Thermal spray coatings protect petro-chemical components from corrosion.

APPLICATION NOTE

Metal spray steel to protect it from corrosion has long been recognized as a long-term solution for many industries. UK based Metallisation Ltd. (Dudley, West Midlands) has been providing anticorrosion solutions since the very beginning of the metal spray process in 1922 to a diverse range of industries all around the world. Users of the metal spray process commonly include the offshore, oil and gas, and marine industries for the protection of structures, vessels, pipelines, and storage tanks. Corrosion is a major problem for these industries.

The variety of metallised coatings is vast, but can be broken down into two main categories. These include finishing coatings such as anticorrosion or decorative coatings, and engineering coatings such as wear resistant and thermal barrier coatings.

The application of metal spray coatings requires some special equipment and expertise. Metallisation's expertise.from involvement in the industry from its inception allows the company to supply high quality, reliable metal spray equipment that meets the needs of its clients.

Advantages of Metal Spray

Major advantages of the metal spray process over traditional painting or galvanizing are that the coatings are available for almost instant use, with no drying or curing times. There is no risk of thermal distortion of the component and virtually no limit to the size of the structure that can be coated, unlike galvanizing where the size of the dipping bath limits the size of the structure. As many marine and offshore structures are very large, the metal spray process is particularly suited to corrosion protection of platforms and process equipment such as flare booms. The metals can also be sprayed in a wider range of climatic conditions (temperature and humidity) than paints, the adhesion of the coatings to steelwork is much better, and the finished surface is much more robust than paint. Metal spray coatings are often used as a base coat prior to painting steelwork, as the paint adheres well to the bond coating. This can also increase the life of the painted surface, because the metal spray surface protects the steel from corrosion, which in turn protects the paint from corroded steel.

The Metal Spray Process

There are four commonly used processes in metal spray: flame spray, are spray, plasma spray, and high velocity oxygen fuel (HVOF). Only two of these, flame spray and are spray, are normally used for anticorrosion coatings in marine, offshore, and petro-chemical environments.

All methods of metal spray involve the projection of small molten particles onto a prepared surface, where they adhere and form a continuous coating. Upon contact, the particles flatten onto the surface, solidify, and mechanically bond, intially onto the roughened substrate, and then onto each other as the coating thickness is increased (Fig.1). Coating thickness can range from around 50[mu]m up to several hundred microns or even millimeters for some metals. Typically, metal spray corrosion protection coating vary from 100 to 350 [mu]m.

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The Are Spray Process

In the are spray process, the raw material in the form of a pair of metallic wires is melted by an electric are. The molten material is atomized by a cone of compressed air and propelled towards the workpiece (Fig.2). Aluminum is particularly suited to are spray as this process can give coating bond strengths in excess of 15 MPa. Combined with the excellent performance of aluminum in saltwater environments, are spray aluminum is a very common process/material combination in the marine, offshore, and petro-chemical industries.

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The Flame Spray Process

In the flame spray process, a wire is fed by a driven roller system through the center of an oxygen-propane flame where it is melted. An annular air nozzle then applies a jet of high-pressure air, which atomizes and projects the molten material onto the surface (Fig. 3). Adhesion values with flame spray are generally less than are spray for many materials, but for zinc and aluminum, a respectable 5-7 MPa is achievable.

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In general terms, zinc is used in reasonably low-corrosion applications such as water tanks, some bridges, and general structural steelwork. Aluminum is used in harsher corrosion applications such as saltwater and splash-zone areas. In addition, aluminum is used in high temperature applications such as flare booms on offshore oilrigs as the aluminum effectively diffuses into the substrate when exposed to high temperatures (aluminizing). Zinc/aluminum alloy is used in environments where the corrosion resistance of zinc is borderline.

Benefits of Thermal Spray Aluminum in Platform Applications

Metal spray is also sometimes referred to as thermal spray, which has been used in this particular application. The main reason for using thermal spray aluminum (TSA) coatings in areas subject to atmospheric exposure is to reduce the need for future maintenance. Maintenance costs for existing constructions have been increasing rapidly due to the fact that many painted platforms in the North Sea are now around 20 years old and require extensive surface preparation maintenance. This combined with longer design lifetimes for many recent installations (e.g., flare booms, crane booms, and steel underdecking) has generated increased interest in TSA coatings as a way to reduce future maintenance costs.

Major Offshore Operation in the North Sea

Anti Corrosion Protective Systems (APS) has regularly used Metallisation equipment to spray offshore installation. Various oil and gas platform structures have benefited from the application of TSA coating. Possibly the most corrosive environment of all is the area on a platform called the splash zone. In this area, the steelwork is subjected to very high corrosion levels due to the elevated level of moisture, oxygen, and chlorides (salt). A thick coating of aluminum can be thermal sprayed locally to the splash zone to offer exceptional corrosion protection in this highly corrosive area.

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On one particular installation for the North Sea, 2,000 [m.sup.2] of splash zone on a platform was grit blasted to cleanliness specification of SA 3, then sprayed with 300 [mu]m of aluminum using Metallisation Are 140 and Are700 systems. The coating was finished with 350 [mu]m of polyurethane paint.

APS has also completed a major project at a new petrochemical processing site in Saudi Arabia. Three vessels, totaling 3,000 [m.sub.2], were blasted to SA 2.5 and coated with 275 [mu]m of thermal spray aluminum. A subsequent 30[mu], of high-temperature silicone aluminum sealer was applied using the Arespray 700 and Arespray 140 systems. The Arespray 140 system was fitted with a 20-m supplies package and was used for areas of difficult and restricted access.

Another example of metal spray in a petro-chemical application is at the Esso refinery at Fawley. The refinery, owned by ExxonMobil, is the largest in the UK and one of the most complex installations in Europe. Situated on Southampton Water, it is a modern marine terminal that handles around 2,000 ship movements and 22 million metric tons of crude oil and other products every year. Using the Metallisation MK73 Flamespray equipment, ExxonMobil Chemical applied thermal spray aluminum to combat corrosion under insulation problems. Equipment can be coated with TSA when process units are shut down and also when units are live, depending on customer requirements.

These are just a few of many offshore petro-chemical companies that have successfully used metal or thermal spray processes. A selection of North Sea platforms has also used the TSA application including Gullfaks A/B/C, Snorre, Troll, and Heidrun. For more information, visit www. metallisation.com.