Page 493 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 7 Central heating systems

2 the formation of black oxide sludge and sediment because of electrolytic
corrosion; black oxide sludge can be prevented by the use of chemicals called
‘inhibitors’, which stop the sludge forming, or by the use of magnetic filters
that use magnetism to attract the metallic black oxide sludge.
Corrosion can attack a system very quickly. As soon as the system is filled with
water, corrosion begins to work to break down certain elements within it.
Air infiltration is a constant problem with some systems, especially those
that are open vented. Central heating systems last longer once the water in
the system has lost all of its oxygen. Without oxygen, rust cannot occur. Air
infiltration happens for a number of reasons:
l micro leaks that let air in but do not show as a water leak; these are
extremely hard to trace and usually occur around the packing glands of
lockshield radiator valves and air release valves; they always occur on the
negative pressure side of any system
l air being sucked down the vent pipe due to poor system design
l the constant see-sawing of water within the F and E cistern aerates the
water
l small leaks introduce fresh aerated water into the system.

Electrolytic corrosion
Within central heating systems, there are a number of metals; steel radiators,
brass valves (brass contains zinc), copper tubes and stainless steel heat
exchangers. On older systems there may also be cast iron boilers or parts
containing aluminium. All of these metals lie at different points on the
electromotive series of metals (see Chapter 3, Scientific principles, page 157)
and, once they are connected via water (an electrolyte), corrosion begins
immediately. This problem is accelerated when the water becomes hot. The
net result of this reaction is that the steel of the radiators begins to be eaten
away, with the fine particles of steel falling to the bottom of the radiator
as a sediment, which forms a magnetic black sludge. As a by-product, the
radiator may also fill with hydrogen that requires constant venting. The
sludge not only blocks pipework and finds its way into all of the low points
of the system, but also causes boiler noise and creates pitting corrosion in
the radiators. Figure 7.54 shows some of the problems that can result from
system sludging.

Corrosion inhibitor
Corrosion inhibitor must be added to the system to comply with the
manufacturer’s warranty. Corrosion inhibitor stops corrosion from occurring
and black sludge from forming, and helps to lubricate pump bearings and valves.
Once added, corrosion inhibitor does not need to be replaced except when the
system is drained down.

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