CoatingsPro Magazine

MAY 2015

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Page 26 of 92

26 MAY 2015 COATINGSPROMAG.COM Second, the answer is not to add more thinner; it is to use a slower evaporating thinner. Your manufac- turer normally recommends two diferent types of solvent. Under your circumstances, they may even recom- mend a special blend to allow the proper reaction of the silicate, zinc, and carbon steel. Te same recommendations apply for intermediate and fnish coats. Apply during nighttime hours, regardless of the type of epoxy and/or polyurethane topcoats, and use a slower evaporat- ing thinner. Coating for steel tunnel Q: A new 540-m-long (164 feet) land transportation tunnel is being constructed of steel in Japan. As we do not have experience construct- ing a steel tunnel in the past, we are unsure about specifying the protective coating system for the structure. Te drawing just indicates that a coating system is required to provide corro- sion protection, abrasion resistance, fre resistance, and hardness (fgures not specifed). I am concerned about using polyurea, polyurethane (PUR), or other resins, because although the thermal decomposition of these coatings are said to be fre-resistant, they may produce toxic gases, such as hydro- gen cyanide. For the interior tunnel coating, please provide comments on the following coating system: One coat inorganic zinc (IOZ) for all steel walls and ceiling (with organic zinc-rich coating for repairs) without topcoat, and a polyurea coating for the driver's eye attention zone ~3 m (10 feet) from the wall bottom line for both wall sides. (Note that the drawing specifes that the driver's eye attention zone is to be coated.) A: W hile I have never built a steel tunnel, I would begin by considering it the same as a pipeline, only larger. Te exterior would need a coating with good corrosion resistance and good cathodic disbondment resis- tance. I assume impressed current cathodic protection will be employed. Tere are numerous 100 percent volume solids epoxies that will perform well in this service, many of which can be applied directly to metal in one coat. For the interior, I would consider a system of IOZ and 100 percent volume solids semi-gloss epoxy. Te epoxy would have to be chosen carefully, in consultation with the manufacturer, to obtain an epoxy with the lowest fame- spread rating. Some people might wish to use high-gloss PUR as a topcoat for ease of cleaning, but it would have to have a decent fame-spread rating. Another possibility for the interior would be a coat of IOZ followed by one or two coats of polysiloxane. Again, this requires consultation with the manufacturer to make the best selection. I would highly recommend getting one of the major coatings manufactur- ers to participate in this design. A: I would be more concerned about the thermal decomposition byproducts of PURs and polyureas than I would about epoxies, but this is the type of information that should come from the coatings manufacturer's test results or test results from indepen- dent tests. Tere should be very little concern about thermal decomposition byprod- ucts with uncoated IOZ and the long-term corrosion protection would be excellent. So would thermal-sprayed zinc. Te only possible negative I can think of is that zinc flms are rather porous, so they would collect partially burnt hydrocarbons from gasoline and diesel engine fumes. Tis would result in a decrease of visibility in the tunnel unless power washed on a sched- uled basis. Inorganic zinc thickness Q: I am going to work as a paint contractor for quality control on a rig live extension project. Te client has given the following specifcations: • 50 µm (2 mils) inorganic zinc (IOZ) • 50 µm (2 mils) epoxy polyamide • 200 µm (8 mils) build coat However, the data sheet of this IOZ specifes that it is a shop primer applied at 15 µm (0.6 mils) dry flm thickness (DFT). I wish to have your views on this specifcation, knowing that the tank is a very complex struc- ture and the guarantee is fve years. A: Something is amiss here. Normally, IOZ shop primers are not intended to be applied in excess of 15 µm (0.6 mils). Tey will mud crack in corners, welds, etc., when applied at excess thickness. Since this is a live extension project, I have to assume there is a coating system already in place. If that system is an organic coating system, the IOZ will not adhere to the organic system. To apply the specifed system would require abrasive blasting to bare metal. You might wish to ask for a conference with the owner's represen- tative to clarify that specifcation. A: Tis approach is fairly common. It sounds like they are ofering a preconstruction primer at a thickness consistent with a weldable primer. Typically this is spot-blasted on weld seams and abraded areas and swept over the remaining area. Tere will be very little zinc primer left after this process. Te primary function of the zinc is to preserve the blast profle. If there is no zinc left, it is of little concern. I would suggest a little more epoxy for the system if life expectancy is a concern. A thickness of 250 µm, or 10 mils, is fairly common for immer- sion, but there are systems available that go to 500 µm (20 mils) in two coats quite easily. A: Wet flm thickness (WFT) gauges correlated to actual DFT for the IOZ coating will reduce but not elimi- nate inaccuracies. Correlate wet to dry thickness for every IOZ batch and diferent manufacturers. Variations in batch consistency and coating design prevent accurate assumptions about wet to dry thickness ratios. CP Notes From the Blog

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