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Facts about Hot-Dipped Galvanizing
The process of using zinc to protect steel from corrosion (hot-dipped galvanizing) is a 150 year old practice. Hot-dipped galvanized steel lasts longer today than it did 20 years ago. This is due to the enforcement of our environmental laws, which keep our air cleaner and less contaminated with corrosive emissions.
The galvanized surface (zinc coating) is self-sacrificing which means that the coating slowly sacrifices itself by galvanic action to protect the base steel. This sacrificing action continues as long as any zinc remains in the immediate area.
How is it done?
- Receiving & Inspection
Material loaded on racks
Inspected for proper venting & bracing
- Cleaning and Degreasing
8-12% sodium hydroxide
3-4% sodium metasilicate
160 to 180°F
Removal of paints/grease
10-15% Hydrochloric acid
Removal of full range of iron oxides
Bare metal exposed to react to zinc bath
15-25% ammonium chloride, 140 - 180°F
Forms ferric chloride & ammonia to prevent
- Dry & Preheat
350° air dryer
Evaporate all water
Preheat steel surface prior to zinc bath
- Molten Zinc bath
Immersion in 850°F molten zinc
Galvanic reaction is complete when base metal
reaches zinc temperature
Pure zinc outer coating deposited when removed from molten zinc bath
- Inspection & Shipping
Coating thickness & visual quality
Mark with drawing number
Features & Benefits
(SOURCE : American Galvanizers Association, Inc.)
- Hot-dipped galvanizing requires vent holes in all closed channel or pipe prior to dipping, which equates to galvanization occurring both inside and out.
- The metallic coating produced by the hot-dipped galvanizing process is the result of a metallurgical reaction called diffusion. The finished product consists of four layers adhered to the steel. The outer layer is pure zinc and the three inner layers are separate intermetallic layers that are metallurgically bonded to each other and the steel.
- The galvanized coating literally becomes part of the steel substrate, thus creating an adhesion bond on the order of several thousand pounds psi between the two providing excellent abrasion resistance.
- Hot-dipped zinc galvanizing provides cathodic protection to the underlying steel. This means that in the event of a scratch or chip (1/4" length and/or width) the zinc will corrode instead of the steel.
- Hot-dipped galvanization substantially increases the useful life of steel.
Touch Up & Repair
Hot-dipped galvanized areas damaged by welding, flame cutting or during handling, transport or
assembly and are larger than 3/16" on flat surfaces or 1/10" on cut ends should be repaired by the
following recommended method.
Cold Galvanizing Compound
- Surfaces to be reconditioned with zinc-rich paint shall be clean, dry and free of oil, grease and
- Areas to be repaired shall be power disc sanded to bright metal. To ensure that a smooth reconditioned coating can be affected, surface preparation shall extend into the undamaged galvanized coating.
- Touch-up paint shall be an organic cold galvanizing compound having a minimum of 65% zinc dust in the dry film.
- The paint shall be spray or brush applied in multiple coats until a dry film thickness of 4 mils minimum has been achieved. A finish coat of aluminum paint shall be applied to provide a color blend with the surrounding galvanizing.
- Where galvanized steel is to be welded, adequate ventilation must be provided. If adequate ventilation is not available, supplementary air circulation must be provided. In confined spaces a respirator will have to be used.
- Welding shall be performed in accordance with the American Welding Society publication D19.0-72, Welding Zinc Coated Steel.
- All uncoated weld areas must be touched up after welding in accordance with the Touch-Up procedure listed previously.
Hot-Dipped Galvanized Equipment Maintenance
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- The galvanized finish of your new implement does not require periodic cleaning. As you know, painted implements such as cutters require below deck cleaning to keep the decks from rusting through. Cutter decks usually rust through from the bottom up due to the wet grass and fertilizer packed under the
cutter decks. Since hot-dipped zinc galvanizing is a self-sacrificing surface, rust through problems are virtually non-existent.
- Be sure to use the proper touch up procedures for any external damages to the galvanized surface. By following these procedures, your Modern hot-dipped galvanized equipment will look great for many years to come.