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| Sulphuric Acid on the WebTM | Technology Manual | DKL Engineering, Inc. |
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Knowledge for
the Sulphuric Acid Industry Introduction
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In the simplest terms possible, thermal spray coating involves heating a
material, in powder or wire form, to a molten or semi-molten state. The material is
propelled using a stream of gas or compressed air to deposit it, creating a surface
structure on a given substrate. The coating material may consist of a single
element, but is often an alloy or composite with unique physical properties that are only
achievable through the thermal spray process. The metallizing process is employed in sulphuric acid plants in areas where carbon steel is exposed to high temperature and oxidizing conditions that would cause it to scale. The formation of scale results in metal losses and blockages of tubes in heat exchangers or catalyst beds. If used, metallizing is recommended on process equipment and ducting where temperatures exceed 371°C (700°F) in the presence of O2, SO2 and SO3. The metal applied to carbon steel is aluminum and is applied is 4 step process.
The base material will generally be carbon steel in the 'mill descaled' condition. The metal should be free of rust, paint, oil, etc. Sand blasting, flame cleaning torches, and/or solvent should be used to achieve a clean surface. A bonding coat is then applied to the surface using a suitable wire type metallizing gun. The bonding coat must be applied soon after the surface is sand blasted (within 2 hours). The aluminum coating is then applied using a suitable wire metallizing gun with acetylene and oxygen as the fuel. The aluminum coating is generally applied to a thickness of 0.25-0.31 mm (10-12 mils). A small amount of the aluminum is oxidized during the spraying process which helps the aluminum adhere to the steel surface. The applied aluminum coating is porous and would allow the underlying steel to rust if the pores are not fill. Therefore, the final step is the application of silicone aluminum paint by brushing or spraying it on. Two coats are generally required. Failures of the coating occur when there is a lost of adhesion between the coating and the steel surface. The most common mode of failure is oxidation of the steel caused by pinholes, poor surface preparation, mechanical damage to the coating, thermal shock, contamination of the steel, or permeation. It is unlikely that the coating will be damaged by high temperatures. Aluminum melts at 660°C (1220°F) which is well above the normal maximum operating temperature in a contact section of an acid plant. Sulzer Canada Inc. Sulzer Metco (US) Inc.
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