April 21 - April 25, 2014
I have several 20-year-old carbon steel tanks for storing demineralized water. The original coating (type unknown) has started to blister. What material, at what DFT, should be used to refurbish the coating?
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Keith Gabbard of TCR Coatings on
May 14, 2014:
As Steve alluded, the osmotic gradient in demineralized water service is the primary concern. The osmotic pressure associated with "pure" water through a semi-permeable (coating) film is at it's maximum because ANY soluble contaminant left on the prepared substrate sets up the "optimum" blister scenario. The solute concentration differential between the demin water and contaminated substrate is exacerbated due to the fact that demin water is theoretically free of mineral contaminants. The contaminant could also includes solvent, some of which have partial solubility. Therefore, critical components of successful systems in demin service include SP-5 followed by thorough salts removal and testing; use of a thick-film, 100% solids lining with low permeability; use of low-embedment abrasive, and pinhole detection and repair.
William Gusnard of Southern Company Services on
May 12, 2014:
I work in the power industry. Demin water is my worst agent. Been working with demin water for almost 30 years. I agree with what Steve has said. Also, do not use sand as your blast material and make sure the coating gets a proper cure for immersion service. I had a tank lining not cured properly in cold weather. When it warmed up, the coating continued to cure. The VOCs given off messed up the organic compound readings in my demin water. The coating then started peeling off. It was blasted with sand and not cleaned up very well. Then the silica started messing up my water chemistry, as well. Had to dump 2.5 million gallons, blast off the failed coating and reapply a new coating. This plant was down for about 1 month during peak season to get the repairs done. Choose the coating carefully and make sure all the application rules on the data sheet are properly followed.
Curtis Goad of GOAD COMPANY on
May 3, 2014:
Many variables come into play in a coatings application, which, if compromised, may result in premature failure. Bonded linings of either rubber or weldable thermoplastic may be a better long-term solution. If the vessel lining could experience a vacuum-type pull, a vulcanized rubber lining would provide the better adhesion. However, plasticized PVC linings will cost less, have a strong bond, and have performed in demineralized water service for 30+ years. Both linings are thicker (1/8" to 1/4") than coatings and are spark-tested at much higher voltage (12,000 to 24,000 volts) to assure that no pin-holes or voids exists. Many years later, if damage has occurred, both linings have proven track-records of successful repairability.
Tom Schwerdt of Texas Department of Transportation on
April 24, 2014:
First, does the coating actually need to be refurbished? If the blisters are intact, the steel isn't corroding and the water isn't being contaminated, I would leave the original coating in place, but schedule more frequent inspections. Sometimes, blisters are just an aesthetic issue, which shouldn't matter inside a tank after a 20-year service life.
Steve Brunner of WPC Technologies on
April 22, 2014:
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Demineralized water is claimed to be the most corrosive water. (I'll exclude salt water.) One claim is this is due to an osmotic gradient.
OK, at 20 years of age the best fix is to remove the existing coating by abrasive blasting. Next, inspect the metal for imperfections, repairing if necessary.
There are several coatings that will provide resistance to this material, including baked phenolics and epoxy phenolics, which are generally applied to a thickness of about 6 mils and require heat of 350-400°F to cure. Two-component epoxy coatings can also be considered. Here, I suggest a novalac or a Bis A cured with an amine adduct or cycloaliphatic amine adduct. Generally, the thickness will be in the range of 15 mil.
In short, there are several possibilities for coatings designed to resist demineralized water. Check with your coating manufacturer for the best fit. Whatever you choose, remove the existing coating first.
Demineralized Water Tanks (Demin Tanks);
Dry Film Thickness (DFT);
Maintenance coating work;
Tanks and vessels
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