Weathering steel, introduced in the 1960's, is high strength steel containing low amounts of chromium and other alloys which offers improved corrosion resistance compared to carbon steel. It was generally put into use without painting. In recent years it has been recognized that severe corrosion of weathering steel can occur in areas of high humidity and condensation, and where chlorides can accumulate. Highway bridges or pedestrian bridges that are treated with de-icing chemicals that contain chlorides are good examples of locations where weathering steel requires protection in order to prevent deterioration.

Painting of uncontaminated weathering steel is generally not considered a problem since conventional coating systems such as oil alkyds and epoxies will perform comparably on weathering steel and carbon steel if the surface preparation is equivalent.

The problem occurs with weathering steel that has corroded in the presence of chlorides and other contaminants. Conventional cleaning techniques such as power tool cleaning does not remove the chlorides, and the performance of conventional coating systems over chloride contaminated steel is unsatisfactory.

The following guidelines address the method(s) of surface preparation and coating of corroded weathering steel in severe environmental exposure, including areas that receive deposit of salt and high humidity, or frequent wet/dry cycling.

Cleaning Methods: Abrasive blast cleaning to near-white metal (SSPC-SP10) is mandatory for cleaning corroded weathering steel because it removes the chlorides and corrosion products in the pitted areas of the steel. After cleaning, the surface should be evaluated for visual and chemical cleanliness. No rust, mill scale or foreign matter is permitted, and the amount of chloride ion present on the surface should be measured to be less than .001 grains per sq. in.

Coating Systems: The recommended coating systems all utilize a primer coat, intermediate coat, and a topcoat. The topcoat is for aesthetic purposes or to provide UV protection for the intermediate coat. The three acceptable coating systems are as follows:

1. Ethyl Silicate Zinc / Epoxy / Urethane

• Primer: Two-component ethyl silicate solvent-borne inorganic zinc-rich coating.
• Intermediate Coat: Two-component high-build epoxy polyamide, by the same manufacturer of the primer.
• Topcoat: Two-component aliphatic urethane.

• Primer: Two-component epoxy polyamide zinc-rich coating.
• Intermediate Coat: Two-component high build epoxy polyamide.
• Topcoat: Two-component aliphatic urethane.

• Primer: Thermally sprayed metallic zinc coating.
• Intermediate Coat: Low build (thin film) epoxy polyamide to provide an additional barrier over the zinc.
• Topcoat: Two-component aliphatic urethane.