Spray Processes

For the chromatographic column, flow of solution from the narrow inlet tube into the ionization/desolvation region is measured in terms of only a few microliters per minute. Under these circumstances, spraying becomes very easy by application of a high electrical potential of about 3-4 kV to the end of the nanotube. Similarly, spraying from any narrow capillary is also possible. The ions formed as part of the spraying process follow Z-shaped trajectories, as discussed below. 

Thermal spray processes can be used to give coatings of chromium carbide or nickel chromium for erosion resistance, copper nickel indium for fretting resistance, tungsten carbide cobalt for wear and abrasion resistance, and even aluminum siHcon polyester mixtures for abradabiHty. 

Thermal or Flame Spray Process. The earliest experiments in metal spray used molten metal fed to a spray apparatus, where it was dispersed by a high speed air jet into tiny droplets and simultaneously blown onto the surface of the part to be covered. The metal solidified on contact. Modem processes use a more convenient source than premelted metal. Spray heads using a flame or an electrical arc to melt metal wires or powders directly are much more convenient. These are the only types used on a large scale in the United States. 

Surface cleanliness of the substrate is important for all thermal spray processes. Degreasing, which formerly often reHed on freons or chlorinated... 

The laser spray process uses a high power carbon dioxide laser focused onto the surface of the part to be metallized. A carrier gas such as belium blows metal particles into the path of the laser and onto the part. The laser melted particles may fuse to the surface, or may be incorporated into an aHoy in a molten surface up to 1-mm thick. The laser can be used for selective aHoying of the surface, for production of amorphous coatings, or for laser hardening. 

Each type of metallic coating process has some sort of hazard, whether it is thermal energy, the reactivity of molten salt or metal baths, particulates in the air from spray processes, poisonous gases from pack cementation and diffusion, or electrical hazards associated with arc spray or ion implantation. 

Property Spray process DBM RoUer process DBM Dry whey, extra... 

Dried buttermilk is made by either the dmm or spray process. Buttermilk is usually pasteurized before drying, even though the milk was previously pasteurized before churning. Dried buttermilk is used primarily for baking, confectionery, and dairy products. 

Flame spraying is no longer the most widely used melt-spraying process. In the power-feed method, powders of relatively uniform size (<44 fim (325 mesh)) are fed at a controlled rate into the flame. The torch, which can be held by hand, is aimed a few cm from the surface. The particles remain in the flame envelope until impingement. Particle velocity is typically 46 m/s, and the particles become at least partially molten. Upon impingement, the particles cool rapidly and soHdify to form a relatively porous, but coherent, polycrystalline layer. In the rod-feed system, the flame impinges on the tip of a rod made of the material to be sprayed. As the rod becomes molten, droplets of material leave the rod with the flame. The rod is fed into the flame at a rate commensurate with melt removal. The torch is held at a distance of ca 8 cm from the object to be coated particle velocities are ca 185 m/s. 

Arc wire utilizes two continuously fed 1.6-mm dia intersecting wires with a d-c arc maintained between the wire tips as they meet. Compressed gas (usually air) strips the molten metal from the tips and forms a directional spray stream. This process is widely used to spray most metals. Arc wire is the most economical process because of the wire feedstock. Moreover, it utilizes - 10% of the thermal energy of the other spray processes (0.4 vs 6.6 kWh/kg using stainless steel) because of the direct arc heating of the wire tips. 

The volume of thermosetting powders sold exceeds that of thermoplastics by a wide margin. Thermoplastic resins are almost synonymous with fluidized-bed appHed thick-film functional coatings whereas thermosetting powders are used almost exclusively in electrostatic spray processes and appHed as thin-film decorative coatings. 

Minimize moisture hiiildiip losses. Avoid formulations which exhibit adhesive characteristics with respect to process walls. Maintain spray nozzles to avoid caking and nozzle drip. Avoid spray entrainment in process air streams, and spraying process walls. 

With damage of a large area, the concrete is removed down to the uppermost layer of the reinforcement then the first layer of sprayed concrete is applied (see Fig. 19-3a). The anode is fixed to this layer and is followed by a second layer of sprayed concrete. With large areas of old concrete which are still solid, the anodes can be attached to this and finally embedded with sprayed concrete (see Fig. 19-3b). The anodes must supply the required protection current and be structurally robust. The bond between the old concrete and sprayed concrete must not be damaged during the spraying process or in operation. 

Despite the progress outlined in this chapter, much work remains to be done in the metal surface preparation arena. For example, there is still no ideal surface preparation method that does for steel what anodization processes do for aluminum and titanium. The plasma spray process looks encouraging but because it is slow for large areas and requires rather expensive robot controlled plasma spray equipment, its use will probably be limited to some rather special applications. For more general use, the sol-gel process has potential if future studies confirm recently reported results. 

The coating of the adsorbent layer with the reagent solution is more homogeneous than with even the most carefully carried out spraying process. 

In all metal spraying processes the particles emerge from the nozzle in a conical stream, and although the particles near the centre are molten, those at the periphery have solidified. In the powder process there are in addition solid particles which have not melted. The solid particles tend to become entrapped in the coating, making it porous. The effect is more pronounced in the powder process owing to the larger number of solid particles present. 

In recent years, the spraying process has been adapted for hard facing, using the chromium-nickel-boron alloys which have become known as Colmonoy. More recently still, the cobalt-base Stellite alloys have also been used. These materials in powder form are sprayed on to the surface in the usual way. The deposit is afterwards heat treated by a torch, so that fusion takes place. The process is often known as spray-welding. Such coatings are primarily used for hard facing under wear conditions, but as the Anal surface is nickel-chromium or cobalt-chromium they exhibit very high anticorrosive properties. 

The spraying process provides a method of treating steel with coatings of zinc or aluminium, which can afterwards be painted. A combination of such a metallic primer and a good peunt system is the most effective means of combatting corrosion of constructional steelwork at a reasonable cost that is yet known. 

Airless Spraying the process of atomisation of paint by forcing it through an orifice at high pressure. This effect is often aided by the vaporisation of the solvents especially if the paint has been previously heated. The term is not generally applied to those electrostatic spraying processes which do not use air for atomisation. 

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