Galvanizing process

Galvanizing, or galvanization, is a process of coating steel or iron with zinc to offer corrosion-resistance to the metal. Galvanized steel is the most popular steel types owing to its extended durability, strength and formability, in addition to the corrosion-resistance of the zinc-iron coating. The zinc protects the base metal by acting as a long-lasting barrier to corrosive elements. This makes the metal extremely versatile and finds application in several industries, including agriculture, solar, automotive, construction, etc.

The Process

Galvanizing process (Source: Galvanize it)

The major steps involved in the galvanizing process are as follows:

• Surface Preparation
  • Steel is first cleaned in a degreasing solution, a hot alkali or acid solution
  • After cleaning, steel is pickled by being lowered into a vat of diluted hot sulfuric acid to remove any scales and rust buildup
  • The steel is then fluxed in an aqueous solution (typically zinc-ammonium chloride), which eliminates oxides while simultaneously creating a protective coating for the prevention of the formation of any additional oxides.
• Galvanization
  • After the flux, the steel is galvanized through immersion in molten zinc. The metal is then withdrawn, drained, and cooled off.
  • Post-treatment coating is often applied to the metal. This process is called quenching. The steel is dipped in mostly water and some chemicals to create a protective layer. Finishing steps may also include grinding off zinc drips for a smooth finish.
• Inspection
  • The steel is later inspected for consistency and a complete coating by analysing coating weight, apprearance and finish.

Benefits of Galvanizing steel

1. Lower initial cost compared to most treated steels.
2. Galvanized steel does not require additional preparation of the surface, inspections, painting/coatings, etc. and thus is immediately ready to use.
3. Higher durability. Galvanization ensures an lifespan of more than 50 years in average environments to finished steel, and can last over 20 years with severe water exposure.
4. No maintenance required. The increased durability of the steel’s finished product also increases the product’s reliability.
5. Higher corrosion-resistance. The sacrificial anode ensures that any damaged steel is protected by the surrounding zinc coating. It doesn’t matter if the steel section is completely exposed; the zinc will still corrode first. The coating will corrode preferentially to the steel, creating a sacrificial protection to the areas that are damaged.

Methods of galvanizing steel

Following are various processes for galvanizing steel:

• Hot-Dip Galvanizing: This is the most popular method for galvanization. In this method, steel or iron is dipped in a molten pool of zinc maintained at a temperature of around 860 °F (460 °C). This molten bath develops a metallurgical bond between the zinc and the receiving metal at the interface. Once the metal is pulled from the bath, it reacts at being exposed to the atmosphere, and the pure zinc mixes with oxygen to form zinc oxide. The zinc-oxide further reacts to carbon dioxide and forms zinc carbonate, which makes up the final protective coating on the material. This method is one of the most economical one that can be quickly executed on both simple and complex shapes. The new coated material can be worked and machined in a similar fashion to uncoated materials. Galvanized steel can be used in high-temperature applications up to 392 °F, but use in temperatures exceeding that level will cause the zinc-carbonate layer to peel off.
• Galvannealing: Galvannealing combines the processes of annealing and hot-dip galvanizing, in order to produce a specialized coating on steel with a matte gray finish. Galvannealed steel is a zinc-iron alloy product, where the base metal is coated by the hot-dip process, then heated to induce alloying between the molten zinc coating and the steel. Galvannealed steel is conducive to welding and the surface is excellent for paint adhesion.
• Pre-galvanizing: Pre-galvanizing is a process that involves rolling the sheet metal through a cleaning agent to quickly prime the material for galvanizing. Then, the metal is passed through a pool of molten liquid zinc and is immediately recoiled. The primary advantage of this method is that coils of steel sheet can be rapidly galvanized on a large scale with a more unified coating than the traditional hot-dipped method.
• Electrogalvanizing: The most unique of these outlined methods, electrogalvanizing does not involve dipping the material in a molten vat of zinc. Instead, an electric current is introduced to an electrolyte solution that is applied to the steel, which reduces positively charged zinc ions to zinc metal – which is then deposited on the positively charged steel. Like pre-galvanizing, this method is typically done at the first stage of production.

External Links

• Galvanizing Process | GAA.com