(Zincs, epoxies and urethanes)
Coatings provide protection to steel by one or a combination of three mechanisms yielding the following coating types:
- Sacrificial coatings, which are rich in zinc. Whenever a scratch or other damage occurs to the zinc to expose the steel, then the zinc acts as a sacrificial anode and corrodes to protect the steel surface.
- Barrier coatings, which keep moisture away from the steel surface. This removes one of the elements of the corrosion cycle, the electrolyte, thus preventing corrosion.
- Inhibitive coatings, (usually only primers) which, in addition to acting as barriers, actively assist in the control of corrosion by using pigments which can provide an inhibitive effect (similar to corrosion inhibitors). Examples include red lead and zinc phosphate. These pigments react with the absorbed moisture in the coating then react with the steel to passivate it, thus decreasing its corrosive characteristics.
Therefore, 1) zinc coatings would be considered a sacrificial coating, 2) epoxies would be a barrier coating (however, epoxies will chalk over time due to ultraviolet light), and 3) urethanes provide color and gloss to protect chalking of epoxy coatings (urethane is not a barrier coating, as moisture can penetrate its surface).
There are two major types of spray application equipment:
Conventional airspray: The coating is atomized by a stream of compressed air and semi-floats to the surface on a current of air. Both the air and coating enter the gun through separate passages, are mixed and are driven through the air cap in a controlled spray pattern.
Airless spray: The coating is atomized without the use of compressed air. The coating is pumped under high pressure to an airless spray gun, where it is forced (under high pressure) through a precisely shaped and sized opening at the front of the valve, called the orifice or spray tip, as it is being driven to the surface.