Page 209 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION PROBLEMS IN HOME APPLIANCES 187

The heating coils in water heaters are exposed to the water in the heater tank.
Common water contaminants such as chlorides, fluorides, and sulfites can cause cor-
rosion of the heating coils, the water heater connections, the tank wall, and the tank
frame. The elevated temperature of the water in the heater is further likely to increase
the internal corrosion rate.
Boilers are heat exchangers constructed of carbon steel to produce hot water or
steam by being heated with an oil or gas burner. The hot water or steam is transferred
to radiators to provide heat. After releasing heat, the cool water or steam condensate
is returned to the boiler for reheating.
A common problem in boilers is the occurrence of calcium oxide build-up on the
heating elements. This is not a corrosion problem in itself, because it is caused by a
chemical reaction in the water at high temperatures. However, a scale deposit present
on a metal surface may cause corrosion under the deposit. This type of underdeposit
corrosion can be aggravated when corrosive species such as sulfides and/or chlorides
are present in the water. While scale deposits reduce the thermal conductivity of the
steel, and thereby increase energy costs, corrosion of the heating element can lead to
a catastrophic tubing failure, which requires costly repairs.

3.30.1 High-Efficiency Furnaces

Corrosion can occur in furnaces when condensation occurs, which can corrode
the internal metal surfaces. Condensation is a problem in high-efficiency furnaces,
because operating at high efficiencies means that the appliance must operate in
a condensing mode. At present, these furnaces are designed with a maximum
annual flue utilization efficiency (AFUE) of 90% compared to a standard minimum
efficiency of 78%. To operate a high-efficiency furnace, the flue gas must be cooled
to a temperature below the dew point, by which the combustion-generated moisture
is condensed in the heat exchanger, and the latent heat of vaporization is recovered
for utilization.
Research in the mid-1980s on the corrosion of materials used in condensing heat
exchangers in furnaces indicated that the greatest probability of corrosion occurs
when the appliance goes through the transition from wet to dry conditions. This is
because the acidity of condensate increases as the water evaporates. The flue gas
generated is a mildly acidic liquid that is corrosive to type 304 and 316 stainless
steels commonly used in heat exchanger furnaces. The corrosivity of the condensate
can increase because of airborne contaminants in particular chlorine-bearing com-
pounds, present in indoor environments, and carried into the burner by the combustion
process.

3.30.2 Air Conditioners
Aluminum and copper are the materials in air conditioners. Coils and cooling fins are
made from aluminum, and piping is usually made of copper. Aluminum is susceptible
to galvanic corrosion when in contact with copper components. Galvanic corrosion
can occur when two dissimilar metals are in electrical contact in an electrolyte.