ZINC-RICH COATINGS – HOW THEY WORK AND HOW TO CHECK IF THEY'LL WORK

John Repp & J. Peter Ault Elzly Technology Corporation

835 Wesley Avenue Ocean City, NJ 08226

Andrew Sheetz

Naval Sea Systems Command - Carderock Division Marine Corrosion Branch, Code 613

9500 MacArthur Blvd. W. Bethesda, MD 20817-5700

ABSTRACT

Zinc-rich paints are a commonly used method of corrosion control. The basic premise is that these paints provide sacrificial cathodic protection to the steel substrate by using zinc particles as the pigment within the coating. For this protection to occur zinc particles must touch each other and the steel substrate providing electrical contact; as particles are consumed the protection is reduced until all the zinc is consumed or electrical continuity is lost. Anecdotally, the optimum zinc loading is 90% (or greater) by weight, providing a film with sufficient zinc particles and contact to provide corrosion protection. In an atmospheric environment there are additional factors that can affect performance, such as: periodic wetting and re-wetting of the surface, moisture coverage, topcoating, etc. This project investigated the performance of zinc-rich coatings on steel surface with varying loadings (percentage by weight) of zinc through electrochemical and atmospheric testing. Methods to quantify zinc loading and performance were also investigated to ascertain film quality and ability to provide sacrificial cathodic protection. These results are being used to better understand the mechanisms by which zinc-rich coatings provide corrosion protection, factors that may impact that protection and methods to perform Quality Assurance (QA) testing on the as-cured film. The results to date are summarized in this paper. Keywords: zinc-rich, coatings

INTRODUCTION Zinc-rich coatings are used by many industries for the corrosion protection of steel substrates. Uses include highway and infrastructure, industrial and automotive applications. These coating materials have demonstrated improved performance when compared to carbon steels protected only by spray applied organic coatings (such as an epoxy-urethane coating system).

The use of zinc for sacrificial corrosion protection has been performed in automotive applications since the 1970’s, primarily in the form of hot-dip galvanizing and galvanized sheet steel. Zinc-rich paints provide lesser protection than galvanizing, although their protection is typically improved as compared to organically coated carbon steel. In a manufacturing environment the use of galvanized components or hot-dip processes are usually preferred. Such processes provide a metallurgically bonded, pure zinc coating to protect the substrate. However, zinc-rich coatings are better suited for repair and touch-up operations (including repair during manufacturing). While they will not provide the same level of protection as galvanizing, zinc-rich coatings can help restore the desired corrosion performance. Zinc-rich coatings are described in a number of military and industry specifications. SSPC Paint 20 is an industry specification that categorizes zinc primers according to four vehicle types and three zinc levels. Type I-A includes water-soluble inorganic post-curing vehicles such as alkali metal silicates, phosphates, and modifications thereof that must be subsequently cured by application of heat or a curing solution. Type I-B includes water reducible inorganic self-curing vehicles such as water-soluble alkali metal silicates, quaternary ammonium silicates, phosphates, and modifications thereof. Type 1-B coatings cure by a reaction among the zinc, silicate, steel substrate, and naturally occurring carbon dioxide during and after evaporation of water from the coating. Type I-C coatings include solvent reducible inorganic self-curing vehicles such as titanates, organic silicates, and polymeric modifications of these silicates. These systems are dependent upon moisture from the atmosphere to complete hydrolysis, forming the titanate- or polysilicate-zinc reaction product. Type II coatings involve organic vehicles which may be chemically cured or may dry by solvent evaporation (heat may also be used under certain conditions). Common vehicles for Type 2 coatings include epoxies and moisture cure urethanes. SSPC Paint 20 defines 3 levels of zinc in the dried film: Level 1 is equal to or greater than 85%; Level 2 is equal to 77% to 85% and Level 3 is equal to 65% to 77%. By this definition, coatings with greater than 65% zinc by weight in the dried film are considered “Zinc Rich.” It is important to note that zinc content is weight percent (since the binder is considerably lighter than the zinc, volume percent will be much lower) and that such percentages are determined in the dried film, so the wet product will likely have a lower zinc content. Finally, note that zinc particle size and purity are also issues which may impact performance but are not addressed in the specification. There are a number of military specifications for zinc rich coatings including DOD-PRF-24648, Primer Coating, Zinc Dust Pigmented for Exterior Steel Surfaces (16 July 1985, Canceled 14 January 2008); DOD-P-23236, Type 3 (circa 1982); MIL-DTL-24441, formula 159 (Epoxy zinc rich with approximately 91% zinc in dry film by weight) and A-A-59745, Commercial Item Description (CID), Zinc-rich Coatings. A-A-59745, Commercial Item Description, Zinc-rich Coatings is most commonly used on military ground vehicles. The CID lists several pertinent characteristics for zinc rich coatings including 90% or greater zinc content, ability to be applied by spraying or brushing and compatibility with CARC primers. The CID also outlines several performance requirements including the ability to resist underfilm corrosion in a cyclic accelerated corrosion test, minimum pull-off and cross-cut adhesion and the ability to resist cracking when bent around a mandrel. Most products supplied under this CID incorporate a moisture cure urethane binder. Organic zinc-rich coatings have been used for ground vehicle systems within the DoD. An example of the use of zinc-rich coatings includes the Medium Tactical Vehicle Replacement (MTVR). This system uses zinc-rich paints on steel frame members for corrosion protection. This is performed during manufacturing on cargo bodies by the Original Equipment Manufacturer (OEM). Another example is the recent Naval Message by the USMC Corrosion

Prevention and Control (CPAC) Program Office, which now allows for the use of zinc-rich coatings on ground weapon systems for upgrade and / or the repair of damaged galvanized sheet steels. Both of these applications of zinc-rich coating are based on products that meet the salient characteristics of the TACOM CID A-A-59745 for zinc-rich paints. In the 2002 Tri-Service Corrosion Conference paper, “Evaluation of Zinc-rich Primers for use on Army Vehicle Systems”, two (2) zinc-rich paints overcoated with the complete CARC system were evaluated for use on military vehicles. Test samples were evaluated through 120 cycles of GM9540P and in natural marine atmospheric exposure along the Atlantic Seaboard (Sea Isle City, NJ and Cape Canaveral Air Force Station, FL). Zinc-rich coatings were applied to pre-rusted steel panels cleaned to a SP-3 condition and virgin steel panels cleaned to a SP-10 condition. All samples were rated using the TACOM stages of corrosion (0 = no corrosion, 4 = complete perforation). Figure 1 shows an example of the stages or corrosion.

 Figure 1. Example of Stages of Corrosion on Steel

At the completion of the GM9540P testing all of the zinc-rich samples over a SP-10 surface with scribes had corrosion ratings of 0 (no deterioration). The zinc-rich samples over a SP-3 surface with scribes had ratings of 1, with one system having a rating of 2. On samples with chipping damage, stage 2 corrosion was observed on all zinc-rich samples, except one system applied over a SP-10 surface. However, the performance of these samples was comparable or better than the control samples with only a CARC coating (where the control samples achieve a stage 2 rating sooner than most zinc-rich samples). In marine atmospheric exposure similar results were observed on chipped samples through six (6) months of testing (no deterioration was observed on scribed samples). Samples where the zinc-rich coating was applied over a SP-10 surface had better performance than those applied over a SP-3 surface. In both environments all zinc-rich and control samples had some corrosion after six (6) months, although corrosion occurred sooner on the control samples and the samples where the zinc-rich paint was applied over a SP-3 condition. At the Cape Canaveral Air Force Station (CCAFS) site (in the paper shown as KSC) the zinc-rich paint applied over a SP-3 condition was rated as having stage 2 corrosion at the completion of testing. Figure 2 provides copies of the corrosion data collected during this program.

Corrosion Rating - GM9540P, Scribed Panels

0 20 40 60 80 100 120

Control

Galvanized

SP-10/Zinc-A/Carc

SP-3/Zinc-A/Carc

SP-3/Zinc-A/Epoxy/Carc

SP-10/Zinc-B/CARC

SP-3/Zinc-B/CARC

SP-3/Zinc-B/Epoxy/CARC

Sys

tem

Cycles Completed

Stage 0

Stage 1

Stage 2

Stage 3

Stage 4

Corrosion Rating - GM9540P, Chipped Panels

0 20 40 60 80 100 120

Control

Galvanized

SP-10/Zinc-A/Carc

SP-3/Zinc-A/Carc

SP-3/Zinc-A/Epoxy/Carc

SP-10/Zinc-B/CARC

SP-3/Zinc-B/CARC

SP-3/Zinc-B/Epoxy/CARC

Sys

tem

Cycles Completed

Stage 0

Stage 1

Stage 2

Stage 3

Stage 4

GM9540P Corrosion Results – Scribed Samples

GM9540P Corrosion Results – Chipped Samples

Corrosion Rating - KSC, Chipped Panels

0 1 2 3 4 5 6

Control

Galvanized

SP-10/Zinc-A/Carc

SP-3/Zinc-A/Carc

SP-3/Zinc-A/Epoxy/Carc

SP-10/Zinc-B/CARC

SP-3/Zinc-B/CARC

SP-3/Zinc-B/Epoxy/CARC

Sys

tem

Months of Exposure

Stage 0

Stage 1

Stage 2

Stage 3

Stage 4

Corrosion Rating - SIC, Chipped Panels

0 1 2 3 4 5 6

Control

Galvanized

SP-10/Zinc-A/Carc

SP-3/Zinc-A/Carc

SP-3/Zinc-A/Epoxy/Carc

SP-10/Zinc-B/CARC

SP-3/Zinc-B/CARC

SP-3/Zinc-B/Epoxy/CARC

Sys

tem

Months of Exposure

Stage 0

Stage 1

Stage 2

Stage 3

Stage 4

CCAFS Corrosion Results - Chipped SIC Corrosion Results - Chipped Figure 2. Corrosion Data from "Evaluation of Zinc-rich Primers for use on Army Vehicle

Systems (2002 Tri-Service Conference) These results are similar to the results obtained in the bridge industry and suggest that zinc-rich paints provide at a minimum a similar, if not an improved corrosion resistance versus using an organic coating alone. However, that performance can be reduced when this system is applied over a minimally prepared substrate, but is still similar or better than an organic system alone.

CORROSION CONTROL FOR ARMOR GRADE STEEL Experimental Approach A current US Marine Corps project includes testing to demonstrate the ability of zinc rich coatings to provide corrosion protection to armor grade steel where coating damage (e.g., a scribe) has occurred. Painted test panels are exposed in natural atmospheric exposure (Ft. Lauderdale, FL) while a duplicate set has completed 120 cycles of exposure in an accelerated corrosion test (GM9540P). Test samples are 4-inch by 6-inch by 1/8-inch MIL-A-46100 armor steel panels. Panels were prepared by abrasive blasting to a nominal 1-mil (25.4-µm) profile prior to coating. The control coating was MIL-P-53030 epoxy primer with MIL-C-64159 CARC topcoat. The primary series of test panels was prepared to investigate the effectiveness of a zinc-rich coating conforming to Commercial Item Description (CID) A-A-59745. Three products meeting

the CID were evaluated untopcoated and as part of the CARC system. All three products employed a moisture cure urethane binder. The zinc-rich primer was applied direct to metal with a CARC system applied over the primed surface (MIL-DTL-53030 epoxy primer and MIL-DTL-64159 CARC topcoat). A second series of test panels was prepared with untopcoated alternatives to the zinc rich products meeting the CID. Specifically, a commercially available ethyl silicate inorganic zinc coating and a zinc rich epoxy meeting the requirements of MIL-DTL-24441, formula 159 were compared to the CID products in an untopcoated condition. Zinc powder tends to settle out of zinc rich coatings in their liquid state. This phenomena sometimes results in a situation where coating is applied with less than the designed zinc content. A third series of test panels were prepared to understand the potential impact of low zinc content in a coating meeting the CID. For these test panels, a coating was mixed with less than the designed level of zinc powder. Specifically, the coating was mixed with 100%, 90%, 75% and 50% of the designed zinc content. Test panels were prepared as described above in both topcoated and untopcoated conditions. During this testing all samples were visually evaluated at regular intervals for visible corrosion (ASTM D610), coating blistering (ASTM D714) and scribe creepage (mm of visible underfilm cutback from the scribe. For natural exposure samples this is nominally every quarter (three months) through one (1) year of exposure. For accelerated corrosion samples this was every 20 cycles through 120 cycles of testing. At each inspection interval samples will be photographed. The natural exposure samples are still in test; this paper will focus on the accelerated test results. At the completion of accelerated testing the panels were destructively evaluated for corrosion beneath the coating. The destructive inspection included removing coating and corrosion product such that the extent of metal loss could be measured with a pit gage. Following is a discussion of the data from the accelerated corrosion test. Synergistic Effect of Zinc Rich Primer with CARC System Figure 3 shows a representative test panel before and after cleaning for analysis. The extent of undercutting was determined by measuring the extent that the coating could easily be removed using a controlled amount of pressure. The maximum pit depth was the deepest pit measured in the corroded area using a Pit & Crack Depth Gauge with a resolution of 0.5 mils. Figure 4 shows the maximum measured pit depth and maximum measured undercutting respectively for the control Epoxy/CARC system, the untopcoated zinc rich coatings meeting the CID and the same zinc rich coatings with the control epoxy/CARC system applied over the CID primer. Note that the zinc rich primer with or without a topcoat resists undercutting and pitting better than the control system. Deepest pits were 2-3 times deeper under the control versus the systems with zinc rich primer.

   

Figure 3. Test panels after exposure (left) and cleaned for evaluation (right).

 Figure 4. Corrosion observed on test panels after 120 Cycles GM 9540P exposure.

Figure 5 shows the maximum measured pit depth and maximum measured undercutting respectively for the control Epoxy/CARC system and the three classes of zinc rich coatings without a topcoat. As would be expected, the inorganic zinc coating performed considerably better than either type of organic zinc material. However, both organic zinc materials performed better than the control system. Finally it is worth noting that the zinc rich meeting the CID performed better both from an undercutting and pitting perspective than the zinc rich epoxy.

 Figure 5. Corrosion observed on test panels after 120 cycles of GM 9540P exposure.

MECHANISMS OF PROTECTION

While the sacrificial protection mechanism of zinc rich coatings are often highlighted, it is important to recognize that the protection mechanisms are far more complex. Figure 6 schematically shows three stages in the life of a zinc rich coating. After application, the zinc rich coating should be a continuous film which effectively isolates the substrate from the environment. At this stage, the coating is behaving as a barrier. Once a defect is made in the coating which exposes the substrate, a certain amount of sacrificial protection is provided by the zinc rich coating. This protection depends on the amount and type of moisture present, the electrical connectivity of the zinc particles to each other and the substrate, the purity of the zinc and other factors. After some finite time, the zinc will be depleted and sacrificial protection will cease. Some have suggested that there is still some type of protection provided to the steel after this sacrificial phase, perhaps due to the inhibitive effects of surface films.

 Figure 6. Schematic of zinc rich coating protection mechanisms.

Experiments are being performed to better understand the practical level of protection afforded by zinc rich coatings. In an effort to understand the behavior of the zinc rich coatings meeting the CID, anodic polarization curves were run on replicate test panels with various products and compared to galvanizing. Figure 7 shows selected curves run in seawater. Note that the potential of the galvanizing does not shift much over a wide range of current values. This behavior is expected of an effective anode. The curves for the zinc rich coatings tend to polarize at lower current values, indicating that they are less effective anodes. Figure 8 attempts to explain the observed phenomena with steel and zinc polarization data from the literature.1 The polarization curve for zinc agrees fairly well with the galvanized curve shown in Figure 7. A second curve has been developed representing the effect of a smaller zinc to steel area ratio (1:100 versus 1:1). The area ration change has the effect of shifting the zinc polarization to the left by two decades of current. This curve is consistent with the measured data for the zinc rich coatings. The analysis suggests that zinc rich coatings have the capacity to engage a much smaller quantity of zinc in the protective behavior.

                                                            1 Based on data in "Atlas of Polarization Diagrams for Naval Materials in Seawater," Harvey P. Hack, April 1995 

 Figure 7. Anodic polarization curves for various zinc rich coatings and galvanizing.

 Figure 8. Interaction of couples with different zinc:steel area ratios.

An experiment was designed to quantify the level of sacrificial protection provided to steel by a zinc rich coating. The test setup essentially involved assembling a zinc coated and bare steel test panel with a clear section of PVC pipe between them such that it could be filled with an electrolyte and the electrical current flowing from the zinc coated panel to the steel was monitored. Provisions were made so that the protected and unprotected electrochemical potential of the steel panels could also be determined. Figure 9 shows a schematic of the test setup.

 Figure 9. Schematic of galvanic experimental setup.

Figure 10 shows the current provided by each of the coatings as a function of time. Figure 11 shows the potential of the couple as a function of time. At the end of the 10-day exposure, the steel panels coupled to the Inorganic zinc, MC Zinc A and MC Zinc B had minimal corrosion while the panels coupled to the other two coatings had corrosion over the entire steel surface.

 Figure 10. Galvanic current provided by various zinc rich coatings.

 Figure 11. Couple potential versus time for various coating-steel couples.

QUALITY ASSURANCE FOR ZINC RICH PRIMERS It is clear from the data presented in this paper that zinc-rich coating performance can vary greatly with coating chemistry and zinc content. However zinc content alone is not sufficient to guarantee effective protection. Other formulation (e.g., zinc particle size) and application parameters (e.g., effective mixing) are critical to ensuring that the zinc particles are effectively distributed in the film. Proper formulation can be validated in laboratory testing during product development. Future work is being directed toward QA practices that can be used in the field to validate that the product as applied will provide the protection that was originally designed into the coating.

CONCLUSIONS

1. Zinc rich coatings offer improved corrosion protection when added underneath a traditional epoxy-polyurethane CARC coating system. The benefit can be measured both as reduced undercutting from a scribe and reduced depth of corrosion penetration.

2. Zinc rich primers offer corrosion protection through both sacrificial and barrier mechanisms. The relative contribution of these and other protection mechanisms is not generally understood.

3. The performance of zinc rich primers can be affected by formulation and application variables. More work is required to determine appropriate quality assurance techniques that can be used in the field to validate that the coatings have been properly applied.