Green Alternatives to Using Zinc Potable Water Systems Authored by Steve Harrison, Don Futch, and Mitch Connor Prelude During the mid 1990’s, several prominent coatings manufacturers started to promote and endorse the use of zinc‐rich coatings as a liner for potable water immersion service. This was considered a new frontier and was met with resistance by the remaining coatings manufacturers that chose not to participate in this new arena for zinc‐rich coatings. At that point in time, the water tank industry was in the midst of removing lead‐based water tank linings. Some coatings manufacturers felt that reintroducing another “heavy metal” back into this environment was a future recipe for disaster and refused to enter this fight. Others felt that the science of this technology didn’t make sense and therefore avoided the fight altogether. Over time, some manufacturers reluctantly entered this market with an offering to compete for business, concerned that they might miss out. Other coatings suppliers that had entered the market suddenly withdrew their offerings over technical issues and left this controversial fight. One thing that has come from this event is that every company, inspector, and professional organization that is involved in this arena has a strong opinion and has an explanation to defend or make their case. In this article, we will be presenting some background about the zinc‐in‐immersion technology itself while addressing some of the important questions where the two opposing sides commonly clash. A discussion over failure modes; reasons for failure; and alternate technologies will be discussed. Additional information will detail what factors and features are needed to develop a good lining to protect steel water tanks. The paper will conclude by discussing alternative coating options that exist (including “greener” versions) and the expected performance levels these coatings are anticipated to provide. Introduction The galvanic corrosion protection afforded when using zinc metal coupled to carbon steel is well documented with decades of successful performance. One need only look at the history of galvanized structures and the revolutionary development of “spray‐on” zinc primers to see the effect that zinc has had in helping industry save billions of dollars that would otherwise been spent on corroded asset replacement. Indeed, Mother Nature has provided some wonderful properties in zinc metal. Our own bodies need a certain amount for survival. When the world’s first self‐curing inorganic zinc primer was commercialized in the late 1950’s (Patented in 1962 by Lopata and Keithler) the painting of steel structures has never been the same. Large steel members used in the construction of the world’s infrastructure (bridges, power plants, refineries, chemical plants; the list is endless) were easily protected with easy to use spray‐on zinc primers. Their performance has been undeniably proven over the years. In more recent years some suppliers have promoted the use of zinc‐rich primers (topcoated with other organic films like epoxies) for potable water immersion service. The appeal of using zinc rich primers for use as a corrosion resistant coating is well understood and quite appealing. Their effectiveness in preventing corrosion and their undercutting resistance is outstanding. They have been considered “permanent” primers for structures exposed to weathering for literally decades. They have been shown to be as effective as galvanizing in longevity; and have actually outperformed galvanizing in salt‐laden environments. The economics of zinc‐coated steel for corrosion protection is based on many years of actual field performance. Extending their use from atmospheric exposures (structural steel, tank exteriors) and moving them from outside the tank to inside the tank can be very intoxicating … even seductive. If their performance is so good on the outside of the tank; it is logical to assume they will do just as well on the inside. While they have been used quite successfully for immersion and storage of all types of solvents (inorganic zincs only) it has always been untopcoated. This paper will look at the potential problems when zinc primers are topcoated for immersion service in water exposures and suggest alternate methods and systems to protect steel tanks from corrosion and maintain water purity through the use of “green” alternatives in lining selection and application. Corrosion Protection from Zinc Primers (General) Zinc‐rich primers have been excellent choices for the protection of steel in atmospheric exposures. Zinc metal has inherent properties that are perfect for this use. Zinc is an active metal, and in most atmospheric and water immersion condition, it is anodic to steel. Zinc primers are designed to “react” with their environment, and to sacrifice themselves to protect the underlying steel substrate from corrosion. This protective process is a form of cathodic protection. In moist neutral or alkaline conditions, and in the presence of oxygen, this protective process results in the formation of zinc hydroxide, as depicted in the reaction below. 2Zn + 2H2O + O2 = 2Zn(OH)2 In the presence of water and oxygen, oxidization of the zinc occurs, and each atom of zinc releases 2 electrons. Subsequently, an oxidation‐reduction reaction occurs, and with the electrons donated from the oxidation of the zinc atoms, form hydroxyl ions (‐OH). These hydroxyl ions are quickly consumed in reaction with the previously formed zinc ions, to form the zinc hydroxide, as depicted in the balanced formula above. The additional presence of contaminants (chlorides; sulfides) such as typically found in an industrial environment can and will accelerate this process, form other zinc salts and compounds resulting in further degradation of the zinc metal. This is all designed purposely to consume the zinc rather than the steel substrate. The zinc hydroxide will continue to react with carbon dioxide in the atmosphere and create zinc carbonate compounds (a passivating and water insoluble salt). This is a naturally occurring phenomenon. These zinc carbonate compounds will in effect “seal” any break in the film and prevent undercutting. Thus, this galvanic action of zinc coupled with the carbon steel substrate will prevent corrosion of the steel. The mechanism for their protection is well understood. Variables affecting the corrosion rate of zinc rich primers include water; pH levels (too high or too low); dissolved oxygen and carbon dioxide content; water purity; and salt (chlorides) content. Increasing levels of these variable can and will accelerate the corrosion rate of zinc. The presence of chlorides will react with zinc to create zinc chloride salts that are hygroscopic (the ability of a substance to attract and hold moisture from its environment). Inorganic Zinc Primers Inorganic zinc primers readily provide cathodic protection because there is “exposed” zinc metal in intimate contact with the steel substrate and with the other zinc particles. They form an electric pathway to provide cathodic protection. These primers are outstanding for atmospheric exposures and will “seal” any break in the film and prevent undercutting. They perform extraordinarily well for this type application. If inorganic zinc primers are topcoated (with say epoxies) and placed in immersion service, water does find its way through the epoxy film, eventually. All coatings are semi‐
permeable membranes which allow moisture; albeit small; to pass through the film. The zinc salts that form in the presence of moisture and oxygen and eventually carbon dioxide will pull water through the film and cause osmotic blistering. This is a very powerful driving force. In the end, what was an advantage for IOZ primers in atmospheric exposure, is now a disadvantage for them in immersion service when topcoated. There is no stopping it, zinc is an active metal and will react with its environment. This is why zinc primers work so well in atmospheric exposures with or without topcoats … but not necessarily for immersion when topcoated. The pictures below depict the differences in the films between an organic and inorganic zinc primer. The inorganic film is filled with porosities and shows intimate and numerous zinc to zinc metal contact. By contrast the organic zinc film shows very few porosities and much more “encapsulation” of zinc particles with the organic polymer resin. Organic Zinc Inorganic Zinc Organic Zinc Primers Some suppliers are promoting organic zincs for immersion when topcoated with epoxies. Organic zincs are also commonly used as primers for atmospheric systems. Again they do very well in atmospheric exposures. Organic zincs do provide “some” cathodic protection … but how? If we look at a film of an inorganic zinc primer, we see that the zinc particles are in intimate contact with one another, as well as the steel substrate because there is barely enough binder to glue them together. In the organic zinc film each zinc particle is encapsulated (in theory) by its organic resin system. And as long as it is encapsulated, it should (in theory) not exhibit any zinc to zinc particle contact or contact with the steel substrate; and therefore not provide any galvanic protection. Of course we know that in fact organic zinc rich primers do in fact provide some cathodic protection as long as the zinc content is sufficiently high. Figure 3 depicts the conductivity of inorganic versus organic zinc primers. One can see that the percent zinc in the dried film for an organic zinc primer must be higher than an inorganic zinc primer in order to create an electrical pathway (less resistance) to occur. The choice of resin and other formulation parameters will also affect the conductivity and ultimate effectiveness of the zinc primer. When sufficient conductivity occurs, it translates into a cathodic protection mechanism. So, in reality there is enough zinc to zinc particle contact and zinc to steel contact to at least provide some cathodic protection afforded by the zinc with organic zinc systems. If not, the zinc would be an expensive filler to use ‐ without benefit. Figure 3: Conductivity of inorganic and organic zinc films as a percent of zinc by weight in the dried film. So, rather than being shielded from the environment; as one would have expected; the organic zincs do in fact have sufficient zinc “available” so that some cathodic protection is achieved. The amount of exposed zinc in organic binders is obviously less than with inorganic binders and consequently reacts to a lesser extent with its environment. As such, organic zincs do ultimately allow moisture, oxygen, and carbon dioxide to reach and then react with the zinc metal albeit in a more restrictive manner than inorganic zinc primers. Why Use Zinc­Rich Primers for Immersion Service in Potable Water? Direct to metal epoxy systems (also called self‐priming epoxy systems) have been used successfully for over 60 years, long before zinc‐rich primers were introduced into this market. Some of the earlier versions of potable water tank linings did contain lead in the primer (and sometimes in the finish as well), but lead‐free epoxy, polyurea and polyurethane coatings have been providing 30+ years of satisfactory service life since the 1970’s, when lead coatings were declared a threat to the public health. The question then becomes, why reinvent the wheel? What are the benefits to be gained by installing zinc‐rich coatings in potable water service? The reactive mechanism that zinc undergoes in atmospheric exposure (with the resultant cathodic protection mechanism) is often applied to immersion exposures as well. Let’s examine this argument. We know that zinc will react with its environment; after all zinc is a reactive metal. When exposed, it will react with its environment and form zinc corrosion by‐products. The by‐
products (at least in atmospheric exposures) serve to “seal” any break in the film. In theory, the zinc beneath the film lays “dormant” until called upon to react with its environment. This is the logic …… but there is a flaw in the argument. Osmosis: The diffusion of water through a semi permeable membrane. More specifically, it is the movement of water across a semi‐
permeable membrane (the topcoat) from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration). Electro­Osmosis: The movement of water through a semi‐permeable membrane as a result of a potential gradient. Volumetric Expansion: The zinc salts that are your coatings ally when un‐top coated are your coating systems enemy when top‐coated. Under the right conditions the zinc salt reaction products, Zn(OH)2, ZnO, ZnCO3, and ZnCl develop at the zinc primer/topcoat inter‐
phase and are the primary cause for topcoat blistering by osmosis and or volumetric expansion. So what happens when the organic zinc primers are topcoated, with say an epoxy? Epoxy formulations are typically great barriers against chemicals; including water. They are chemically cured resins with a high degree of crosslinking making it difficult for chemicals to penetrate the film or break the bonds between the molecules within its polymer matrix. That said, these films are nevertheless semi‐permeable films that in fact do allow water and gases to pass in (and out) of a film. Depending on the formula of the specific lining this movement can be relatively rapid or very slow. (Lining formulations will be discussed later in this paper). As long as there is nothing in the film to “hold” water, the moisture and gases flow into and out of a film. When moisture flows in at a rate greater than the rate going out, a blister occurs. The question then becomes, “Under what conditions will moisture “linger” in a film or be held within the film?” There are several conditions where moisture is “pulled or trapped” within a paint film. 1) If the steel surface beneath the film has contamination such as chlorides, salts, dirt, (worker sweat); contaminated rinse water; etc; water is pulled into the film by osmosis. Water wants to dilute the salts and will drive through the film to try and equalize the concentrations across the membrane (film). 2) The backside steel temperature has a sufficient temperature gradient (colder) than the commodity carried inside the tank. This phenomenon is known as the cold‐wall effect and explains why some applications fail by blistering (water wants to condense on the cold wall like a soda can sweating on a warm summer day); or 3) Contaminants or water soluble compounds are present within the coating that pulls and holds water in the film by osmosis (similar to #1). When zinc primers are topcoated and placed in water immersion the potential for blistering is present. We know that zinc is an active metal and will react with its environment. We know that films are semi‐permeable membranes that allow moisture and gases to pass through. We know that the zinc metal is “available” to react with moisture and oxygen regardless whether the zinc is in an inorganic binder system or an organic one. With the formation of zinc hydroxide and other zinc salts, a powerful osmotic cell is created. With the continued movement of water through the topcoat film due to osmosis, blisters form in the topcoat. Volumetric expansion of these zinc compounds causes the topcoat to disbond, blister, and eventually delaminate. If the zinc primer functions as intended, then blistering will occur. Otherwise, the zinc is not needed. This process is inevitable; it is not a question if blisters will occur, only when. In spite of this potential failure by blistering; suppliers have reported projects where they have thus far performed successfully. Claims of doubling the expected service lives of linings using zinc primers are not uncommon. The practice of using topcoated zinc in immersion for potable water linings goes back approximately ten years. So the claims about longevity are far from reality since many 2‐coat epoxy linings have been known to last 15‐20+ years. Nevertheless, some users continue to use this approach even today. In practice, when epoxy coating systems are applied in the field, variations in film thickness are commonplace and hard to avoid. Thinner areas offer less barrier protection. Breaks or discontinuities (pinholes) allow moisture to easily penetrate. The ability of a lining system to retard or resist moisture penetration is directly related to its formulation, film thickness applied, and degree of cure. Lining systems that contain solvent can be particularly problematic if the solvents have not totally left the film prior to service. Knowing all these potential problems only serves to emphasize the importance of specifying and applying a lining system so that these problems do not occur which might otherwise lead to lining failure. Lining Failures What constitutes a lining failure? The simple answer is … whenever it fails to perform its intended function of protecting the steel from corrosion or protecting the cargo. There may be some linings that partially disbond from the surface; but do not cause corrosion or impart any negative impact to the cargo. No harm done … no failure … at least yet! At some point the lining may continue to disbond; corrosion beneath the film begins; and sooner or later the cargo is affected by pieces of lining or corrosion products from the substrate in the commodity. The most common types of lining failures are; •
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delamination between coats; delamination from the substrate; blistering; cracking; surface breakdown (chemical attack); and Contamination of the cargo. One can grade the performance of lining systems by evaluating the presence of or degree of rusting (ASTM D610), blistering (ASTM D714) both size and frequency, delamination, surface attack, softening, or discoloration of coating film or commodity. In addition; one can also conduct weight gain/loss evaluations on the coating film; wet adhesion (post exposure adhesion compared to pre‐test conditions); or a chemical analysis of the commodity (looking for contamination). This last evaluation is particularly important for food‐grade solutions where contamination, taste, or odor is of prime importance. The chart below is a commonly used performance evaluation to determine suitability of lining systems that takes into account a “weighting” of more important performance elements. Higher score represents better performance. Coating Perfect Performance Coating A Coating B Coating C Penetration or Blisters to substrate Surface Attack or topcoat blisters Softening 40 25 15 10 10 100 40 20 40 25 25 0 12 15 12 9 8 10 9 6 5 95 74 67 Solution Coating Discoloration Discoloration Coating A would be acceptable if very slight coating discoloration or solution discoloration is acceptable. Coating B would not be acceptable since the solution penetrated to the substrate resulting in blistering. Coating B would not be acceptable since surface attack was evident even though it did not penetrate to substrate. Total What Makes a Good Tank Lining? Good performing tank linings share some common characteristics or combinations of characteristics. Many provide excellent barrier protection against the commodity or cargo. The degree of crosslinking (in the case of epoxies) helps define or quantify the level of barrier protection. The choice of resin and curative will determine the overall chemical resistance of the polymer and whether it is suitable for the service. The selection of fillers and pigments has a dramatic effect on performance. Water soluble pigments would obviously not be a wise choice for linings in water service. “Barrier” pigments that may be flake‐like or lamellar that stack together in a film can be desirable to slow down the penetration of water or other cargos. Fillers like mica or micaceous iron oxide (MiO) are inert, non‐reactive, flake‐like pigments that do not react with water, oxygen or carbon dioxide; are excellent choices to coatings in these type exposures. Zinc by contrast, is an active metal and spherical in nature. It is just the opposite of what one would choose to formulate a tank lining product. Topcoating it for immersion, creates a potential problem that will eventually surface. Throughout history, water tank linings were formulated as a barrier coating. Barrier coatings are
designed to limit the permeability of water and oxygen to the substrate which could cause
corrosion of the substrate. What makes a good formula for a water tank lining? Coating
formulations consist of four parts: binder (resin), pigment, additives and solvents. Each of these
parts is critical to a good formula. Let us look at each part:
Binder is normally associated with resins. Resin provides a large portion of the physical
properties in a coating. The choice of resin provides properties that include cure rate, hardness,
impact resistance, flexibility, chemical resistance, adhesion, recoatability, water absorption, and
permeability. Resins normally used in water tank linings are typically epoxies but there has been
a growing interest and use of polyurethanes, hybrid polyurethanes and polyureas. In general, a
formulator is looking for a resin that provides both application properties desired by an
applicator and physical properties desired by the ultimate customer, the tank owner.
Pigments have multiple purposes in a formulation. Those purposes include hiding, color, fillers, reinforcing, and corrosion inhibiting pigments. The choice and the amount pigments used are important depending on the properties desired. Filler and reinforcing pigments can improve certain physical properties and improve barrier properties. Pigments you might want to avoid are water soluble pigments or pigments to could react with water. Inhibitive pigments can be water soluble and have been known to cause osmotic blistering. Pigments, like zinc dust, are used in primers to provide protection of the steel. The problem is that all topcoat coatings are permeable to some degree so when water and oxygen penetrates the film it reacts with the zinc pigment. This reaction forms zinc hydroxide and creates an osmotic cell. That cell is known to cause osmotic blistering in the presence of water. Additives are substances that are added in small quantities to usually improve certain properties. Normally additives are not added unless they are needed. Properties modified by additives can be flow, wetting, film build and dry time. Solvents are liquids that are volatile and are used to lower the viscosity, regulate application properties and control the consistency of the finish. Due to regulations that limit VOC and HAPs in coatings, paint formulations are moving towards higher and higher solids. The choice on the selection of a particular solvent involves the evaporation rate, the water solubility, and its strength (ability to “cut” the resin). A key factor in its selection is whether or not it will remain in the film for long periods and thus influence the threshold limit when extraction testing is done for potable water evaluation. Normally a formulator will avoid slow and water soluble solvents. A well formulated lining for water tanks will provide a good barrier that is stable over the service life. Is Zinc a “Heavy Metal?” We are not talking about Metalica or Blue Oyster Cult. A common argument for opponents of the zinc in immersion theory has been the re‐introduction of a heavy metal (zinc) into drinking water components – right after our society has just about removed all of the lead primer off of the water tanks. Proponents make the claims that galvanized metal has been used in potable water tanks for decades (although we used lead as well). All grades of zinc dust contain levels of lead and other trace heavy metals. Some manufacturers show them on MSDS sheets, whereas others choose not to reveal this because the levels are very low and are not mandated by the Federal Government to provide this information to the consumer or unsuspecting coatings applicator. In recent years, we have seen our Nation’s VOC (Volatile Organic Compound) emission laws become more stringent, especially in the State of California. As our Nation and the world become more aware and sensitive of the environment, will this constricting trend lead to tighter controls over our water supplies? If so, it is reasonable to assume that having some of these “contaminants” in the presence of our drinking water supplies may be frowned upon and removal, encapsulation, or remediation of these coatings may become mandatory. There is a long list of “alternative” coatings that can be used in lieu of zinc‐rich coatings that provide what could be viewed by some as green alternatives. Green Alternatives What makes a liner Green? A lining system can be considered a “green alternative” to a zinc rich coating if it: •
Is considered a solvent‐free lining. •
Has a lower amount of solvents and/or requires a lower amount of solvent to thin or clean up equipment during and/or after application. •
Contains alternative filler(s) that provide the same or similar features and benefits of a zinc‐rich coating without the detrimental side effects. •
Can be applied in a fewer number of coats (preferably one) which reduces labor costs and energy expended during installation. •
Has a higher film thickness, which will provide a longer lasting coating system, thereby reducing the frequency of relining. •
A combination of two or more of these conditions. Solvent­free linings The ideal green coating would be a lining free of any chemicals or solvents. However, it is impractical to line water tanks with desert sand or forest mulch, so solvent free linings are the next best choice. This type of lining system typically consists of 2 components that have liquidity before they are mixed together. When mixed, they react and their liquidity is converted into a solid, solventless film. This reaction can take seconds, as in the case with many polyureas, or several hours, as with many 100% solids epoxy linings. There may be a trace amount of solvent (usually less than 1%) present during the reaction that evaporates or is consumed during the reaction, but these coatings are still classified as solvent‐free linings. An additional benefit to the environment is the little if any VOC’s emitted from these coatings during installation. Besides the 100% solids epoxies and polyureas already mentioned, elastomeric polyurethanes and polyurethane‐polyurea hybrids also fall into this category. Lower Solvent Versions Traditional epoxy linings had hovered around the 50‐60% volume solids range for decades. This type of coating had high VOC levels and contained 40‐50% solvent. In addition, up to 30% solvent would be added to the coating mix. All of this solvent did not make this type of coating green! In the past few decades, efforts by coatings companies created lower solvent coatings that also required little if any additional solvent for their application. Pushed by governmental mandates to lower VOC limits, coating companies increased epoxy lining’s volume solids levels from the 50% level all the way up to 100%. The coatings that fall in this category‐
primarily epoxy lining systems‐ typically hover around the 75‐90% volume solids level. Some of the 100% solids materials require the addition of some solvent (3‐5%) to apply the coating properly. Plural component applied epoxies, polyureas, and polyurethanes require some solvents to flush equipment. These are typically small amounts and pale in comparison to the levels of solvent required to install the typical zinc rich primer and subsequent epoxy topcoat(s). Non­Reactive Barrier Pigments (MiO) Micaceous Iron Oxide (MiO) is a greener solution to the zinc in a potable water lining. MiO has been widely used overseas, particularly in Europe, as an alternative (and as an additive or a hybrid) to zinc. The benefits of using MiO over zinc are numerous: •
MiO has been used in paints to protect steel for over a century, twice as long as zinc. •
MiO‐based primers (and intermediates) have been found to exhibit improved adhesion of epoxy topcoats than over zinc‐based primers •
MiO provides superior resistance to blistering as opposed to zinc‐rich coatings •
MiO is a naturally occurring mineral whereas zinc metal has to be mined, refined and processed. •
MiO is insoluble in water, Zinc is not. •
MiO is inert and non‐conductive, unlike zinc. •
MiO will not increase the current demand of a cathodic protection system unlike zinc primers •
MiO is non‐toxic and non‐oxidizing; the same cannot be said about zinc. •
MiO non‐corrosive, and non‐flammable; Zinc dust is. •
MiO is lamellar flake, which provides superior barrier protection to the spherical zinc dust found in the zinc rich coatings. This provides 2.5‐3 times the amount of barrier protection. A lining’s purpose is to provide a barrier between the cargo and the substrate to protect cargo purity and/or prevent deterioration of the substrate. By controlling the chemistry at the coating‐steel interface, linings can provide long term service lives. Having a reactive pigment (zinc) at this interface does not offer this control. Non‐reactive pigments (like MiO) do. Single­coat systems (labor savings) The argument that single‐coat systems are less expensive due to fewer coats (labor) is intuitive provided they are just as easy to apply, inspect and repair as multi‐coat systems. Products that are commonly recommended as single‐coat systems are typically higher solids (less VOC) and therefore “greener” than multiple coat systems. They are also typically high film build and offer more “barrier” protection. These higher solids resins have shown excellent flow and leveling that minimizes pinholes typically encountered with some lower solids formulations and therefore fewer repairs. While there is still some reluctance by some owners to move away from multi‐coat systems, the single coat systems are gaining ground. Thick­film systems (longevity) In order to get extra thickness in a tank an extra coat had to be applied. Three‐coat systems provided that extra thickness but at the expense of an additional labor cost. Thick‐film systems provided that extra barrier protection resulting in longer coating lives than thinner lining systems. The longer a coating system can protect the steel substrate, the “greener” it is considered. Re‐lines cost money, energy, labor, and downtime. The newer high solids or solvent‐free systems are thick‐film linings often applied in a single coat. On owner can get the longevity he wants in fewer coats with less cost. Summary The facts about zinc‐rich coatings topcoated for potable water service have been clearly presented. A logical argument has been made regarding their effective use in this regard, but the logic is flawed. Their reactive nature is ideal for atmospheric exposures but that same reactivity poses a liability when topcoated for water immersion. There are many suitable, “greener” alternatives available today that have proven performance. These greener alternatives are better for the environment and provide outstanding protection of steel water tanks in which they are used. Non‐reactive barrier pigments, solvent‐free linings, single‐coat linings, and thicker film coatings offer longevity of service, ease of application, and greener advantages to tank owners.