The heat exchanger is a vital part of your home's furnace. It conducts the heat from the burner chamber to the blower so that the warm air can be spread around your home. Corrosion can lead to cracks, which will destroy the heat exchanger.
There are two main causes when it comes to rust and corrosion on the exchanger: condensation and vibration.
Condensation will most likely damage the outside surfaces of the tube bundles that make up the main body of the furnace heat exchanger. Moisture develops inside the area due to condensation from the exchange of hot air into what was previously a cool environment. As the air heats, more moisture condensates out and settles on the tubes. This type of corrosion is often highly visible because it is on the exterior of the heat exchanger. You will see rust stains and possibly even holes that have been eaten into the metal from the rust and condensation.
Vibration causes an issue called fretting. It is most likely to affect the heat exchanger at the point where the tubes that make up the exchanger meet the baffle that forms the barrier around the exchanger. Vibration weakens the metal while also allowing friction damage as the two metals move together. Over time, this results in the formation of corrosion on the outer or inner surface of the baffle tubes, where it is not as visible during a casual inspection. Often, this type of corrosion is discovered by lightly tapping the baffle tubes with a hammer. If the tubes are quite shaky from the impact, then they are weakening and there is likely corrosion occurring.
Prevention is more difficult, as part of the causes are due to conditions created by the normal operation of the furnace.
Impingement plates are small circular metal devices that are designed to minimize condensation and water impingement into the heat exchanger. Not only does the plate block moisture from seeping between the exchanger tubes, but it is also a sacrificial device that will rust before the rest of the exchanger is affected by the moisture. Impingement plates should be installed on your heat exchanger, as well as inspected and replaced as necessary.
The single best way to prevent corrosion from destroying your heat exchanger is to schedule an annual furnace inspection and tune-up. The inspection will catch loose parts early so they can be tightened to minimize vibration. Corrosion and other issues can also be discovered before the damage becomes irreversible so that repairs and adjustments can be made to extend the life of the heat exchanger.
Contact a heating service in your area for more information.