Material Description

To effectively protect reinforcing steel against corrosion, a coating must provide a continuous film that will: resist penetration by salt ions; resist the action of osmosis, adhere to and expand/contract with the steel substrate; resist breakdown from weathering and exposure; and be flexible and durable enough for handling.

Fusion-bonded epoxy coating satisfies all of these requirements. It is a thermoset material, meaning that once it is cured, the coating will not tend to soften with higher temperatures. It achieves its beneficial properties as a result of a heat catalyzed chemical reaction. Epoxy coating starts out as a dry powder. The powder is produced by combining organic epoxy resins with appropriate curing agents, fillers, pigments flow control agents. When heated, the powder melts and its constituents react to form complex cross-linked polymers.

Epoxy coatings are environmentally friendly materials. Unlike many paints, the fusion-bonded epoxy coatings used for steel reinforcement do not contain appreciable solvents or other environmentally hazardous substances. Systems used to apply the coating are very efficient, resulting in little material loss to the atmosphere and little waste disposal.

Manufacturing Process

The process of applying fusion-bonded epoxy coating to steel reinforcement involves four major steps: surface preparation, heating, powder application and curing. In most North American epoxy-coating plants, bars are first coated in straight lengths and then fabricated (i.e., cut to length and bent to shape). A few facilities have the capabilities to coat reinforcing steel (both bars and welded wire fabric) after it has been fabricated.

Surface Preparation

Proper surface preparation assures that maximum adhesion will develop at the interface between the steel and the coating. Reinforcing steel is blast-cleaned to a near white metal finish using abrasive grit. This cleans the steel of contaminants, mill scale and rust. It also roughens the surface in order to give it a textured, anchor profile (i.e., the microscopic peaks and valleys on the steel surface). The surface roughness "keys" the coating to the steel and provides mechanical anchorage. Texturing the surface also facilitates adhesion by increasing the exposed surface area of the steel and by providing more opportunity for the coating to chemically bond.

Chemical pre-treatments have been used to supplement blast cleaning.

Heating

After blast-cleaning, the bars are heated to approximately 450 degrees F. Using electrical induction heaters.

Fusion Bonding

The heated bars are then passed through a powder spray booth where dry epoxy powder is emitted from a number of spray nozzles. As the powder leaves the spray nozzle, an electrical charge is imparted to the particles. These electrically charged particles are attracted to the grounded steel surface providing an even coverage of the coating. When the dry powder hits the hot steel, it melts and flows into the anchor profile and conforms to the ribs and deformations of the bar. The heat also initiates a chemical reaction that causes powder molecules to form the complex cross-linked polymers that give the epoxy coating its beneficial properties.

Curing

Following powder application, the coating is allowed to cure for a short period (approximately 30 seconds) during which it hardens. To facilitate handling, the curing period is often followed by an air or water quench that quickly reduces the bar temperature.

How Epoxy Coating Protects

Fusion-bonded epoxy coating principally protects against corrosion by serving as a barrier that isolates the steel from the oxygen, moisture, and chloride ions that are needed to cause corrosion. Epoxy coating also has a high electrical resistance, which blocks the flow of electrons that make up the electrochemical process of corrosion. In addition to serving as a circuit breaker, the coating protects in way that is less obvious: coating reduces the size and number of potential cathode sites, which will limit the rate of any corrosion reaction that could occur. In order for macrocell corrosion to take place, a large area of steel surface is needed to serve as the cathode where oxygen reduction can occur.