The installation of thermal spray coatings (TSC) or metallized coatings is becoming more common to protect new and existing structural steel members on locks and dams, bridges, storage tanks and other industrial structures. While installation of thermal spray coatings is generally more labor-intensive and more expensive than traditional liquid-applied coating systems, life cycle costs can be lower. According to the Federal Highway Administration, when used properly, these coatings offer superior long-term performance compared to more traditional paint systems, especially in more severe coastal and salt-rich environments.
Case in point : Originally built in 1974, the Perdido Key Bridge has two lanes (connecting mainland Florida to Perdido Key) and is the route to Key, so traffic demand is high. The structure is in good condition, but the existing coating system is in poor condition. The structure is exposed to the harsh salt (coastal) environment and is expensive to maintain due to access, traffic control and mobilization. Due to the high costs associated with maintaining painting, the Florida Department of Transportation (FDOT) has mandated the use of thermal spray systems in an attempt to reduce maintenance costs for decades. In 2016, the structural steel was blast cleaned and thermally sprayed, followed by a 100% solids epoxy penetration sealer,
The terms "thermal spray" and "metallization" are often used interchangeably; however, metallization is more commonly used when the starting material is metal (as opposed to plastic or ceramic powders). There are three basic forms of thermal spraying, including flame spraying, arc spraying and plasma spraying. Arc spraying is the most common (and effective) of the three methods, and is commonly used in shops or on-site to metalize new and existing industrial structures. Different materials can be thermally sprayed, including ceramics, plastics and metals. For the protection of industrial structures, metal is used. Typically, a wire (also known as feedstock) is fed into a coating device (Spray Gun), melted, then atomized and blown onto the surface of the structure using compressed air. The raw material (wire or powder) can be made from a variety of metals and is chosen based on the environment of use and the expected level of performance (in the same way as a liquid-applied coating is chosen). Wire forms are commonly used for metallizing industrial structures and can be composed of zinc, aluminum or zinc/aluminum alloys (typically 85% zinc/15% aluminum).
Flame Spray Metallization - Flame Spray Metallization uses an oxygen-acetylene generated flame to melt the wire as it exits the Spray Gun. Once the wire is melted, compressed air atomizes the molten metal and blows it onto the surface. The spray fans are only about 2 inches wide and the spray distance is typically maintained at 4 inches, making flame spray applications labor intensive and often cost prohibitive for large projects. Flame spraying is economical for metallizing small areas and for repairing/rebuilding metal parts subject to cavitation corrosion.
Arc Spray Metallization - Arc spray metallization uses a relatively high electrical amperage to melt the feedstock rather than a flame. Two wires (of the same chemical composition) are fed into the Spray Gun from each spool. The reverse charge is between 375 and 400 amps per wire. As the wires exit the Spray Gun nozzle, they touch, arc and melt. Compressed air atomizes the molten wire and blows it onto the surface. Fan mist is significantly wider than flame spray and has a much higher application rate.
pre-surface preparation
SSPC CS 23.00 / AWS C 2.23M / NACE No. 12 lists three "preliminary" requirements:
Environmental Conditions: Verify surface temperature is at least 5°F above dew point and rising.
Abrasive Cleanliness: Vial testing according to ASTM D7393 [2] confirms that the abrasive is visibly free of oil contamination and has levels of water soluble contaminants <1,000 μS/cm per SSPC-AB 1 [3], AB 2 [4] and AB 3 [5], tested according to ASTM D4940 [6].
Compressed Air Cleanliness: Confirms that compressed air used during blast cleaning and removal of surface dust by blowdown is clean and dry when tested according to ASTM D4285 [7].
In addition to these three, it is important to remove (usually by grinding) any surface hardening (case hardening; carburization) caused by torch cutting. The extreme heat along the cut line hardens the surface of the steel so that subsequent blast cleaning does not produce the same depth of profile as the adjacent surface, even though it all appears uniform to the eye. Once thermally sprayed steel is put into service and expansion/contraction or mechanical bending of the steel occurs, the lack of sufficient surface profile on the heat exposed areas can lead to loss of adhesion. Removing this case hardening by grinding prior to blast cleaning will generally result in a more uniform surface profile depth.
In addition, visible grease and oil shall be removed by solvent cleaning in accordance with SSPC-SP 1, and weld spatter, lamination and surface salt contamination shall be removed as required by contract documents.
Surface treatment method
As mentioned earlier, TSC is not surface resistant and needs to be applied to blast cleaned surfaces. Other surface preparation methods, such as hand or power tool cleaning, water blasting or chemical stripping, are not suitable for TSC.
surface cleanliness
Minimum level of surface cleanliness, post-abrasive blasting is SSPC-SP 10/NACE No. 2, near white blast cleaning, allowing up to 5% contamination per 9 square inches of fabricated steel. This level of surface cleanliness is acceptable for mild atmospheric exposure. More commonly, designers require SSPC-SP 5 / NACE No.1, white metal blast cleaning for immersion service or corrosive environments, which requires the surface to be free of any contamination. Both standards automatically adjust for abrasive cleanliness, compressed air cleanliness, and removal of visible grease contamination prior to blast cleaning.
surface profile
The surface profile blast cleaning process not only cleans the surface, but also creates anchors for the TSC by creating a series of peaks and valleys in the steel, effectively increasing the surface area. The abrasive used to create the surface profile needs not only to be clean (as previously stated), but also to be angled and of sufficient size to form a 2.5-5 mil (63-125 μm) surface profile or anchor pattern (note that KTA prefers 3.5 mil [89 μm] surface profile for thermal spray coatings) using round abrasives (such as steel shot) to create a similar profile depth will result in reduced adhesion; at the same depth, the surface area is relatively small due to the reduction in peak density smaller.
Surface Profile Depth Verify surface profile depth according to ASTM D4417, [8] Method B (depth micrometer) or Method C (replicating tape). The SSPC/NACE/AWS standards reference SSPC-PA 17 [9] for the frequency and acceptability of surface profile measurements. Another surface property that can be quantified using replica tape and replica tape readers is peak density. Studies conducted by Roper, Weaver and Brandon [10] showed that an increase in peak density improves the adhesion (adhesion) of the liquid coating to the prepared metal, the depth of the surface profile and inhibits corrosion undercutting if the coating is damaged in service . Currently, SSPC/NACE/AWS standards do not address peak density measurement, and the industry is still evaluating its significance.
Verification Test
The SSPC/NACE/AWS standards include three tests that can be used to verify proper surface preparation and installation of a TSC: adhesion (tensile pull and hammer-chisel cut test) and flexibility (bend test). Each of these tests is an indicator of the initial adhesion of the TSC to the prepared steel; the bend test is also a qualitative indicator of equipment setup and application parameters such as spray distance, amperage, etc. These tests were rarely performed on liquid coating systems, which further demonstrates that TSC is very sensitive to surface quality preparation.
Tensile adhesion testing using a self-aligning adhesion Tester (hydraulic or pneumatic) is a mandatory requirement of the standard and needs to be performed according to ASTM D4541 [11]. An average of three tests (performed on a production piece or mating panel) is required to meet the minimum requirements based on wire type as shown in the table below.
Mandrel bend testing is not mandatory, but contract documents may require it. The diameter of the mandrel is based on the applied thickness of the TSC as shown in the table below. While minor cracking in the bend area is acceptable, cracking that would cause lifting/stripping is not permitted.
Cutting tests are also optional but can be invoked through contract documents. Briefly, the handle of a 1.5" wide chisel is held at an approximately 60° angle to the surface and struck with a 3 lb hammer. When performed in triplicate, there was no evidence of delamination along the width of the cut edge.
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