ANTICORROSIVE COATINGS WITH CONTENT OF NON TOXIC Ca-TITANATE PIGMENT

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1 ANTICORROSIVE COATINGS WITH CONTENT OF NON TOXIC Ca-TITANATE PIGMENT Ing. David Veselý, Ph.D. Ing. Pavel Veselý University of Pardubice, Institute of Chemistry and Technology of Macromolecular Compounds, Faculty of Chemical Technology, Studentská 573, Pardubice, Czech Republic Abstract Calcium titanate was synthesized from titanium dioxide and calcium carbonate at high temperature. The obtained pigment was characterized by means of X-ray diffraction, particle size distribution measurement, and scanning electron microscopy. The obtained pigment was further characterized with regard to the parameters required for paint formulation; its specific mass was determined along with oil consumption and critical pigment volume concentration (CPVC). The synthesized calcium titanate was used to prepare epoxy coatings with varying contents of the anticorrosion pigment. The coating was tested for physical-mechanical properties and in corrosive atmospheres. The results were compared with titanium dioxide that served as a starting material for calcium titanate preparation. Calcium titanate was prepared from materials that do not add any impurities to the anticorrosion properties of the pigment. It was identified that calcium titanate of perovskite structure is a highly efficient anticorrosion pigment for paints. Keywords: corrosion inhibitor, calcium titanate, perovskite, anatase, anticorrosion coating, paint. INTRODUCTION Organic coatings currently provide the most widespread method of material protection [1]. The anticorrosion coatings present a number of specific requirements for ensuring perfect efficiency in metal base protection [2, 3]. The anticorrosion efficiency of protective coatings depends on the perfect pre-treatment of the metal surface [4], the type and concentration of the anticorrosion pigment [5, 6], the method of film creation [7], the adhesion of the coating to the metal base [8], and on the mechanical properties of the whole coating system. The anticorrosion pigments of inhibitive kind have been in the centre of attention in recent years mainly with regard to their toxic [9-11] and environmental effects. Various pigments with diverse anticorrosion properties were employed during the development of anticorrosive paints. Some of the pigments, such as chromated or plumbous pigments were often used because of their excellent anticorrosion efficiency [12]. However, their use in organic paints was gradually discontinued owing to ecological reasons. The last ten years witnessed a nearly complete replacement of chromates and plumbous anticorrosion pigments with phosphate anion-based pigments of which zinc orthophosphate found most applications. To enhance its anticorrosion efficiency, zinc orthophosphate was modified in several ways or combined with other effective components, including molybdates, organic inhibitors of corrosion and oxides modifying a system s ph. Zinc orthophosphate was most widely used in water-borne paints. However, some negative aspects of the use of zinc compounds in paints have been revealed in the course of time. The biggest problem is the content of zinc salts in wastewater that is generated during the production of paints and supplied to purification plants where it interferes with a treatment procedure. Although zinc is classified as the least harmful heavy metal with regard to its adverse ecological effects, it is necessary to consider future legislation regarding its application in protective coatings. At present, the application of pigments whose inhibiting effects are based on the exchange of ions appears to be the most promising option in the area of anticorrosion paints. The exchange mechanism consists of affecting the corrosion process developing on an anode while Ca 2+ is replaced with the H + ions and of subsequently passivating the base metal. This mechanism is conveyed, for instance, by calcium silicate whose commercial version contains 6 % of calcium. The attempts to increase the calcium content led to the synthesis of calcium titanate whose perovskite structure comprises many more calcium ions. EXPERIMENTAL Preparation of CaTiO 3 with a perovskite structure Titanium dioxide of anatase kind and calcium carbonate were used at a molar ratio of 1:1 to prepare CaTiO 3 of perovskite structure acting as an anticorrosion pigment. Both starting substances were homogenized in a planetary spherical mill for two hours. The subsequent procedure completed the high-temperature calcinations of the homogenized mixture. The mixture was calcinated at 1100 C, with a maximum holding period of two hours, and a heating rate of 5 C per minute. A sample was taken from the prepared product to be subjected to an X- ray diffraction analysis that identified the complete reaction of the starting substances. The product was transferred to a suspension in ethanol and then ground in the planetary spherical mill for six hours. 99

2 (a) (b) fig. 1 CaTiO 3 crystal structure (a) Cubic structure of CaTiO 3 crystal - perovskite (spatial group: Pm3m, structural type C5, symmetry: cubic); (b) Imaging of Ca atoms and TiO 6 octahedrons in the perovskite structure. (a) (b) fig. 2 SEM electron micrographs of CaTiO 3 at magnification (a) x; (b) x The obtained product was dried in a laboratory drier at 110 C. Using scanning electron microscopy (SEM), pictures of particles of the prepared pigment were taken and the distribution of particle sizes was measured. Comparison TiO 2 pigment with the anatase structure Titanium dioxide of anatase kind was chosen as a comparison pigment (Precheza a.s., Přerov, Czech Republic). This pigment was simultaneously used as a starting component during CaTiO 3 synthesis. Formulation of paints containing a synthesized pigment Paints were prepared by dispersing pigments in aqueous epoxy emulsion. Dispersion was first carried out in a dissolver and the obtained paste was later exposed to dispersion proper in a pearl mill. The CaTiO 3 to a binder ratio was defined so as to create a concentration line of 0-35 vol. % (PVC = 0, 5, 10, 15, 20, 25, 30 and 35 vol.%). The formulation of the paints containing TiO 2 employed the same concentration line PVC = 0-35 vol. %. Epoxy resin. This is an aqueous emulsion of the solution of medium-molecular epoxy-diane resin (manufacturer, Spolchemie a.s., Ústí nad Labem, Czech Republic). Epoxy resin hardener. This is an adductive hardener consisting of epoxy resin with aliphatic polyamines emulsified in water (manufacturer, Spolchemie a.s., Ústí nad Labem, Czech Republic). Epoxy resin and a polyamine hardener were mixed at a ratio of 100: 27. Testing paint preparation Testing paints were prepared on steel, cold rolled sheets (by the Q-panel company) provided with a standard finish. For the purpose of selected tests, the paints were prepared on glass boards (measurement of paint hardness). The paints were performed by means of an applicator so that the final thickness of the dry film was 80±3 μm. Upon producing the paints, the samples were conditioned in an air-conditioned box at 23 o C and a relative humidity of 50% for 30 days. 100

3 (a) (b) fig. 3SEM electron micrographs of TiO 2 at magnification (a) x; (b) x Methods of measuring mechanical properties Paint thickness measurement The thickness of the paints on steel panels was measured by means of an electromagnetic thickness gauge according to ISO Paint hardness measurement Paint hardness was determined by means of a pendulum apparatus, type Persoz, in accordance with CSN ISO EN The test is based on measuring the decay of the oscillation of the pendulum that is induced by the paint. The final value of hardness is related to a glass standard. Measurement of the adhesion of coatings to the base Determination was carried out according to ISO 2409, using a special cutting blade that creates a lattice in the paint when completing a second perpendicular section. Adhesion is evaluated according to standards, 0 being the best and 5 being the worst. The other employed method of measuring the adhesion of the coating to its base is that of determining the force required for peeling off the coating from the base metal in accordance with CSN ISO Measurement of the resistance of the coating against cupping This measurement determines the resistance of the coating during the continuous deformation of film and base panel induced by a penetrating ball with a diameter of 20 mm. According to CSN ISO 1520, the result of this measurement indicates the depth to which the steel ball is pressed into the panel at the moment of the first interference with the coating. Measurement of the resistance of the coating during impact deformation This method consists of determining the height of the free fall of a weight (1000 g), in which the fall of the weight onto the coating does not yet induce failure. The method is based on CSN ISO Determination of the resistance of the coating during bending This method consists of deforming the coating along with the base steel panel while it is being bent over pins of varying diameters. According to CSN ISO 1519, this determination identifies the diameter of the pin whose occurrence has not yet led to obvious defects of the coating. Methods of measuring and evaluating the corrosion tests of the coatings Corrosion test in the atmosphere of condensing water vapor During this test corrosion processes developing under the coating are accelerated in a condenser chamber by the effects of condensed humidity at a temperature of 38 o C. Corrosion test in the atmosphere of SO 2 and condensing water vapor During this test corrosion processes are accelerated by the effects of condensed humidity with the SO 2 atmosphere at an increased temperature of 35 o C. The test takes place in cycles: 8 hours of humid condensation with SO 2 and 16 hours of drying. Corrosion test in NaCl mist During this test corrosion processes under the coating are accelerated by the effects of humidity and the sprayed mist of the 5 % NaCl solution. The test proceeds in cycles in salt spray cabinets (6 hours of the NaCl mist at 35 o C, 2 hours of drying at 23 o C and 4 hours of water vapor condensation at 40 o C). Measurement of the resistance of the coating against UV radiation In this test coating samples are continuously exposed to UV-A radiation in a QUV test chamber. 101

4 Methods of evaluating the coatings after the corrosion tests Upon completion of the corrosion tests the test panels were evaluated with regard to the occurrence of corrosion. The degree of blistering was evaluated according to ASTM D The failure of coatings in an artificially prepared cut was evaluated according to ASTM D The corrosion of steel panels after the removal the studied coatings was evaluated according to ASTM All of the methods of evaluating corrosion behavior are contained in a formula for the calculation of total anticorrosion efficiency. RESULTS AND DISCUSSION Calcium titanate affects the physicalmechanical properties of epoxy coatings. Thanks to the shape and morphology of their particles as well as thanks to the possible ferroelectric properties displayed by M II ntio 3 -type substance, the mechanical properties of pigmented coatings are better than those of the binder alone, which applies mainly by the cupping test. Throughout the entire PVC concentration line = (0;40 vol.%, the mechanical properties are either substantially better than or fully comparable to a pure non-pigmented coating (PVC = 0 vol. %). For the purpose of comparison, Table 1 indicates the results of the mechanical properties of epoxy coatings pigmented in the same concentration line with titanium dioxide of anatase type. The results are fully in harmony with the knowledge of the deterioration of the mechanical properties of the paints during their pigmentation with isometric particles, particularly in the area of CPVC. As the obtained results imply, calcium titanate has highly positive effects on the adhesion of the coatings to the steel substrate. These lattice adhesion test results are shown in Table 1. The hardness of the pigmented coatings was measured by means of a pendulum apparatus, type Perzos, as the time dependence of hardness on the time of film hardening-through applicable to varying concentrations of the studied pigments. Calcium titanate reduces the hardness of the coatings and the pigmented films are even less hard than the binder alone. In case if pigmentation within a range of 0 30 vol.%, the hardness of the coatings diminishes; whereas, the same hardness is enhanced if PVC is 30 vol.% or higher. The dependency obviously indicates that calcium titanate is a highly efficient anticorrosion pigment while titanium dioxide is an inert pigment. A decrease in the anticorrosion efficiency of TiO 2 as compared to the epoxy binder proper is caused by making water diffusion easier thanks to a film at the pigment binder interface. Calcium titanate is a highly efficient anticorrosion pigment with a maximum anticorrosion efficiency in a range from 15 to 20 vol. %. Calcium bonded in the lattice of perovskite is released by controlled hydrolysis to react on the surface of the substrate with the products of a cathodic corrosion reaction, thus forming a highly efficient passivation layer. In general, corrosion tests performed in salt spray cabinets provide a highly exacting test for waterborne coatings. tab.1 Results of the mechanical test of the paints containing calcium titanate and titanium dioxide of varying concentrations (DFT = 65±3 μm) PVC vol.% Lattice adhesion /peel off [dg.]/[mpa] / < / < / < / < / < / / / / TiO / < / / / / / / / / Cupping [mm] Bend [mm] Impact [cm] 102

5 PVC = 0 % PVC = 10 % PVC = 20 % PVC = 30 % Hardness [%] time [d] fig. 4 Time dependence of the hardness of the coatings pigmented with calcium titanate to PVC = 0-30 % CaTiO3 TiO2 50 Hardness [%] PVC [%] fig. 5Anticorrosion efficiency of the coatings pigmented with CaTiO 3 and TiO 2 in dependence on pigment concentration (PVC) following 400 and 800 hours of exposure in a condenser chamber 103

6 100 Anticorrosion efficiency [st.] CaTiO3-300 hours CaTiO3-500 hours TiO2-300 hours TiO2-500 hours PVC [%] fig. 6 The anticorrosion efficiency of the coatings pigmented with CaTiO 3 and TiO 2 depending on pigment concentration (PVC) after 300 and 500 hours of exposure in a NaCl salt spray chamber This is why the test panels were evaluated after completing only 300 hours of exposure, which exposure time was long enough to exhibit obvious differences between a non-pigmented coating (PVC=0%), a pigmented coating with the CaTiO 3 content and a coating containing inert TiO 2 anatase. 500 hours of exposure in a salt spray cabinet unambiguously demonstrated the highly efficient inhibiting properties of the coatings pigmented with calcium titanate. The exposure of the test panel in the UV chamber where the coatings were exposed to 200 hours of permanent ultraviolet radiation provided a research material for further corrosion tests in the condenser chamber. UV radiation has degrading effects on the binder whose surface is impaired, thus allowing corrosive substances to pass easily through the coating to the steel base. The low value of the anticorrosion efficiency of the coating without the content of a pigment at PVC = 0 vol. % derives from the aromatic nature of the epoxy binder that shows a strong tendency towards degradation by UV radiation. The pigmentation of the epoxy with calcium titanate enhances UV radiation resistance in proportion to the content of titanate in the coating. The higher the concentration of stable inorganic pigment in the paint film, the better the protection of the binder from degradation and the better the results of the subsequent corrosion test. The S-shaped curve of the dependence of the anticorrosion efficiency of the CaTiO 3 pigmented paints on PVC clearly shows that a pigment concentration of at least 20 vol. % is required for the efficient protection of paints from UV radiation. The corrosion test results illustrate the role of pigmentation in securing efficient binder protection from the effects of UV radiation. During the pigmentation of the paint with titanium dioxide of anatase type, the concentration of this pigment in paint film is increased, which leads to intense photocatalytic reactions that result in the degradation of the binder at the pigment polymer interface. As a result, the surface of the film is degraded and pigment particles are released. When exposed to the corrosive environment of the condenser chamber again, the impaired surface is very susceptible to steel base corrosion. The illustrated dependencies indicate that in this test the optimum concentration of the paints pigmented with calcium titanate is between 15 and 20 vol. % of the effective substance. Like in the previous tests of corrosion stress, TiO 2 anatase does not have positive effects on the anticorrosion properties of the epoxy coatings. 104

7 Epoxy binder (PVC = 0%) TiO 2 (PVC = 20 %) CaTiO 3 (PVC = 10 %) CaTiO 3 (PVC = 20 %) fig. 7 Anticorrosion effects of the paints pigmented with CaTiO 3 and TiO 2 in dependence on pigment concentration (PVC) after 200 hours exposure in an UV test and subsequent 400 hours exposure in a condenser chamber. CONCLUSION Synthesized calcium titanate provides a very good anticorrosion pigment. In corrosion tests, epoxy coatings containing vol. % of this pigment exhibit high anticorrosion efficiency. The physical-mechanical properties of the paints with the CaTiO 3 content have more advantages than in pigmentation with titanium dioxide. The properties of the paints containing calcium titanate are favorably affected mainly by the convenient shape and distribution of the size of the particles. The hottemperature method described in this research paper was employed to formulate an anticorrosion pigment that finds appropriate applications especially in water-borne paints. Acknowledgment This work was supported by the Ministry of Education of the Czech Republic under project MSM and by the grant of the Ministry of Industry and Trade of the CR MPO FT-TA4/

8 Anticorrosion efficiency [st.] CaTiO3-400 hours CaTiO3-600 hours TiO2-400 hours TiO2-600 hours PVC [%] fig. 8 Anticorrosion efficiency of the paints pigmented with CaTiO 3 and TiO 2 in dependence on pigment concentration (PVC) after 400 and 600 hours of exposure in a condenser chamber with the SO 2 content. References [1] Bacova V. and Draganovska, D. (2004), Analyses of the qality of blasted surfaces Mater. Sci., Vol. 40 No. 1, pp [2] Bethencourt, M., Botana, F.J., Marcos, M., Osuna, R.M. and Sánchez-Amaya, J.M. (2003) Inhibitor properties of green pigments for paints Progress in Organic Coatings Vol. 46 No. 4, pp [3] Blustein, G., Deyá, M.C., Romagnoli, R. and del Amo, B. (2005), Three generations of inorganic phosphates in solvent and waterborne paints: A synergism case, Applied Surface Science in press. [4] Brezinova, J. (2005), Utilization of peening technology for deposition of zinc coatings Mat. Eng., Vol.2 No. 1, pp [5] Caprari, J.J., Di Sarli, A.R. and del Amo, B. (2000), Zinc phosphate as corrosion inhibitive pigment of waterborne epoxy paints used for steel protection, Pigment & Resin Technology, Vol. 29 No. 1, pp [6] del Amo, B., Romagnoli, R., Vetere, V.F. and Hernández, L.S. (1998), Study of the anticorrosive properties of zinc phosphate in vinyl paints, Progress in Organic Coatings, Vol. 33 No. 1, pp [7] Deyá, M., Vetere, V.F., Romagnoli, R. and del Amo, B. (2001), Aluminium tripolyphosphate pigments for anticorrosive paints, Pigment & Resin Technology, Vol. 30 No. 1, pp [8] Deyá, M.C., Blustein, G., Romagnoli, R. and del Amo, B. (2002), The influence of the anion type on the anticorrosive behaviour of inorganic phosphates Surface and Coatings Technology, Vol. 150 No. 2, pp [9] Emira, H.S. (2005), A novel approach to the synthesis of a non-toxic, platy pigment for anticirrosive paints, Pigment & Resin Technology, Vol. 34 No. 3, pp [10] Emira, H.S. and Abdel-Mohsen F.F. (2003), The dependence of the corrosion protection of water-borne paints on the concentration of the anticorrosive pigment Pigment & Resin Technology, Vol. 32 No. 4, pp [11] Emira, H.S., Abdel-Mohsen, F.F., Hana, S.B., Schauer, T and Greisiger, H. (2002), Anticorrosive paints based on magnesium ferrite European Coatings Journal, No.11, pp [12] Hana, S.B., Abdel-Mohsen, F.F. and Emira, H.S. (2005), Preparation and characterization of magnesium and calcium ferrite pigments Inter. Ceram., Internationqal Ceramic Review, Vol. 54 No. 2, pp