Spray Wires and the Spray Metal Coating

The three metal wires used for metalizing structural steel are zinc, aluminum, and the 85% zinc / 15% aluminum alloy. Coatings of these metals protect steel in three ways.

  • First, the sprayed-metal is a barrier coating that separates the steel substrate from the air and moisture in the environment24 (See AWS C2.14 “Corrosion Tests of Flame-Sprayed Coated Steel” in the section Sprayed-Metal Coating Thickness regarding minimum thicknesses.)
  • Second, these coating metals are anodic to steel and protect the steel galvanically.25
  • Third, zinc and aluminum corrosion products develop in the pores and on the surface of sprayed-metal. This corrosion layer shields the metallic coating from the corrosive atmosphere slowing the consumption (oxidization) of the galvanic coating metal. See the reference to NAVSEA at Sealed Metal Coating below.

Spray Wires for Coating Structural Steel

The three metals used to protect steel in atmospheric or immersion service are 99.99% zinc, aluminum in the range of 99.0-99.5%, and the zinc aluminum alloy composed of 85% zinc and 15% aluminum. The following is general information on each of these sprayed-metal coatings.

Zinc (99.99%)

Metalized zinc coatings are used in rural, industrial, and marine environments. The three mechanisms of protection described above are most pronounced with zinc, and it is by these three mechanisms, barrier, galvanic action, and corrosion product development that zinc and zinc alloy coatings protect steel.

Zinc is the most active of the three coating metals and provides a high degree of cathodic protection to the underlying steel. Zinc’s corrosion resistance is unique; it has a high corrosion rate, which is offset by the formation of the aforementioned corrosion products on the metal coating’s surface.

  • Sprayed zinc and 85/15 are commonly specified for metalizing bridge steel in part because of their relative ease of application and adhesive strength. This is especially important for on-site work and will be a factor when repairing previously metal-coated steel.
  • Zinc’s galvanic protection of steel reaches beyond the coated surface to protect uncoated areas for example where there is mechanical damage.26
  • Sprayed zinc coatings do not degrade in sunlight.
  • Zinc coatings react sufficiently with concrete to form a mechanical bond, which is important when metalizing bridge expansion joints or end dams27
  • Zinc metalizing is used to coat bare, partially galvanized steel girders when those girders cannot fit properly into a galvanizing kettle. Sprayed zinc is the best way to zinc coat bare, partially galvanized steel, and to repair damaged galvanized steel. (See ASTM A780 Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings.)28
  • Cathodic protection of rebar set in concrete. Metalized zinc forms the anode in cathodic protection systems that protect rebar set in concrete e.g. the reinforcing steel in bridge substructures, parking garages, etc. There are two systems that use sprayed zinc; one relies on an impressed electrical current, and the other is passive or galvanic.29
  • Zinc pH range is 6 to 1230
  • The tensile adhesion strength of metalized zinc on blast cleaned steel is about 500 psi.31
  • Service temperatures up to about 140° F (60° C)32
  • Density: .258 lb per cubic inch
 
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