Geopolymer suspension and method of the preparation thereof

Abstract

The invention relates to the geopolymer suspension for insoluble heat- and corrosion-resistant coatings for the surfaces of Fe alloy products that contains 20 to 25% wt. of the aqueous solution of 80% to 90% wt. phosphoric acid, 25 to 30% wt. of aluminosilicate, 4.6 to 5.2% wt. of graphite, and the remainder is isopropyl alcohol. It further relates to the method of preparing the geopolymer suspension, wherein 20 to 25% wt. of the aqueous solution of 80 to 90% phosphoric acid is added to 100 ml of isopropyl alcohol and 25 to 30 g of kaolin is gradually added while stirring at laboratory temperature for 8 to 15 minutes. After 9 to 15 minutes, 4.6 to 5.2 g of graphite as an additive is added at once, and the resulting mixture is homogenised for another 18 to 25 minutes.

Description

Geopolymer suspension and method of the preparation thereof.[Technical Field

[0001] The invention relates to a geopolymer suspension for insoluble heat- and corrosion-resistant coatings for surfaces of Fe alloy products and a method for the preparation thereof.Background Art

[0002] Inorganic coatings on metallic materials are prepared by several physical, electrolytic, or chemical methods. There are, among others, hot- or plasma- deposition technologies. While these are highly effective as a top protective layer, their application is highly energy-intensive, and some materials may suffer structural effects due to heating of the material during application; therefore, they are not suitable for such applications. In addition, there are sophisticated techniques for coating formation by diffusion processes or vapour-phase deposition of layers. The most common electrolytic methods include the galvanisation process. Various technologies of applying aqueous suspensions followed by firing are used for common surface treatments of metallic materials with coatings and enamels.

[0003] The clear disadvantages of all the above processes include technology- and, especially, energy-intensity. The technological intensity is due to the need for expensive equipment, and the energy intensity is reflected in the price of products, where the profitability of production usually requires large-scale series production.

[0004] The simplest and cheapest method to prepare coatings in a limited range of applications consists of applying them directly by painting or spraying suspensions. Using this method, the organic coatings are mainly prepared, but they have limited temperature stability and a very short expiry time. However, their production and subsequent disposal are highly environmentally, and therefore financially, demanding. It is necessary to take care of protecting the health of persons both during production and especially during application. Inorganic coatings are prepared in this simple method for aesthetic and corrosionprotection purposes especially. Inorganic coatings for high-temperature applications are usually prepared on a silicate or silicone base. Siloxane-based silicone paints are easy to apply with typical applications up to max 400 to 500 °C; however, their cost is high compared to other inorganic coatings.Summary of Invention

[0005] The drawbacks mentioned above are largely eliminated by a geopolymer suspension for insoluble heat- and corrosion-resistant coatings for the surfaces of Fe alloy products according to the present invention. The summary thereof is that the suspension contains 20-25% wt. of an aqueous solution of 80-90% wt. phosphoric acid, 25-30% wt. of aluminosilicate, 4,6 -5,2% wt. of graphite and the remainder is isopropyl alcohol.

[0006] Preferably, the aluminosilicate is washed kaolin and / or metakaolin. In a preferred embodiment, the ratio of the amount of phosphoric acid to the amount of aluminosilicate is 1.1 to 1 .35.

[0007] A further object of the invention is a method of preparing the aforementioned geopolymer suspension according to the present invention. The summary thereof is that 20 to 25% wt. of the aqueous solution of 80 to 90% phosphoric acid is added to 100 ml of isopropyl alcohol, and 25 to 30 g of kaolin is gradually added while stirring at laboratory temperature for 8 to 15 minutes. After 9 to 15 minutes, 4.6 to 5.2 g of graphite as an additive is added at once, and the resulting mixture is homogenised for another 18 to 25 minutes.

[0008] The summary of the invention is the formation of an aqueous or alcoholic suspension comprising phosphoric acid, powdered aluminosilicate and powdered graphite as an additive, in which selected additives in the form of powdered graphite can be readily dispersed to prepare functional inorganic coatings on metal substrates. Suspensions for the preparation of combined Fe alloys-based materials and functional inorganic coating are characterised in that the suspension is formed by the inorganic coating precursor and the additive - powdered graphite.

[0009] Aqueous suspensions of a mixture of phosphoric acid with selected aluminosilicates represent a cheap, environmentally friendly and very simple surface protection application suitable for the preparation of functional coatingson metallic materials. A wide range of additives can be easily dispersed into the suspensions, which then provide the coatings with variability in the properties thereof. This flexibility in coating formulation gives the resulting composites new functional properties, particularly in the fields of adhesion and tribology, as well as allows for a broader range of applications.

[0010] Therefore, the suspension can be used as a base component to which various additives such as powdered materials made of metallic and non-metallic elements, metal alloys, metal oxides, nanotubes, etc., can be mixed prior to application, thus formulating a resulting mixture with properties suitable for the specific application.

[0011] The suspensions exhibit adhesion to metallic materials and can be applied by painting or spraying without the need for firing. After the application, a so-called setting, a conventional geopolymer must be dried and stabilised in an oven at a certain temperature. This emulsion eliminates the need for heat stabilisation, and the layer is stabilised simply by drying the coating.

[0012] The resulting coatings do not need to be fired and are designed for repeated long-term, i.e., at least 50 hours, heat load up to at least 500 °C and short-term protection, at least 30 minutes, to 800 °C. The suspensions are environmentally friendly, long-term stable and affordable. In particular, the coatings on metal substrates allow surface modifications of the sliding and tribological properties of the resulting surfaces, thus expanding their functional, i.e., application possibilities.

[0013] The great advantage of geopolymer emulsion is the long shelf life thereof, which is up to 10-times longer compared to other emulsions. The suspension does not tend to sediment and separate and degrade the individual components. The guaranteed expiry time is at least 5 months at room temperature. The expiry time of geopolymer suspensions can be further extended through suitable storage at a low temperature of about 4 °C or by forming individual components that are mixed into the final mixture just before application.

[0014] Pre-treatment of material surfaces consisting of degreasing or mechanical cleaning is essential for successful applications. Furthermore, the application itself, where it is necessary to apply the suspension in a thin layer, which is alsovery preferable, as the consumption of the suspension is very low compared to conventional and organic coatings. The options for application to the material surface are the same as for conventional organic compounds, i.e., painting with conventional brushes, air spraying with a spray gun or atomiser, etc. The application by spraying will further enhance the aesthetics of the resulting visual surface. Therefore, the suspensions can be used for direct visual application and can be pigmented with dyes according to the desired colour shade to increase their utility value. After suspension application and drying, the surface does not need further treatment and is maintenance-free. Another positive feature is the thickness of the resulting geopolymer layer, which is very low compared to conventional organic coatings and, therefore, does not significantly affect the final dimension of the component.Brief Description of Drawings

[0015] The geopolymer suspension for insoluble heat- and corrosion-resistant coatings for the surfaces of Fe alloy products according to the present technical solution will be described in more detail on specific examples of embodiments referring to the attached drawings, wherein Figure 1 shows a homogeneous layer with a uniform, cohesive intact layer of geopolymer emulsion after application and drying, 500 times magnification. Fig. 2 shows the areal EDS analysis of the geopolymer layer and the profile GDS analysis of the geopolymer layer, which shows that the layer exhibits elements originating from the materials used in the production of the geopolymer, i.e., Al, Si, a,C P, O, P. Fig. 3 shows an uncoated specimen with visible corrosion damage in a section of 200 times magnification. Fig. 4 shows a specimen protected by the geopolymer coating that prevented corrosion from penetrating through the coating into the substrate, section of 200 times magnification. Fig. 5 shows a specimen protected by the geopolymer coating that prevented corrosion from penetrating through the coating into the substrate, section of 200 times magnification. Fig. 6 is the graph showing that the geopolymer U3 reduced the temperature on the uncoated side by 5 °C on average in the first minute, 11 °C in the second minute, and 12 °C in the third minute. Fig. 7 shows the resulting substrate surface after 3 minutes at high burner performance, and Fig. 8 shows it after 6 minutes at high performance. The suspension shows resistance to prolonged high temperature, as shown in Fig. 9,showing an intact surface exposed to 500 °C for 50 h. Fig. 10 and Fig. 11 show the detail and layers and layer thickness (SEM), which is around 5 pm and is compact.Examples of the Invention Embodiments

[0016] The exemplary geopolymer suspension for insoluble heat- and corrosion- resistant coatings for the surfaces of Fe alloy products contains 22% wt. of the aqueous solution of 85% wt. phosphoric acid, 25% wt. of aluminosilicate, 5% wt. of graphite, and the remainder is isopropyl alcohol. The aluminosilicate is a washed kaolin. The ratio of the amount of phosphoric acid to the amount of aluminosilicate is 1 .1 to 1 .35.

[0017] The exemplary method of preparing the geopolymer suspension for insoluble heat- and corrosion-resistant coatings for the surfaces of Fe alloy products comprises the adding 22% wt. of the aqueous solution of 85% wt. phosphoric acid into 100 ml of isopropyl alcohol; the gradual adding 25 g of kaolin under stirring at laboratory temperature for 10 minutes; and, after 10 minutes, 5 g of graphite as an additive is added at once; and the resulting mixture is homogenised for further 20 minutes.

[0018] Another exemplary geopolymer suspension for insoluble heat- and corrosion- resistant coatings for the surfaces of Fe alloy products contains 25% wt. of the aqueous solution of 80% wt. phosphoric acid, 20% wt. of aluminosilicate, 5.2% wt. of graphite, and the remainder is isopropyl alcohol. The aluminosilicate is a washed metakaolin. The ratio of the amount of phosphoric acid to the amount of aluminosilicate is 1 .1 to 1 .35.

[0019] Another exemplary method of preparing the geopolymer suspension for insoluble heat- and corrosion-resistant coatings for the surfaces of Fe alloy products comprises the adding 25% wt. of the aqueous solution of 80% wt. phosphoric acid into 100 ml of isopropyl alcohol; the gradual adding 25 g of kaolin under stirring at laboratory temperature for 10 minutes; and, after 10 minutes, 5 g of graphite as an additive is added at once; and the resulting mixture is homogenised for further 20 minutes.

[0020] The coated specimens were subjected to corrosion test in a corrosion chamber at intervals of 5-10-15 days of corrosion loading. With visible corrosionprogression to the surface, the coating protects the coated substrate ideally, which was evident in the cross-sectional images compared to the coated and uncoated specimens.

[0021] Under the heat load of up to 500 °C, when the geopolymer specimens were subjected to 500 °C for 50 hours with an initial ramp rate determined to be 7 °C / minute, the geopolymer layer showed no signs of failure. Also, when exposed to 800 °C for 3 hours with an initial ramp rate of 7 °C / minute, the geopolymer layer showed considerable stability. Furthermore, a simplified heat transfer and temperature resistance test was performed with the geopolymer applied on a steel substrate. The specimens were exposed to an open flame with a flame temperature of 1000 °C ± 50 °C.

[0022] The flame heated the side with the applied geopolymer, and the highest temperature reached on the other uncoated side was measured as a function of heating time. The temperature was measured using a Testo 872 thermal imaging camera with a set temperature range of 0 °C to 650 °C. Due to the temperature range of the thermal imaging camera and the more accurate measurement results, the burner performance was experimentally chosen such that the heating of the uncoated side of the specimen from 25 °C to about 650 °C occurred in 3 minutes. The achieved temperature was recorded at each minute, at 1st min, 2nd min and 3rd min according to Table 1 .

[0023] Table 1

[0024] The next test was a very high-temperature test where the burner was set at full performance, and the flame was in direct contact with the surface. The test was running for 3 minutes, then documented and evaluated. The same specimen was again subjected to the same procedure for another 3 minutes, demonstrating that the geopolymer layer can be used as a safe coating to prevent burn-through or heat transfer.

[0025] The geopolymer suspension was stored at room temperature at 23 °C in a sealed laboratory container. At regular time intervals, the stored geopolymer was checked, and the differences from the original newly formed suspension and the possibility of the application thereof were monitored. First of all, any changes in the solution’s state of matter and especially the quality of the applied layer or the differences thereof from previously applied coatings were monitored.

[0026] After 3 months under the given conditions, there was no observable change in the suspension. After 4 months under the given conditions, smaller but still soluble deposits were formed. The emulsion can be homogenised by a simple mechanical stirring of the solution.

[0027] After 5 months, there has been no significant degradation of the emulsion, and it can still be applied to steel surfaces. The expiration time of the emulsion can be guaranteed for a period of 5 months, which is another significant advantage over other emulsions.

[0028] An important aspect is the quality of the steel surface treatment prior to the geopolymer application. As mentioned, it is highly advisable to mechanically treat the surface of steel components by brushing or blasting before coating. The possibility of coating an imperfectly treated surface was observed under laboratory conditions, where an oxidation layer was deliberately left on the steel specimen surface. The image of the oxide layer on the substrate surface showed that the coating was homogeneous and cohesive, even in this case. Therefore, it is possible to tolerate to some extent imperfections in the preparation of the steelsurface prior to the geopolymer suspension application, which leads to a further reduction in the cost and simplification of the application.

[0029] A possible method of suspension application:

[0030] The steel surface is mechanically treated by brushing or blasting before applying the suspension. Chemical pre-treatment of the surface is also possible.

[0031] Application of the precursor suspension is done either by coating or by spray application, using e.g., atomiser or spray gun. Application by painting is carried out in a thin layer. It is also possible to apply multiple layers of one type of geopolymer to the surface or to create multi-layer systems with different geopolymers, wherein the combination of the different properties of different geopolymers is preferable, e.g., 1 st layer with good adhesion to the material, 2nd layer with improved anti-corrosion properties, etc.

[0032] The thickness of the layer itself after drying is up to 10 pm and is compact.

[0033] The suspension is mainly used as an anti-corrosion surface protection as an alternative to conventional organic coatings or other anti-corrosion protection. Another use is as thermal protection of machine part surfaces, etc. Here, the low layer thickness can be used preferably, which does not significantly affect the final dimensions of a component. The thermal protection can be further improved by the appropriate choice of additives added to the suspension to create a coating with application-specific properties. Another option is fire protection of construction materials in the building industry, e.g., steel beams, etc., in emergency scenarios or of the machine components themselves. The solution thus improves the mechanical properties of machine components.Industrial Applicability

[0034] Surface-treated composites can be used to reduce steel surface adhesion in high-temperature applications, heat and corrosion protection of metal structures against temperature in construction and engineering, etc.

Claims

Claims

1. !A geopolymer suspension for insoluble heat- and corrosion-resistant coatings for the surfaces of Fe alloy products characterised in that the geopolymer suspension contains 20 to 25% wt. of the aqueous solution of 80% to 90% wt. phosphoric acid, 25 to 30% wt. of aluminosilicate, 4.6 to 5.2% wt. of graphite, and the remainder is isopropyl alcohol.

2. The geopolymer suspension according to claim 1 , characterised in that the aluminosilicate is washed kaolin and / or metakaolin.

3. The geopolymer suspension according to claim 1 or 2, characterised in that the ratio of the amount of phosphoric acid to the amount of aluminosilicate is 1 .1 to 1 .35.

4. A method for preparing the geopolymer suspension according to any one of claims 1 to 3, characterised in that 20 to 25% wt. of the aqueous solution of 80 to 90% phosphoric acid is added to 100 ml of isopropyl alcohol and 25 to 30 g of kaolin is gradually added while stirring at laboratory temperature for 8 to 15 minutes, and then after 9 to 15 minutes 4.6 to 5.2 g of graphite as an additive is added while stirring at once, and the resulting mixture is homogenised for another 18 to 25 minutes..!

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