Thermal-spray coatings in corrosion
Thermal-spray coatings in corrosion
Keywords Coatings, Corrosion prevention,Thermal spray
Many engineers think thermal-spray coatings are strictly for high-end applications such as jet engines and power-generation gas turbines. While the technology was originally developed for such detailed components, it can also be used for less-complex situations.
Thermal-spray coatings are produced by heating a powder or wire material to a molten or semi-molten state and spraying it on components. Manufacturers shoot the molten spray onto parts using gas or compressed air. The coatings may consist of a single element, but are often an alloy or composite tailored to provide the best combination of physical qualities. The combination of materials is nearly limitless. Coatings can be metallic, plastic, or a combination that addresses specific performance needs.
Manufacturers apply thermal-spray coatings using several different processes, including fuel combustion, plasma spray, and electric arc equipment. Devices operate in normal air environments as well as in controlled atmospheres or even under water. Both manual and automated equipment are available.
Thermal-spray coatings are used to extend product life, increase performance, and reduce production and maintenance costs. Engineers take advantage of thermal-spray coatings to address design issues including:
Coatings boost hardness and surface-finish conditions on mating surfaces to resist abrasive wear,adhesive wear, and fretting. Applications include screw-conveyor bearings used in concrete mixing equipment, compressor seal rings on turbine engines, and piston rings on marine engines.
Thermal coatings bring worn components back to original dimensions by providing a variety of thicknesses, finishes, and compositions. Sometimes reworked pans actually outperform original designs because the coatings are better suited than the substrate to resist corrosion, oxidation, and mechanical wear. Applications include fuser rolls for photocopiers, flow valves used in hydroelectric dams,and turbocharger shafts for train motors.
High temperatures cause surfaces to degrade from oxidation and chemical attack. Thermal coatings improve heat-transfer qualities and slow down the effects of corrosive environments. Applications include jet-engine combustion chambers, casting troughs used to form metals, and automotive-engine valve seats.
Thermal coatings protect substrates from harsh liquids, gases and particulate solids that corrode and erode metal surfaces. When these fluids move at high speeds, they prevent the metal surface from forming a protective oxide layer and therefore lead to erosion. Cavitation results from pressure changes which causes vapour bubbles to form and collapse. The bubbles produce shock waves that attack metal surfaces. Particulates within the fluid damage the surface even further by acting much the same as grit blasting materials. Applications include automotive oxygen sensors,waste-water pump pistons, and rotary-pump impellers.
Thermal-spray coatings can produce near-zero clearances to increase sealing efficiency on rotating components. By letting the component wear its own seal, abradable, sacrificial coatings help improve devices such as compressor-blade seals in turbine engines,fan blades in gas turbines used to generate power, and compressor seals for natural-gas and petrochemicals equipment.
Gaseous and particulate pollutants now top the list of environmental pollutants that attack engineering structures. Thermal-spray coatings resist corrosion in applications such as bridge structures, ships and fishing boats, and water-storage tanks.
Conductive alloys can be tailored to control electrical and thermal qualities to increase conductivity,create a dielectric or thermal barrier, or shield electronics from electromagnetic impulses. Applications include satellite antennas, capacitor solder connection on plastic substrates, and for grounding trains through their axles.
