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 erected without painting. In recent years, however, it has been recognized that in areas of high humidity and condensation, and where chlorides can accumulate, severe corrosion, scaling, and pitting of weathering steel can occur. Bridges in these environments require corrosion protection by painting to avoid potential metal loss. Painting of new uncontaminated weathering steel is generally not considered a problem. T est fence and laboratory data developed by the paint industry have indicated that conventional coating systems such as oil alkyds and epoxies will perform comparably on weathering steel and on carbon steel if the degree of surface preparation is equivalent. The major problem faced by highway departments and other owners of weathering steel structures is protecting weathering steel that has corroded in the presence of chlorides and other contaminants. Conventional cleaning techniques such as dry abrasive blasting do not remove the chlorides, which apparently penetrate the bases of pits in the steel. The performance of standard highway coatings such as oil alkyd, epoxies, and zinc-rich systems over chloride contaminated steel has not been satisfactory. The problems posed by maintenance of weathering steel structures have been the subject of numerous studies. A comprehensive literature survey was conducted as part of the interim report of this work, additional pertinent material has subsequently been issued under the National Cooperative Highway Research Program.
The prinCipal objective of this program was to establish techniques, procedures, and guidelines for maintenance cleaning and coating of bridges constructed of weathering steel. The first step was to review the existing literature to determine the extent and nature of the problem and to identify the chemical and physical processes and factors associated with this phenomenon. Because of the critical importance of the surface condition to coating durability, a major effort was directed at the surface cleanliness of the steel. In particular, the role of chlorides in accelerating metallic corrosion and the paint degradation was studied. Two aspects were considered: analysis of surface cleanliness and method of surface cleaning. A technique for detecting and measuring the amount of chloride or other soluble salt on the surface was developed. The goal was to provide a method which can be readily used in the field. It was first necessary to establish a standard laboratory reference technique to verify the accuracy and precision of any field technique. The cleaning methods considered included standard surface preparation techniques such as dry abrasive blasting, various forms of wet and water blasting, along with other special techniques such as chemical or heat treatment of the surface. Each method was evaluated for its effectiveness in removing chloride as well as its practicality and suitability as a field cleaning technique. Both of the above aspects of this research effort were reported in detail in the interim report describing the laboratory testing efforts. Although it would be desirable to remove all, or essentially all, of the chloride from the surface, this may not be economically feasible for most structures. Of principal interest was how the chloride remaining on the surface affects the adhesion and durability of the coating system. The next step was, therefore, to evaluate the performance of candidate coating systems applied over surfaces containing varying levels of chloride. Coatings were selected on the basis of commercial availability, demonstrated performance under adverse circumstances, and practicality for field application.