CoatingsPro Magazine

JAN 2013

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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Page 83 of 100

Ab ove It is important to hang steel pieces on the racks in the correct orientation to allow proper drainage during all the stages of the HDG process. main shop of a HDG plant, airborne acid fumes from the pickling process can be deposited on the HDG surface, especia lly if large overhead loading doors for raw- or galvanized-steel are open for prolonged periods, or if the preparation process is downwind of the acid pickling process. Finally, the surface preparation process may remove too much zinc as a result of blasting with an abrasive that is too hard, at too high pressure, at too close distance, or if the dwell time is too long. (Note: Using power tools can also remove too much zinc.) If too much zinc is removed, the surface preparation process can expose several alloy layers of the HDG and even the steel, creating three of the four elements for a corrosion cell. After the prepared surface is coated over, and moisture permeates through the film for several months, the fourth and final element of the corrosion cell results in rapid corrosion at times. duplex coating process; whereas in other facilities, the physical plant layout and lack of proper climate control make it difficult, if not impossible, to produce consistent results when performing the HDG process and the duplex coating process. Most specifiers, owner's engineers, coating manufacturer's representatives, and coating inspectors will likely at some point become involved with at least one galvanizing project involving a duplex system. How can he or she determine, before a job is awarded to a HDG shop, that the prospective shop is capable of consistently and successfully applying a duplex coating system? Below is a list of specific items to check when performing pre-award visits to HDG shops to determine whether they are properly suited for applying the HDG and the duplex coating system. It is best to conduct this visit when the shop is performing all the processes that are required for the upcoming project. • How are the ambient conditions (relative humidity and air temperature) controlled in the facility, including the area where the coating system is applied and where any surface preparation is performed? • Is the climate-controlled area large enough to handle the number of pieces that will be processed on a daily basis for your project and other projects the shop is running? • Is the area of the shop where surface preparation occurs climate controlled? Is it isolated from exposure to acid fumes from the pickling process, especially when the loading doors are open at the shop? • If they are performing surface preparation prior to applying the coating system, what method are they using? Are they measuring the dry film thickness (DFT) of the HDG before and after the preparation method to ensure that too much galvanizing is not removed? • How long does the hot-dip galvanizing remain exposed in the shop during and after the surface preparation process, before it is painted? Understanding what is required for application of the duplex coating system in a HDG shop will help eliminate shops whose processes or whose physical layout simple does not provide the proper set-up for successful finished product. Cp The next article in this series will discuss proper surface preparation for galvanizing prior to coating and the proper coating systems to apply on HDG. It will contain several case histories of practices resulting in expensive premature coating failures with duplex coating systems. DeterMining if A hDg shop is sUiteD for Applying DUplex systeMs Based on this author's experience visiting galvanizing facilities located in North America, Africa, and Asia, considerable variations exist between HDG plants. Some plants are properly designed to handle both the HDG process and the RIGHT New HDG guardrails leaving the kettle, where they were immersed in molten zinc until the proper amount of zinc accumulated on the steel. January 2013 g 83

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