THE PROBLEM
The hot water tank was exhibiting heavy corrosion and perforations which were preventing its continued use. The tank is used for a process called ‘Burtonisation’ which replicates the excellent water for brewing found around Burton Upon Trent – hence the large numbers for breweries in the area. The tank can operate at temperatures close to 100 Degrees Centigrade, meaning that a high temperature and food safe tank lining was required.
SOLUTION
Initially all laminate corrosion was removed using vibration reduced breakers and needle guns to expose the steel substrate. This revealed several significant perforations in the tank structure that would require plating prior to the application of the tank lining.
Following removal of the laminate corrosion as per best tank lining practice the tank was washed with fresh water prior to preparation by abrasive blasting. The reason for this is that abrasive blasting can force soluble salts into the surface profile of the steel substrate.
The unfortunate consequence of the ‘Burtonisation’ process is that a large amount of sulphates are added to the process water in the form of Gypsum. This would explain the extreme corrosion exhibited as Sulphates increase the conductivity of water, which in turn accelerates the electrochemical corrosion process.
Following washing the tank was prepared by method of low dust sponge blasting due to concerns from the client about dust from preparation works. This innovative preparation technology is 95% dust free. More information on this can be found below;
When applying high temperature tank linings a high standard of surface preparation is required due to the stresses elevated operating temperatures place on the adhesion of the tank lining. The steel was prepared to SA2.5 with a minimum surface profile of 100 microns as tested using a surface profile needle gauge.
Unfortunately the high concentration of Sulphates present beneath the corrosion present as tested using a soluble salt conductivity meter as per best tank lining practice meant that further washing after preparation was required. To achieve the specification required for further washes were required in order to achieve permissible soluble salt levels.
The presence of soluble salts is something that is much over looked as they are not visible to the naked eye. If soluble salts are left in place they can quickly lead to the failure of tank linings as a result of osmotic blistering caused by the transmission of moisture through the coating film.
The additional washing caused the flash rusting of the prepared steel. Normally this would necessitate the further preparation of the steel by the original preparation method – in this case more sponge blasting. However in this instance we employed the use of Corr-Ze 200. This product both cleans and removes flash rusting and is followed by the application of Corr-Ze 100 to passivate the steel substrate.
Some areas did require minimal preparation by mechanical methods to ST3 as per ISO8501-1 in order to comply with the manufacturers recommendation, but this was still quicker and more environmentally friendly than repeating the sponge blasting operation.
To ensure the correct climatic conditions for coating application dehumidification equipment was introduced and climatic conditions monitored and recorded as all tank lining applicators should.
All surfaces were primed using Chemco RE500P to a thickness of 200 microns. This material is itself designed to be applied to water jetted surfaces, so can tolerate a less than perfect surface, giving extra reassurance especially given the poor condition of the tank prior to lining.
To ensure the primer was worked into all pits and crevices it was applied using brushes only – time consuming but highly beneficial.
Perforations in the tank were sealed using galvanized steel plates bonded using thixotropic high temperature epoxy adhesive.
To assist with the fluid tightness of the tank all seams were then sealed using a high temperature polysulphide sealant, given the bolted and degraded nature of the tank in case of future movement in the structure occurring. It is not possible to apply rigid epoxy coatings over flexible sealants without reinforcement, so in these areas as part of the stripe coating procedure reinforcing scrim was incorporated for additional strength.
All angles and edges were also stripe coated using Chemco RW500 as per good tank relining practice to achieve full tank lining thickness in these areas where tank linings pull thin as a result of gravity and surface tension.
Once cured, Chemco RW500 was applied in two coats at a target thickness of 250 microns per coat. Chemco RW500 is FDA approved for food contact and can also tolerate immersed temperatures up to 125 degrees centigrade, making it the perfect tank lining material for this application.
After each coat the thickness was measured using a dry film thickness gauge and any areas below thickness marked for rectification.
As per our standard tank lining quality assurance the new tank lining was checked for porosity using a DC holiday spark tested and any pin holes or ‘holidays’ found made good using the same material applied by brush.
This was probably the most corroded sectional tank that we have ever relined and was a challenging application given the extreme level of soluble salt contamination. However this again demonstrates why a rigorous testing and inspection regime is required for every tank lining application as these could have easily been overlooked and lead to tank lining failure.
