changing the engine coolant. If it's still working, everything must be OK, right? Wrong. Corrosion is a silent killer of vehicles. By the time we can see cooling-system corrosion, costly repairs are often needed.

A cooling system is a complex assembly of components and materials. Aluminum, cast iron, steel, stainless steel, copper, brass, rubber, plastic -- an engine coolant must be compatible with all of them. The coolant must also transfer heat efficiently, stop corrosion and provide lubricant for water-pump seals.

This task doesn't sound too hard unless you realize that any time two dissimilar metals (such as different aluminum alloys) are placed in contact with an acidic liquid (used antifreeze), you have a crude battery. Electricity generated by the cooling system causes accelerated corrosion of the materials. Changing the coolant when required will prevent excessive acidic build-up in the system and help prevent corrosion.

Three types of antifreeze are commonly used in light-duty vehicles. Ethylene glycol has a sweet taste, but even a small amount can be fatal to small animals and children so care must be used when storing or draining it.

Propylene antifreeze is less toxic and has a slightly bitter taste, so it's less attractive to animals, but care should still be taken to store it safely. Both ethylene glycol and propylene glycol contain silicates, phosphates and/or borates as corrosion inhibitors to keep the coolant solution alkaline. These antifreezes are typically green in colour.

A third type of antifreeze is also ethylene glycol-based, but contains organic acids that protect the engine from corrosion. General Motors started using this antifreeze under the Dexcool name in 1996. It has a five-year/160,000 km lifetime versus the two-year life of most other antifreezes. This "long-life" antifreeze is orange in colour, but don't be fooled by orange or red coolants in non-GM vehicles. Check the owner's manual to see if it should be changed at five-year or two-year intervals.

Don't mix long-life and regular antifreeze together or the life of the antifreeze will be shortened to two years. If you have to add antifreeze, mix it with water in the ratio shown on the container. Some come pre-mixed, but most recommend a 50/50 mixture (some cheaper ones are 60/40).

A low coolant level indicates a leak. If there are no visible stains or corrosion on the outside of the engine or components, the coolant could be leaking internally into the engine oil. This can damage engine bearings quickly, so have it checked out as soon as possible by a mechanic.

The most environmentally friendly way to change antifreeze is to take the vehicle to your local repair shop. Most shops now use antifreeze recycling machines to flush the cooling system, clean your old coolant and fill the vehicle with rejuvenated coolant.

During a cooling-system flush, the old coolant is removed from the vehicle into the recycling machine, and then water is forced throughout the system and drained. Next, the old coolant in the machine is filtered and tested for its strength (freezing point) and its pH balance. New antifreeze is added if necessary to increase the coolant's strength.

An additive is used to balance the pH level of the coolant so it's no longer corrosive and a chemical package containing extra corrosion-protecting materials and water-pump lubricant is added. Then the recycled coolant is pumped back into your cooling system.

While antifreeze coolants do protect the engine from freezing in cold weather, their ability to provide heat transfer and raise the boiling temperature in hot weather are just as important to engine durability. Compared to water, with its 100 C boiling point, a 50/50 mixture of antifreeze and water has a boiling point near 130 C.

Clean, quality antifreeze mixed in the correct proportions with water will protect your cooling system from corrosion and help protect your engine from overheating. So don't overlook vehicle maintenance that is at the bottom of your "to do" list.


Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists' Association of Canada.