Sep 8 2004
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Several techniques are available to apply zinc coatings and each will determine the thickness of the coating and its ultimate durability in a particular environment.
The most widely used types of zinc coatings are:
- Mechanical plating
- Zinc electroplating
- Continuously galvanized wire
- Continuously galvanized sheet
- Zinc metal spraying
- Galvanized tube and pipe
- General or hot-dip galvanizing
A short description of each application process and the properties of the coating developed is given in the below sections.
In zinc electroplating, the items to be coated are immersed in a solution consisting of zinc ions, and an electric current is applied to evenly coat the surface.
Coating characteristics: Zinc-electroplated coatings are bright coatings that are thin, usually about 5-10 µm, and are not appropriate for external use where durability is needed. Heavy chromate coatings are often applied to zinc platings to enhance their durability, particularly for fastener applications. The coating is completely made of pure zinc and does not have the hard alloy layers of the hot-dipped coatings.
In mechanical plating, the items to be coated are tumbled in zinc powder with special reducing agents and glass beads to bind the zinc particles to the surface of the steel.
Coating characteristics: Zinc or alloy coatings are applied to fasteners and small parts using the mechanical plating process. The zinc particles are typically in lamellar form, and durability equivalent to hot-dip coatings can be attained in a uniform coating that is mainly suitable for hardened TEK type screws and threaded fasteners that are not appropriate for hot-dip galvanizing. These coatings typically have a thickness of 15–20 µm.
In sherardizing, the items to be coated in zinc powder are heated to about 400 ○C at which temperature diffusion bonding of the zinc with the steel takes place.
Coating characteristics: Sherardized coatings are said to be diffusion coatings whose thickness can be significantly altered up to over 300 µm and whose components can be customized by adding inorganic compounds or other metals to the zinc powder. The sherardized coatings are almost completely composed of iron-zinc alloy phases. The extended cycle times for the process make the application expensive. Currently, it is not often used.
Continuous Strip Galvanizing
In continuous strip galvanizing, coil steel is passed through a bath of molten zinc in a controlled reducing atmosphere at high speed (180 m/minute).
Coating characteristics: The thickness of the zinc coating is carefully controlled in the manufacturing process by air wiping the sheet as it comes out from the galvanizing bath. The coating thickness ranges from an average of 7 µm on the ZI00 sheet to 42 µm on the heaviest Z600 sheet. In addition, the coating has an extremely thin zinc-iron alloy layer that gives it its flexibility for pressing and forming.
Continuously Galvanized Wire
Cleaned steel wire is passed through a lead/zinc bath at high speed (180 m/minute) to produce a continuously galvanized wire.
Coating characteristics: It is comparable to those of continuously galvanized sheet. The thickness of the coating changes according to the coating grade and diameter of the wire from 3 µm in the thinnest standard gauge to 43 µm in the thickest (8 mm) heavy galvanized grade.
Galvanized Pipe and Tube
Galvanized tube and pipe is manufactured by two techniques. One is semi-continuous where the tube’s stock lengths are decontaminated and passed continuously through a bath of molten zinc at 450 ○C. The other technique is continuous where the strip is developed into the tube from the coil and the tube is subsequently passed through a bath of molten zinc at 450 ○C. The second technique coats only the tube’s exterior.
Coating characteristics: The semi-continuously applied coating is a traditional galvanized coating having a coating thickness generally about 65 µm which comprises mostly of zinc-iron alloy layers as the free zinc layer is largely removed via air wiping during the process. A bright coating is produced by the continuous tube galvanizing process which is nearly free of zinc with extremely thin alloy layers, giving good forming properties to the product. The thickness of the coating is generally 12–25 µm on the exterior of the tube only.
General or Hot-Dip Galvanizing
In general or hot-dip galvanizing, work is prepared by acid pickling on jigs or in batches and subsequently dipping the work into a molten zinc bath.
Coating characteristics: The typical galvanized coating ranges from 65 µm to more than 300 µm based on material thickness, steel analysis, and the immersion time in the galvanizing bath. Usual coating thickness on most general galvanized products is 80–100 µm.
Zinc Metal Spraying
In zinc met metal spraying, the steel surface needs to be cleaned to a Class 3 level and subsequently, zinc powder or zinc wire is sprayed onto the surface using a plasma flame gun or oxy-acetylene.
Coating characteristics: Zinc metal spraying creates a comparatively porous coating that can be applied in any preferred thickness but is normally 75–200 µm.
It is used where the item is unsuitable for hot-dip galvanizing because of its shape or size. Due to the availability of larger galvanizing baths, zinc metal spraying is not being used widely except for repairing galvanized coatings.