Surface characterization of the chromium hydroxide and basic calcic deposition layers formed on the steel cathode in the chlorate production process
Sammanfattning: During technical service, the surface of the steel cathodein the chlorate production process changes with respect tochemical composition and morphology. Deposition of insolublespecies from the electrolyte and corrosion products from thebase metal accumulate and form surface layers that influencethe cathode activity with regards to hydrogen evolution, theprimary reaction, as well as to various parasitic reactions.Improving the activity on the cathode side could reduce theproduction cost of chlorate significantly. Surface analysis hasbeen performed on the thin film formed by reduction of chromateand the layer consisting of basic calcium and magnesiumspecies.Small amounts of sodium chromate are added to the chloratebrine in order to improve the current efficiency of theprocess. Chromate is reduced and forms a thin film withselective permeability that hinders parasitic reduction ofhypochlorite and chlorate ions.Ex situsurface analysis with ESCA and GD-OES were usedin combination within situspectroscopic ellipsometry. Several experimentaldifficulties had to be overcome in order to performellipsometric measurements. The surface film was identified asa hydrated chromium(III) hydroxide, Cr(OH)3'x H2O. The film grows homogeneously on the substrateand attains a maximum thickness of 1-10 nm, depending onchromate concentration, cathodic polarization and substratematerial. The thickness of a monolayer is approximately 0.5 nm.The chemical composition and structural arrangement areconstant during growth and dissolution of the surface productsignificant.Industrial brine contains ppm levels of calcium andmagnesium impurities and carbonate and sulfate ions,<1 g l-1Na2CO3and 10-20 g l-1Na2SO4. Hydrogen gas evolution produces hydroxide ionsand a steep gradient with elevated pH forms in the vicinity ofthe cathode, where various calcium and magnesium saltsprecipitate and form a thick surface layer, 1-1000 µm. Thecalcic deposit is nonconductive and causes cathodicoverpoteniallosses. Characterization using SEM/EDS shows thatthe layer consists of a mixture of calcium hydroxide/carbonate,magnesium hydroxide and sodium sulfate species, which wasconfirmed by an independent kinetic model. The model treatscomplex chemical processes in great detail and employsliterature data for model parameters. To the author'sknowledge, this is the first published attempt at modeling thechemical aspects of the formation of the calcic layer. Thecalcium/magnesium ratio is 3-5, which is higher than predictedby the model, due to the superior ability of calcium to formsoluble complexes and precipitate close enough to beincorporated in the surface layer. High calcium concentrationspromote distinct crystals and dense deposits. Magnesium reducesparticle size and increases the insulating properties of thelayer. Carbonate and sulfate ions contribute to increasingstructural disorder. Precipitation of sodium sulfate onlyoccurs above a critical concentration of sulfate, estimated to25 g l-1Na2SO4. Sodium sulfate precipitates as Na2SO4or as NaClO3'3Na2SO4at high chlorate concentrations.Key words: chlorate cathode processes,chromium(III)hydroxide film, calcic deposit, ESCA, GD-OES,SEM/EDS, spectroscopic ellipsometry, kinetic modeling
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