Surface Preparation of Steel Prior to Application of Thermal Spray Coatings

Introduction

The installation of thermal spray coatings (TSC) or metalized coatings is becoming more common for protection of new and existing structural steel members on locks & dams, bridges, tanks and other industrial structures. While the installation of thermal spray coatings is typically more labor intensive and more costly than traditional liquid-applied coatings systems, the life cycle costs can be lower. According to the Federal Highway Administration, when applied properly, these coatings have shown excellent long-term performance when compared to more conventional paint systems, especially in more severe coastal and salt-rich environments[1].

Case Study: The Perdido Key Bridge, originally constructed in 1974, carries two lanes of traffic (connecting the Florida mainland with Perdido Key) and is the only route on and off the Key, so traffic demands are high. The structure was in good condition, but the existing coating system was in poor condition. The structure is exposed to a harsh salt (coastal) environment and maintenance costs are high due to access, traffic control and mobilization. Because of these high costs associated with maintenance painting, Florida Department of Transportation (FDOT) specified the use of a thermal spray system, attempting to reduce the maintenance costs for decades. In 2016, the structural steel was abrasive blast cleaning and a thermal spray coating was applied, followed by a 100% solids epoxy penetrating sealer, an aliphatic polyurethane (blue) and an aliphatic polyurethane clear coat for greater protection from solar radiation (sunlight).

The terms `thermal spray coating’ and `metalizing’ are frequently used interchangeably; however, metalizing is more commonly used when the feedstock is metal (versus a plastic or ceramic powder). There are three basic forms of thermal spray, including flame spray, arc spray and plasma spray. Arc spray is the most common (and most productive) of the three methods and is typically the method employed in the shop or field for metalizing both new and existing industrial structures. Different materials can be thermal sprayed, including ceramic, plastic, and metal.  For the protection of industrial structures, metals are used. Generally, a metal wire (also called feedstock) is fed into an application device (spray gun), melted, then atomized and blown to the surface of the structure using compressed air. The feedstock (wire or powder) can be made from a variety of metals and is selected based on the service environment and intended level of performance (the same way liquid-applied coatings are selected). Metal in wire form is most commonly used for metalizing of industrial structures, and may consist of zinc, aluminum, or a zinc/aluminum alloy (typically 85% zinc / 15% aluminum).

Flame Spray Metalizing – Flame spray metalizing employs an oxygen-acetylene generated flame to melt a 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 fan is only approximately 2-inches wide, and the spray distance is typically maintained at 4-inches, making flame spray application very labor intensive and oftentimes cost-prohibitive for large projects. Flame spray can be economical for metalizing small areas and for repairing/rebuilding metal components subject to cavitation erosion.

Arc Spray Metalizing – Arc spray metalizing uses relatively high electrical amperage to melt the feedstock, rather than a flame. Two wires (of the same chemical composition) are fed from individual spools into a spray gun. Each wire is given an opposing electrical charge between 375 and 400 amps. As the wires exit the gun nozzles, they contact, arc and melt. Compressed air atomizes the molten metal wire and blows it onto the surface. The spray fan is substantially wider than that of flame spray and the application rate is much higher.

Joint Standard/Standard Practice, and Contractor Qualification

SSPC: The Society for Protective Coatings, NACE International, and the American Welding Society (AWS) developed a joint standard/standard practice (SSPC CS 23.00/AWS C 2.23M/NACE No. 12), titled “Specification for the Application of Thermal Spray Coatings (Metalizing) of Aluminum, Zinc, and Their Alloys and Composites for the Corrosion Protection of Steel.” The latest publication date is May 2016. The standard lists feedstock (wire) requirements, surface preparation requirements, application equipment and processes, inspection, processes for developing a Jobsite Reference Standard (JRS), topcoating and a few other miscellaneous items. More information on the content of this standard is available in an article prepared by Greg Richards of KTA and posted to KTA University in the Spring of 2017. The article is titled, “Highlights of Revisions to SSPC-CS 23.00/AWS C223M/NACE No. 12, Specification for Thermal Spray Coatings (TSC).”

The SSPC Painting Contractor Certification Program (PCCP) Qualification Procedure 6 (QP 6), “Standard Procedure for Evaluating the Qualifications of Contractors Who Apply Thermal Spray (Metalizing) for Corrosion Protection and Steel and Concrete Structures” qualifies contractors performing metalizing work. Facility owners considering the use of TSC should consider invoking SSPC-QP 6 as a bid requirement, as the metalizing process is quite different than application of liquid coatings.

Heightened Awareness of Surface Preparation Requirements

Thermal spray coatings are not forgiving, are not designed for marginally prepared surfaces, and can fail catastrophically if the surface is not prepared properly. The bond of the TSC to the prepared steel is essentially all mechanical, and creating that bond is paramount to achieving the life cycle cost expectations of the installed coating. The remainder of this article focuses on the need for heightened awareness during both design (specification preparation) and construction to carefully control the quality of workmanship during surface preparation. 

Pre-Surface Preparation

SSPC CS 23.00/AWS C 2.23M/NACE No. 12 lists three “pre-preparation” requirements:

• Ambient Conditions: Verify the surface temperature is a minimum of 5°F above the dew point temperature and rising.
• Abrasive Cleanliness: Verify the abrasive is visibly free of oil contamination using the vial test according to ASTM D7393[2], and that the level of water soluble contaminants is <1,000 µS/cm, per SSPC-AB 1[3], AB 2[4] and AB 3[5], when tested according to ASTM D4940[6].
• Compressed Air Cleanliness: Verify the compressed air used in the abrasive blast cleaning process and to remove 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 (typically by grinding) any surface hardening (case hardening; carburization) caused by torch cutting. The extreme heat along the cut lines hardens the surface of the steel such that subsequent abrasive blast cleaning will not yield the same profile depth as adjacent surfaces, even though it all appears uniform to the eye. The lack of sufficient surface profile on the heat-exposed area can cause loss of adhesion once the thermal spray-coated steel is placed into service and expansion/contraction or mechanical flexing of the steel occurs. Removing this surface hardening by grinding prior to abrasive blast cleaning will typically result in a more uniform surface profile depth.

Also, visible grease and oil should be removed in accordance with SSPC-SP 1, Solvent Cleaning, and weld spatter, laminations and surface salt contamination must be removed according to the requirements of the contract documents.

Surface Preparation Methods

As mentioned previously, TSC is not surface tolerant and must be applied to an abrasive blast cleaned surface. Other methods of surface preparation such as hand or power tool cleaning, water jetting, or chemical stripping are not appropriate for TSC.

Surface Cleanliness

The minimum level of surface cleanliness, post-abrasive blast is SSPC-SP 10/NACE No. 2, Near-White Blast Cleaning, which allows up to 5% staining on each 9 square inches of prepared steel. This level of surface cleanliness is acceptable for mild, atmospheric exposures. More commonly, the designer will 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 staining. Both standards automatically invoke abrasive cleanliness, compressed air cleanliness, and removal of visible grease/oil contamination prior to abrasive blast cleaning.

Surface Profile

The abrasive blast cleaning process not only cleans the surface but creates an anchor for the TSC by creating a series of peaks and valleys in the steel, effectively increasing surface area. The abrasive used to generate the surface profile must not only be clean (described earlier) but must be angular and of sufficient size to create a 2.5-5 mil (63-125 µm) surface profile or anchor pattern (note that KTA prefers a minimum of 3.5 mils [89 µm] surface profile for thermal spray coatings). Creating a similar profile depth using a round abrasive like steel shot will result in loss of adhesion; while the depth is the same, the surface area is comparatively less due to a reduction in peak density.

Surface profile depth is verified according to ASTM D4417,[8] Method B (depth micrometer) or Method C (replica tape). The SSPC/NACE/AWS standard invokes SSPC-PA 17[9] for the frequency and acceptability of surface profile measurements. Another surface attribute that can be quantified using replica tape in conjunction with a Replica Tape Reader is peak density. Research conducted by Roper, Weaver and Brandon[10] revealed that an increase in peak density improves the bond (adhesion) of liquid-applied coatings to the prepared metal and inhibits corrosion undercutting if the coating becomes damaged while in service. Currently the SSPC/NACE/AWS standard does not address peak density measurements and the industry is still evaluating its importance.

Verification Testing

The SSPC/NACE/AWS standard includes three tests that can be used to verify the proper surface preparation and installation of the TSC: Adhesion (tensile pull-off and hammer-chisel cut test) and flexibility (bend test). Each of these tests is an indicator of the initial bond of the TSC to the prepared steel; the bend test is also a qualitative indicator of equipment set-up and application parameters like spray distance, amperage, etc. Rarely are these tests performed on liquid-applied coating systems, which is further evidence that TSC is very sensitive to the quality of surface preparation.

Tensile adhesion testing using a self-aligning adhesion tester (hydraulic or pneumatic) is a mandatory requirement of the Standard and must be performed according to ASTM D4541[11]. The average of three tests (conducted on a production piece or companion panel) must meet the minimum requirements based on the type of wire, as shown in the table below.

Mandrel bend testing is nonmandatory but may be required by the contract documents. The diameter of the mandrel is based on the applied thickness of the TSC, as illustrated in the table below. While minor cracking at the bend area is acceptable, no cracking that results in lifting/spalling is permissible.

The hammer-chisel cut test is also nonmandatory but may be invoked by the contract documents. Briefly the handle of a 1.5” wide chisel is held at approximately 60° angle to the surface and impacted with a 3-pound hammer. There can be no evidence of disbonding along the width of the cut edge, when performed in triplicate.

Summary

Thermal spray coatings can provide an economical solution to long term corrosion protection. While the installation costs are comparatively higher than a traditional 3-coat liquid-applied system, the life cycle costs over the life of the structure can be lower. However, to realize these long-term cost savings, the surface must be properly prepared and the TSC properly installed. This article focused on the pre-surface preparation steps, the importance of proper surface preparation (cleanliness and profile), including the direct and indirect requirements of the surface cleanliness standards, as well as the verification testing that can be invoked to provide evidence of the quality.

[1] US DOT Federal Highway Administration (FHWA) Bridge Coatings Technical Note: Metallized Steel Bridge Coatings, January 1997

[2] ASTM D7393, Standard Practice for Indicating Oil in Abrasives

[3] SSPC-AB 1, Mineral and Slag Abrasives

[4] SSPC-AB 2, Cleanliness of Recycled Ferrous Metallic Abrasive

[5] SSPC-AB 3, Ferrous Metallic Abrasive

[6] ASTM D4940, Standard Test Method for Conductimetric Analysis of Water Soluble Ionic Contamination of Blast Cleaning Abrasives

[7] ASTM D4285, Standard Test Method for Indicating Oil or Water in Compressed Air

[8] ASTM D4417, Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel

[9] SSPC-PA 17, Procedure for Determining Conformance to Steel Profile/Surface Roughness/Peak Count Requirements

[10] The Effect of Peak Count of Surface Roughness on Coating Performance; Hugh J. Roper, Raymond E.F. Weaver, Joseph H. Brandon; Journal of Protective Coatings & Linings, Volume 21, No. 6; June 2005

[11] Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers