Page 305 - Failure Analysis Case Studies II

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Pit Mains water
Fig. 7. The Pourbaix diagram for iron at 25 "C, showing the hydrogen-reduction reaction

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the condition of the surface, and the nature of the environment. Polarizations of ~0.24.3 are
common when hydrogen is reduced on the surface of iron. As shown in Fig. 7, the practical line for
the hydrogen-reduction reaction probably lies below the corrosion potential of iron in mains water.
In theory, therefore, steel radiators should not corrode by the hydrogen-reduction reaction.
In spite of this, hydrogen is often given off in central heating systems. The usual symptom is that
one or more of the radiators suddenly goes cold for no apparent reason. This is because hydrogen
gas has collected at the top of the radiator, and has blanketed the top ends of the vertical waterways.
Heating engineers often claim that this is caused by dissolved air coming out of solution. In fact,
when the gas is vented off it can usually be lit with a match! Provided there is no risk of explosion,
having to vent radiators in this way is just a nuisance. However, the corrosion process produces
magnetite, which can deposit in the waterways and clog the pumps.
Why does this hydrogen reduction occur? If the temperature is high enough, a reaction called the
Schikorr reaction can take place [4, 71. This has two steps as follows.
Fe + 2H,O = Fe(OH), + HZ, (2)
3Fe(OH), = Fe304 + 2H,O + H,. (3)
The Schikorr reaction is catalysed by copper ions: when these are present, it can take place above
60°C. In systems where the water is aggressive to copper, the concentration of Cu2+ which is
released into the circulating water can be large enough to provide the required catalyst. A survey
carried out on one central heating system gave the results shown in Fig. 8. Below 63 "C, there was
no detectable evolution of hydrogen. However, at 70 "C, the radiators had to be vented every week;
and at 80 "C they had to be vented every day. An obvious way of suppressing the reaction, therefore,
is to keep the temperature in the system below 60 "C.

4.1. Hydrogen and bacterial corrosion
Heating systems can become contaminated by bacteria [3,8]. The most common are the anaerobic
sulphate-reducing bacteria (SRB) such as Desulfovibrio which live in oxygen-starved conditions. As
part of the metabolic cycle, the organisms convert sulphate ions into sulphide ions. Sulphide greatly
speeds up the hydrogen-reduction reaction, and this allows steel to corrode even in neutral oxygen-
free solutions. Desulfooibrio grows in the pH range 5-10, and the temperature range 5-50 "C. Some
SRB can survive to even higher temperatures. If aerobic bacteria are present, these can colonize
deposits and produce anaerobic conditions under the deposit where SRB can thrive under nominally
aerobic conditions. The bacteria require nutrients, which are usually present in the form of hydro-