Metalizing is one of several thermal spray processes in which a metal is melted, atomized by a stream of compressed air, and sprayed onto a blast-cleaned steel surface to form a coating. The American Welding Society (AWS) defines the broader category thermal spray as
. . . a group of processes in which finely divided metallic or nonmetallic surfacing materials are deposited in a molten or semi-molten condition onto a substrate to form a spray deposit.7
Dr. Max Ulrich Schoop (1871-1956), was a Swiss physicist, teacher, and inventor, who has been credited with the invention of the metalizing gun in the early 1900’s. A multi-billion dollar thermal-spray industry has evolved from Schoop’s simple gas flame, wire-fed gun and today that industry concentrates on the application of very sophisticated coating materials by oxy-fuel flame-spray, electric arc-spray, and the more advanced atmospheric-plasma-spray (APS), and high-velocity-oxygen-fuel (HVOF) processes. In addition to corrosion control, the uses for these coatings include machine element repair, wear resistance, thermal barrier, and many other industrial applications.
Metalizing For Corrision Control™ deals with only one of the many applications of metal spraying – the deposit of galvanic metal coatings of zinc and aluminum onto iron and steel to prevent their corrosion.
The two types of wire-fed systems used to spray zinc and aluminum onto steel are the older oxy/fuel flame-spray, and the innovative high throughput electric arc-spray. We refer to both of these spray processes as metalizing, and the protective coatings that they deposit as metalized or sprayed-metal coatings.8
Metalizing with zinc is a better way of coating structural steel than painting. Metalized zinc’s galvanic protection is qualitatively better than that of zinc paint, and the sprayed-metal coating has a longer life to first maintenance than paint. These qualities make metalizing the ideal protective coating for structural steel, specifically for bridges, and especially for bridges exposed to the most corrosive environments. Zinc, and the 85% zinc and 15% aluminum alloy are the spray materials best suited to bridge metalizing.
A forgotten number of steel bridges in the United States were metalized as early as the late-1920’s. Occasionally, there is a mention here or there of the Kaw River Bridge or the Broadway-Woods Weather viaduct in Kansas City which were both metalized with zinc; but the details of their metalizing, and most probably, the bridges themselves have disappeared. One notable example of that early metalizing survives though, the SEPTA rail bridge over Ridge Avenue in Philadelphia that was metalized in the mid-1930’s by Metalweld, Inc. Remarkably, when that bridge was inspected in 1987, some of the original metalized zinc coating remained, still providing galvanic protection.
J.C. Bailey and F.C. Porter in their 1972 report
Sprayed metal coatings for the protection of structural steel – a fresh appraisal, wrote,
Over 100 important bridges and structures in various parts of the world, including tropical countries are recorded as being satisfactorily protected by metal spraying with zinc or aluminum during the last 30 years, and there are very many lesser structures not fully documented.
In the early 1980’s, the Federal Highway Administration acknowledged metalizing’s value as presented in the AWS research report
Corrosion Tests of Flame-Sprayed Coated Steel, dated 1974. FHWA then financed bridge metalizing trials beginning with Acorn Street in Providence, Rhode Island in 1984 and then by the Ohio Department of Transportation at Morrow County Route 229 in 1985. Ohio followed this first project by metalizing several other bridges including two on I-71 north of Columbus, the first at Mile 185 using 85/15, and then the bridge at Mile 135 using 99.99% zinc. This toe in the water approach laid the foundation for a wider use of metalizing across the U.S.
Over these past thirty-five years, a number of transportation organizations, state departments of transportation, tunnels, and turnpikes have used metalizing, and while metalizing is still underutilized in the United States, it is encouraging that these structure owners have used the process on girders, expansion joints and even for the cathodic protection of steel rebar set in concrete. The fact that owners now specify metalizing on a modest number of bridges is reason for optimism; after all, it wasn’t so long ago that most specifiers had never heard of metalizing. Today, all bridge engineers know about metalizing and the coatings industry has followed the lead of these several state departments of transportation and FHWA.
AASHTO S8.2, Specification for Application of Thermal Spray Coating Systems to Steel Bridges10
Specification AASHTO S8.2/ SSPC-PA 18 was issued in 2017. It may be thought of as the steel protective coatings industry’s imprimatur for bridge metalizing
The purpose of AASHTO S8.2 is given in its introduction, “The primary objective of this specification is to achieve quality and value in the application of metallic thermal sprayed coating (TSC) systems.” AASHTO S8.2 puts metallizing into a bridge coating framework and tailors the AWS/SSPC/NACE Joint Standard to bridge metallizing. 11 12
In the early 1980’s the lack of an industry specification was an impediment to metalizing steel structures, bridges, potable water tanks, etc. Metalizing has cleared that obstacle and the periodic updates of the AWS/NACE/SSPC Joint Standard on metalizing and the publication of the AASHTO specification mark a turning point in the metalizing story.
Atmospheric Corrosion Testing of Metallized, Metallized and Painted, and Painted Steel,Haagenrud and Klinge cited the 19-Year Report as follows:
NSB [Norway] thus wanted very good reasons for changing their practices. The results from the AWS 19-years tests of zinc- and aluminium-coated steel and the unambiguous experience gained through the last two decades by using metal spraying for the protection of Norwegian road bridges against corrosion together with the constantly growing demand for non-toxic treatment procedures have now convinced the NSB of the advantages of metal spraying.(9th International Thermal Spraying Conference, 19-23 May 1980.
Specification for the Application of Thermal Spray Coatings (Metallizing) of Aluminum, Zinc, and Their Alloys and Composites for the Corrosion Protection of Steel