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Synthesis and chemical durability of enamel coating for law melting pipe

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  • Save American Journal of Materials Science 2016, 6(2): 45-48 DOI: 10.5923/j.materials.20160602.01 Synthesis of Law-Melting Pipe Enamel Coatings and Their Chemical Durability I. G. Berdzenishvili Georgian Technical University, Georgian Engineering Academy, Georgia Abstract The new compositions of nickel-free enamel protective coatings for flexible direct-on enameling technology of steel pipes having high operational properties have been presented and discussed in this work. Based on the requirements of competitiveness we have developed the new composition of special complex adherence promoter, containing different transition metal oxides such as cobalt and copper that facilitate adhesion at glass / steel interface. The coatings forming mechanism on steel substrate and the role of adherence agents - cobalt and copper oxides were discussed basing on X-ray diffraction and metallographic analyses. It has been found that the synthesized coating system in certain parameters meet the requirements of standards for enameled steel pipes. Keywords Enamel, Protective Coatings, Pipeline, Direct-On Enameling, Corrosion Rate, Adherence Strength, Transitive Layer 1. Introduction requirements. Corrosion is the single most costly problem in oil pipelines. No matter how well these pipelines are designed, they are subjected to severe corrosion effects in service, destroying miles of steel pipes annually. The accident and leaks from pipelines create the risk of economic and environmental damage. The proper use of enamel protective coatings is the extremely effective means of preventing pipeline deteriorations. They are cost effective for long service life of pipelines and don’t cause environmental pollutions [1-4]. Common an enamel surface is of two coats, base and cover, each of which had to be applied and fired separately on steel substrate [2-6]. However, in accordance with the newest trends cheaper technologies and materials are sought. This process is so-called 1C/1F (one-coat / one-fire) enameling process. 1C/1F mode is a process of application and firing of direct-on enamel coating. This flexible enameling technology of pipes is implemented requiring a minimum of energy resources, raw materials and number of operations [4-8]. For this reason, the aims of the present work were the synthesis in a cost-effective way of upgraded one-coat low-melting glass enamels for line pipes satisfying various * Corresponding author: (I. G. Berdzenishvili) Published online at Copyright © 2016 Scientific & Academic Publishing. All Rights Reserved 2. Experimental To achieve the above task we developed a series of experimental compositions of nickel-free nonfluorine enamel frits. The chemical composition of one of the developed frits (denoted SL-3) is as follows (wt.%): ∑(SiO2+ B2O3 ) 58; ∑(ZrO2 + TiO2 ) 7,6; Al2O3 2,5; ∑(SrO + CaO + MgO) 9,9; ∑(Na2O + K2O) 20,2; ∑(Со2O3+ СuO) 1,8 [7]. In this study we have chosen Co2O3 and CuO as an adherence agent, which intensely imparted a blue color to the enamels. The glass frits were melted at 1250-1300°C for 60-90 minutes. The synthesis of enamels was performed by the slip-firing technology on sample pipes from steel 9XГС. The slips were prepared by milling of the frits in a ball mill with adding of 5% clay, 0,75% electrolytes and fine-grained filler (marshalite (SiO2) and other) in the amount of 15-18%. Glass coating was applied and fired directly on the steel at 760-770°C. After firing they form single-layer, glossy, defect-free coatings 450 µm thick. The corrosion resistance of the glass materials was estimated in 20 % HCl and 4% NaOH solutions according to ISO 2743 and ISO 2745. The corrosion rate in hydrosulphuric solution of enameled samples has been tested according to standard NACE TM 0177-96. X-ray diffraction (XRD) analyses were carried out to determine the transition phases on the steel / enamel contact surface. 46 I. G. Berdzenishvili: Synthesis of Law-Melting Pipe Enamel Coatings and Their Chemical Durability 3. Results and Discussion The determination of the optimum ratio of active cations in enamels made it possible to obtain coatings, which service and technological parameters meet the requirements imposed on inner protective coatings for steel pipelines. Such properties of enamel as high resistance to the effect of aggressive media, wide firing interval are formed by addition of filler (marshallite (SiO2) and other) (Fig.1). Results showed that adding of marshallite in the slip’s composition with a dosage up to 15-18% not only increases corrosion resistance of enamels by 35-40% and the firing temperature range expanding up to 120°, but also changes the mechanical properties of glass-coatings and increases the impact strength up to 5,1-5,6 j (further 15 SL-3 and 18SL-3 are the coatings with addition of 15% and 18% marshallite). Experimental tests of obtained coatings on hydrosulphuric corrosion have shown positive results. The application of the basic SL-3 coating (without mineral filler) on steel by at least one order of magnitude reduces the corrosion rate of steel samples in hydrosulphuric solution. Corrosion rate in the case of 15SL-3 and 18SL-3 coatings decreases from 0,4 mm/year to 0,011 and 0,008 mm/year (Fig.2). After test the adhesion of one coat enamel to metal, which is one of the important properties of corrosion prevention coatings, remained at baseline values. Impact strength also has not changed, before and after the experiment is within 5,1-5,6 j, compressive strength - 1500-1600 MPa. The mechanical properties and corrosion rate are influenced by the adhesion of coatings to the steel surface. We can conclude from measuring series the best result of bending test (more than 95%) according to EN 10209 [4] is observed when the integrated content of Co2O3 and CuO oxides in an amount 1.8 mass % (stable adhesion presented in the case e.g. of 15 SL-3 glass coating). In fact, the adhesion at enamel/steel interface increases with a heightening of adherence agents’ concentration and has reached maximal magnitude at the joint content 1.8 mass %. That means that this optimal concentration of special adherence promoter increases the reaction activity of copper containing melts on steel, helps to improve the conditions of wetting and allows active interaction in the contact zone. parameters 18% SiO2 15% SiO2 10% SiO2 8% SiO2 5% SiO2 basic 0 20 40 60 80 100 120 140 firing temperature range, acid resistance, % Figure 1. Dependence of acid resistance and firing temperature range of glass covers on filler content corrosion rate, mm/year 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 steel SL-3 15 SL-3 18 SL-3 corrosion rate, mm/year Figure 2. Decrease in corrosion rate of enamel coatings compared to the initial value of steel samples adherencestrength,% American Journal of Materials Science 2016, 6(2): 45-48 47 B 100 90 80 70 60 50 40 30 20 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 mas.%, CoO+CuO 23 Figure 3. Effect of promoter oxides on adherence strength of enamel 15 SL-3 to steel Figure 4. X-ray investigation of the boundary layer It is known, that adherence of coatings to the metal substrate is the result of a complex physicochemical process depending on a combination of many factors [4, 8, 9]. Further studies are underway to better understand the coatings forming mechanism on steel substrate and to develop a more effective protection against corrosion of one-coat enamels in service. The metallographic study of enamel / steel interface established that during firing as a result of the formation of new products and diffusion processes is formed a transitive layer approximately 20-22 µm thick. X-ray pattern performed at the phase boundary reveals the presence of ferrites and iron oxides as the main phases (Fig. 4). The crystallization of CoFe2O4 and CuFe2O4 ferrites is due to the fact that adherence activators interact with Fe2O3 (FeO). The boundary layer in almost all samples contains spinels, which can provide a stable adhesion of enamel to metal. The presence of iron oxides – hematite (Fe2O3) and magnetite (Fe3O4) at contact layer is due to a thermally induced oxidation of wustite (FeO) upon firing. The iron oxides dissolve in the glass melt and thereby could also favor the adhesion. It should be noted that with increasing contact time increases the migration of metal cations to the interface enamel / metal. This diffusion process has a favorable effect on the adhesion of the glassy phase to steel. Thus, the stable adherence of the enamel layer leads to durability of finished product, including chemical, mechanical, heat resistance and resistance to abrasion. The results of complex tests of steel samples coated with obtained single-layer coatings for their compliance with the specifications are presented in Table 1. 48 I. G. Berdzenishvili: Synthesis of Law-Melting Pipe Enamel Coatings and Their Chemical Durability Table 1. Parameters of Pipe Glass Coatings № Properties 1 Acid resistance, ISО 2743 2 Alkali resistance, ISО 2745 3 Resistance to boiling water, ISО 2744 4 Heat resistance, DIN 51167 5 Impact resistance, GOST 3-17-48-98 6 Wear, DIN 51152 Parameters 0,12-0,14 mg/cm2 0,21-0,32 mg/cm2 0,003-0,004 mg/cm2 230-250°С 5,1-5,6 J 0,7-0,8 mg/m2 REFERENCES [1] Gerhardus H. Koch, Michiel P.H. Brongers, and Neil G. Thompson. Corrosion — a natural but controllable process. PUBLICATION No. FHWA-RD-01-156, 2010, p. 3,4. [2] Shubin V.S., Ryumin Yu.A. Reliability of equipment of chemical and oil-refining industries. – M.: Chemistry, 2006. – 359 p. Developed direct-on pipe enamels can provide 40-45 years of service due to their excellent durability and ductility at all service temperatures of oil pipelines. Thus, novel approach such as the preparation of competitive one-coat law-melting non-nickel enamel coatings for flexible enameling technology will provide chances of commercial success in the manufacturing process of enameled steel pipes. [3] Proskurin E. Protective coatings. Quality and service life of pipes// National Metallurgy, 2003, No. 5, p. 69-78. [4] Technology of Enamels and Protective Coatings. Eds. L.L. Bragina and A.P. Zubekhin.– Kharkov: KPI, 2003,–484p. [5] Kovziridze Z.D., Berdzenishvili I.G. Multiple profiles protective coatings for pipelines and vessels. Fourth International Conference on Shaping of Advanced Ceramics, November 15-18, 2009, Madrid, Spain, _ p. 2-11 4. Conclusions [6] Theory, Practice and Prospects of the Use of Enamelled Pipes. / Proceedings of the Conference. – Penza: House of Knowledge, 1999, p.3-24. The present results confirm, that the strong adherence is due to the formation of transitive layer as a result of complex heterogeneous interaction on enamel / steel contact surface. A combination of enhanced adhesion of one-coat glass enamels to steel with other desired properties provides high performance of pipes direct-on enameling technology in a cost-effective way. Experimental data allow to considering the new glass materials as having a high potential for the application as a protective coatings for line pipes under a wide variety of service conditions. [7] Berdzenishvili I.G. A Frit for Direct-on Enamel Coating. Georgian Patent No. P 4765. //Invention Bulletin, 2009, 08, 25, #16. [8] Protasova L.G., Kosenko V.G., Farafontova E.P. Corrosion of steel under enamel coating // Glass and Ceramics, 2003, No. 7, pp. 31-32. [9] Shirasaki Masahiro. Reactions and adherence mechanism of Co and Ni on border between metal and enamel // Chiba Kogyo Deigaku, Kenkay Hokou, 2000, No. 47, p.340.

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