What is a Zinc Filled Coating?

The corrosion of steel or iron is an electrochemical phenomenon wherein the base metal reverses to a lower, more stable more energy state.  If the corrosive environment is water or brine, then the coating product formed is commonly known as rust.  In the case of other chemicals, such as alkalis or acids, other combinations of iron salts are formed as part of the corrosion product.  The electrochemical corrosion process may be retarded or stopped by the proper use of protective coatings.  One preventive method provides an insulation barrier between the corrosive environment and the metallic substrate.  This type of protection is exemplified by the painting of structural steel with organic coatings such as epoxies and vinyls.  An even more effective method is to use a more reactive metal such as zinc.  A conductive zinc-filled coating projects the metal by galvanic protection.  Alternatively, the zinc can be applied by dipping the piece of steel into molten zinc; this is called galvanizing.  The zinc sacrifices itself and corrodes in preference to the steel.  This method of protecting the steel will continue until the effective available zinc is used up.  A local corrosion attack on the zinc, also proceeds simultaneously with the sacrificial reaction.

The rate that these reactions proceed determines the useful life of the zinc coating.  The ideal condition is to keep the local zinc corrosion at an absolute minimum while the sacrificial reaction proceeds just fast enough to provide protection.  In almost any condition, if the local corrosion were allowed to proceed unabated, zinc coatings would prove most uneconomical.  However, the corrosion of zinc is uniquely fitted for use as a sacrificial metal.  In atmospheric exposure, zinc reacts with the moisture and oxygen from the air to form zinc oxide and zinc hydroxide.  These salts are fairly soluble and if this were the only reaction that occurred, the life of zinc coated surfaces would be quite short.  However, a second reaction takes place with carbon dioxide from the air to form an insoluble zinc carbonate.  These salts take up more volume than the original zinc they came from.  This basic zinc carbonate is a tightly adherent, semi-permeable barrier that protects the remaining zinc from corroding too rapidly, while providing sufficient permeability for the necessary electrical current to keep steel from corroding.

In some cases however, insoluble zinc salts are not formed and a local corrosion rate becomes so rapid that zinc coatings are impractical.  This is particularly true in strong acid for base service.  Practice indicates that for immersion service of zinc coatings, the electrolyte pH should be between 5 and 10 for the local reaction to be kept to a minimum.

Is Zinc Rich Coating rust resistant?

The function of a zinc rich coating is to prevent corrosion of the substrate beneath the coated surface.  Topcoats are frequently applied over zinc rich primers to lengthen the life of the system by preventing atmospheric corrosion of the zinc.  This will permit the zinc to be latent and held in reserve.  It would be called upon to sacrifice only when damage to the coating system is severe enough to expose bare steel.

Before discussing zinc rich coatings, let’s briefly cover molten zinc coatings.  They fall into two types as follows:

1. Galvanizing
2. Thermal spray metallizing

These both apply to the steel surface a relatively pure continuous layer of zinc metal.  These zinc films, when properly applied, do not contain voids.  Therefore, all of the chemical reactions occur at the surface of the zinc.  Normally, this is primarily a local corrosion attack on the zinc.  The steel underneath is protected initially by the barrier property of the layer of zinc.  Of course, any steel exposed by damage to the zinc layer will be protected galvanically.  The reactions occurring at the surface form soluble salts that have a tendency to be eroded away before much zinc carbonate can form.  What zinc carbonate is formed is right at the surface where it is vulnerable to erosion.  As time passes, the film becomes thinner and thinner until finally steel is exposed.  At first the exposed steel is protected by the remaining zinc but by the time steel is exposed, very little zinc is left.  Since the film was pure zinc, there was nothing else there to hold the salts that were formed together.  These films don’t have a nonmetallic, non corrodible binder.

The salt formation at the surface of these zinc films causes problems when they are topcoated.  It is all but impossible to remove all the soluble salts at the surface.  Normal topcoats do not adhere very well to these pure zinc films.  Any traces of soluble salts can cause osmotic blistering.  Normally, a conversion coating is used to form a zinc phosphate layer for improved adhesion.  This involves an extra labor step for tiecoat that requires greater than normal care in application.  Many of these problems have been mitigated by the use of zinc-dust filled coatings.

Before we discuss the different types of zinc-rich coatings, let’s assume we are the initial laboratory people trying to develop a zinc rich coating.  What characteristics should we build into that coating to give effective cathodic protection and long service life?

1. The coating should have the ability to carry a high volume of zinc and maintain good film integrity.

2. The coating should permit electrical contact between the zinc particles and between the zinc particles and the metal substrate.  The binder or vehicle must not surround and insulate individual particles of zinc; nor insulate the zinc particles from the steel at the surface interface.

3. The zinc must be allowed to sacrifice freely without destroying the integrity of the film.

4. The binder must have fairly good overall alkali resistance, as the zinc salts that are formed are alkaline.

5. A binder must be used that will “glue” the zinc particles to each other and the substrate.

6. The film should be porous.  This point is somewhat controversial since it goes against the conventional way of thinking for protecting metallic substrates.  However, if you will remember the mechanisms of cathodic protection, three items are necessary to set up a galvanic cell – an anode, a cathode, and an electrolyte.  The porous nature of a film will allow the easy penetration of electrolyte to set up a good galvanic cell.  The porosity also allows for the internal formation of salt and the resulting additional solid volume without the destruction of the integrity of the film.

Basically, we can divide the types of zinc coatings into two broad classes by the type of resins or binders used.  One class is called inorganic zinc.  These are zinc-rich coatings that have as their binder a substance of inorganic nature, i.e., based on any substance other than carbon.  The other type is organic zinc-rich primers.  These are zinc coatings that have as a binder an organic resin, i.e., based on compounds of carbon.

The different types of Inorganic Zinc Filled Coatings 

Alkali Silicates: Post-Cured 

These are water-based materials.  The application of this type of inorganic vehicle is made in two coats.  The first coat is an alkali silicate gel containing zinc powder.  An example of the alkali silicate might be sodium silicate.  After the application of this coat to the surface being protected, an additional coat of curing solution must be applied to set and precipitate the gel.  This is the oldest of all inorganic zinc coatings.  It was developed in the 1940’s and the first famous use of this type was on a pipeline in Australia.  Although a recent inspection found it in excellent shape, the majority of the pipeline is in a non-corrosive desert environment.  Until the curing solution has been applied, the silicate gel is extremely sensitive to moisture and can be readily removed from the surface by unexpected rainfall or even condensation.  Adhesion is poor to surfaces other than white metal.  SSPC-SP5, abrasive blasted steel.  Hardness is very good.  Since the curing solution must penetrate completely through the film to cure, film thickness is limited, otherwise more than one coat of curing solution is required.  If the curing solution does not completely penetrate the film a weak steel/coating interface is formed that can be subsequently dissolved by water.  Cracking and poor cure will result if film thicknesses are excessive.  Before topcoating, the surface must be scrubbed to remove salts formed from the curing agent.  These salts are alkaline in nature and may cause topcoat blistering in wet environments.  If properly applied, however, this type of coating will provide excellent galvanic protection.  This type of product is three-package (binder, zinc dust and curing solution components), but more importantly, requires extra labor in the form of two separate application steps.

Alkali Silicates, Self-Cured  

These are also water-based materials.  After much use of the post-cured type silicates, it became apparent that the tolerance to application variables was less than optimal.  Part of the reason was the importance of the curing solution.  Work began to eliminate this application step.  The resultant type of inorganic vehicles is actually a more recent version of the post-cured type.  They contain modifications that allow the coatings to be self-cured.  The binder may be based on ammonium silicate, lithium silicate, etc.  This type material is insensitive to moisture within hours after application.  Like the post-cured material, adhesion to other than white-metal blasted steel is inadequate.  Also, when applied under very high humidity, drying and curing will be prolonged and water sensitivity will be increased.  The lower the humidity, the faster they will dry.  Again, because of the alkali nature of the binder, topcoated systems using this type of self-cured inorganic zinc may exhibit topcoat blistering in wet environments.  The performance of properly applied self-cured alkali silicates will be excellent.  These type products are two-package (binder and zinc dust components).

Silicate Complexes, Solvent-Based, Self-Cured  

The next major step in the development of inorganic zincs was the elimination of the criticality of field application.  The required white-metal blasts and application technique limitations for the water-based alkali silicates (post-cured and self-cured) had to be overcome.  A product was developed that used alkyl silicate as the starting material for the binder.  This allowed the use of alcohols for the solvent.  Because of the nature of the polymerization reaction, water is required for cure.  These two factors allowed a rapid cure under high humidity conditions and yielded adequate adhesion to less than a white-metal blast.  The coating itself, after being applied to a substrate, becomes water insoluble within a matter of minutes.  Adhesion to commercial-blast cleaned surfaces is excellent.  Because the major solvents are alcohols, it may be applied below 32°F and under very high humidity conditions.  This is a major advantage over water-based inorganic zincs.  After application of the primer on the steel surface, water from the atmosphere then completes the curing reaction.  Contrary to the curing conditions previously discussed for the alkali silicates (post-cured and self-cured), the solvent based silicates cure best under high humidity.

High-Solids Silicate, Solvent-Based, Self-Cured 

When solvent-based, self-cured silicate primers were developed, naturally they were optimized for cost, ease of application, and performance.  In doing this, they turned out to be relatively low in solids.  Recently, solvent emission regulations for VOC (volatile organic content) have been written which caused standard solvent-based self-cured silicate zincs to be non-compliant. There was a need for raising the solids of these materials while keeping all of their positive benefits over water-based inorganic zincs (which by their very nature contain essentially zero VOC).  Products of this type have been and are being developed.  Raising solids in any coating usually entails significant formula modifications.  There will in all likelihood be a number of different approaches to the problem.

Inorganic Zinc Weld-Through Primers  

The use of inorganic zinc primers as weld-through primers was introduced over fifteen years ago and is now standard practice.

The advent of automatic centrifugal wheel abrasive cleaning coincided with the introduction of these materials.  This combination of automatic blasting and a weld-through primer has resulted in reduced overall labor costs.  Acceptance has been widespread, especially in the marine, nuclear, chemical and petroleum industries.

Single Package Inorganic Zinc Primers – The main advantage of the single package silicate was, of course, convenience.  Instead of blending a zinc filler with a silicate base, one simply re-mixes the paint as supplied, things as necessary, and applies the material.  The reaction:

Zn + 2H₂O –> Zn (OH)₂ + H₂

precludes the existence of a water based single package silicate and is part of the consideration when formulating the solvent-based single package.  This type of zinc primer has essentially disappeared from the marketplace.  Three vehicle types were commonly available.  The first involves amine-initiated hydrolysis and colloidal suspension in solvent.  The second type consists of an alkyd silicate in combination with an alkali metal alkoxide.  The third type vehicle is a polyol silicate or a polyol hydrocarbon ether silicate.

One limitation of single package zinc silicates was the incompatibility of certain topcoats that would normally be compatible with two package solvent-based zincs.  This problem exhibited itself through non-adhesion and blistering of topcoats in certain wet environments and is due to the residual alkaline material present in the dry inorganic zinc film.

Performance, non top coated, over commercial blast has been found to be similar to a standard two-package system at the same zinc loading in the dry film.

Stability was a problem.  Careful control over manufacturing is required for excluding excessive moisture to prevent gassing upon storage.  Careful formulation can provide materials with a minimum of settling and caking of the powder, even after long storage.

Because of the handling of the zinc component during manufacturing, which accounts for most of the weight, the packaged cost is usually greater for a single-package inorganic zinc than a conventional two package with equal zinc content.  This, plus the limitation in topcoat ability with some single package zincs under certain conditions, must be weighed versus the convenience of a single package.  Other advantages of single package zincs that have been promoted are less storage space required, easier to use, and increased labor savings.

The different types of Organic Zinc Rich Coatings

Essentially, most types of organic binders that are used in protective coatings have been used as a zinc-rich binder at one time or another, although only a few generic types are widely used.  A brief discussion of these types of organic zinc-rich primers being used follows:

Epoxy

The most common type of epoxy zinc is the polyamide cured because of its reduced tendency to react with zinc powder.  Because a curing agent is necessary, the epoxy polyamide zinc-rich primers are available both as two component and three component materials.  The zinc dust is a dry powder in three component materials.  The advantages of the epoxy polyamide vehicle are the same as those discussed earlier on the maintenance coatings.

Phenoxy 

This type of vehicle is considerably different from the conventional type epoxy resin.  The thermoplastic epoxy is an extremely high molecular weight binder and dries to a tough film through solvent evaporation without the addition of a curing agent.  In this respect, it is really thermoplastic rather than thermosetting.  These are commonly known as phenoxies.  High molecular weight thermoplastic epoxies are available as a single package and two component materials.  The fats that they dry quite rapidly, become quite tough, and a curing agent is not required, make this material a very popular binder for use in zinc-rich coatings, even though cost per mil square foot is usually higher than a conventional epoxy.

Chlorinated Rubber

The advantages of chlorinated rubber are rapid dry, easy application, and being single package.  This type of material has been used as a binder for zinc-rich coatings.  The very high VOC of this type of product has greatly reduced its use in the United States.

Epoxy Ester

Despite the fact that this vehicle is sensitive to alkaline environments, epoxy ester zinc-rich coatings are quite widely used in the automotive industry because of the suitability of this type of material for application on the assembly line.

Urethane

Moisture cured urethane resins are used as binders for zinc-rich coatings.  They are available in both single package and two package versions.  They cure very rapidly except under low humidity conditions and have better solvent resistance than either phenoxies or chlorinated rubbers.

Miscellaneous

For various specialized applications, vinyl resins and silicone resins have been used as binders for zinc rich coatings.

Comparison of Inorganic Versus Organic Zinc Rich Coatings

Organic vehicles are film formers and tend to encapsulate and insulate individual particles of zinc.  These zinc filled organic coatings usually provide some initial galvanic protection because the high loading of zinc used affords some zinc to zinc contact.  It is found that they tend, after being in service for some time, to function only as a conventional type primer with the zinc particles acting only as an inhibitive pigment rather than providing true galvanic protection.  Let’s examine why an inorganic zinc primer performs much better than an organic zinc primer in severe demand areas.  To do this, please refer back to the original list of required properties of an effective zinc rich coating that we developed.  Inorganic zinc rich primers have a greater ability to carry a high volume of zinc than do organic zinc primers.  This can be explained by the high density of the silicate binder.  Look at the following example comparing the zinc loading of an organic versus an inorganic zinc.

	Organic		Inorganic
	Pounds	Gallons	Pounds	Gallons
Binder	10	1	10	0.45
Zinc Dust	90	1.54	90	1.54
	100	2.54	100	1.99

Note that both types have 90% by weight zinc in the dry film.  However, the inorganic coating has 77.4% (1.54 divided by 1.99) zinc by volume, while the organic coating has only 60.6% (1.54 divided by 2.54) zinc by volume.  Again, the difference results from the difference in density of the typical organic binder compared to a typical inorganic binder.  A typical organic binder has a density of about ten pounds per gallon, while SiO₂ has a density of 22.3 pounds per gallon.  Thus, an inorganic zinc primer has a higher volume percent of zinc in the dry film than does an organic zinc primer and an organic zinc primer has a higher volume percent of binder in the film at the same zinc loading by weight.  This fact then leads us to say that an inorganic zinc primer allows more intimate zinc to zinc contact and zinc to steel contact than does an organic zinc primer at the same zinc loading by weight.  We can seeca see this by looking at a graph of film resistance versus the amount of zinc by weight in the dry film.  This is shown in Chart 1.

What are the advantages with Organic Zinc Coatings?

Organic zinc coatings are good for field touch-up, faster to topcoat, and easier to apply.  They have better adhesion to unblasted substrates and some are recommended over SSPC-SP2, power tool cleaned surfaces.  They are generally easier to topcoat due to lower porosity.  They generally have better physical properties such as flexibility and impact resistance.  These advantages are especially observable when comparing the organic zinc rich primers to the water-based alkali silicate inorganic zinc primers.  It is not quite as apparent when comparing the organic zinc primers with the alkyl silicate based inorganic zinc primers.  In other words, the properties of the alkyl silicate primers fall somewhere between the organic properties and the alkali silicate properties.

When do you use inorganic and organic zinc coatings?

Thus, to sum up, inorganic zincs should be used where high performance is necessary.  In less severe environments, organic zincs may be used to provide good protection with better physical properties.  An excellent use for organic zinc coatings, because of their adhesion to unblasted surfaces, is that of field touch-up of shop primed systems that have used inorganic zincs.  In the field, the rapid cure and adhesion of an organic zinc is more desirable where it may be impractical to blast damaged areas and recoat with the inorganic zinc.

Topcoating Inorganic Zinc Primers 

Inorganic zinc primers should always be topcoated if exposed in areas where the zinc will tend to be sacrificed too quickly.  Typical high demand areas are immersion or splash and spillage of salts or water, as encountered in marine areas; and chemical plant exposures where acid and alkali fumes are present.  The function of the topcoat is to insulate the zinc from the environment, thus limiting the need for sacrificial action to small areas caused by mechanical damage or thin spots in hard-to-cover areas like welds, complex shapes and edges.  When the topcoat is damaged, the zinc sacrifices forming zinc salts and may even seal itself with these salts if the area of break is too large.  If no conductive zinc primer were present, attack on the base steel would cause undercutting corrosion, resulting in more and more coating loss as corrosion progressed at the coating/steel interface.

The following discussion is of a very general nature.  A word of warning should be made against loosely applying these topcoat considerations before thoroughly checking with the coatings manufacturer for a specific topcoat recommendation.  For example, while it generally can be said that silicones can be used as topcoats over inorganic zinc primers, it is also possible that a certain silicone product may be incompatible with zinc primers.  There are many ways a coating manufacturer may modify a topcoat to achieve certain properties.  Perhaps a chemist chooses to modify a vinyl coating with an alkyd.  If this modification is substantial, it will make the vinyl topcoat incompatible with the inorganic zinc primer because saponification will take place between the alkyd and the zinc primer.

Also, it is the recommended practice not to use a primer made by one coatings manufacturer and a topcoat by another manufacturer.  The reasons are numerous and obvious.  Again, always rely on a specific recommendation from the coating manufacturer.

High build epoxy topcoats are very useful to insulate zinc primers in areas where electrolytes like water or brines are present.  Epoxy coatings are excellent insulating barriers that can be formulated easily to give high film characteristics.  Epoxy tars are also excellent topcoats and are relatively inexpensive.  The problem with the epoxy tars is that they are sensitive to wet, low temperature applications and can result in adhesion problems with the zinc primers.  Epoxies have proven useful for topcoating inorganic zincs in such areas as offshore drilling structures, ships and barge hulls, buried pipelines, cooling water towers, bridges, etc.

Polyurethanes are also very good topcoats for zinc primers.  Normally, they are not put directly onto a zinc primer in severe wet environments due to potentially poor adhesion between the coats.  However, polyurethanes can be formulated to work well directly over zinc primers in mild environments such as non-coastal weathering.  These types of polyurethanes are formulated to have high film build characteristics for good appearance over inorganic zincs.  In more severe environments, a tiecoat is required or recommended.  Commonly, an epoxy is used as a tiecoat between zinc primers and urethane topcoats.  This type of three-coat system works very well in many environments because of its overall chemical resistance and weatherability.

Both low and high build vinyl coatings and chlorinated rubbers have been used in the past as topcoats for inorganic zincs.  Their use was discontinued in the United States because of high VOC’s and non-compliance with VOC regulations.  Not all vinyls adhere well to these primers, so care must be taken for proper selection.  Tiecoats may be required.  Vinyl topcoated, inorganic zincs are excellent for general non-immersion marine and chemical plant exposures.  Vinyls tend to chalk less than epoxies when exposed to sunlight, but are generally inferior to epoxies in abrasion resistance, solvent resistance and resistance to severe splash and spillage of water, brines, and other chemicals.  Vinyls are generally inferior to urethanes in these same areas.

Acrylic coatings, whether they are solvent-based or the latex type, are useful topcoats for inorganic zincs.  They offer excellent outdoor durability because of their resistance to ultraviolet waves present in sunlight.  They are normally applied in thin films; therefore, for high zinc demand areas, an intermediate high build coat of epoxy or vinyl should be considered.  The typical low viscosity of these products allows easy penetration of the porous inorganic zinc film, greatly aggravating the common problem of topcoat bubbling.

Silicone topcoats are useful for high temperature applications such as stacks and breechings.  Aluminum filled silicones provide protection up to 1,000°F.  At temperatures above 750°F, the zinc and the inorganic primer fuse to the steel, the silicone vehicle volatizes and the aluminum pigment fuses to the zinc, leaving a dense, hard protective film.  The main advantages of this system are the high temperature resistance of the inorganic binders and the galvanic protection from the zinc if moisture condenses when the stacks are down.

Inorganic topcoats may be applied over inorganic zincs.  Generally, these types of coatings use the same binder as the self-curing, solvent-based inorganic zinc – i.e., an alkyl-silicate binder.  Therefore, these topcoats retain the excellent solvent and temperature resistance associated with inorganic zinc primers.  When used in tank lining systems over inorganic zincs, these inorganic topcoats act as a barrier to prevent any loose zinc from contaminating the product without reducing the basic corrosion protection of the inorganic zinc primer.  Two package inorganic topcoats containing aluminum or stainless steel pigmentation are available.  Besides having the advantages listed above, these materials also provide a significant degree of additional corrosion protection.  Remember that inorganic topcoats are useful only over inorganic zinc primers since they wet or soak into the top layer and otherwise are porous themselves.  They are poor film formers and have limited adhesive characteristics of their own.

The above discussion has referred to organic zinc topcoating in general.  There are differences among the types of inorganic zincs available which may alter the topcoat recommendations.  For instance, we discussed that the water-based alkali silicate post-cured and self-cure may exhibit some problems of topcoat blistering in wet environments.

Minimizing Topcoat Bubbling Over Inorganic Zincs

The single greatest complaint about inorganic zincs concerns the topcoat bubbling that exists during application.  Inorganic zinc primers form a porous structure that permits the diffusion of gases and electrolytes vital for their cathodic protection.  This porosity is also responsible for causing topcoats to bubble shortly after they are applied to the zinc as the coating displaces some of the air from the pores.  There are various methods being employed to overcome this topcoat bubbling.

Some modification can be made to the formula of the topcoat.  Basically, there are two theoretical approaches.  The topcoat can be formulated to have a low viscosity and be slow drying.  When applied over an inorganic zinc, this topcoat will bubble vigorously, but the bubbles break and the coating will flow back together, forming a smooth and uniform film.

On the other hand, the topcoat can be modified to be very thixotropic.  When applied over an inorganic zinc, this approach yields little penetration of the topcoat, reducing the bubbling.  The above are theoretical approaches.  Realistically, it is not so simple.  Formulators do not have the liberty of adjusting the rheology of the topcoats without drastically affecting some other important property of the product.

Many times topcoat bubbling over inorganic zinc is greatly aggravated by the presence of loose zinc on the primer.  The loose zinc may be formed by dry spray or overspray.  In this case, the loose zinc should be removed from the surface by wiping, brushing, or hosing with fresh water.

A common means of reducing topcoat bubbling in the field is to use a tiecoat.  Its function is to seal off the voids of the inorganic zinc primer to reduce the penetration and bubbling of the final coat.  Typically, the tiecoat is applied at 1.5 to 2.0 mils dry film thickness.

Another commonly used method in the field is the mist-coat technique.  This consists of a fast pass of the spray gun to seal the surface, followed shortly thereafter by a full wet coat.  This mist coat penetrates and fills the top layer of the voids, reducing the bubbling of the full coat.  Sometimes it works better if the material used for the mist coat is thinned 1:1 by volume.  This greatly reduces viscosity and allows easier penetration of the voids.  If possible, the inorganic zinc primer should be allowed to weather in order to allow the zinc to salt, decreasing the void space.  These salts will seal the surface quite effectively, reducing the tendency of the topcoat to bubble.  This is one reason the alkali silicate post-cured tends to exhibit less topcoat bubbling than other types.  Any loose salt build up on the surface should be removed, however, before topcoating.

None of these methods alter the high performance characteristics of the inorganic zinc primer.  There still remains the option of the formulator to modify the inorganic zinc primer.  The concept of this approach is very simple and straightforward.  A reduction of topcoat bubbling will occur with a reduction of the porosity of the film.  This can be accomplished with the addition of binder or organic materials.  There exists some major tradeoffs that occur when organic resins are incorporated into the inorganic zinc primer.  As the organic content is increased, the solvent resistance and temperature resistance decreases.  More importantly, the performance of the zinc primer is affected.  Any modifications to reduce the porosity with an organic material will give properties of intermediate character between the inorganic zinc and the organic zinc.  Thus, to obtain reduced topcoat bubbling by increasing the organic content of the zinc primer, you must be willing to sacrifice performance and material costs.  It is left to the customer to weigh the advantages and disadvantages of this approach.

How much does Inorganic Zinc cover? 

A solid gallon will cover 1,604 square feet at one mil.  For conventional coatings, a product that has 50% volume solids will get a coverage of 802 mil square feet per gallon.  However, this calculation is valid only for coatings that form a continuous film with no porosity present.  The calculation for coverage of inorganic zincs is somewhat more complicated.  Remember that a good deal of porosity is present in inorganic coatings.  Therefore, this air space extends the coverage that is obtained over steel with an inorganic zinc primer.  If an inorganic zinc coating is 50% volume solids, its coverage may be obtained by multiplying the theoretical volume solids times 1604 and dividing by 1-minus the fraction void content.  This formula is shown below.

Note that for most normal organic coatings the void content is zero, so the bottom part of the fraction becomes the number one leaving the more common version of the equation.

Suppose the theoretical volume solids of an inorganic zinc primer is 50%.  Further, assume that the void content in the film is measured to be 20%.  The coverage is then calculated as follows:

[ .5 x (1604) ] / (1-.2) = 1002.5 mil ft.² / gallon

ASTM D2697 is a test method for measuring the volume solids of coatings.  To determine the dry volume of a coating in this test water is used as a displacement liquid.  This will not work with an inorganic zinc coating, because the water will penetrate into its porosity.  This would result in an incorrect value for the apparent volume of the coating.  The porosity is part of the apparent thickness of the dry coating film and is included in dry film thickness measurements.  ASTM D2697 does allow for the use of alternate displacement liquids.  Because of the porosity in inorganic zinc coatings the displacement liquid used should be mercury.  The surface tension of mercury is high enough that it does not penetrate significantly into the pores of the coating.  This allows for an accurate measurement of the apparent coverage of inorganic zinc coatings.