Issue 44 winter 24 - Journal - Page 84
Cathodic V Traditional
Treatment of Steelwork
In the early 20th century construction techniques moved away from traditional load-bearing masonry
towards masonry clad steel framed buildings
forms of electrochemical corrosion prevention: sacrificial
cathodic protection or impressed current cathodic
protection (ICCP).
Sacrificial cathodic protection is the older method and
consists of connecting a more corrosive metal like zinc or
magnesium to the steel. The zinc or magnesium corrodes
in preference to the steel and generates a protective
current that prevents corrosion of the steel in contact
with the sacrificial anode material. It is worth noting that
sacrificial/galvanic protection has been shown to be
unsuccessful on stone buildings.
This marked the evolution of the modern high-rise
building. Corrosion problems associated with these early
steel framed buildings are inherent due to the nature of
the early design and engineers and architects of the time
failing to appreciate the destructive nature of corrosion.
External masonry tightly notched around the steelwork
with crudely in-filled cavities allows moisture to collect
and come into contact with the steel, making the onset
of corrosion inevitable. Moisture can penetrate through
porous cladding materials, open or degraded mortar joints,
cracked masonry, or faulty or degraded services such as
cracked rainwater downpipes and gutters. Often the
condition of cracking and spalling stonework is referred
to as Regents Street Disease due to the number of
buildings along London’s famous street that have suffered
from this.
ICCP involves the use of an inert non-corrosive anode
material being placed in the structure or soil and the
protective current being forced onto the steel structure
through a power supply. It works on the principle of
preventing corrosion by passing a current from an
externally placed anode material, through a connecting
electrolyte (facing and mortar) to the corroding steel
work. As a result, electrochemical reactions are prevented
at the steel surface through the prevention of ferrous ion
formation on the steel surface and the removal of reactive
negatively charged ions.
Treatment
There are two practical methods of treatment for
corroded steelwork in buildings: cathodic protection or
traditional treatment of the steelwork.
Cathodic Protection
This describes a general area of technology covering two
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Conservation & Heritage Journal
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