CORROSION OF ELECTRODE AND STRUCTURAL MATERIALS DURING EVAPORATION OF CAUSTIC SODA SOLUTIONS IN APPARATUSES WITH DIRECT ELECTRIC HEATING

Communication 1

V. P. Pishchulin, V. I. Kosintsev

(Presented by the scientific seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production)

At the Department of Processes and Apparatuses of Chemical Production, KhTF TPI, a highly intensive method has been proposed and is being developed for carrying out the process of evaporating caustic soda solutions to high concentrations using direct heating by alternating electric current of industrial frequency (50 Hz) in electrode-type apparatuses [1, 2]. An important task in studying this process is the selection of electrode and structural materials. A review of reference literature [3, 4] showed that the following materials are the most corrosion-resistant in caustic soda solutions: nickel, copper, brass, chromium-nickel steels, shaped carbon, and graphite, fluoroplast-4; data on electrode materials are absent from the literature. This communication presents the results of corrosion tests of nickel, copper, brass, Kh18N10T steel, steel-3, and electrode carbon as electrode and structural materials in caustic soda solutions under conditions of direct electric heating.

Studies of the corrosion rate of electrode and structural materials were carried out in cells made of fluoroplast-4. In each cell, two electrodes of the tested material with a working surface of 2 cm² and two specimens of the same material were installed to determine the rate of chemical corrosion. The studies were carried out at a constant temperature of 100°C and at an industrial-frequency (50 Hz) alternating sinusoidal current density of 2 A/cm². Regulation and control of the current density were performed by connecting an ammeter, voltmeter, and autotransformer into the electric circuit. The concentration of the caustic soda solution was kept constant during the test and in the first series of experiments was 30%, and in the second, 50% NaOH. The corrosion rate of the electrodes and specimens was determined by the gravimetric method. The reproducibility of the results was ±3%.

The results of the study showed that in both series the weight loss of the electrodes and specimens increases linearly with time, which indicates constancy of the corrosion rate of the materials.

It was established that the corrosion rate of the “phase” electrode is somewhat greater than the corrosion rate of the “zero” electrode; thus, for nickel electrodes the corrosion rate of the “phase” electrode exceeds that for the “zero” electrode on average by 6–10%, for copper and brass by 10–15%, for Kh18N10T steel by 8–12%, and for steel-3 by 15–20%. Studies of the threshold voltage drops of alternating electric current also confirmed the non-equivalence of the threshold drop.

the voltage at the electrodes. At the “phase” electrode the limiting voltage drop is somewhat higher than the limiting voltage drop at the “zero” electrode. Such inequality of the limiting voltage drop indicates a different degree of occurrence of electrode reactions at the electrodes, which is manifested in different rates of electrode destruction.

The corrosion rate of electrode materials in a 30% caustic soda solution is approximately 4 times higher than the corrosion rate of constructional materials for all the tested metal specimens; thus, for example, for nickel the corrosion rate of the working surface of the electrodes was \(1.28\ \text{g}/\text{m}^2\cdot\text{h}\), and of the constructional specimen \(0.31\ \text{g}/\text{m}^2\cdot\text{h}\); for steel-3, respectively, \(5\ \text{g}/\text{m}^2\cdot\text{h}\) and \(1.3\ \text{g}/\text{m}^2\cdot\text{h}\). With an increase in the concentration of the caustic soda solution from 30 to 50%, the corrosion rate of the electrodes increases on average by 2–4 times; thus, for example, for nickel from 1.32 to \(4.9\ \text{mm}/\text{year}\), for steel Kh18N10T from 2.33 to \(9.82\ \text{mm}/\text{year}\), for steel-3 from 5.6 to \(13.8\ \text{mm}/\text{year}\). The corrosion rate of the electrodes in a 50% solution is already 8–12 times higher than the corrosion rate of constructional materials.

Thus, the most resistant, both as electrode and as constructional materials, among the metals and alloys studied are nickel and copper, whose corrosion rates in a 50% NaOH solution at \(100^\circ\text{C}\) are, for electrodes, less than \(5\ \text{mm}/\text{year}\), and for constructional specimens \(0.3\text{–}0.5\ \text{mm}/\text{year}\); for brass and steel Kh18N10T the corrosion rates under these conditions are, respectively, 7.5; \(9.82\ \text{mm}/\text{year}\) and 0.7; \(0.62\ \text{mm}/\text{year}\), and for steel-3 they are already 13.8 and \(1.75\ \text{mm}/\text{year}\).

Carbon electrodes are practically not subject to corrosion under the given conditions and completely retain their original mechanical strength.

Visual observations of the condition of the inner surfaces of the cells indicated the high corrosion resistance of fluoroplastic-4 under the given conditions.

Conclusions

1. Corrosion tests of electrode and constructional materials were carried out in caustic soda solutions under conditions of direct electric heating.

2. It was established that the corrosion rate of electrodes significantly exceeds that for constructional specimens; the “phase” electrode is destroyed faster than the “zero” electrode; an increase in the concentration of the NaOH solution accelerates corrosion processes.

3. The following of the investigated materials are recommended for the construction of electrode evaporating apparatuses:

as electrodes—electrode carbon;

as a constructional material—steel Kh18N10T;

as lining and electrical insulation—fluoroplastic-4.

LITERATURE

1. N. I. Kozlova, V. P. Pishchulin. Application of direct electric heating for evaporating a NaOH solution. Report at the conference of young scientists, Tomsk. Collection of reports. Tomsk, 1971.

2. V. P. Pishchulin, V. I. Kosintsev, G. M. Izmailov, N. P. Konkov. Improvement of the process of evaporating caustic soda at the Kemerovo nitrogen-fertilizer plant. Communication 1 (this collection).

3. I. Ya. Klinov. Corrosion of chemical equipment and corrosion-resistant materials. Moscow, Mashinostroenie Publishing House, 1967.

4. A. A. Lashinskii, A. R. Tolchinskii. Fundamentals of the design and calculation of chemical equipment. Handbook. Leningrad, Mashinostroenie Publishing House, 1970.