THE PROBLEM
The water tank at Didcot Railway Centre was exhibiting heavy corrosion and leaking. The tower was constructed in 1932 also incorporates a coal stage beneath for refueling steam trains.
Tank Lining and Protective Coating Didcot
This Grade II listed Water Tank and Coal stage required significant refurbishment to allow its continued use serving the Great Western Society and its fleet of steam engines.
Internally the tank was lined with a thin film bitumen coating. Externally a coating system had been applied in the 1980’s, which had incorporated a red lead primer. This presented a health hazard that needed careful consideration given the close proximity of the museum and Didcot Parkway train station.
A particular and unique challenge for this tank lining and protective coating contract was that all plant, equipment and materials had to be brought into site by train as there is no access by road.
THE SOLUTION
We proposed the removal of all existing coatings, followed by the application of Eon Coat – a revolutionary steel protective coating that permanently protects steel and iron assets by alloying the surface of the steel.
Because of the presence of lead containing coatings and also the inability to fully encapsulate the tower we proposed the removal of the existing coatings by method of ultra high pressure water jetting. On aged steel with sufficient surface profile ultra high pressure water jetting is the perfect preparation for Eon Coat, as the material requires some surface corrosion present for optimum bonding to the steel substrate.
Following removal of the existing tank lining and protective coatings we did however uncover something very unusual that we had not experienced before. Both the tank internals and externals exhibited corrosion and delamination of layers of steel as shown below.
After some investigation and consultation with industry experts and engineers it appeared that the water tank may have been constructed from recycled armour plate from the first world war. This if formed by laminating layers of steel together and then forging them together. The layering process prevents the penetration of bullets. To ensure an effective tank lining and protective coating application these sharp edges needed to be removed to expose the corroded steel beneath and provide a thickness of steel sufficient to protect.
Initially we explored removing these areas using an abrasive wheel, however during the trial it was found that this was too aggressive and the heat energy imparted caused further delamination between the layers of steel.
Fortunately we had some new steel plating which had been installed that required abrasive blasting to remove mill scale prior to the tank lining application. With the equipment already transported by train into site a trial was undertaken to see if it were possible to ‘feather’ back these areas of delamination.
By experimenting with abrasive blasting aggregate size, flow and air pressure we were able to carefully feather back these areas to a point that delamination was no longer occurring and the corrosion deposits removed from the steel surface.
As part of our standard quality assurance the climatic conditions were tightly monitored to ensure compliance with Eon Coats recommendations. Unusually for a tank lining project we had to closely monitor the wind speed, as specified by Eon Coat to prevent the material drying out too quickly.
Installing the Eon Coat tank lining
One of the key benefits of Eon Coat for this tank lining and protective coating application was that it likes being applied to corroded steel – it actually needs a degree of surface rusting. This was perfect for a tank lining which was exposed to the elements and could not be controlled with dehumidification equipment.
Eon Coat was applied as a tank lining internally and as a primer to the tank externals. Eon Coat is applied by plural spraying due to the fast gel time of the material. It was perfect for application to the highly pitted internals as the chemical reaction with the steel surface gets to the deepest parts of the surface profile, unlike conventional protective coatings and tank linings.
Eon coat is applied to a thickness of 500 microns, which was checked during application using a wet film thickness gauge. This was then confirmed following cure using a dry film thickness gauge as per best tank lining practice.
Intricate areas were further detailed using the cartridge gun system, particularly around the backs of brackets and stays.
As part of the refurbishment works a new flashing detail was fitted to the base of the water tank externally. Because the bottom of the tank which is supported by the brick walls could not be accessed this area was treated using Corroless CCI 400, containing Corroless VCI technology.
Corroless VCI technology works by emitting a vapour which forms a microscopic layer on metal surfaces. This layer has considerable anticorrosive properties and prevents steel and other metals from corroding. Therefore, not only surfaces which are in direct contact with the Corroless product, but also areas in contained void or airspaces above the product, will be protected from corrosion.
The new flashing detail was installed by bolting through the water tank, to avoid damaging the protective coatings and tank lining by welding. As part of this installation we assisted with the sealing operation by wet fitting the flashing using a polyurethane sealant. Once cured each fixing was then further sealed by applying two coats of Corroless ACO locally. Corroless ACO is a flexible polyurethane tank lining that is ideal for sealing operations.
To provide the decorative finish required Sherwin Williams Acrolon 7300 Acrylic Polyurethane finish was applied in two coats, with the first coat being diluted to aid adhesion to the Eon Coat. The decorative border was carefully masked from measurements taken prior to preparation and applied using the same material.
We cannot take credit for the sign writing as this was undertaken by the museums own volunteers using the same material provided by ourselves.
This was one of the most interesting and unusual tank relining and protective coating projects we have ever been involved in. It was not without its challenges given the rail access to site only, and the exceptionally hot 2022 summer. We believe thought the results speak for themselves and the use of a pioneering technology in the form of Eon Coat will provide corrosion protection for many years to come.