The two Glass Fused to steel sludge tanks were exhibiting perforations and corrosion which had meant that they had been removed from service. As can be seen in the images previous repairs had been made by fixing in place additional glass fused to steel plates, however the repairs required were now too extensive for this method and a longer term solution was required.
The weakness in many glass fused to steel tank systems is that the glass enamel coating does not extend to all panel edges and bolt penetrations. This means that in these areas the polyurethane jointing compound forms the corrosion protection, which if damaged or degraded allows corrosion of these unprotected areas to commence.
Further to this the glass enamel if chipped for damaged allows moisture to ingress into the enamel matrix and corrosion of the steel substrate to commence. More information on this can be found here.
THE SOLUTION
We proposed the application of our proven glass fused to steel tank lining in the form of Specialty Products Incorporated (SPI) Corrolastic HT pure polyurea. This polyurea tank lining has an expected service life in excess of 25 years.
Once the tank had been pumped out and handed over to us free from water and sewerage we first removed the existing polyurethane mastic sealant in order to expose all steel surfaces. This is an essential part of the glass fused to steel tank lining process as the mastic is often concealing unseen corrosion that must be treated in order to prevent sub film corrosion and ensure maximum longevity from the new tank lining.
Once all the existing sealant had been removed the glass fused to steel was first cleaned by method of hot washing, incorporating an emulsifying degreaser. This ensured that contaminants such as fats and chlorides were removed from the surface.
The effective removal of chlorides was confirmed using Chlor Tests. It is best tank lining practice and imperative to ensuring the longest possible life space from the new tank lining to ensure that soluble salts present on the surface are within the parameters set by the tank lining manufacturer. The reason for this is that all organic chemistry coatings are semi permeable membranes, meaning that if chlorides and other soluble salts are left on the steel surface they can draw moisture through the coating film, causing osmotic blistering and allowing corrosion to recommence.
Removing salts if present prior to abrasive blasting prevents them being forced into the surface profile created by preparation.
Once we were satisfied that all surfaces that the new tank lining was to be applied to were sufficiently clean we prepared for abrasive blasting of the tank internals. Prior to blasting commencing the purity of the air supply was confirmed by the conducting of a blotter test – wherein the abrasive blasting air is directed at a clean sheet of paper. If contamination is present this will be apparent visually on the paper. This is a highly valuable pre check to prevent the recontamination of the now cleaned surface with grease or oil from a faulty compressor, which would prevent optimum adhesion of the new tank lining.
The abrasive blast media was also tested for the presence of soluble salts to again prevent recontamination of the substrate.
The pressure of the air supply was reduced in order to prevent removal of significant amounts of the glass enamel where sound and intact.
Following abrasive blasting all spent abrasive was contained for removal from within the tank. All surfaces were vacuumed clean and the effectiveness of this confirmed with dust tape tests.
Ensuring that all surfaces are sufficiently clean before the application of tank linings is essential to ensuring proper adhesion and preventing the tank lining being applied to a film of dust which would lead to premature failure of the tank lining.
To prevent flash rusting from occurring and ensure the correct climatic conditions for tank lining application heating and dehumidification were installed. The climatic conditions were tested and recorded to ensure compliance with the manufacturers recommendations as part of our standard tank lining quality assurance.
All surfaces to be coated were then primed using Corrolast DSP – our specifically developed epoxy primer for difficult substrates like glass fused to steel. To ensure maximum adhesion of the polyurea tank lining a quartz aggregate was broadcast into the uncured primer to provide mechanical as well as chemical adhesion between the two layers of the tank lining. This was applied by brush and roller in order to better work the epoxy primer into the surface profile created by abrasive blasting.
Perforations in the glass fused to steel tank sheets were sealed by bonding in place galvanized steel sheets with a thixotropic epoxy adhesive.
Once all preparation a priming had been completed the polyurea tank lining could be applied. First all bolt heads and awkward areas received a stripe coat in order to ensure full film build and coverage in these areas where coatings pull thin through gravity and surface tension.
The beauty of applying fast setting tank linings such as polyurea is that this stripe coat can be applied in the same shift as the main tank lining.
The polyurea tank lining was then applied throughout, with others coming behind and visually checking for any gaps and voids in the tank lining. This are filled using a fast curing polyurethane mastic prior to making good using the same polyurea material.
Once set the polyurea tank lining was checked for porosity using a DC holiday spark tester. This is an essential part of the tank lining / relining process as a single pin hole is a point of failure from which corrosion can recommence and undermine the new tank lining.
Again as part of our standard tank lining quality assurance dry film thickness readings were taken and recorded throughout the tank to ensure that the required tank lining thickness had been achieved.
Any defects identified either from the spark test or the dry film thickness readings were marked. These were made good made using the same material. A significant benefit of using SPI Polyurea materials is that they contain their patented Ultrabond molecule. This overcomes one of the major constraints of conventional polyurea tank linings which is the very short overcoat /recoat window – which can be less than 6 hours. In a tank lining application such as this completing all quality assurance checks to a high standard within 6 hours would be almost impossible. This would mean that any repairs to be made would have to be abraded, which would be extremely challenging to complete 100% satisfactorily around the tightly packed bolts.
Because SPI Ultrabond allows their material to bond tenaciously to aged polyurea indefinitely provided that it is clean and has not oxidised we simply had to respray them, giving 100% confidence in any making good.
Finally the previously removed mastic seal at the concrete/steel interface was reinstated using a pourable polyurethane jointing compound and the top edge sealed using a gun grade material. This protects the edges of the tank lining and prevents creep in these areas.
