thermal spray coatings

Sealer and Topcoat Systems for Thermal Spray Coatings (Metallizing)

Thermal spray coatings (TSC), also known as thermal spray metallizing (TSM) of steel is widely recognized for its superior galvanic protection properties. It can also be used to protect concrete structures. While bare uncoated metallizing is known to provide ‘good’ corrosion protection properties, it is porous; therefore, the application of conventional liquid-applied coatings over metallizing is often used to improve barrier protection, prolong coating system service life, and impart aesthetics.  This article focuses on the benefits of applying seal coats and/or topcoat systems (paints) to TSC, including a discussion of the generic product types that are commonly used. For further reference, the recognized industry standard for TSC is SSPC-CS 23.00/AWS C.2.23/NACE 12, Specification for Application of Thermal Spray Coatings (Metallizing) of Aluminum, Zinc, and their Alloys and Composites for Corrosion Protection of Steel. This joint standard/standard practice provides detailed information on TSC, including sealing and topcoating. The most recent version of this standard is dated September 2016. An article written by Greg Richards highlighting the primary changes incorporated into the “tri-society” standard was published on KTA University on March 31, 2017.

Basics of TSM

            In simple terms, TSM is a process by which a metallic feedstock material is melted and then injected into a compressed air stream.  The compressed air “atomizes” the molten metal into fine droplets, which are propelled to the blast cleaned steel surface where they cool rapidly, fuse and solidify into a protective metallic film.  Although powdered metal feedstock is sometimes used, the most common feedstock in the industrial coatings market is metal wire consisting of zinc, aluminum, or an alloy of the two (85% zinc and 15% aluminum), referred to as 85/15 wire.

Benefits of TSM

            TSM creates a relatively thin (typically 12-15 mils, or 305-380 µm), abrasion resistant metallic layer that protects the steel galvanically (sacrificially), similar to zinc-rich primers; however, the TSM does not create an intermetallic layer the way that hot dip galvanizing does. When used as a stand-alone coating, TSM provides relatively good weathering properties, temperature and sun light resistance, and exhibits good adhesion properties when applied correctly. However, bare TSM can have a very mottled, rough appearance (very similar to abrasive blast cleaned steel) and does not have the same smooth appearance produced by spray-applied liquid coatings.  In addition, areas of TSM touchup and spray applications are often visible (See Figure 1) and bare TSM will oxidize.  While the oxidation will sometimes minimize the contrast in appearance, in other instances, it can result in a worse appearance (See Figure 2).

Figure 1
Figure 2

            In terms of longevity or “service life,” TSM is only surpassed by hot dip galvanizing. The performance and service life of TSM can be significantly improved and lengthened by sealing the surface or applying a conventional high-performance coating over the TSM.

Benefits of Sealing and/or Topcoating TSM

Improved Corrosion Protection: As long as the exposure environment is suitable for metallic coatings, bare, uncoated TSM provides relatively good corrosion protection.  However, as the exposure environment becomes increasingly aggressive, the TSM performs as designed and galvanically sacrifices itself (i.e., becomes the anode) to protect the steel (i.e., the cathode).  This increase in galvanic action results in the more rapid depletion (sacrificing) of the metallic layer. In such cases, the corrosion protection attributes of the TSM can be improved by creating a “barrier” between the TSM and the exposure environment using sealer coats and/or topcoats applied to the TSM.

            Exposure environments that typically require the use of sealers and or topcoat systems applied to the TSM include:

  • Strong acidic or alkaline exposures (i.e., pH less than 4 or greater than 12)
  • Chemical exposure
  • Frequent wetting by splashing/spraying moisture
  • Abrasive environments

Sealing and Topcoating TSM

Clear Sealers for TSM

            When TSM is applied the atomized molten metal cools rapidly upon impact and does not remain fluid enough to completely flow together. As a result, the TSM film contains interconnected porosity throughout the film. In addition, the surface of the metallizing is often coarse, uneven and contains numerous peaks and valleys similar to abrasive blast cleaned steel. The purpose of the sealer coat is multifold; (1) it penetrates and is absorbed into the interconnected pores to seal off porosity, (2) it drives out air that can cause outgassing, (3) it helps prevent oxidation, (4) it covers the coarseness (i.e. peaks and valleys) of the TSM while still maintaining sufficient surface roughness to assist with adhesion of subsequent topcoats (when applied), and (5) it extends the service life of the TSM.  The thickness of the sealer is not typically measured, but instead is applied at a coverage rate that would be equivalent to applying a 1 to 2-mil (25-50 µm) coating film on smooth steel.

            The most commonly used sealers (when additional application of finish coats is not specified) consist of clear single component moisture cured urethanes (MCU) or clear two component (2k) aliphatic polyurethanes. These products are typically thinned to the maximum amount allowed by the manufacturer (while still maintaining compliance with the VOC regulations) to improve penetrability into the TSM.  One proven approach to applying sealers to TSM is to use a multi-step process.  This process typically involves applying a mist coat (thinned to the maximum amount permitted) that is allowed to flash-off for approximately 20 to 30 minutes, followed by a second coat (thinned approximately 10%).  The sealer should be applied as soon as possible after the TSM is applied (usually within 8 hours) and before it has a chance to visibly oxidize. If oxidation occurs it can be removed with pressure washing and wire brushing as needed.

Sealers with Topcoats for TSM

            Pigmented (colored) paint systems are also used over TSM to extend the service life, impart a specific color and improve appearance.  When pigmented topcoats are to be applied, the most common sealer for TSM include 100% solids, low viscosity, epoxy penetrating sealers and MCU penetrating sealers.  These products have sufficiently low viscosity to provide excellent “wetting” and penetration into the TSM film.  However penetrating sealers can have very short recoat windows that make their use on larger projects more challenging.  Therefore, in lieu of penetrating sealers, highly reduced epoxies are often used to seal the TSM.  Similar to clear urethanes, the key to sealing with a highly-reduced epoxy is to select products that can be thinned enough to penetrate the pores of the TSM while maintaining compliance with VOC regulations.

            Once the clear sealer or highly-reduced epoxy sealer has been applied, the most common topcoats consist of one or two coats of epoxy (if no sunlight exposure is anticipated) or aliphatic polyurethane (if direct exposure to solar radiation from sunlight is anticipated). Polyaspartic, polysiloxane or fluorourethane finish coats are also durable and will withstand exposure to sunlight.

            If very aggressive, corrosive exposure environments are anticipated, it may be advantageous to specify an additional coat of epoxy (i.e., a four coat system consisting of the sealer coat [penetrating sealer or reduced epoxy]), two full coats of epoxy and a full coat of UV-resistant topcoat.

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