The Use of Vapour Phase Inhibitors for the Protection of Reinforcement Steels within Concrete from Corrosion
This paper has been prepared by H.D. Hosgood and L. Banks
Introduction
Steel
reinforcement within concrete is normally well protected from corrosion
because of the highly alkaline nature of the surrounding concrete
matrix. Whilst this environment is maintained, and in the absence of
any destabilising influences, such as the presence of Chloride ions
from salt contamination or the use of Calcium Chloride Accelerators
during the concrete placing, no rusting of the steel should occur.
As
intimated above, Chloride ions, which may arise from contamination with
salt from marine environments, from the use of de-icing salts, or
Calcium Chloride use, will promote rusting even in an alkaline
environment. Furthermore, the alkalinity cannot be considered to be
permanent, since it is gradually destroyed by a phenomenon known as
Carbonation. This is a result of the following chemical reaction :-
Ca(OH)2 + CO2 = ► CaCO3 + H2O
Where
Calcium Hydroxide (Lime from the Cement), reacts with carbon Dioxide
from the atmosphere, to produce Chalk, with a neutral pH and water. The
result is that a carbonation front gradually penetrates the concrete
and will eventually reach the reinforcement, when rusting can then be
expected. The time needed for the above to take place depends upon a
number of factors, such as thickness and density of the concrete cover,
original cement content, atmospheric humidity, etc.
When the
reinforcement does begin to corrode, the salts produced, (shown below),
increase the volume requirement, exerting pressure on the surrounding
concrete, until eventually the pressure is sufficient to crack it. The
course of the rusting will then further accelerate, since Oxygen and
Water can penetrate more easily.
The rusting of iron
General
conditions:- Oxygen and Water are essential, carbon Dioxide and
Metallic Salts and Acidic materials accelerate, Alkalis inhibit,
including Organic Alkalis, such as Amines.
It is essentially an
electrolytic process; because the surface is not uniform, area with the
lowest electrode potentials form anodes, with the remainder of the
surface forming a cathode. This means that effectively moist iron
functions as a short-circuited cell, with the following electrochemical
reactions :-
Anode: Fe (Iron Metal) – 2e (2 electrons) = ► Fe++ (Result, Iron dissolves)
Cathode: 2H2O = ► 2H+ + 2OH-
2H+ + 2e = ► 2H2 ∆ ( Hydrogen gas evolved )
In
the absence of Oxygen, the evolved Hydrogen polarises the cell and
corrosion halts. In the presence of Oxygen, Hydrogen is oxidised to
Water and the iron passes continuously into the ionic state at the
anode, where the following further reactions can take place:-
Fe++ + 2OH- = ► Fe (OH)2 (Ferrous Hydroxide)
Fe(OH)2 + CO2 = ► FeCO3 (Ferric Hydroxide) + H2O
FeCO3 + O2 = Basic Ferric Carbonate, which is hydrolysed to Ferric Hydroxide, Fe (OH)3, which is dehydrated to give Fe2O3 + H2O which is hygroscopic and results in acceleration of rusting, since it maintains the iron surface in a moist condition.
Vapour Phase Inhibitors (VPI’s)
VPI’s are Organic corrosion inhibitors which can volatilise at normal temperatures, producing vapour pressures in the range 10-2 to 10-7
mm.Hg. and will condense onto surfaces, producing a surface film which
produces corrosion-resistant properties. It has been shown that such
vapours pass fairly easily through concrete and will condense on the
surface of the steel reinforcement. They are, therefore, able to retard
and arrest active corrosion where it is taking place.
VPI’s
work, by first forming a bond on the steel surface and then by forming
a barrier to aggressive ions such as Chloride Ions. They are Secondary
Electrolyte Type, possessing sufficiently high vapour pressures to
allow significant vapour transport of the active inhibitor component.
They adsorb onto the steel surface and can ionise, resulting in
electrolytic protective action, as well as forming a physical barrier
to Oxygen and Water. It is known that the most effective VPI’s are the
reaction products of a weak volatile base and a weak volatile acid, an
example being Amine Nitrites, (A component of the Margel™ VPI product),
which are susceptible to hydrolysis and produce inhibition.
R2NH2NO2 + H2O = ► R2NH2+ + OH- + H+ + NO2-
Laboratory
investigations utilising Headspace Gas Chromatography, carried out on
periodic samples taken from typical concrete beams, showed passage of
vapours through 1 metre of concrete within 1 week at an ambient
temperature of 20°C. The particular VPI used was one which volatilises
rapidly, one of the components of QED’s Margel™ 580 products. Margel™
incorporates three differing components, with different volatilities,
thus giving a spectrum of protection ranging from immediate to very
long term.
Site History, CEGB site at Fawley, Hampshire
This
was one of the early uses of VPI’s for protection of in-situ concrete
from reinforcement corrosion. The test area was a suspended floor in
the power station, subjected to frequent inundation with wash waters,
therefore in a permanently saturated state.
The floor was
constructed using 100mm thick reinforced concrete slabs, 1.8m x 0.6m.
The slabs had longitudinal main steel, with some stirrups supporting
top and bottom bars. There was extensive spalling, resulting from
reinforcement corrosion. The main areas of spalling were along
longitudinal joints between panels.
Two slabs were chosen for
the trial; the first had heavy longitudinal corrosion, whilst the
second one was one in an area not exposed to the water, exhibiting no
spalling or evidence of corrosion.
Measurements of corrosion
were made using a device which sampled corrosion potential in the
customary method using one connection to the steel and the other to a
half cell on the concrete surface. However, a third electrode
(stainless steel) was employed in order to be able to superimpose an
artificial corrosion rate by driving the steel in the direction of
corrosion. The state of the steel within the concrete could then be
assessed by switching off the current and then measuring the recovery
rate, as the half cell potential should return to its initial value if
there were no active corrosion taking place.
Background readings
were taken from both slabs over several days, to establish their
behaviour. Having obtained the base values from each slab, Margel VPI’s
were inserted and further measurements made over several months.
It
was found that the dry slab maintained a steady state throughout,
whilst the wet one with active rusting taking place gradually altered
after the VPI insertion and slowly approached the values for the dry
slab. It would not be expected to achieve exactly the same value, since
saturation with water results in lower potentials.