Metalizing with either zinc or aluminum is a versatile and cost-effective way of stopping steel corrosion. A metalized coating of pure zinc or an alloy of 85% zinc and 15% aluminum protects steel for a longer time than paint whether that metalized coating is applied to new steel in-shop, or applied as a maintenance coating to existing structures on-site. Because of this longer service life, metalizing’s life-cycle cost can be lower than that of paint, and life-cycle cost is the objective way to compare the cost of one coating type versus another.
Whether shop or field applied, the metalizing process includes three essential steps:
Proper surface preparation is the indispensable first step in the metalizing process. Surface preparation includes cleaning the steel (removing dirt, rust, millscale, etc.) and roughening the steel surface to create the surface profile or anchor tooth pattern to which the sprayed-metal adheres.
The blast cleaning abrasive (grit) shall be of the size and grade that will produce a uniform angular surface profile depth of 2.5 to 5 mils. In general, one can say that the rougher the surface, the higher the bond strength. The following is taken from an early version of the SSPC-CS 23.00, the forerunner of the current Joint Standard on metalizing.
The depth of surface profile required depends upon thermal sprayed coating thickness. When specifying greater than 12 mils (300 microns) of thickness, a minimum of 3 mils (75 microns) surface profile (not to exceed 1/3 total coating thickness) shall be achieved to ensure coating adhesion. NOTE: Adhesive strength tends to increase with surface roughness.14
(See the reference to
excess profile depth at Service Life & Repair under the heading AASHTO S8.2/SSPC-PA 19 – Section 11 & Appendix A.)
SSPC Specifications SP5 White Metal (NACE 1), SP10 Near-White Metal (NACE 2), and SSPC- SP16 Brush-Off Blast define blast cleaning. For more details on surface preparation for metalizing, see AWS C2.23 (Joint Standard), Clause 5 and specifically 5.2 Surface Cleanliness, 5.2.2, guidance on abrasive selection, and 5.3 Surface Profile.
The two methods for spraying zinc and aluminum onto structural steel are gas flame-spray and electric arc-spray; either an oxy-fuel flame or an electric arc is used to melt the zinc or aluminum spray wire(s).15 The oxy-fuel gun produces temperatures in the range of 5,000-5,600°F; the arc-spray temperature may be twice that of the gas flame and both temperatures are well above the melting points of zinc (787 °F) and aluminum (1220 °F).
The metalizing gun feeds the spray wire(s) into its heat source, a flame, or an arc, where the tip(s) of the metal wire(s) melt. Compressed air atomizes the molten metal stripping loose particles of that metal and propelling those particles to the roughened steel surface. The metal particles impact on the steel, flatten, freeze, and build up particle upon particle to form a coating.
The sprayed-metal establishes a close mechanical bond with the steel and, unlike zinc paint, there is direct contact between the galvanic metal and the steel. The sprayed-metal coating is somewhat porous and over time corrosion products of the zinc or aluminum form in the porosity and on the surface of lamellar structured metal coating.16 These corrosion products, oxides, hydroxides, carbonates, fill voids in the coating and close any through porosity to the substrate. The buildup of oxides and other corrosion products adds another dimension to the coating by its shielding the sprayed-metal itself from the atmosphere, slowing its galvanic consumption.
The gradual galvanic consumption of the coating metal is the primary mechanism by which the sprayed zinc metal protects steel. (See Sealing & Topcoating.)
Sprayed-metal contains no VOC’s. Sprayed zinc and sprayed aluminum contain no volatile organic compounds (VOC's), and require no drying or curing time.
No cure time. The sprayed-metal cools immediately on impact and is then ready to accept a liquid sealer. The complete process, blasting, metalizing, and sealing may be completed in just one day meaning that in fabricating and coating shops, workspace is freed up. On-site, scaffolding can be moved as soon as sealing is completed.
No application temperature limit. Zinc and aluminum may be applied nearly year ‘round in almost any shop or field environment. Since ambient temperature does not limit the use of the process, as is the case with painting, the coating season can be extended by several months in most regions allowing for key bridges, water tanks and other structures to be coated on-site when demands on those facilities are low.17 (See the Joint Standard, paragraphs 5.1.1 “Ambient Temperature” regarding dew point, and 8.3 “Spraying in Low Temperature Environments” regarding condensation. Also, refer to the additional coating temperature information in the galvanizing information below.)
Very long service life. According to two national standards, British Standard 5493 and Canadian Standard G189, metalized coatings provide very long service lives even in the most corrosive environments.18 In the British Standard for instance, only sealed metalized coatings are said to provide a very long service life in “Sea water splash” or “Sea water immersed” service. Not one of the dozen or so paints types listed in 5493 is ranked in the very long service life category in those two environments. The Canadian Standard proposes metalizing service lives of 40 years or longer in industrial and marine environments.19