Dave Plowman (News) wrote:

This is a brief summary of the accumulated knowledge/ignorance
(delete as you think appropriate) I have acquired in the course of 20+
years in building services. I'm not a chemist, there's a lot more
to corrosion science, but this is adequate for most purposes.


There are 2 main varieties of corrosion which affect heating systems;
dissolved oxygen and galvanic corrosion.

DISSOLVED OXYGEN
Dissolved oxygen will corrode the inner surface of radiators and
reduce them to black, stick magnetite sludge. The oxygen can get into
the water by a variety of routes, e.g., the exposed water surface in a
feed & expansion tank, or leaks through valve glands caused by the low
partial pressure of the oxygen in the water, mentioned by AG above. A
water leak will introduce dissolved oxygen in the fresh make-up water.
fresh water with dissolved oxygen. 'Pumping over' will introduce
large quantities of oxygen into the water and will ruin the system
within a few years.

Plastic pipes are permeable to oxygen and some oxygen will enter the
system through the pipe walls. This is it relevant with car cooling
systems, which generally have a lot of flexible, non-barrier hoses.
Pipes with an oxygen barrier will reduce the permeability of the pipe
walls to a negligible level, but they do completely stop the oxygen
absorption.

The inhibitor solution deals with the dissolved oxygen by having an
oxygen scavenger component. The scavenger reacts with the oxygen before
the oxygen reacts with the steel radiators. The scavenger can be a
number of chemicals, typically sodium sulphite. The sulphite should be
maintained at nlt 20 to 50 ppm; titration teat kits are available to
monitor the concentration.
O2 + 2SO3= 2SO4

The rate of the sulphite depletion indicates the rate at which oxygen
is being absorbed


GALVANIC CORROSION

Galvanic/ bi-mettalic corrosion normally takes place between the copper
pipes and the steel radiators. A corrosion cell will exist between any
two metals that are electrically connected and immersed in a suitable
electrolyte. A voltage will be created between the two metals and ions
will be stripped from the anode, the less-noble of the two metals, like
electro-plating in reverse. Hydrogen is generated at the anode (I
think), the steel radiator in this instance. The hydrogen is insoluble
and can be identified by collecting it in an upturned glass and
igniting it. The oxygen is dissolved and attacks the radiators, see
above.

The galvanic corrosion is dealt with by ensuring that the
electrolyte/water is alkaline pH7. Typically this is done by adding
caustic soda to neutralise any acids and adjust the pH to 8 or 9.

The rate of galvanic corrosion will be greatly increased if the water
is acidic. The main source of the acid, in new systems, is active flux,
which contains hydrochloric acid. This should be thoroughly flushed out
of the system but often the pre-commisioning flushing is inadequate or
is completely omitted. Excessive flux and the careless use of flux will
add to the problems.

Another source of acidic compounds is anti-freeze (AH please note). The
glycols in anti-freeze will degrade in the presence of heat and
dissolved oxygen to form acidic compounds. Bad anti-freeze can cause
truly spectacular corrosion. This is very relevant with car cooling
systems. Once the residual alkalinity is consumed, the coolant will
become increasingly acidic and galvanic corrosion will attack the metal
engine components. Usually the head gasket fails.


Chemical corrosion inhibitors are cheap. The steel components of a
heating system are costly and internal damage cannot be repaired.
Neglecting or omitting the corrosion inhibitors is the mother of all
false economies. Inhibitors will not stop existing corrosion problems
(leaks, pumping-over). You can overdose with inhibitors, but I don't
know what the effect would be.

my CORGI man (been in business 30+ yrs) told me he NEVER adds inhibitor, it is
not needed and just a marketing ploy.

He is not competent, he should stick to gas.

Interesting view form a professional.

He may be a technician, he is not a professional.

....if you really want to enhance the lifetime of an old system..POWER FLUSH IT

No. You'd only need to powerflush it if it had been neglected.
Powerflushing is unlikely to remove all the corrosion products. You
should maintain it and ensure that it never needs flushing. The above
strange opinions add to my suspicions about Corgi-registered
technicians.