The metalized coating may be left bare or as-sprayed; it may be sealed with a low viscosity penetrating sealer or, in a limited number of circumstances, the sealed-sprayed metal coating may be topcoated with paint.
As-sprayed zinc is a standalone coating that can be supplemented and improved by the application of a sealer. Sealing the sprayed metal lengthens the coating’s service life for a very modest cost.50 The addition of paint over the sealed sprayed-metal does not necessarily improve the coating’s service life in the same way that sealing does.
We rely on the British Standard and other sources to explain the application, function, and benefits of sealing and topcoating.
As previously explained, the bare metallic coating is both a barrier and a galvanic coating. Zinc, and to a lesser degree aluminum, protects steel by corroding in preference to the underlying steel.
When the bare galvanic coating is consumed, maintenance will be to blast clean the steel to the required cleanliness and profile, followed by the reapplication of the sprayed-metal
The low viscosity penetrating sealer as defined below penetrates and closes-off the sprayed-metal coating’s porosity or as ISO 2063 reads,
The purpose of sealing is to reduce inherent porosity of the sprayed coating.51 Moreover, sealing preserves the sprayed-metal and prevents its galvanic consumption.
Periodic maintenance, i.e. reapplication of the sealer, preserves both the sprayed-metal and the underlying surface preparation.52
Metallic zinc and aluminum react with air and moisture in the atmosphere and corrosion products, oxides, hydroxides, and carbonates of the coating metal form on the coating’s surface, and in its porosity.53 This buildup of oxide and other byproducts closes off any through-porosity to the steel substrate, and the corrosion layer shields the sprayed-metal from the corrosive atmosphere extending the coating’s service life.
Over time, a period of years, the sprayed-metal’s reaction with the atmosphere consumes the coating. Instead of leaving the coating exposed to oxidize, an artificial or liquid sealer may be used to preserve the galvanic metal as well as the underlying steel.
A low viscosity liquid sealer as defined below may be applied to the sprayed-metal to artificially close off its porosity and preserve the galvanic metal coating. This sealer will eventually deteriorate and expose some of the sprayed-metal. The exposed galvanic metal coating then corrodes in preference to the underlying steel. The alternating consumption of sealer and galvanic metal prolongs the life of the two coating materials. (See the NAVSEA comment below.)
AWS C2.18-93 defines the liquid sealer as
TSC sealers are low-viscosity, clear or colored (pigmented) paints, lacquers, and vinyls formulated to flow over and be absorbed into the natural pores of the TSC. The pigment particle size for colored sealers must be small enough to flow easily into the pores of the TSC, nominally 5-fineness grind per ASTM D 1210, Test Method for Fineness of Dispersion of Pigment-Vehicle Systems.54
The definition itself explains how the sealer’s
pigment particle size affect the function of the sealer. The sealer’s chemistry, especially as it relates to VOC’s, must also be considered.
The January 1984 Naval Sea Systems Command (NAVSEA) report on thermal sprayed coatings in marine environments also speaks about sealers giving some further clarification on how they work.
The purpose of the sealer is to physically close the pores by impregnation, thus reducing the exposed surface. Sealers retard the accumulation of dirt and enhance the appearance
of the coating, making the surface easier to maintain. Coating life is, thus, extended….The sealant/coating system is effective in protecting the substrate and limiting the formation of corrosion products to only exposed areas of the coating.
55 [Emphasis added.]
Standard 5493 favors sealing sprayed-metal coatings and, in most cases, prefers not to paint them. (We’ve placed more excerpts from the Standard that deal with both sealing and paint topcoating at the end of this section.)
There are two issues here: 1) how do paints perform over these metallic zinc coatings and 2) what are the implications for future maintenance. These four quotes summarize BS 5493 on the use of paint topcoats.
Life between subsequent maintenances. After galvanized or metal-sprayed structures have been painted, subsequent maintenance will be of the paint scheme.56
Paints “usually have a longer life on metal coatings than on bare steel.57
Even though paint performs better over sprayed-metal than on steel, it has a shorter service life than the sprayed-metal coating and will require maintenance long before the sprayed metal. These next two points are often overlooked or ignored – they are essential.
Painting of sprayed-metal coatings is seldom the preferred treatment except when colour, and inert barrier or abrasion resistance is required.58 [Emphasis added.]
. . . sealed-sprayed-metal is usually preferable to painted sprayed-metal. 59 [Emphasis added.]
There are different opinions on the use paint topcoats, but in general, paint topcoats do not improve the performance or maintenance interval of sprayed-metal coatings. For example, BS 5493 gives the following guidance for applying a topcoat to the sprayed-metal
Metal-coated steel is painted only when:60 [Emphasis added.]
(a) The environment is very acid or very alkaline (i.e. when pH-value is outside the range 5 to 12 for zinc or 4 to 9 for aluminium); or
(b) The metal is subject to direct attack by specific chemicals (see table 3); or
(c) The required decorative finish can be obtained only by paint; or
(d) When additional abrasion resistance is required.
Generally one or two coats of paint may be sufficient except in abnormally aggressive environments.
Similar guidance appears in the 2018 version of the AWS/NACE/SSPC Joint Specification, Annex C.
Consider applying a sealer and topcoat to the thermally sprayed surfaces if any of the following conditions apply:61 [Emphasis added.]
(a) The environment is very acidic or very alkaline (normal pH range for pure zinc is 6 to 12 and for pure aluminum, 4 to 11).
(b) The metallic coating is subject to direct attack by specific chemicals.
(c) A particular decorative finish is required.
(d) Additional abrasion resistance is required; or
(e) Frequent saltwater spray, splash, or immersion service.
At first glance, the two standards seem to say the same thing, but they have very different meanings about when paint topcoats are to be used. The Joint Specification’s meaning is that sealer and topcoat may be, but would not necessarily be used when any of its five listed conditions exist.62
On the other hand, the British Standard recommends a paint topcoat
only when any one of its four conditions exists. The inference is that there is a benefit to topcoating in 5493’s four circumstances, but topcoating is not necessary when those conditions are absent.63
Whenever paint topcoat(s) are used, the sprayed-metal coating should first be sealed.
The Joint Standard should cover the use of sealers separate from paint topcoats and should recommend sealing sprayed-metal coatings in most applications. The Joint Standard’s inclusion of “saltwater spray, splash, or immersion service” is a sound addition.
Sprayed-metal coatings are not simply a substitute for zinc containing paint primers that have to be topcoated with multiple layers of epoxy and urethane. While sprayed-metal may be used in combination with sealers, and occasionally with paint topcoats, sprayed-metal is not dependent upon multiple layers of paint to provide long term corrosion control.
The grey color of the sealed-sprayed-metal coating or a preference for a certain color will sometimes have bridge owners topcoat the sealed metalized coating. A common practice and efficient way of dealing with this aesthetic preference is to paint only those surfaces that attract attention, in most cases only the fascia girders. The sealed-sprayed-metal will protect the rest of the structure.
Each of Table 3’s ten parts describes an environment and categorizes various coatings according to their expected service lives or lives to first maintenance in those environments. The first mention of sealers and the use of paint with sprayed-metal coatings is in the introduction to Table 3.
The following lists of systems, classified by environment are typical times to first maintenance, indicate the options open to the specifier. . . . 64 [Emphasis added.]
The recommendations indicate minimum requirements to ensure protection; thus combinations of metallic zinc or aluminium with paint are limited to sealed, sprayed-metal or metal with relatively thin paint coatings, although it is recognized that, for decorative purposes, additional paint coatings will often be specified.
And later in the notes to Table 3, the Standard introduces the idea of maintaining the metalized coating by the periodic reapplication of sealer
Sealing is particularly desirable when it is desired to retain the sprayed coating when the surface is eventually to be maintained, and such maintenance requires only the renewal of the sealer.65
Next, the function and application of the sealer
The appearance and life of sprayed-metal coatings is improved by sealing. . . .
There is no requirement for a measurable overlay of sealer but sealers shold be applied until absorbtion is complete. . . . 66
Sealers should be applied immediately after spraying the metal coating. . . .67
Later in Section 11 under the heading
Characteristics of metal coatings, the phrase
renewal of the sealer first used in the introduction to Table 3 is repeated.
At the risk of being redundant, this is the quote from Section 11.
A sealer . . ., which fills the metal pores and smooths [sic] the sprayed surface, improves the appearance and life of a sprayed-metal coating.
It also simplifies maintenance, which then requires only the renewal of the sealer.
68 [Emphasis added.]
We will close by repeating this very certain statement about the use of paint with sprayed-metal coatings
Painting of sprayed-metal coatings is seldom the preferred treatment except when colour, and inert barrier or abrasion resistance is required.69 [Emphasis added.]
Thermal sprayed steel should be sealed and/or topcoated under any of the following conditions. . . .This is more in keeping with the meaning of the British Standard.