Background There are a number of methods of applying zinc coatings and each will determine the coating’s thickness and its ultimate durability in a specific environments The most commonly encountered types of zinc coatings are:
· Zinc electroplating
· Mechanical plating
· Sherardising
· Continuously galvanized sheet
· Continuously galvanized wire
· Galvanized pipe and tube
· General or hot dip galvanizing
· Zinc metal spraying
A brief description of each application process and the characteristics of the coating formed are provided in the following sections.
Zinc ElectroplatingZinc electroplating involves immersion of the items to be coated in a solution containing zinc ions and applying an electric current to uniformly coat the surface.
Coating characteristics: Zinc electroplated coatings are bright coatings that are thin - typically around 5-10 microns and are not suitable for exterior use where durability is required. Heavy chromate coatings are frequently applied to zinc platings to improve their durability, especially for fastener applications. The coating is all pure zinc and lacks the hard alloy layers of the hot dipped coatings.
Mechanical PlatingMechanical plating involves tumbling the items to be coated in zinc powder with glass beads and special reducing agents to bond the zinc particles to the steel surface.
Coating characteristics: The mechanical plating process is used to apply zinc or alloy coatings to fasteners and small parts. The zinc particles are in lamellar form and durability equivalent to hot dip coatings can be achieved in a uniform coating that is particularly suited to threaded fasteners and hardened TEK type screws that are unsuitable for hot dip galvanizing. These coatings are typically 15 - 20 microns thick.
SherardizingSherardizing involves heating the articles to be coated in zinc powder to approximately 400 400oC at which temperature diffusion bonding of the zinc with the steel occurs.
Coating characteristics: Sherardised coatings are diffusion coatings whose thickness can be varied considerably up to over 300 microns and whose constituents can be modified by adding other metal or inorganic compounds to the zinc powder. The sherardized coatings are almost entirely made up of iron-zinc alloy phases. The long cycle times for the process make application costly. It is now rarely used.
Continuous Strip GalvanizingContinuous strip galvanizing involves passing coil steel through a bath of molten zinc in a controlled reducing atmosphere at high speed (180 m/min).
Coating characteristics: The zinc coating thickness is closely controlled in the manufacturing process by air wiping of the sheet as it emerges from the galvanizing bath. The coating thickness varies from an average of 7 microns on ZI00 sheet to 42 microns on the heaviest Z600 sheet. The coating has a very thin zinc-iron alloy layer which gives it its flexibility for pressing and forming.
Continuously Galvanized Wire Continuously galvanized Wire is produced by passing cleaned steel wire through a lead/zinc bath at high speed ( 180 m/min).
Coating characteristics: Similar to those of continuously galvanized sheet. Coating thickness varies depending on the diameter and coating grade of the wire from 3 microns in the thinnest standard gauge to 43 microns in the thickest (8 mm) heavy galvanized grade.
Galvanized Pipe and Tube Galvanized pipe and tube is produced by two methods; one is semi-continuous where stock lengths of tube are cleaned and passed continuously through a bath of molten zinc at 450 degrees centigrade.
The other method is continuous where strip is formed into tube from coil and the tube then passed through a bath of molten zinc at 450 degrees centigrade. This second method coats the exterior of the tube only.
Coating characteristics: The semi-continuously applied coating is a conventional galvanized coating having a coating thickness typically around 65 microns which consists largely of zinc-iron alloy layers as the free zinc layer is largely removed through air wiping during the process. The continuous tube galvanizing process produces a bright coating which is almost all free zinc with very thin alloy layers, giving the product good forming properties. Coating thickness is typically 12-25 microns on the exterior of the tube only.
General or Hot Dip GalvanizingGeneral or hot dip galvanizing involves preparing work by acid pickling in batches or on jigs and then dipping the work into a bath of molten zinc.
Coating characteristics: The typical general galvanized coating ranges from 65 microns to over 300 microns depending on the steel analysis, thickness of material and immersion time in the galvanizing bath. Typical coating thickness on most general galvanized products is 80-100 microns.
Zinc Metal Spraying Zinc met metal spraying requires the steel surface to be cleaned to a Class 3 level and then zinc wire or zinc powder is sprayed onto the surface with an oxy-acetylene or plasma flame gun.
Coating characteristics: Zinc metal spraying produces a relatively porous coating that is able to be applied in any desired thickness but is typically 75-200 microns.
It is used where the size or shape of the article make it unsuitable for hot dip galvanizing. The availability of larger galvanizing baths has resulted in it being little used for other than repairs to galvanized coatings.