NOVEMBER nswer Curtis Peacock Dixon Engineering Inc - PDF document

A thin film epoxy polyamide systemis generally used to line potablewater storage tanks for immersionwet interior of a water stor-effect of soluble salts, and Water Tank Linings Under what conditions should small diameterblisters (Ú2repaired/repainted, and should the blisters bebroken and the substrate examined? Copyright ©1997, Technology Publishing Company planned, one should answer several¥How long has the tank been inservice, and how much longer until¥Is corrosion currently associated¥What is the consequence of signif-Generally, it is common to find afew small blisters in water tankssoluble contaminants on the sub-strate or between coats. This condi-tion is especially true in overheadwater promotes blistering throughosmotic forces. (Osmotic pressure is inversely proportional to the concentration of dissolved species condensate, a distillate, tends tocontain less dissolved salt, increas-ing the local osmotic pressure andmay, however, not be cause for sig-In these cases, if the extent of blis-total area and is isolated (not scat-tered over the entire coating sur-able that the coating remains an ef-fective barrier to the rapid ionictransport necessary to promote cor-rosion. A pseudo-equilibrium mayalso exist between the coated sub-Should the blisters be widely scat-at hand. If the total (actual) blisteredarea remains small, it may be ac-ceptable to defer maintenance untilIn either case, if there is little blis-growth, especially with scatteredlikely, complete recoating will bephotographs and notes of the blis-blisters and subsequent corrosion,blistering/corrosion is localized, onlyPROBLEM SOLVING FORUM Copyright ©1997, Technology Publishing CompanyNOVEMBER 1997 PROBLEM SOLVING FORUM such areas need to be maintained. Ifwidespread, the entire surface willneed maintenance. In addition, anew specification or quality assur-the consequences of failure. In apotable water storage tank, somesome blisters is not a major reasonnation from ruptured blisters maythe substrate is problematic. Somethe liquid is alkaline, little steel cor-rosion is expected. While this maysubstrate corrosion is more con-trolled by ionic permeability thanbe investigated by field-electrochem-resistance of the coating. Coatingsmain effective (corrosion) barriersDuring destructive blister inspec-most cases with blistered yet intactfindings are not a large concern be-cause corrosion is not propagating.Interestingly with some high-hard-under-coat corrosion in the absencecorrosion than the absence of blis-sponse of a coating after it is locallyacted upon by osmotic or internalThe single biggest advantage to adestructive examination is one oflocal contaminants (e.g., salts, sol- Copyright ©1997, Technology Publishing Company vents) within the blister may reflecton the application process. In suchmay provide some insight into thetering is common to the rest of the coating, significant rework mayIn all cases, any destructive in-The decision to repair/repaint areasof blistering depends on the situa-tion and the conditions. If the blis-coated tank and is within the coat-ing warranty period, any areas ofblistering noted should be repairedcurred in a tank where the coatingsters should be broken to determineif corrosion or metal loss has oc-substrate but no metal loss is noted,the broken blisters should be re-tank should be returned to serviceand evaluated on a regular basis toThe only exception to this guide-blisters could contaminate the waterconsideration should be given to re-pairing/repainting the affected sur-faces in the near future. It should bePROBLEM SOLVING FORUM Copyright ©1997, Technology Publishing CompanyNOVEMBER 1997 PROBLEM SOLVING FORUMRegardless of when repair work iscommon possible reasons for blister-¥Improper thinning or mixing ofare not thoroughly mixed from thebottom of each container prior toor the components are not thor-¥There is inadequate ventilationduring and after the coating opera-tions, resulting in solvent entrap-¥The substrate was contaminatedto adhere properly to the substrate.Substrate contamination can resultfrom improper surface preparationor the presence of soluble salts on¥Improper operation of the cathod-ic protection system can also lead ing type, tank material, type ofwater contained in the tank, andwhether the tank has supplementarycathodic protection. My answer ap-applied lining rather than a rubberor plactic sheet-applied lining. Myformed during tank manufacture andIn general, blistered tank coatingsobvious indications of corrosion ac-tivity. The repair of tank coatingscan be costly and can result in moreThe level of tolerance to break-down or deterioration of protectivecoatings for water tanks largely de-¥If the coated tank is made fromcarbon steel and used to contain seawater, produced water, or brackishwater, then rapid corrosion of thefective, especially if the water iswarm or aerated. The problem may¥Tanks containing potable waterwill corrode less rapidly beneath a Copyright ©1997, Technology Publishing Company ¥Tanks fabricated from corrosion-tolerant materials (e.g., stainlesstion and maintenance of these coat-A blistered coating may still besound. If there is no evidence ofthe coating is probably intact andter at its extremity. If fluid seeps out,and the underlying substrate is atarea without breaking the blisters.rosion has probably occurred. If ul-devised and implemented to moni-If the blister is porous and ultra-removal of a blister will be neces-sary if the tank condition beneaththe blistered area must be estab-sample of blisters should be brokenand the coating removed untilsound, uncorroded metal surface isfound. The diameter of the exposedshould then be gauged against re-If no further action is required,blister must be re-coated. The exist-(preferably by grit blasting), and acompatible tank coating repair sys-The original coating manufacturerBlistering of the paint system usedin a steel water tank indicates thatgrease, rust, dirt, or similar contami-nants were not removed during sur-face cleaning or that non-visible,PROBLEM SOLVING FORUM 106 / or hydrophobic materials. For exam-ple, application of thick layers of sol-rosion of the substrate is probably verycondition is particularly true when thecomes discolored with rust stains,then the coating should be repaired,either by spot repair or completeblistering is widespread. In eithercase, the reason for the initial blister-ing should be ascertained to ensure PROBLEM SOLVING FORUM Reader Response To This