The Complete Guide: Spraying Multiple Coats - Powder Coating

Figure 3 Illustration of Two-Coat Inorganic Corrosion Protection System on Steel

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The finish can be applied within hours of primer application because it is composed of similar resin chemistry. Also, the finish does not contain zinc dust and consequently can be pigmented to various matte colors. While the typical three-coat system offers a 20-30 year service life (8), single-coat and two-coat inorganic systems offer well beyond that. This extended service life places them alongside other long-life corrosion protection systems such as galvanizing and thermal spray metalizing.

As noted earlier, assessing performance of long-life corrosion protection systems for steel continues to be a debated challenge. When reviewing standards for galvanizing and thermal spray metallizing, it is difficult to find any true performance assessments other than predictive life-cycle calculations. Most of the related standards and techniques revolve around preparation and application (Table 2).

TABLE 2 Standards for Galvanizing and Thermal Spray Metalizing

StandardDescriptionCategory ASTM A123 Spec for HDG Coating of Iron and Steel Quality of Application ASTM A143 Practice for Safeguarding Against Embrittlement (HDG) Quality of Application ASTM A384 Practice for Safeguarding Against Warpage and Distortion (HDG) Quality of Application ASTM A385 Practice for Providing High-Quality Zinc Coatings Quality of Application SSPC-CS 23.00 Spec for the Application of Thermal Spray Coatings (Metallizing) of Aluminum, Zinc, and Their Alloys and Composites for the Corrosion Protection of Steel Quality of Application SSPC-QP 6 Procedure for Evaluating the Qualification of Contractors Who Apply Thermal Spray Quality of Application ASTM A780 Practice for Repair of Damaged and Uncoated Areas of HDG Coatings Quality of Application ASTM E376 Practice for Measuring Coating Thickness Quality of Application

ASTM B 117 Salt Fog testing has been used for decades to test paints and coatings for their resilience against corrosion. The salt fog test is a constant fog of neutral salt (5% NaCl) at 35 degrees Celsius. The test panels typically are scribed, exposing bare steel, and are tested over the course of hundreds or thousands of hours. While this test has not been sufficiently correlated to real-world ageing (9), it does offer the formulator a powerful screening tool for identifying prototypes that will likely offer corrosion protection properties. Figure 4 shows results of a two-coat inorganic system after 14,954 hrs salt fog testing.

METHODS

Description of Systems Tested

Five corrosion protection systems were tested by an outside, independent laboratory: a two-coat inorganic system, a low-VOC two-coat inorganic system, galvanizing, thermal spray metallizing, and thermal spray metallizing with a urethane clear sealer. Table 3 describes the samples.

TABLE 3 Corrosion Protection Systems

System IDPanel IDDescriptionDFT Coat 1DFT Coat 2Cure A 40A Low-VOC Inorganic Zn Primer/Low-VOC 2.8 3.6 Minimum 7 days ambient A 44B Low-VOC Inorganic Zn Primer/Low-VOC 3.4 2.8 Minimum 7 days ambient A 48B Low-VOC Inorganic Zn Primer/Low-VOC 3.3 3.1 Minimum 7 days ambient A 61B Low-VOC Inorganic Zn Primer/Low-VOC 3.3 4.2 Minimum 7 days ambient A 62B Low-VOC Inorganic Zn Primer/Low-VOC 3.0 5.1 Minimum 7 days ambient B 4B Inorganic Zn Primer/Inorganic Finish 2.6 3.6 Minimum 7 days ambient B 5B Inorganic Zn Primer/Inorganic Finish 2.7 3.6 Minimum 7 days ambient B 6B Inorganic Zn Primer/Inorganic Finish 2.9 4.4 Minimum 7 days ambient B 7A Inorganic Zn Primer/Inorganic Finish 2.5 3.8 Minimum 7 days ambient B 9B Inorganic Zn Primer/Inorganic Finish 2.8 4.7 Minimum 7 days ambient C 1 Hot-Dipped Galvanizing 3.3 ---- ---- C 6 Hot-Dipped Galvanizing 3.4 ---- ---- C 7 Hot-Dipped Galvanizing 3.6 ---- ---- C 10 Hot-Dipped Galvanizing 3.4 ---- ---- C 13 Hot-Dipped Galvanizing 3.5 ---- ---- D 1 Thermal Spray Metallizing 13.8 ---- ---- D 3 Thermal Spray Metallizing 14.0 ---- ---- D 7 Thermal Spray Metallizing 12.4 ---- ---- D 9 Thermal Spray Metallizing 13.3 ---- ---- D 10 Thermal Spray Metallizing 13.0 ---- ---- E 1 TSM/Urethane Sealer 19.0 19.0 Minimum 7 days ambient E 6 TSM/Urethane Sealer 22.0 22.0 Minimum 7 days ambient E 9 TSM/Urethane Sealer 18.0 18.0 Minimum 7 days ambient E 10 TSM/Urethane Sealer 19.8 19.8 Minimum 7 days ambient E 11 TSM/Urethane Sealer 20.1 20.1 Minimum 7 days ambient

Preparation of Galvanized and Thermal Spray Metallizing Panels

Hot-dipped galvanized panels were prepared according to ISO . Application of galvanizing was on carbon steel panels with an MEK wash prior to application. Thermal spray metallizing panels were prepared according to ISO with 100% zinc. Application of thermal spray metal was on ASTM A36 carbon steel panels with a surface prep of G50 grit blast to SSPC-SP5 with a 1-3 mil anchor profile.

Application of Painted Samples

Painted samples were spray-applied to ASTM A36 carbon steel with a surface prep of G50 grit blast to SSPC-SP5 with a 1-3 mil anchor profile. Spray application utilized airless spray (60:1) with a 517 tip. Pressures varied between the various coatings ranging from 2,000 to 2,500 psi.

Additional Conditions

Table 4 highlights additional details surrounding the conditions around sample panel preparation. Environmental conditions and time to test are of particular note. Inorganic silicates cure with moisture in the atmosphere. Relative humidity is an important variable to factor when using this coating chemistry. Also, age of the panel when tested is of consideration as all of the systems tested use reactive zinc metal. As the coating ages zinc will react with atmosphere and form corrosion products. The degree of this reaction may have an impact on final performance. As all systems were applied and tested by an outside lab, conditions were not necessarily kept controlled.

TABLE 4 Additional Application Conditions

SystemTemp. (Degrees Celsius)% RHDays Prime to TopcoatDays Application to Test System A Primer, 22.5; Topcoat, 24 Primer, 24; Topcoat, 27 3 33 System B Primer, 24.5; Topcoat, 29.5 Primer, 42; Topcoat, 42 1 27 System C 21 days elapsed from receipt of galvanized panels to test start System D 2 days elapsed from receipt of metallized panels to test start System E 11 days elapsed from receipt of sealed metallized panels to start of test

RESULTS

ISO -9 Annex B Cyclic Ageing rates panels on blistering, rusting, cracking, and flaking. It also requires a measurement of scribe creep. Scribe creep was assessed and measured based on the visible rust observed at the scribe. Since the ISO -9 standard requires an assessment for measurement under the scribe, the scribe creep should be considered an estimate. The passing requirement for ISO CX is shown in Table 5.

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TABLE 5 ISO -9 CX Passing Criteria

Blistering (ISO -2)Rusting (ISO -3)Cracking (ISO -4)Flaking (ISO -5)Scribe Creep* 0(S0) Ri0 0(S0) 0(S0) < or = 3mm

Corrosion under the scribe was not assessed after 4,200 hours as it is destructive. The panels were placed back into testing after the aforementioned assessments. Corrosion under the scribe will be analyzed at the conclusion of the 50 cycles or 8,400 hours testing. Results are listed in Table 6.

TABLE 6 Results of ISO -9 Annex B Cyclic Ageing

*Estimated scribe creep, coating not removed as test is ongoing, **Stained from edge rust or grit contamination

SystemPanel IDBlistering (ISO -2)Rusting (ISO -3)Cracking (ISO -4)Flaking (ISO -5)Scribe Creep* A 40A 0(S0) Ri0** 0(S0) 0(S0) 0.36 A 44B 0(S0) Ri0 0(S0) 0(S0) 0.00 A 48B 0(S0) Ri0** 0(S0) 0(S0) 0.00 A 61B 0(S0) Ri0** 0(S0) 0(S0) 0.29 A 62B 0(S0) Ri0 0(S0) 0(S0) 0.00 B 4B 0(S0) Ri0 0(S0) 0(S0) 0.00 B 5B 0(S0) Ri0 0(S0) 0(S0) 0.00 B 6B 0(S0) Ri0 0(S0) 0(S0) 0.00 B 7A 0(S0) Ri0 0(S0) 0(S0) 0.00 B 9B 0(S0) Ri0 0(S0) 0(S0) 0.00 C 1 0(S0) Ri4 0(S0) 0(S0) 8.19 C 6 0(S0) Ri4 0(S0) 0(S0) 7.40 C 7 0(S0) Ri4 0(S0) 0(S0) 4.25 C 10 0(S0) Ri4 0(S0) 0(S0) 16.75 C 13 0(S0) Ri4 0(S0) 0(S0) 9.64 D 1 0(S0) Ri0 0(S0) 0(S0) 6.33 D 3 0(S0) Ri0 0(S0) 0(S0) 1.85 D 7 0(S0) Ri0 0(S0) 0(S0) 1.42 D 9 0(S0) Ri0 0(S0) 0(S0) 1.74 D 10 0(S0) Ri0 0(S0) 0(S0) 0.38 E 1 2(S3) Ri0 0(S0) 0(S0) 0.48 E 6 2(S3) Ri0 0(S0) 0(S0) 0.31 E 9 2(S3) Ri0 0(S0) 0(S0) 0.30 E 10 2(S4) Ri0 0(S0) 0(S0) 0.19 E 11 2(S4) Ri0 0(S0) 0(S0) 0.21

ISO standards use "0" to signify high-performance. For blistering, ISO uses the designation Density (Size) where, for density, 0 is none, 2 is few, and 5 is dense. For size, S0 is none, S1 is small, and S5 is large. Cracking is rated for quantity, 0 being none and 5 being many, and size from S0 where no cracks are visible under magnification of 10X, S1 is only visible under 10X, S2 is just visible with normal vision, S3 is clearly visible with normal vision, S4 is large cracks up to 1 mm wide, and S5 is very large cracks greater than 1mm wide. For flaking, ISO uses % flaking (size) where 0 is no flaking, 1 is 0.1%, 3 is 1%, and 5 is 15%. The flaking size scale starts at S0 under 10X magnification, S1 up to 1mm, S2 up to 3mm, S3 up to 10mm, S4 up to 30mm, and S5 larger than 30mm. Rusting is measured for % area and is rated as Ri0 being 0 rusted area, Ri1 being 0.05%, Ri3 being 1%, Ri4 being 8%, and Ri5 being 40%.

Photographs after the panels were removed from the 25 cycles of testing are shown in Figures 8 through 12.

DISCUSSION

The low estimated scribe creep indicates that the two-coat inorganic systems and the thermal spray metallizing systems offer continued cathodic protection overexposure, with the System B two-coat inorganic system showing no scribe creep on any of the five panels tested. None of the systems showed any cracking or flaking, and only the thermal spray metallizing system with the urethane sealer showed blistering. Field or plane rusting was not seen on the two-coat inorganic systems or the thermal spray metallizing systems. The low-VOC inorganic system did appear to have notable rusting at the scribe in two of the panels tested. Previous internal lab testing does not show this. It is not certain whether this variance is within the expected noise of the test or if it is indicative of a true performance difference between the low-VOC and the traditional inorganic system. Further testing will be performed to make a better assessment. The scribe creep measured is far below the allowable scribe creep for passing the test and significantly better than traditional three-coat, as shown in Figures 5 and 6.

It is theorized that the silicate binder allows complexing between the silicate binder, zinc, and iron substrate, allowing for excellent adhesion and contact of the zinc with the steel substrate. The very low to virtually no corrosion at the scribe seems to emphasize the cathodic protection afforded by this technology. This type of performance has been seen with single-coat inorganic zinc, but now it appears that an inorganic finish coat enables the same level of protection. Unlike organic epoxies and urethanes, the silicate-based inorganic finish maintains a level of vapor permeability that permits oxygen, water, and carbon dioxide to reach the outer layer of zinc particles within the zinc-rich primer layer. Zinc corrosion products will form over time and are thought to fill in any pores creating an impermeable layer or patina. The inorganic chemistry of the binder prevents it from degrading in UV exposure. The results of the ISO -9 cyclic ageing test also suggest that the inorganic finish may help to maintain aesthetics by masking the corrosion products that are visible in the thermal spray metallizing panels.

Both the inorganic zinc-rich primer alone and the two-coat inorganic system of inorganic zinc-rich primer with the inorganic finish are rated class B for slip, meaning no roughening and no masking is required when installing with bolted connections. (Tested according to the Research Council on Structural Connections Specification for Structural Joints Using High-Strength Bolts, Appendix A. Class B Certificates are system and manufacturer-specific. Certificates are available upon request from Carboline.) Because inorganic finishes are based on the same inorganic resin as the inorganic zinc-rich primer, topcoating is possible within hours rather than days. Both of these factors improve schedule time compared to the traditional inorganic zinc-rich primer, epoxy, urethane system.

Inorganic coatings use different raw materials than organic coating systems that primarily rely on petroleum feedstocks. The cured silicate-based film is more glass or rock-like and is not thought to contribute to microplastics in the environment.

The galvanized panels exhibited notable rusting in the plane and the scribe. It is unclear if this can be attributed to the galvanizing process for these samples or whether the salt cycling is too aggressive to allow the proper mix of protective corrosion products to form on the outermost zinc layer.

While performance is paramount, cost and schedule are critical to a successful bridge project. Current three-coat system may utilize an inorganic or organic zinc-rich primer. Often, if the DOT or guiding specification allows, an organic zinc primer will be used. Overcoating an organic zinc primer with an epoxy midcoat can be done within the same day or shift even. Applying an organic epoxy midcoat over an inorganic zinc primer requires a minimum of 18-24 hour wait time in order to allow moisture curing of the inorganic zinc primer. In contrast, topcoating of the inorganic zinc primer with an inorganic finish can be done within two hours. The moisture permeability of the finish allows for continued cure of the primer after topcoating. This puts two-coat inorganic coating systems at an advantage over three-coat organic zinc-based systems.

When comparing painting to schedule for galvanizing and thermal spray metallizing, different factors should be considered. Typically, galvanizing is done outside of the fab shop, which can extend schedule depending on the availability and capacity of local galvanizers. Thermal spray metalizing can be done in the fab shop, so that is often not a consideration. What is of consideration is the longer cleaning cycle times used by fabricators to ensure a proper surface for thermal spray coating application.

When reviewing cost, many factors are to be considered. The NSBA published results of a cost survey at the World Steel Bridge Symposium (11). Using uncoated weathering steel as the baseline, the NSBA study calculated the following relative cost comparison. See Table 7.

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