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3 . Corrosion
3.1 Deposit corrosion
Deposit corrosion is an electro-chemical phenomenon
caused by the presence of foreign bodies (sand, rust,
etc.) in the water mass. These solid substances generally
form deposits (sludge) in the bottom of the boiler, in the
tube heads and in the joins of tube bundles.
These parts of the boiler can therefore be affected by
micro-corrosion caused by the electrochemical poten-
tial difference created between the metal parts in con-
tact with impurities and other metal parts around them.
3.2 Stray current corrosion
Stray current corrosion is caused by the different electri-
cal potentials of the water in the boiler and the metal
body of the boiler or piping. Stray current corrosion is
easily identified by the regular tiny conical holes it le-
aves.
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All metallic parts should be grounded by an effi-
cient earth cable for this reason.
4 . Eliminating air and gas from central heating sy-
stems .
If oxygen enters a circuit continuously or even intermit-
tently (e.g. in under-floor heating systems whose pipes
are not protected by impermeable synthetic sheaths, in
circuits with open expansion vessels, or in circuits that
require frequent top-ups) always separate the boiler's
water circuit from the central heating circuit.
Mistakes to avoid and precautions.
From what has been said above we can see that it is
essential to avoid two main factors that could lead to
corrosion. These are contact between system water and
air and the continuous addition of fresh water to the
circuit.
To eliminate contact between system water and the air
(thus avoiding oxygenation of the system water), we
need to:
− ensure that the expansion vessel is a closed ves-
sel, and of the correct size and pre-charge pres-
sure (the pressure to be checked periodically);
− ensure that the system is always kept at a pres-
sure higher than atmospheric pressure at all
points (including the pump suction side) and
under all operating conditions (precisely because
the seals, gaskets and joins in a water circuit are
designed to resist pressure from within, but not
to resist a vacuum within);
− ensure that no part of the system is made from
materials that are permeable to gases (e.g. pla-
stic pipes with no oxygen barrier destined for
under-floor heating systems).
9
Finally, never forget that boiler failures caused by
encrustation and corrosion are not covered under
the terms of the warranty .
25
INSTALLER
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