Page 303 - Failure Analysis Case Studies II

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Vented
system
,

0 50 IO0
T ("C)
tig. 6. Rate of corrosion of mild steel in aerated mains water as a function of temperature.

suffering from this defect [4]. To avoid air intake, the static head h must be greater than the flow
resistance of the boiler by a suitable safety margin; a typical value for h is 1.5m.
In order to guarantee that oxygen is excluded from the system, the trend in most modern
installations has been to replace the traditional open expansion tank with a sealed air cushion
expansion tank. The system water expands into a polymeric bag which is contained in an outer tank
of pressurized air.

2.2. Corrosion rates
When the water is saturated with oxygen, the steel will corrode rapidly. The rate of corrosion is
controlled by the oxygen-reduction reaction, which is in turn controlled by the rate at which oxygen
diffuses through the water to the surface of the steel. Because diffusion is a thermally-activated
process, the rate of diffusion will increase exponentially with temperature. So also will the corrosion
rate, as shown by the rising curve in Fig. 6. As one approaches 80°C, the curve divides into two
branches [3, 51. If the system is pressurized (so none of the dissolved oxygen can escape), the curve
carries on upwards. But if the system is vented (as most central heating systems are), the oxygen
can come out of solution as the water warms up. As the oxygen concentration falls below 3 mg I-',
the oxygen-reduction reaction slows considerably, and at 100 "C the steel is corroding no faster than
it would have done in aerated water at 25 'C.

2.3. Diagnosis
It is relatively easy to find out whether excessive oxygen has been getting into a heating system.
When oxygen is plentiful, the iron corrodes rapidly at a relatively high potential. The reaction takes
place in the Fe203 field of the Pourbaix diagram (see Fig. 3), and a red sludge of hydrated ferric
oxide collects at the bottom of the radiators. When oxygen is scarce, the iron corrodes slowly at a
relatively low potential. The reaction generates ferrous ions, and a thin black coating of hydrated
magnetite (magnetic iron oxide) forms instead. Both oxides are 5 times as dense as water: although
oxide particles can be carried around with the circulating water, most of the sludge remains in the
radiators. Contrary to the view commonly held by many heating engineers, oxide sludge cannot be
removed by flushing water through the radiators (although it can be removed by circulating a warm
solution of inhibited phosphoric acid through the system).

3. OXYGEN REDUCTION-SOME COMPLICATIONS

There are several things which can make steel corrode faster in the presence of oxygen. These are
summarized as follows.