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
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 volatalise at different rates, thus giving a spectrum of protection ranging from near-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.
This initial early-stage testing proved the catalyst for further development of the Margel product range. Later independent testing was carried out by Geomaterials in tests spanning 8 years in both their lab and in the field, proving that Margel both migrates as a vapour through dense concrete structures and also provides a protective coating layer on reinforcing steel.