Offshore Corrosion Protection With Thermal-Sprayed Aluminum

ABSTRACT Thermal-sprayed aluminum (TSA) coatings offer distinct advantages for long-term corrosion protection of offshore steel structures and facilities over conventional coatings. This coating system, which has been seldom used in the offshore oil industry, is effective for immersion, splash zone, and atmospheric service. An overview of past experiences, the results of recent laboratory tests, and Conoco's field experiences on offshore platforms with TSA coatings are summarized. Procedures found to be effective for coating application and quality control are discussed along with future applications where TSA coatings offer unique advantages. INTRODUCTION Corrosion protection of offshore structures and facilities is essential for safe long-term operation of these facilities. Unfortunately, the achievement of this goal can sometimes be difficult and expensive. For the past few years, we have pursued a wide ranging research program to evaluate the potential application of thermal sprayed aluminum coatings to provide long-term corrosion protection of steel on offshore structures and facilities. The result of these studies have shown that thermal-sprayed aluminum (TSA) coatings offer distinct advantages for long-term corrosion protection over conventional coatings for marine service. This TSA coating system, which has seldom been used in the offshore industry, is effective for immersion, splash-zone, and atmospheric services. In particular, we have found that this coating offers a tough barrier coating, it is economical, it provides effective cathodic protection to bare steel exposed by damage to the coating, and finally, it is the only coating system recognized to provide 20-yearmaintenance-free service.l This paper will present a brief description of thermal spraying and an overview of field and research experiences by Conoco and others with TSA. Costs, application and quality control procedures, and suggestions for future applications of TSA, will also be discussed. THERMAL SPRAYING The process for thermal spraying metal coatings on a substrate was developed in 1910. This procedure which is also called metallizing has been more widely used in Europe than the United States. Most applications of thermal-sprayed coatings for corrosion protection has been for bridges, towers, and other steel facilities subject to atmospheric and tidal zone exposures. Applications for immersion service in fresh and ocean water has been very limited. The thermal-spraying process consists of melting a material into tiny droplets and spraying them onto a substrate where they fuse together and solidify to form a coating. The coating material in the form of a wire or powder is fed into a heat source where molten droplets are formed and propelled by a gas jet to the substrate being coated. The heat source may be an oxygen-hydrocarbon flame, an electric arc, or a plasma arc with the overall procedure being called, respectively, flame spraying, arc spraying, or plasma spraying. It is essential that the substrate be very clean and have an anchor pattern profile that will facilitate bonding between the coating and the substrate. A special advantage of thermal-sprayed coatings are that they dry immediately and no waiting before further handling is required.