All-Inorganic Ceramic Performance Coatings
Ceramic coatings have been developed that can provide the benefits of polymer coatings without their inherent shortcomings.
In the coatings world, all products are polymer based. In fact, it is almost impossible to think of coatings without polymers. Whether architectural or performance, all coatings are produced from some kind of polymer emulsion. Polymer emulsions are easy to pump and spray, and are easy to store without component separation. When applied, polymer emulsions exhibit a smooth, glass-like appearance that is appealing. These coatings are impermeable to water and resistant to commonly used chemicals.
Unfortunately, polymer coatings are also known to produce volatile organic compounds (VOCs), are generally flammable, produce hazardous emissions when they burn, have a high carbon footprint, and are known to damage the ozone layer. Therefore, it is time to search for alternative coatings that provide the benefits of polymer coatings without their inherent shortcomings.
Chemically Bonded Ceramics Development
Chemically bonded phosphate ceramic (CBPC) technology was developed to address these challenges. CBPCs are formed by reacting inorganic minerals and/or oxides with acid-phosphate solutions. Their pastes can be pumped or sprayed like polymer coatings, provide excellent corrosion protection to the substrate in a wide range of environments, protect the substrate from heat with heat-reflective and non-flammable surfaces, and can be produced in different colors.
While products are entering markets for several applications in normal conditions, efforts are underway to produce coatings for specialty applications in harsh environments, such as in high-temperature and acidic conditions created by the presence of hydrogen sulfide and sulfuric acid, for deep and saline water applications for steel structures (e.g., on oil rigs), or to protect against corrosion in a marine environment due to the attack of marine organisms and attachment of barnacles. For example, pipelines that are thousands of miles long connect the Arctic to the Gulf and are exposed to hot and cold conditions, along with abrasive flying particulates and objects. CBPC coatings, with their hard ceramic surface and excellent freeze-thaw performance, are ideal for this application. Chemical attack on the interior surface of tankers carrying highly acidic or alkaline chemicals leads to the need for specialty ceramic coatings for long-term performance. With their tolerance to a wide range of pH values, these coatings may be an answer to such extreme chemical conditions.
The durability of natural minerals in harsh conditions is common knowledge. Glass and ceramics, which are made of these minerals, have the ability to withstand extreme chemical, thermal, and biological attack. The concept of all-ceramic coatings has evolved over the years at the Argonne National Laboratory in Lemont, Ill. One researcher and his group showed that it is possible to produce CBPCs through a room-temperature chemical reaction, as an alternative to the high-temperature sintering used in conventional ceramics.1
The first CBPCs were developed for the immobilization of nuclear waste stored within the U.S. Department of Energy (DOE) complex. CBPCs are now being used by the DOE for the safe storage of nuclear materials. These coatings have also made their way in the commercial market as rapid-setting cements and building components. A natural next step now is to produce coating products. All-inorganic ceramic coatings are the result of this evolution.
Ceramics are produced by packing inorganic powders and firing the monolith at high temperatures until the particles fuse with each other and form a hard ceramic. This process, though highly successful in producing intricate ceramic objects, is not useful for coatings, which need to be sprayed as paste and allowed to set like polymer coatings. Since this ambient-temperature-setting was not possible with ceramic powders in the past, there was no attempt to develop all-ceramic coatings.
CBPCs solved this problem. They are synthesized using two aqueous solutions and by chemical reaction in ambient conditions, so it is possible to employ this process to produce all-ceramic coatings. The two pastes can be pumped and mixed in a static mixer and allowed to react prior to spraying. Subsequently, the reacting paste can be atomized and sprayed on any substrate prior to completion of the reaction. The initial sprayable CBPC was demonstrated in Argonne National Laboratory and then improvised into a product by small businesses.2
The basic components in these coatings are magnesium oxide (MgO) and monopotassium phosphate (KH2PO4). The paste formed by these two powders sets into a ceramic.* The reaction is given by:
MgO + KH2PO4 + 5 H2O = MgKPO4·6H2O
The reaction product is the binder for particles that forms phosphate ceramic, or a ceramic coating. Since a range of phosphate ceramics can be produced by acid-base reaction, it should be equally possible to produce a variety of ceramic coatings. Alternatively, it should be possible to produce specialty ceramic protective coatings tailored to specific harsh conditions with a correct choice of the oxides or oxide minerals.
X-ray diffraction analyses show that CBPCs are composed of unreacted oxides or oxide minerals bonded by the phosphate reaction products. The phosphate binders are inherently stable in any conditions. Therefore, the choice of the oxides will guide the coatings’ performance in harsh environments.
This selection of oxides and oxide minerals is aided by the electrochemistry of aqueous solutions. For example, if corrosion is due to acid attack, an oxide should be selected that is resistant to the acid but can be slightly dissolved and reacted to form the coating at a different pH. If corrosion is a result of a saline environment, an oxide or an oxide mineral suitable to withstand saline water should be selected. Electrochemical phase diagrams of oxides and knowledge of solubility of minerals at different pH and Eh (redox potential) from the mineralogy literature aid this process.3
Interestingly, these phase diagrams also explain why and under what conditions a metal corrodes. All-ceramic coatings are synthesized by using the same chemistry, first to determine under what conditions the corrosion occurs and then to decide which materials will work for the best protection. Because of this, all-ceramic coatings can get to the root cause of the problem and remedy it precisely.
*named Ceramicrete™ by Argonne National Laboratory
Corrosion Protection Mechanism
As shown in Figure 1, when the reacting mixture of the acidic and alkaline pastes is deposited on a substrate (e.g., steel or concrete), it reacts first with the substrate, forming a passivation layer and bonding to the substrate. A subsequent acid-base reaction forms the protective ceramic coat, while the passivation layer is mostly amorphous (or glassy); the protective layer consists of oxide or mineral particles bonded by phosphate reaction products. The two layers bond to each other. The passivation layer provides corrosion protection while the top layer imparts ceramic properties to the surface—all in one application of the reacting paste.
This configuration is distinct from that of polymer coatings, where the coating is only “skin deep.” There is no passivation layer underneath, and a primer needs to be applied prior to applying the polymer layer. All-ceramic coatings do not need a primer, which also eliminates the time needed to apply and dry the primer. This is an economic advantage of all-ceramic coatings in field applications.
Additional advantages are evident in performance. While the topcoat has properties equal to or better than equivalent polymer coats, all-ceramic coatings do not exhibit osmotic blistering; if the coating is breached at a certain spot, the corrosion front does not creep under the remaining coating. The breached spot can be simply repainted and, because the ceramic coating bonds chemically to itself, repair is easy. Most polymer coatings, being only a physical cover on the substrate, do not have this advantage. In addition, the damaged spot cannot be simply repainted. A significant area would need to be peeled off to repaint it.
Ceramics are inherently non-flammable and good insulators, and some ceramics are good reflectors of heat. As a result, they exhibit excellent fire protection properties. In the standard flame-spread test (ASTM E84), ceramics show zero flame spread, implying that, in the event of a fire, the coating will not burn and the flame will not propagate. This is unlike most polymer coatings, which have a specified duration beyond which the coating will catch fire.
Many oxides, such as titanium dioxide (TiO2) or magnesium oxide (MgO), are excellent reflectors of infrared (heat) radiation. All-ceramic coatings can be produced with such oxides, either in large proportion as the reacting components or as fillers in the coating. Tests carried out with such formulations have shown that it is possible to maintain a temperature difference of about 200°F between the coated surface of a 1-in.-thick steel plate exposed to > 1,000°F heat and an uncoated back surface. Spectrophotometric measurements of the coated surface have shown that as much as 80% of infrared radiation is reflected back to the atmosphere.
When exposed to high temperatures, all-ceramic coatings convert the coatings into refractory ceramic coatings. These refractory coatings are good high-temperature insulators. Their durability, reflectivity to heat radiation, and insulating properties make them ideal for use as refractory liners; heat reflectors on roof shingles in the tropics; and zero-flame-spread coatings for dwellings, offices, and industrial complexes.
Environmental Impact Assessment
Because climate change is here and real, any new product introduced in the market should go through scrutiny of its impact on the environment. At the minimum, its environmental impact should not be worse than the product it is replacing. To test this, greenhouse gas emissions and their effect on ozone layer were estimated and compared to those of conventional polymer coatings.4
Greenhouse gas emissions arise from the manufacture of the raw materials used in the coatings. These raw materials are produced from ore mined from the Earth’s crust, which are then subjected to mechanical grinding, calcining, pulverizing, and in the case of phosphates, reaction to acids. Each one of these uses energy, which is produced from some fuel that gives off greenhouse gases. In addition, the ore itself consists of carbonates, which decompose and emit carbon dioxide.
Using the EPA-suggested model for the estimation of carbon footprint for Portland cement, greenhouse emissions for the raw materials can be estimated.5 The estimates will vary depending on which oxides are used because oxides are produced by different processes; some can be more energy intensive and some less. In addition, the ore itself consists of carbonates, which decompose and emit carbon dioxide. However, mining, crushing, pulverizing, etc. are generally similar in all mineral industries. Greenhouse gas emissions amount to approximately 700 lbs per ton of the product used. This number is roughly the same for solvent-based alkyd paint, but only 30% of solvent-based varnish.
The real environmental benefit of all-ceramic coating, however, comes from the fact that, because the coatings are all produced from inorganic materials, VOC emissions are zero. As a result, there is no ozone depletion due to production or use of all-ceramic coatings. In addition, the absence of VOCs makes the coatings safer for manufacturers, applicators and consumers.
Finally, the disposal of leftover paint or end-of-use coatings has no effect on the environment. At the most, they will only enrich the soil because they are very slow-releasing phosphate fertilizers. These long-term benefits of all-ceramic phosphate coatings and their superior performance and economic advantages gained from the performance should make them very attractive in the market.
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1. Wagh, Arun S., Chemically Bonded Phosphate Ceramics, Elsevier pub., 2004.
2. Wagh, Arun S., and V. Drozd, Inorganic Phosphate Corrosion Resistant Coatings, U.S. Patent 8.557,342, B2, issued October 15, 2013.
3. Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE, Houston, 1974.
4. Swedish Environmental Research Institute (IVL), Lifecycle Assessment of Paint, Summary of IVL, Report B 1338.A.
5. U.S. Environmental Protection Agency, Climate Leaders, Direct Emissions from Cement Sector, Climate Leaders Greenhouse Gas Inventory Protocol Core Module Guidance, 2003.