Metalizing steel bridges in the field

INNOVATIVE PRACTICE
(May, 1995)
Metalizing Steel Bridges in the Field
By Eric C. Lohrey, PE, Connecticut Department of Transportation

During the mid-1980's, the Connecticut Department of Transportation (ConnDOT) participated in a study sponsored by the Federal Highway Administration (FHWA) to evaluate the effectiveness of high performance coating materials for steel bridges. The study was designed to focus on maintenance options that would meet regulatory requirements for removal of lead-based paint from bridges, reduction of volatile organic compounds (VOC) in coatings, and disposal of contaminated blast residue from painting projects.

For this study, ConnDOT chose to evaluate a field-applied coating system that used a thermal sprayed zinc wire as the primer. At the time, thermal spraying, or metalizing, was a well-established technology for the corrosion protection of water storage and distribution facilities, ships, and offshore oil rigs. Although metalizing had performed favorably in these industries, its use was limited on bridges, primarily because it had higher initial coats than conventional painting. In the FHWA study, ConnDOT metalized sections of 4 bridges between 1987 and 1990.

Favorable results from the FHWA research project, together with the positive results of metalizing in other industries, prompted ConnDOT engineers to specify metalizing for an upcoming bridge coating project that included all structural steel of 5 local road bridges over the Connecticut Turnpike (I-95 and I-395) in eastern Connecticut. Originally erected in 1958, all of these bridges contain rolled beam girders.

The use of a metalized coating system on the current bridge project was also supported by reduced metalizing costs in recent years. Plans and specifications for the current project were prepared in- house by ConnDOT bridge personnel. Bids were received in December, and the project is scheduled to begin this spring.

This article will discuss ConnDOT's decision to use field-applied metalizing on its current bridge coating project. It describes the technical aspects of the metalizing process, the service life of metalized coatings, and appropriate applications for the coatings. The article also provides projected installation costs for the current metalizing project, and it compares these estimates to cost data from the FHWA-sponsored project. In addition, the article identifies circumstances that favor the use of metalizing over other bridge maintenance options.
 
When Field-Applied Metalizing is Appropriate

The following factors generally favor the use of field-applied metalizing over other bridge maintenance options such as conventional painting or steel replacement:

• Present bridge size is adequate for future travel demands.
• The deck is new, newly rehabilitated, or in excellent condition.
• The bridge structure is in good condition.
• The girders to be re-coated are in good condition.
• Existing paint is in poor condition.
• Steel is exposed to a harsh salt spray or industrial environment.
• High costs are anticipated for future maintenance operations.
• The bridge either carries, or is over, a heavily traveled roadway.
• The bridge is difficult to access.
• High fixed costs for working at the site (traffic control, mobilization) exist.

Overview of Metalizing

Metalizing is a specific process within the broad term of thermal spraying that has applications in a variety of industries. Technically, both metalizing and thermal spraying are welding processes in which a solid material is liquefied, atomized, propelled to a substrate, and then re-solidified to form a bonded coating. Metalizing specifically refers to the wire combustion, or flame-spray process, where metal wire is fed through and melted in the gaseous combustion chamber of a special gun. The molten metal is then atomized and propelled by the release of compressed air through the nozzle. As the liquid metal particles strike the substrate, they splatter and instantly solidify on the surface. This process, which involves millions of overlapping particles, forms a build-up of the sprayed metal. It can be controlled to produce a uniform coating.

A newer form of thermal spraying is the electric arc process. In this process, 2 wires are electrically charged with opposite polarities and fed through the gun at converging angles.

At the place where the wires meet, an electric arc is formed with sufficient energy to melt both wires while compressed air atomizes and propels the molten metal. Because the 2 wires are fed simultaneously, electric arc guns have higher production rates than flame-spray guns. Traditionally, electric arc guns have been too bulky and cumbersome for field work.

When the experimental installations on the 4 Connecticut bridges in the FHWA-sponsored study were completed in the 1980's, the flame-spray process was used because it was the most practical method available at the time for field applications. Since then, however, equipment improvements are anticipated to reduce costs. As a result, specifications for the current ConnDOT project allow the contractor to choose electric arc guns or flame-spray guns. In this article, metalized refers to either application method.

Zinc and Aluminum Alloy

When metalizing is specified for protecting steel structures from corrosion, zinc and aluminum are the most commonly used materials. The benefits of zinc include excellent galvanic protection of the steel (passive cathodic protection); ductility, which aids in obtaining uniform coverage and provides increased tolerance of surface defects; and low costs (compared to other metals). A lightweight material, aluminum offers abrasion resistance and passivity, which slows consumption of the coating during galvanic protection of the steel. An 85 percent zinc and 15 percent aluminum alloy has been identified as the optimum mixture for the protection of steel in harsh industrial and salt spray environments. Since these conditions are typical for bridges, the zinc and aluminum alloy was specified for the FHWA-sponsored project and ConnDOT's current metalizing coating project.

Table 1

1994 Bid Prices for Field Metalizing Steel Surface Area  Unit Price 
Bridge # Location Lump Sum sq. ft. sq. m $/sq. ft. $/sq. m
00244 Whipporwill Road over I-95 $134,000 12,900 1,200 $10.39 $111.67
00246 Four Mile River over I-95 $107,000 10,600 985 $10.09 $108.63
00247 Route 449 over I-95 $160,000 16,150 1,500 $9.91 $106.67
00249 Society Road over I-95 $108,000 10,200 950 $10.59 $113.68
00303 Moosup Pond Road over I-395 $134,000 13,100 1,220 $10.23 $109.84
TOTAL $643,000 62,950 5,855 $10.21 $109.82

Field Application

Metalizing can be applied to steel bridge components either at the fabrication shop or in the field. The actual metalizing application is similar for both; however, accessibility of the work area and other circumstances are quite different. This article will focus on field application.

The success of any coating system depends on strong adhesion to the substrate, which is promoted by high quality surface preparation. For field metalizing, a sharp, angular finish conforming to SSPC-SP-10, Near White Blast cleaning, is adequate to obtain good mechanical bonding of the zinc and aluminum alloy. These surface characteristics are best attained using hard, angular grit for abrasive blast cleaning. Because most field projects involve the disposal of contaminated blast debris, the use of recyclable grits is necessary to reduce the volume of waste. Ferrous abrasives are more efficiently recycled than non-ferrous abrasives because they can be magnetically removed from contaminated debris. Steel shot, however, is not acceptable for preparing steel surfaces for metalizing because it produces a peened surface that does not provide the profile necessary for good adhesion. To satisfy requirements for both magnetic recycling and angularity, chilled iron or fractured steel grit should be used.

Table 2

1994 Bid Prices for Class 3 Containment and Collection of Surface Preparation Debris Steel Surface Area  Unit Price 
Bridge # Location Lump Sum sq. ft. sq. m $/sq. ft. $/sq. m
00244 Whipporwill Road over I-95 $58,000 12,900 1,200 $4.50 $48.33
00246 Four Mile Road over I-95 $46,000 10,600 985 $4.34 $46.70
00247 Route 449 over I-95 $89,893 16,150 1,500 $5.19 $55.93
00249 Society Road over I-95 $44,000 10,200 950 $4.31 $46.32
00303 Moosup Road over I-395 $57,000 13,100 1,220 $4.35 $46.72
TOTAL $288,893 62,950 5,855 $4.59 $49.34

ConnDOT required a minimum metalizing bond strength of 700 psi (4.8 MPa) to ensure good adhesion of the coating. Adhesion is verified in the field by direct testing in accordance with ASTM D 4541, Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers.

To minimize flash rusting, surfaces must be blast cleaned, vacuumed, and metalized on the same day. As with painting, these operations should not be performed during atmospheric conditions that promote surface moisture. Sprayed metal coating may be applied, however, at lower temperatures than conventional primers. Whereas many conventional primers cannot be applied successfully when the steel or air temperature is below 50o F (10o C), the metalizing specification prepared by ConnDOT for its current metalizing project does not have a temperature restriction, as long as humidity restrictions are met. Unlike conventional coatings, the metalized coatings cure instantly, thus eliminating delays between coating layers.

Typically, one pass of the metalizing gun applies approximately 2-3 mils (50-75 microns) of the coating, making about 3 passes necessary to meet ConnDOT's required dry film thickness of 6-8 mils (150-200 microns). These passes should be made at right angles to each other to maximize density and uniformity of the finished coating, which should have a density of 85-95 percent of the wire alloy density. The metalized coating will have a long service life if left bare. However, application of liquid sealers and topcoats extends service life and reduces the thickness requirement of the metalized primer.

Coating System Specified

The 2 primary forms of protection provided by zinc-based coating systems are barrier and galvanic protection. Generally, the barrier protection prevails and galvanic protection takes over when the barrier is penetrated due to coating damage or aging. Effective primers provide both types of protection, while topcoats are used chiefly as barriers to the environment.

The metalized system used by ConnDOT for the FHWA-sponsored project and for its current metalizing project contains 3 parts. The metalizing primer is 85 percent zinc and 15 percent aluminum. The intermediate coat, or sealer, fills pores in the metalizing, significantly reducing permeability and electro chemical consumption of the primer. The sealer used on the FHWA installations was a polyamide epoxy. For the current project, a high-solids epoxy amido-amine has been specified. ConnDOT made this change to reduce the level of VOC's in the sealer material.

The topcoat for the system is a high-build aliphatic urethane, intended to provide resistance to abrasion and ultraviolet degradation. In addition to extending the service life of the overall system, the topcoat adds color and aesthetic quality to the bridge. As with conventional paint systems, care must be taken to ensure that all materials are compatible.

Contractor Qualifications

Quality workmanship is essential to the success of a metalizing application. To foster quality in the field, ConnDOT requires a two-part qualification process for metalizing applicators. The first part of the specification pertains to the company that will be conducting and supervising the work. It is worded as follows: "The Contractor shall show evidence of having successfully applied metalizing on steel for a minimum period of 3 years prior to the contract." A contractor's list of completed projects, including the name of an owner or a contact person, is usually sufficient for proof of this experience.

The other qualification requirement is directed at the actual operators of the metalizing equipment. It is specified as follows: "The Contractor shall submit to the Engineer evidence indicating that the proposed metalizing applicators are fully qualified in metalizing according to Department of Defense (DoD) Standard 2138 to perform the work." The standard governs the use of metalizing for corrosion-control applications aboard Naval surface ships.

Under the terms of the DoD standard, a metalizing operator must be tested and certified before working on the project. The testing process requires that proposed applicators prepare test specimens using the approved metalizing procedure. The coated specimens are then subjected to a visual examination, a bend test, a bond test, and a shape test.

To pass the visual examination, the coating must not have blisters, cracks, loose chips, contaminants, or pinholes. For the bend test, the specimen is bent approximately 180 degrees around a ˝ in. (13 mm) rod. If any disbonding, delamination, or gross cracking of the coating occurs, it does not pass the test. The bond test requires direct tensile loading, which is monitored until failure. The adhesion and cohesive strength of the coating at failure must exceed a specified minimum. The shape test simply verifies the coating thickness and the operator's ability to apply it uniformly. Thickness of the metalizing is also checked in the field throughout the project.

Once the specimens coated by the applicator have passed these tests, he or she may obtain certification. The certification remains valid for 6 months. These certification requirements are in effect for ConnDOT's current metalizing project. The requirements were also in effect for 3 of the 4 bridges in the FHWA-sponsored study.

In addition to the DoD metalizing qualifications, contractors must satisfy certification requirements for standard painting projects. Contractors performing lead paint removal, containment, and surface preparation must be certified by SSPC under its Painting Contractor Certification Program (PCCP) to SSPC-QP 1 and QP 2. Contractors or subcontractors who apply the sealer or topcoat materials must be certified to SSPC-QP 1.

Metalizing Costs

As with engineering decisions, cost is a primary consideration when determining strategies for corrosion protection. The main goal in selecting coating systems is to minimize total cost over the service life of the structure. In addition to initial expenditure, total cost over the service life of the structure includes future maintenance and "loss of use" costs. These 2 elements have a significant impact on total cost and should not be overlooked.

High installation costs have been the primary deterrent to the widespread use of metalizing on bridges. When the 4 bridges in the FHWA-sponsored study were coated, the cost to install the sealed and top-coated system averaged $15.00/square foot. The unit price was somewhat escalated by the small surface areas involved. Also, the prices for the FHWA-sponsored project do not reflect the improvements in spray equipment mentioned earlier in the article. Because of these circumstances, cost data from ConnDOT's current metalizing project are more accurate for typical metalizing applications. The cost data is listed in Table 1 (available upon request).

The table shows lump sum prices, per bridge, for the bid item "Abrasive Blast Cleaning and Field Metalizing of Structure." This item includes all materials, equipment, and labor for surface preparation and installation of the 3 components of the coating system on the structural steel elements of each bridge. Prices for containment, collection, and disposal of surface preparation debris are paid under separate work items. The bid prices for these items are shown in Tables 2 and 3. All bid prices were provided by the low bidder for the whole project. Thus, they may not be the lowest bids received for any individual item. However, these are the prices that will be paid upon completion of the work. They are consistent with the engineer's estimate prior to bidding.

Based on the ConnDOT surface area estimates shown, the average unit cost for the complete metalizing system on the 5 bridges is $10.21/square foot. These installation prices probably could have been reduced by eliminating the topcoat; however, ConnDOT decided it would use a topcoat to maximize the service life of the coating and to maintain a color finish on the bridges.

Service Life of Metalizing Coatings

The primary benefit of a metalized coating is its expected service life. This benefit typically offsets the high initial cost associated with metalizing. However, variations in the corrosivity of local environments make it difficult to generalize about the predicted service life of a coating system. In addition, localized coating failure on portions of a bridge may or may not constitute failure of the entire installation. These circumstances make it necessary to consider ranges of a service life and to use engineering judgment to make predictions for individual structures.

The maintenance-free service life of an untopcoated zinc and aluminum alloy metalized coating on a typical bridge is predicted to be 25-40 years. Using a sealer and topcoat over the metalized primer is reported to extend the service life 15-20 years. These predictions assume that all surface preparation and coating application procedures are conducted in compliance with industry- recommended specifications. The ranges correspond to bridge exposure conditions that are typical in marine and industrial environments and in environments with exposure to deicing salts.

The service life predicted for metalized coating systems is longer than the expected service life for conventional paint systems. The primary reason for this difference is the superior galvanic protection offered by the pure metal primer. In particular, cathodic protection of the steel is needed in areas where damage to the topcoat may occur and in confined areas where moisture is retained. On bridges, early failure typically occurs under leaking expansion joints, in bearing locations, and at connections. The ability of the metalizing to galvanically protect damaged and anodic locations delays the progress of corrosion and, hence, extends service life.

The coatings applied to the 4 bridges in the FHWA-sponsored project are now 4-8 years old. To date, evaluation results show no sign of coating distress or failure on any of the metalized sections. The oldest of the 4 metalized sections was coated in 1987. The entire section remains in excellent condition, including the bearing areas, which were extensively corroded before the project began. Although many years must pass before the expected service life of these coatings is achieved, early observations support evidence of the long service life observed in other industries.

Table 3

1994 Bid Prices for Disposal of Surface Preparation Debris
Classification of Debris Unit of Measurement Quantity Estimated Unit Bid Price Total Price
Hazardous 55-gallon barrel (bbl) 24 bbl $800.00 $19,200.00
Contaminated cubic yard (cy) 8 cy $1.00 $8.00
Total Disposal Cost  $19,208.00

5-bridge Total 

62,950 square feet Total disposal cost per square foot: $.031
5,855 square meters Total disposal cost per square meter: $3.28
Appropriate Applications for Metalizing
Many factors must be considered when developing a coating maintenance strategy for a particular bridge. First, owners must determine that continued use of the existing structural steel is desirable. In some cases, it has been found that severe structural or functional deficiencies in bridge components have favored the total replacement of structural steel, rather than removal of its lead- based paint. The determining factors in these decisions are usually the physical condition of the bridge and its ability to support traffic. If the deck, structural steel, and substructure elements are new, rehabilitated, or in excellent condition, then re-coating the existing steel is usually favored over removing and replacing it.

Once an owner establishes that the existing steel will remain, the condition of the present coating should be analyzed. In many cases, the existing paint is in poor condition, with low adhesion strength, and it needs to be removed. When this is the case, the next step is choosing a new coating system. The need for future coating work will be minimized if the life expectancy of the new system matches the remaining design life of the bridge. Use of metalizing is favorable when the bridge deck and other components are in excellent condition and only minor repairs are anticipated in the near future. A unique benefit of metalizing is that its expected service life corresponds well to the typical design lives of new, or newly-rehabilitated, structural components.

Other circumstances that favor metalizing over conventional paint systems are corrosive environments and anticipated high costs for future coating maintenance. As stated earlier, metalized coatings have proven durable when exposed to harsh salt spray and industrial environments. High costs for future maintenance can be expected in congested traffic areas where lane closures create costly traffic delays. Difficulty accessing the bridge, due to its height or location, also increases the cost of the routine maintenance required by a less durable coating system.

Conclusions

Although the use of metalizing as a protective coating for steel is well-established, its use by transportation agencies is somewhat limited. The primary reason has been the high cost of installation compared to conventional paint systems.

In recent years, however, developments in the bridge coating industry have made field metalizing a more viable alternative. New technology has made the process more efficient for field applications. The rising costs of lead paint removal, traffic control, and labor have favored the use of low-maintenance materials with a long service life. In addition, it is becoming more desirable to fit the service life of a new coating system to the overall design life of the bridge to minimize the need for future coating maintenance. Based on ConnDOT's research and evaluations in the field, metalizing has proven to be a superior coating system for bridges in highly corrosive environment.

References

M. M. Kasinskas, "Installation of Experimental Coatings on Structural Steel-Construction Report," Report N. FHWA-CT-RD-1084-1-91-5, July 1991.
W. Gardega, V. Lanza, and A. H. Roebuck, "Thermal Spray Coatings," Journal of Protective Coatings & Linings, January 1986, pp. 44-48.
S. J. Oechsle and J. N. Childs, Jr., "Thermal Sprayed Coatings," in Steel Structure Painting Manual, Vol. 1, Good Painting Practice, Third Edition, Steel Structures Painting Council, Pittsburgh, PA, 1993.
K. Duplissie, "Metalizing: Tips for Contractors," Journal of Protective Coatings & Linings, August 1994, pp. 35-40.
"Zinc Metalizing: Long Lasting Protection for Steel Structures (Video)," Zinc Institute, Inc., New York, NY, 1987.
"Corrosion Tests of Flame-Sprayed Coated Steel - 19 year Report," American Welding Society, Miami, FL, 1974.
DOD-STD-2138(SH), "Metal Sprayed Coating Systems for Corrosion Protection Aboard Naval Ships," November 1981.
L. B. Castler, "Replacing Steel vs. Removing Lead Paint: Connecticut's Experience," Journal of Protective Coatings & Linings, July 1994, pp. 54-62.

Acknowledgments

The author wishes to acknowledge the support and cooperation of the Federal Highway Administration during the course of the experimental project and the effects of the following individuals for their input to information provided in the article: Michael M. Kasinskas, Connecticut Department of Transportation (Retired); Peter B. Barlow, and Ralph D. Daily, Jr., Connecticut Department of Transportation.