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Coatings
Thermal Sprayed Coatings
Thermal spraying refers to a process by which a metal wire or powder is melted and sprayed onto a surface to form a coating. In the past, the spraying of molten metal was referred to as metal spraying or metallizing; now thermal spraying is more appropriate. A thermal spray gun is used to apply the coatings. The thermal spray gun heats the metallic wire or powder to a molten state and compressed air or other gas propels it onto the surface to form a coating. The compressed gas also aids in division and atomization of the molten coating.
A white metal blast surface preparation procedure (SSPC-SP-5) is required prior to applying any thermal spray coating. Thermal spray coatings don't have the ability to penetrate, seal, or encapsulate surface contaminants and as such the surface must be clean. Upon cooling the metal particles that have been applied to the surface undergo a thermal contraction that results in residual stresses in the metallic film. As a result, a minimum anchor pattern of 0.08 mm (3 mils) is required on to withstand the shrinkage stresses and to disperse these stresses over a larger interfacial area. Anchor patterns of 0.10 to 0.13 mm (4 to 5 mils) are preferable to the minimum of 0.08 mm (3 mils).
The two metals most commonly applied by thermal spray are zinc and aluminum. These metals and their alloys provide excellent protection in a variety of marine and industrial corrosive environments. The most common methods of thermal spray coating application are: wire flame spraying, powder flame spraying, electric arc spraying, and plasma spraying. The applicable military standard for thermal spray coatings for corrosion control applications is MIL-STD-2138.
Wire Flame Spraying
Wire flame spraying involves the use of a hand-held gun with an air motor that draws the metal wire into an oxygen-fuel gas flame where it is melted. The molten metal is then forced onto the surface by an air blast. Under ideal conditions this type of gun can apply zinc at a rate of 1000 sq. ft per hour and aluminum at a rate of 1250 sq. ft per hour both with a thickness of 1 mil. In the field only about 60 percent of these rates are actually attained because of issues such as changing wires, moving, etc. Wire flame sprayed coatings generally exhibit lower bond strengths, higher porosity, and higher heat transfer to the substrate than other forms of thermal spraying.
Powder Flame Spraying
In the case of powder flame spraying a metal powder is used to form the coating. The powder is stored in a powder hopper mounted on top of the spray gun from which it is carried into oxygen -fuel flame where it is melted. The flame carries the molten coating onto the work piece. Preheating of the substrate is necessary with this coating method. As a result of the lower particle velocities, coatings produced by this method generally have the lowest bond strength and highest porosity of all the flame spray methods.
Electric Arc Spraying
Electric arc spraying is accomplished by creating a continuous electric arc between the tips of two wires through the passage of current. Heat from the electric arc melts the wires and an air jet propels the molten metal onto the surface. The extremely high temperatures in the arc zone result in coatings with high cohesion and strength.
Plasma Spraying
Plasma spraying uses metal in powder form. The plasma that is delivered from the tip of the gun resembles a bright flame and is formed by forcing an inert gas through and electric arc within the gun. Powder is pulled into the plasma arc cavity by the plasma stream, which also melts the powder. The plasma jet propels the molten coating onto the work piece. The plasma coating process produces high quality coatings but the equipment is expensive and difficult to operate. Generally, plasma coatings are best applied in a shop environment and are typically used for restoration of dimension work.
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SSPC-CS 23.00/AWS C2.23M/NACE No. 12, Specification for the Application of Thermal Spray Coatings (Metallizing) of Aluminum, Zinc, and Their Alloys and Composites for the Corrosion Protection of Steel |
07/01/2003 |
This standard is a procedure for the application of metallic thermal spray coatings (TSC) of aluminum, zinc, and their alloys and composites for the corrosion protection of steel. Required equipment, application procedures, and in-process quality control checkpoints are specified. This standard may be used as a procurement document. Annex A presents a fill-in-the-blanks model procurement specification.
Not included in this standard are requirements for design and fabrication, thermal spray equipment qualification, coating selection, and operator and inspector certification.
This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety problems associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices, and to determine the applicability of regulatory limitations prior to use. |
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