By Thursday afternoon our client had received our quotation and we had confirmed that we could attend site to undertake the tank lining works from Monday morning.
Initially following the engineering works all surfaces were cleaned by method of high pressure washing and degreasing to remove any oils and cutting fluids which would be detrimental to adhesion of the new polyurea tank lining.
It is also important to wash new steel to remove soluble salts if present. It is not uncommon for steel to be shipped by sea which can lead to chloride contamination, which if left in place can lead to blistering through osmotic action.
The effectiveness of this washing was confirmed by conducting a soluble salt test.
Following washing the preparation was broken down to two tasks. The first was to abrade the existing polyurea to remove oxidation and provide a mechanical surface profile for the new polyurea tank lining primer to adhere to.
During preparation of the existing polyurea we found extensive blistering – likely caused by the polyurea being sprayed off ratio / out of balance. These were cut back to as sound an edge as possible and feather edged.
More disturbingly where the existing polyurea tank lining had been cut in order to extend the tank we found that the existing polyurea was poorly adhered, with sub film corrosion and what appeared to be mill scale still present.
Removal of mill scale is essential to all coating application and a fundamental task as this poorly adhered mineral layer will eventually de-bond – along within any coating / tank lining adhered to it.
We immediately raised this with our client, however unfortunately they did not have the time due to the critical nature of the project to remove all of the existing polyurea. What this did demonstrate though is the importance of trained applicators and a thorough testing and inspection regime.
The new steel was abrasive blasted to SA2.5 as per ISO8501-1 and the surface roughness tested and recorded using a surface profile needle gauge. This is an essential test to avoid the poor adhesion as witnessed with the existing polyurea coating.
Due to inclement weather being forecast and the tight and critical deadline for the tank lining works we liaised with the client for the erection of temporary weather protection and indirect oil fired heating. Moisture and condensation / dew will prevent the proper adhesion of most coatings and tank linings.
As part of our standard polyurea tank lining quality assurance the climatic conditions were tested and recorded prior to and during coating application to ensure compliance with the manufacturers recommendations.
The bare steel was primed using Corrolast DSP – our own surface tolerant and solvent free epoxy primer. This was scattered with quartz aggregate to provided a mechanical profile for the polyurea lining to adhere to, in addition to the chemical bond
The existing polyurea was primed using Mapei PU60 – a solvented primer specifically recommended by Mapei for priming existing aged and properly prepared polyurea coatings. This included the addition of 1% of their catalysts for accelerated curing.
As part of our standard polyurea tank lining quality assurance the dried film was checked for thickness using a dry film thickness gauge and pin holing using a DC Holiday spark tester. Where defects were found these were marked prior to making good using the same material – importantly within the overcoating window, which is typically very short for polyurea coatings.
If the overcoating / recoat window is exceeded this would necessitate abrading and repriming as per Mapei’s recommendations. One of the benefits of polyurea coatings is that there fast gel time means that tests such as spark testing can be conducted almost immediately.
The tank lining works were completed exactly one week from first receiving the enquiry, which allowed our client to continue with their planned crane lift of the tank into the factory over the weekend in order to allow completion of their multi million pound order on time.
