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CORROSION Hot Dip Galvanized Steel Reduces Plant Maintenance Costs By Joseph P Langemeier . The maintenance of corrosion protection coatings on mining equipment is time consuming, expensive and often neglected. This can lead to catastrophic equipment failures. Due to the triple protection provided by hot dip galvanizing, coating maintenance is virtually eliminated for the life of the equipment, which provides a safer working environment and defers maintenance expenses to other pressing needs. The time spent repairing and replacing corroded steel items is time and money that could be better spent on other activities. It's frustrating for maintenance employees, expensive for mine owners, and very often causes shipping delays that upset customers. Hot dip galvanizing is a maintenance-free coating that protects steel from corrosion for many multiples of practical paint service lives. The Galvanizing Process To truly understand how hot dip galvanizing saves lives, time and money, one first needs to understand how and why it is so effective at preventing corrosion on steel. Hot dip galvanizing is a thermo-chemical diffusion process that takes place in a vat of molten zinc. As with painted steel, surface preparation is key to a successful coating. If the steel is not clean, it will not galvanize. The metallurgical bond between zinc and steel is ensured by thoroughly cleaning the steel before immersing it in molten zinc. As part of the galvanizing process, steel goes through three cleaning steps: • Degreasing—The first cleaning step, degreasing, is often a hot alkali solution that removes organic contaminants like dirt, water-based paint, grease and/or oil. After degreasing, the article goes through a water rinse. Any epoxies, vinyls or asphalt coatings must be removed by mechanical means (grit blasting, etc.) before steel is taken to the galvanizer. • Pickling—Next the steel is moved to the pickle bath, an acidic solution of either ambient hydrochloric acid or heated sulphuric acid, that removes iron oxides and mill scale from the surface of the steel. After pickling, the steel is rinsed again. • Fluxing—Finally, the steel moves into the flux tank. The flux serves two purposes. www.e-mj.com First, the lightly acidic solution cleans any remaining iron oxides; and second, it provides a protective layer to prevent any iron oxide formation prior to immersion in the galvanizing kettle. The true "galvanizing" phase of the process consists of completely immersing the steel in a minimum 98% pure zinc bath. The bath temperature is maintained at 815°F (435°C) or higher. The steel is lowered at an angle by crane hoist. This allows air to escape from tubular shapes or pockets that may be within the design of a fabricated piece and permits the molten zinc to displace the air. Approximately 5 to 7 minutes after complete immersion (depending on the size of the articles), the steel reaches the bath temperature and the metallurgical reaction is complete. The last phase of the process is the final inspection. A very accurate determination as to the quality of the galvanized coating can be accomplished through a visual inspection of the material. As stated earlier, if the steel surface is not properly and thoroughly cleaned, the zinc will not adhere to the steel. Additionally, a magnetic thickness gauge can be used to determine the thickness of the coating to ensure compliance with specification requirements. Coating Structure and Performance During the galvanizing process, the zinc in the kettle metallurgically reacts with the iron in steel to form a series of zinc-iron (intermetallic) alloy layers. The first zinc- iron alloy layer, the Gamma layer, is approximately 75% zinc and 25% iron. The next layer, the Delta layer, is approximately 90% zinc and 10% iron. The third layer, the Zeta layer, is approximately 94% zinc and 6% iron. The last layer (Eta), which forms as the material is withdrawn from the zinc bath, is identical to the zinc bath chemistry, i.e. pure zinc. The Gamma, Delta and Zeta layers form approximately 60% of the total galvanized coating, with the Eta layer making up the balance. Because the galvanizing process involves total immersion of the material into cleaning solutions and molten zinc, all interior and exterior surfaces are coated. This includes the insides of hollow and tubular structures, and the threads of fasteners. Complete coverage is important because corrosion tends to occur at an increased rate on the inside of some hollow structures where the environment can be extremely humid and condensation occurs. Hollow structures that are painted have no corrosion protection on the inside. Additionally, fasteners with no protection on the threads are susceptible to corrosion, and corroded fasteners can lead to concerns about the integrity of structural connections. Three elements (barrier protection, cathodic protection and the zinc patina) are what provide galvanizing its long-lasting protection. Similar to paints, the hot dip galvanized coating provides barrier protection from corrosion. The zinc coating acts as a barrier against the penetration of Figure 1ÑTime to First Maintenance chart shows longevity of zinc protection in various environments. FEBRUARY 2013 • E&MJ; 53