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February 24 - February 28, 2014

What type of coating system (and surface prep) should be used to protect a steel factory roof that is subject to hydrochloric acid vapors and other industrial pollutants and particulates in a hot, humid environment?

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Selected Answers

From virani jagdish of MHI on April 29, 2014:
Affix 2mm-thick, ready-made FRP sheet over the existing sheet using nut and bolts, and then seal the border area using a sealer to eliminate air gaps.

From Nathan Spencer of Nukote Distributors on March 8, 2014:
I would be looking at high-build, highly chemical-resistant polyureas. They have extremely good vapor transmission figures and chemical resistance. Good cleaning and priming is imperative, as stainless steel can be a difficult substrate at the best of times.

From Warren Brand of Chicago Coatings Group on February 28, 2014:
Wow. This is a tough one. I've worked on a problem exactly like this over a pickling line at a steel mil. It wasn't the roof, but the stainless steel duct that was failing. In that case we specified power washing and hand-tooling the duct and applying a unique UV-cure vinyl ester, with pre-impregnated FRP. This system worked very well because it was applied in sheets (like wallpaper) and it adhered to itself. We would not have used this for a roof. For what you've described here, you would have to determine if the roof has movement, then identify an optimal coating solution that fits the logistics and access. The surface prep would then be dictated by the optimal coating identification requirements.

From William Slama of International Paint/Ceilcote Products on February 26, 2014:
Peter van Riet stresses a very important factor that is most important to ANY protective coating application, to take steps to "clean" the surface prior to coating and to assure that the surface is not contaminated with soluble salts - especially chlorides. This is even more important for elevated temperature immersion service. Regarding best coating selection, and in accordance with Peter's selection criteria, we have found that flake-filled vinyl ester coatings, 2 coats at about 0.5 mm per coat, have been very effective in solving this type of corrosion problem.

From Pieter van Riet of Corrocoat SA on February 26, 2014:
The inside of galvanizing and zinc processing plants can be highly corrosive due to acid fumes. HCL is a volatile acid and will evaporate from open tanks and condense in roof spaces, depending on ambient conditions. The corrosion of steel with HCL will form soluble metallic salts such as ferric chlorides, which are electrochemically bonded to the steel, and which are not removed during normal surface preparation activities unless specifically addressed. The less effective your efforts to remove these soluble metallic salts to low levels, the greater the need to select a coating with outstanding (above normal) diffusion resistance, to limit and retard diffusion of water molecules down to the substrate, where it can dissolve these soluble metallic salts to form an effective electrolyte under the coating. When using epoxy-based materials, it is imperative that acid on the surface is decontaminated thoroughly, as residual acid can affect the polymerization of the coating material. This same reason make a typical multi-coat system problematic if it needs to be applied during plant operation, as fumes settling on the coated surface during application can cause coating failure down the line. Ideally you need a single- coat system, or a system that can be applied-wet-on-wet,  a quick drying and curing system that reduces the time it is at risk. The coating should have very good diffusion resistance (low vapor transmission rate) to reduce the number of water molecules that reach the substrate. The coating should be a high build material that can create a thick film (long diffusion path length) quickly. The material should dry to produce a coating film that will not be chemically attacked by the acid fumes condensing or settling on the painted structure. We have had great success using a glass-flake polyester metacrylic coating, applied as a single coat system.

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Tagged categories: Exposure conditions; Exterior industrial pollutant exposure; Metal roof coatings; Roof coatings

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