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Metallizing and Hard Banding

Overview

Metallizing
Metallizing is a metal-coating process in which a metal substrate is coated with another metal by thermal-spray equipment. The metal coating is typically applied with a twin-arc wire gun or a gas-powered gun. The HVOF process has also become popular as a metallizing procedure in recent years and is especially efficient in the applications of ceramic and plastics as well as metal. Any metal that can be made into a wire can be melted and sprayed onto a substrate in the twin-arc and gas-powered thermal-spray procedures; and a wide variety of common and exotic metal substrates can be metallized. Zinc, aluminum, copper, stainless steel, magnesium and various exotic metals are all commonly used in metallizing by Superior Shot Peening. Metallizing can be on-site or in-house and can be performed in almost any weather.

Metallizing combines the advantage of a mechanically-bonded coating that will not deform the substrate with the option of choosing from several aesthetically pleasing metallic finishes. Before being bonded to the substrate, the metal is first atomized by the metal-spray equipment. The atomized droplets are then propelled towards the substrate where they flatten upon impact and begin to bond to the substrate object and to each other. This gentle, low-heat process allows for a strong mechanically-bonded coating of desired thickness to be applied to the substrate with no melting or warping of the target.

 

In many industries, a popular use of metallizing is the use of metallizing for cathodic protection. In this case, a metal that is more likely to lose electrons (and act as an anode) is used to coat a substrate metal that is less likely to lose electrons. When corrosion begins, the corrosion is more likely to form on the sacrificial anode (the metal coating) than the protected cathode (the substrate). This is a highly effective way of protecting a wide variety of metal equipment particularly in soils and marine environments. Some of the applications for metallizing are tension legs for offshore platforms, bridges, underwater concrete structures, and industrial plants where elevated temperatures prohibit standard coatings, such as stacks and mufflers.

Hard Banding
Hard banding is a process by which a rough, wear-resistant, band of tungsten carbide is applied to a metal part to help protect the metal from wear in a specific area. This tungsten-carbide hard banding is commonly used in the drilling tool industry on downhole tools. The band is applied by the metallizing thermal-spray process in an area on the tool that is expected to undergo high friction. As it need only be applied over a limited surface area, it is an economical way to eliminate wear and extend tool life.

Technical Information

The Arc-Wire and Gas-Powered Thermal Spray Procedures
Before metallizing, the chosen substrate must undergo a cleaning and a rough-surface preparation to ensure proper bonding of the metal coating to a clean substrate. Grit blasting is normally the preferred procedure. However, chemical etching and grinding can be appropriate in certain applications.

Superior Shot Peening uses both arc-wire guns and gas-powered guns in metallizing. The gas-powered gun uses acetylene to fuel and oxygen to cool as the metal wire is fed through the thermal-spray equipment. The arc-wire gun involves feeding two wires into the thermal-spray equipment. When the wires meet, an electrical arc is established between them. Compressed air is blown across the intersection, atomizing the metal and propelling it towards the substrate. Of the two procedures the arc-wire spray is almost always the most economically efficient.

Facts About Thermal-Spray Metallizing, Paint and Galvanizing

  • With the proper equipment and procedures, metallizing is most often cheaper than paint.
  • Although both metallizing and paint must be refreshed after a period of time, metallizing can be expected to provide corrosion protection for fifty of more years. Paint must be replaced considerably sooner.
  • Galvanizing (hot-dip metal coating) and thermal-spray metallizing both protect carbon steel by cathodic protection, but the metallizing procedure will extend the life of the substrate far beyond that expected with the galvanizing process.
  • Metallizing will produce a purer coating that galvanizing. The galvanizing procedure often causes iron to be absorbed into the protective coating, giving it a tendency to rust faster than the purer thermal-sprayed coating.
  • Metallizing will produce a more porous coating than galvanizing. This porosity allows a protective sealant to be applied to the substrate that will extend corrosion protection further.

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