VITREOUS ENAMEL COATINGS FOR PROTECTION OF HEAT EXCHANGER STEEL DETAILS
Lyudmyla L. Bragina¹, Gennadiy K. Voronov¹, Victoria V. Reznikova¹, Natalia P. Sobol¹, Oksana V. Shaligina¹ and Igor A. Zuban^2
1National Technical University, Kharkov Polytechnic Institute, Frunze str., 21, Kharkov, 61002, Ukraine
² Trade House, Emal’zavod, Dovgalevskaya Str., 25, Kharkov, 61067, Ukraine

Introduction
The vitreous enamels application for protection non-alloyed and low alloyed steels from corrosion in aggressive environments at heightened temperatures is effective alternative to use of dear metals and alloys with considerable corrosion resistance.
Known advantages of enamel coatings before other anticorrosion coatings is a higher corrosion resistance to solutions of acids, alkalis and salts at rather high temperature (the invariance of operation properties is usual up to 300 ??, in express cases up to 600 ??), during tens years, complete lack of propensity to ageing, reflecting smoothness of a surface stipulating a rather low friction coefficient and in adhesion to it of high-viscosity polymeric substances and solid excretions from oil, ease of clearing, high gigienically, heightened abrasive-resistance, resistance to atmosphere effect.
Actual there is a problem of development of enamel coatings for protection of heat exchangers details. The investigations in this direction are carried on 80th years XX century, when the compositions of chemical and thermo resistant enamels for protection of these details were patented [1-5].
In the present manuscript the results of investigations on a protection of such details from lowcarbon steels working in an atmosphere of a superheat steam and acid mediums with pH from 0 up to 3 at 20 – 300 o?.
Studied an opportunity of direct-on chemical resistant enamel creation for this purpose, and also ground enamel for two-layer chemical resistant coating. In both cases was the provision of high adherence of coatings to steel substrate without use as adhesion promoter cobaltous oxide was as main problem. In this connection the development of complex adhesion promoter consisting of several other polyvalent cations of transition metals was carried out.

Experimental
The model glasses were melted in corundum crucibles in the electrical furnace at temperature 1200-1250 oC. Crystallizing ability of glass melts, wetting ability (Θ₈₄₀, degree), electrical conductivity (æ, Ω^-1⋅cm^-1) and corrosion activity (Δd, %) of enamel melts, linear expansion (α⋅10⁷, K^-1), chemical resistance and adherence strength of enamel coatings to steel (P) was determined.
The values of enamel melts viscosity (η) were calculated on flow distance data according to the Dekker’s equation [6] with corrections of V. Reznikova and L. Bragina [7].
Probability estimation of new phase formation on contact enamel coating–metal was made with by use thermodynamic calculations, identification of these phases realized by XRD. In work have utilized samples and details of industrial heat exchangers from low-carbon steel 08?? of composition (weight %): C – 0.06; Mn – 0.29; Si – 0.006; P – 0.010; Cr – 0.07; Ni – 0.05; Cu – 0.03; Al – 0.08.
For obtaining of complex adherence activator galvanic slimes containing Ni²⁺, Cu²⁺, Fe²⁺, Fe³⁺ have utilized.
Corrosion activity of melts evaluated by diameter change of a steel bars after its one-hour isothermal holding in a melt at 1000 o?. The starting diameter of steel bars was 10 mm. A terminating diameter measured on the metal–melt–atmosphere.

Results and discussions
Developments of complex adherence promoter
According to known ideas about a role of corrosion processes intensity on steel–enamel melt contact a in reaching strong adherence of coating with metal the special role is assigned to variable valence oxides [8]. To the present time the known theoretical investigations in this direction concern into to viewing of separately taken transition metals oxides influence on these processes [8, 9]. Meanwhile in practice of enamelling the combine introduction of several adherence oxides in ground enamels is more and more widely applied. The absence of scientific bases of synthesis such enamels and interactions with steel of their melts precludes from the further development of introducing about a nature of adherence with metal enamel coatings. It has stipulated necessity of examinations about influence of a several transition metals cations complex on made mention processes carrying out. On the other hand, technogenic wastes can be source of this complex, in particular galvanic slag of machine-building productions containing simultaneously few of heavy metals polyvalent cations.
As a fundamental for synthesis of complex adherence promoter the system Fe₂O₃-MnO-CuO-CoOCr₂O₃- NiO was selected. The partial and integrated influence of certain components of this system to electrical conductivity and corrosion activity of model glasses melts in relation to low-carbon steel 08?? in temperature interval 600-1000 o?, and also on adherence with metal substrate of coatings from these melts are investigated.
These glasses synthesized in the field of system R₂O-RO-B₂O₃-Al₂O₃-TiO₂-SiO₂, which restricted to the following content of the basic components, weight %: SiO₂ 40-60; R₂O (R – Li+, Na⁺, K⁺) 10- 25; B₂O₃ 10-20; CaO 3-10; Al₂O3 1,5-5; TiO₂ 0-5. Different alkaline oxides using provided for polycation effect. The numbers compositions of model glasses and their physicochemical properties are given in Table 1, in Fig. 1 – XRD results for an estimation of these glasses crystallization degree during melting: the majority of the glasses were X-ray amorphous.

Fig. 1 XRD investigations of model glasses.

The optimal composition of a model glass ? 4 which were selected for the further examinations answered the following requirements:
:: Absence of crystallization;
:: α in limits (90…110) ·10⁷ K^-1;
:: σ in limits 220 - 250 mN/m;
:: Θ₈₄₀s less 15 degree;
:: T_fusion is less 850 oC
At first was planned to study of partial and integrated influence of NiO, CuO, CoO on corrosion activity of a selected model glass. However previous researches [10] have shown analogy of influence cobalt and nickel oxides to investigated processes. Besides an opportunity of use galvanic sags as the suppliers of adherence promoter and aspiration to exception CoO from ground enamels composition have stipulated necessity of processes examination for melts containing only nickel and coppers oxides. These oxides brought in an amount from 0.5 up to 12-weight %.
The obtained data about partial and integrated oxides content influence on change of steel bars diameter are given in Fig. 2. They show the greatest partial influence on corrosion activity heightening of melts renders a cupric oxide in an amount up to 1 weight %. The effect of melts on a steel increases at the content NiO up to 2-weight %. At the further magnification of MeO (where Me - Cu, Ni) concentration the corrosion processes were retarded on contact surface metal–melt–atmosphere owing to deposition of the reduced metal particles.

The character of coatings adherence strength dependence from the content of adherence promoter in them (Fig. 3) in a major extent comes nearer to corrosion activity of melts: passivity of steel surface reduced to decreasing of active interaction zone between steel and melt.
The corrosion activity of melts with the integrated content NiO and CuO increased with a heightening of adherence promoter concentration and has reached maximal magnitude at the joint content 5 weight % (Fig. 2), maximal adherence of a coating - at 6 weight % (Fig. 3).

Fig. 2 Partial and integrated influence promoter oxides on glass melt corrosion activity.

These data confirm carried out, before researches [10], which linked intensity of interaction between enamel melts and steel with their electrical conductivity and as result with adherence of coatings.

Fig. 3 Partial and integrated influence promoter oxides on coatings adherence.

Definition of electric conductivity (æ, Ω-^-1cm^-1) of experimental glasses melts in temperature interval 600-1000 o? has shown (Fig. 4), that its dependence of the partial content in glasses of Ni and Cu oxides has more complicated character, especially in a case Cu?: a value æ at the beginning increased with magnification of copper cations content up to 1.5 weight %, with increasing of copper cations content up to 4 % were reduced, and then again began to increase.
At the integrated NiO and CuO content character of electric conductivity change is similar to character of corrosion activity change.

Fig. 4 Dependence of glass melts electric conductivity from oxidespromoter content.

The detailed examination æ in a wide temperature interval is carried out for compositions with maximal adherence. The dependence æ = f(T) is illustrated by the curves on Fig. 5? for glasses in temperature interval 200-400 o? and in Fig. 5b for melts in interval 600-1000 o?. The values of activation energy Eæ for melts made: at the content ?uO = 3.8 %; - 56.2 ?J/mol; at the content NiO = 2.2 % - 56.8 ?J/mol; at Σ_NiO+CuO = 6 % - 57.3 ?J/mol.
Like values of E_æ and the conformity of areas of melt corrosion activity maximal values and coatings adherence confirms common regularities of these properties bond argued above. Relative displacement maximum of these properties were for glasses with the integrated content of promoter oxides (Δd_maxat 5 % Σ_NiO+CuO and P at 6 % Σ_NiO+CuO). Apparently, it is bound with contribution of forming products – the results of steel and reduced metals from promoter oxides interaction to development of adherence forces on the inter phase boundary. In this case the formation ferrites is most probable.

Thermodynamic calculations
For probability estimation of ferrites formation on the enamel coating–metal boundary thermodynamic calculations were carried out. The following reactions considered:
NiO + Fe₂O₃ = NiFe₂O₄ (1)
CuO + Fe₂O₃ = CuFe₂O₄ (2)
MnO + Fe₂O₃ = MnFe₂O₄ (3)
2FeO + 0.5O₂ = Fe₂O₃ (4)
The reaction of manganese ferrite formation (3) surveyed because this oxide was included in model glass ? 4 composition in an amount up to 3 weight % for improvement of rheological performances: magnifications flow distance and lowering of enamel viscosity.

The calculation of Gibbs energy values for reactions (1) - (4) at different temperatures of interaction was carried out by precise integration method. The obtained results are presented in the Table 2.

The reaction (4) - formation Fe₂O₃ was considered as an example of interconversions iron oxides on steel substrate. According to the data, presented in Table 2, there is no necessity to investigate the interaction of promoter’s components with FeO: Gibbs energy of oxidation FeO in Fe₂O₃ essentially exceeds similar index for iron oxide (III) interaction with other oxides.
As follows from the data analysis (Table 2), the increasing of coatings fusing temperature is favourable has an effect for formation NiFe₂O₄; the formation of copper ferrite with temperature increasing is aggravated, and the values ΔGT become positive at formation CuFe₂O₄ already after 900 To. According to obtained calculation data (table 2), the reaction of manganese ferrite formation thermodynamically is impossible, that renders of nickel ferrite forming is the main influence to adherence processes.
In a case promoter oxides introduction by one raw component (galvanic slag) was necessary to install influence of simultaneous passing reactions (1) - (2) on equilibrium reaction products composition.
The solution of such problem is reduced to a determination of following equations system:

where ?_eq(?) - equilibrium reaction constant;
[C_i] _eq. - equilibrium concentration of reagents and reaction products.

The deduced dependences of an equilibrium constant on temperature look like the following:
ln?_eq (1) = -10.526 – 5.410·10-2·T + 2513.159 / T + 3.331·lnT (6)
ln?_eq(2) = -3.388 – 5.503·10-2·T + 99.106 / T + 1.239·lnT (7)

On the deduced dependences was is calculated model of equilibrium (stoichiometrical) composition obtained at an initial oxides relation, which theoretically were ensured complete interaction of oxides, which share in reactions (1) - (3) - Table 3.
As it is visible, in production mixture received at different temperatures of heat treatment, the nickel oxide is absent, that is it is completely spent for formation of ferrite. The increasing of coating fusing temperatures aggravates formation of copper ferrite, that confirms outputs made at the analysis of the previous data (Table 2). The diminution of Gibbs energy with temperature increasing in a response (2) explains, why in the reacted stoichiometrical mixture there is a free iron oxide (III).

The data XRD results – Fig. 5 are experimental affirming of ferrites formation on the inter phase boundary enamel coating – metal.

Fig. 5 XRD investigations of the inter phase layer at 820 oC.

The carried out examinations have allowed to define the optimal content of complex adherence promoter components, and also to make an conclusion about prevailing influence of forces of a chemical bond as contrasted to by mechanical and electrochemical bonds on the boundary enamel coating - metal substrate, as adherence, adequate primarily for security, to what testify data of thermodynamic calculations and XRD.

On the basis of model glasses and complex adherence promoter are obtained direct-on and ground enamels for application on heat exchangers steel details. Their physicochemical properties and production characteristics are defined. For glasses containing a different amount of a manganese oxide the viscosity values were calculated by flow distance on data according to the Dekker’s equation with corrections of V. Reznikova and L. Bragina:
lgη = 5.8 + 2.57 lg [(τ₁ - τ₂) / (l₁ - l₂)] (8)
These corrections are made in view of features method definition flow distance of enamel melts on the standard of Russia and Ukraine.
The temperature dependence of viscosity lgη = f(T) is obtained at the different content MnO2 in₂ temperature interval of coatings fusing (T = 700-850 o?) – fig 6. The optimal content of a manganese oxide in composition of a ground is established.

Fig. 6 Viscosity temperature dependence of model ground enamels with different content of MnO2.

The two-layer coatings with obtained designed ground enamel are applied for a protection steel detail of regenerative rotary air heaters. In them flue gases manufacture the preheating of air acting through a furnace of boiler. Corrugated, the Z-figurative or U-shaped surface of these details assembled in a package, has the common area from 11 up to 228 m², width of metal of 0.7 mm, width of an enamel coating from each side - no more than 0,35 mm. Production of these details isc arried out on Kharkov Joint Stock Company “Emal’zavod” for thermal power stations.

Conclusions
Complex adhesion promoter of enamel and steel has been developed. Its use makes it possible to except cobalt oxide from ground and direct-on enamels and to use machine-building,
metallurgical and chemical wastes for providing adhesion. Direct-on chemically resistant enamel and ground for two-layer coating of heat exchanger steel details have been synthesized.

Summary
The possibility of direct-on chemically resistant enamels synthesis in R₂O–RO–B₂O₃–Al₂O₄–TiO₂–SiO₂–MexOy system (R₂O – alkaline oxides, RO – alkali-earth metals, MexOy – complex adhesion promoter) has been investigated. The model glass component relationship by which necessary values of acid resistance, chemical resistance to overheated steam and thermo resistance has been optimised. The Fe₂O₃–MnO–CuO–CoO–Cr₂O₃–NiO system as a base for receiving complex adhesion promoter has been investigated. Partial and integrated effects of this system components on enamel melts and coatings properties have been studied. On the basis of these data the optimal relationship of the complex adhesion promoter components has been found out.
Direct on and ground enamels have been synthesized, their physical and chemical properties and operational characteristics has been determine.

References
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