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Notes From The Blog CS tank and have fve areas of attachment per tube (with welded 3-inch, or 76 mm, tubes). In addition, six solid 1-inch (25 mm) SS rods are welded to the tank (holding cleaning heads). Te amount of SS versus CS is signifcantly greater than would be present with SS stirrers. All of the SS attachments and tubes were coated with the same epoxy lining up to ~3 inches (76 mm) from the wall of the tank. Te interior of the tanks are sprayed approximately once a month with 2 percent sodium hydroxide (NaOH) at 110° F (43° C). Te NaOH was constantly drained and the interior was fushed with rinse water per normal procedures. Would cathodic or anodic protection be warranted? Assuming we have no soluble substrate contaminants and the flm was applied properly and is suitable for the service as described, I would appreciate any of the NACE members' expertise regarding the corrosion potential of the design in the given service. A: Is it safe to use coated steel for such a worthy liquid? I'd have thought corrosion-resistant alloys would be more appropriate for this service. Coatings are not 100 percent impervious and corrosion will occur at holidays. I agree that SS such as Type 316 (UNS S31600), 2205, or 6 Mo should be used for such a service. Tere are health concerns. Te life cycle cost may also justify such material selection. However, if coated CS is to be used, frequent inspection should be established to detect the onset of coating deterioration. Coatings are not impervious and will have holidays. During multilayer coating, any pinholes in the preceding layer may not be sealed right up to the metal surfaces. Te construction features suggest difculty of a thorough inspection. Of course, greater flm thickness does help to retard permeation of aggressive liquor and increase recoating intervals — up to a point. A: A: Also note that the coating is being exposed to acid (pH 4.2) and alkaline conditions (NaOH wash). Tis may be contributing to premature failures as well. Tese conditions are mainly for amphoteric alloys and perhaps not for the resins formulated for the coating. Te fact that only areas of low flm build are blistering is an indication that the coating system can handle the conditions. With immer- A: coating over a period of time, further unearthing defciencies. Without knowing the coating type, curative (polyamide, aliphatic, cyclo, or adduct), and the DFT, it is difcult to postulate a true reason for failure. As for a coating not withstanding 2 percent caustic, it must be a metalized acrylic. Beer should not be a problem either, but if the coatings were cheap polyamide, it could be. Do not make the mistake of throwing flm build at this problem. Te thing that bothers me is that 22 mils (559 µm) of a suitable epoxy should not have blistered. It may be caused by a combination of factors: the unpainted stainless may have gone passive because of the NaOH wash and become more cathodic to the CS, or the flm may have had pinholes, allowing the electrolyte to get to the substrate. Several things may be going on here. I believe a low-voltage holiday detector should have been used, and all application criteria should have been stringently adhered to. I would have coated everything in this tank, even the SS. Tere also may be better processes such as e-coat or powder coating that could have been used. It is not very helpful at this point to get into the fact that the entire tank should have been made out of some other material. We have similar conditions on the ships we build here, and we now coat everything in tanks constructed with mixed materials. We use epoxies with good success on a wide range of materials put into these tanks. Coating the SS is a good idea. However, 20 mils (508 µm) of coating may cause a noticeable change in heat transfer efciency. I would suggest a thinner flm coating for the SS components. I would expect minimal sacrifce in coating performance on the SS. My experience has been that, in general, coatings tend to remain intact on corrosion-resistant surfaces. Tere are limits to this characteristic. You will have to consider the temperature of the efuent in the coils and, of course, testing is preferred to postulation. CP A: sion in aggressive environments, the complete surface must be checked for holidays and the dry flm thickness (DFT) reading must never by accepted if it is under the manufacturer's recommended DFT. I have seen numerous CS tanks in the brewing industry that were coated with ceramic coatings and later had SS stirrers added with no problems. Some were more than 20 years old. A pH of 4.2 is well within the capabilities of some epoxy ultrahigh-build coatings. Your inspection procedures have let you down in this case. Te simplest review of coatings performance in a solution, given the solution is not chemically damaging the coating structure, can be found in Chapter 3 of Munger's Corrosion Prevention by Protective Coatings, Second Edition, "Essential Coating Characteristics." Te major issue regarding blistering is their position in the fuid and the manner by which the coating was applied. Permeability of the coating will unearth surface defects but not necessarily result in coating failure. Similarly, the vapor absorbed by the coating after time will lead to a reduction in the adhesive strength of the A: A: A: COATINGSPRO JANUARY 2014 25