Galvanizing Steps  
 
  Galvanizing is the practice of coating clean, oxide-free iron or steel with a thin layer of zinc to protect the surface against corrosion. The zinc coating provides protection to the iron or steel in two ways: (1) it shields the base metal from the atmosphere and (2) because it is more electronegative than iron or steel, the zinc gives cathodic or sacrificial protection. Even if the surface becomes scratched and the base metal is exposed, the zinc is slowly consumed while the iron or steel remains protected from corrosion.

The hot dip process is adaptable to coating nearly all types of fabricated and non-fabricated products such as wire, tanks, sheets, strip, pipes and tubes, fittings, hardware, wire cloth, hollow ware, and structural assemblies.

Galvanizing consists of three fundamental steps: (1) surface preparation, (2) galvanizing, and (3) finishing.

The preparation step consists of cleaning and pickling operations that free the surface of dirt, grease, rust and scale. Clean, oxide-free work is galvanized by immersion into molten zinc. Finishing operations include removing excess zinc, quenching and inspection.

Surface Preparation

Good surface preparation of metal parts is essential for the production of high quality galvanized coating. Surface preparation usually involves a combination of methods for producing a steel surface which is clean and oxide-free.

A variety of methods is available and more than one sequence of surface preparation steps may be used successfully in a particular situation.

- Mechanical surface preparation
- Abrasive blasting
- Brush and hand finishing
- Alkaline cleaning
- Pickling

Galvanizing

Galvanizing is accomplished by immersion of the work in molten zinc at a temperature of 820-860 F (438-460 C) for a period of about one to five minutes, depending upon the thickness of the pieces to be coated. A layer of molten top flux should be kept on the zinc at the end of the kettle where the articles enter.

The pieces should be withdrawn at the opposite end of the galvanizing kettle, through a clean zinc surface. For carrying the articles to be galvanized into, through and out of the molten zinc, steel tongs, baskets or various types of hooks are used. These may be handled with an air or electric hoist.

The period of time the work is left in the zinc bath varies with the thickness of the steel, the amount of preheat and the thickness of coating desired. The reaction between the clean steel and the molten zinc to give an alloy layer proceeds rapidly for the first one or two minutes and then continues at a decreased rate. The longer the immersion time, the heavier and more brittle the alloy layer becomes.

Finishing

After the work comes from the molten zinc, excess zinc should be removed by shaking or by centrifuging in the case of small items, such as bolts, nuts, etc.

The hot dip galvanized product should meet recognized and accepted ASTM standards for coating weight and distribution, finish and adherence.

The heavier (or the thicker) a zinc coating, the longer the protection to the base steel. However, due to higher costs, brittleness of the alloy layer, poor adhesion, and appearance, a compromise must always be reached.
 
     

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