DD244151A1 - METHOD FOR PRODUCING AND REGENERATING DIMENSIONAL STABILIZED ANODES - Google Patents

Abstract

The invention relates to a process for the preparation and regeneration of dimensionally stable anodes (DSA) for the electrolysis of aqueous solutions, in particular for the chloralkali electrolysis. The aim of the invention are DSA with reduced Korrosionsanfaelligkeit and thus extended life. The invention has for its object to increase by a targeted surface treatment of the strainer, the adhesion of the precious metal or metal oxide cover layer. In accordance with the invention, the cleaned and pickled carriers prepared in a known manner are anodically polarized prior to coating in an aqueous solution containing the same aggressive ions as the electrolyte for which the anodes are provided, until breakthrough is achieved. This treatment is carried out until the occurrence of current density oscillations at low current densities. Subsequently, the substrate is rinsed and treated with a pickling solution to remove the resulting oxide layers and corrosion products from the surface. The application of anodic loading and pickling can be repeated several times.

Description

Field of application of the invention

The invention relates to a process for the preparation and regeneration of dimensionally stable anodes (DSA) for the electrolysis of aqueous solutions, in particular for the chloralkali electrolysis.

Characteristic of the known technical solutions

Dimensionally stable anodes are produced by applying thin electrocatalytically active cover layers of noble metals or metal oxides to a suitably pretreated support (substrate or core) of a less noble or base, chemical-resistant, but highly electrically conductive material, which seals by chemical or electrochemical reactions in corresponding electrolytes , forms adherent, chemically resistant protective coatings.

According to H. B. Beer (DE 200627 [1957] and 23375 [1962]), all metals which can be coated at the sites not protected by the noble metal with a barrier layer inert to the reaction medium are suitable as the carrier material. This applies in a special way to the metals titanium, tantalum, bismuth, aluminum, niobium, nickel. These barrier layers, which are essential for the function of DSA, can be produced electrolytically, chemically and thermally.

For this purpose, the pretreated substrates are oxidized by the noble metal coating in suitable electrolytes by anodic polarization or converted by a chemical or a chemical-thermal treatment in a special atmosphere at the unprotected sites in chemically resistant compounds (oxides, nitrides, fluorides). It is believed that the barrier layers thus formed are uniform, extremely durable, and protect the substrate over time.

A number of recent patents relate to specific processes for the pretreatment of the substrates - especially of titanium, with particular cleaning methods and pickling methods for achieving optimum roughness of the surface with a wide variety of acids, pickling temperatures and pickling times.

In addition, AC treatments in various oxidizing electrolytes and complex compound electrolytic treatment are proposed (SL) -PS 42296 [1974]).

The application of complex images immediately after etching is the subject of DD-WP 158043 (22.12.1982), which prevents the conversion of the electrically conductive titanium oxide interlayers into non-conducting T1O2 and thus improves the life of the anodes.

However, all of these methods of making DSA interfere with substrate oxygenation and aggressive ions of the composite substrate-noble metal or noble metal oxide layer when using the anodes.

Pitting and salt formation lead to detachment of the active layer and thus an increasing partial inactivity of the anode to local current density increases and finally to the ineffectiveness and unusability of the anodes.

Explanation of the essence of the invention

The invention is based on the object by a targeted surface treatment of the carrier, the adhesion of the noble metal or. Metal oxide cover layer to enlarge.

According to the invention, the cleaned and pickled carriers prepared in a known manner are anodically polarized prior to coating in an aqueous solution containing the same aggressive ions as the electrolyte for which the anodes are provided, until the breakthrough has been reached. This treatment is carried out until the occurrence of current density oscillations at low current densities.

The substrate is then rinsed and treated with a pickling solution to remove any oxide films and corrosion products from the surface. The application of anodic loading and pickling can

if necessary, be repeated several times. .. .- ·

It is found that the breakdown potential of the substrates is increased by the inventive method as a result of a physical-crystallographic and thus energetic homogenization of the substrate surface, which leads to a more uniform formation of the protective layers and thus occurring according to the previously applied methods locally very different protective effect against eliminated aggressive ions.

The advantage of the method according to the invention can be further enhanced by additional measures, such as by special production methods of the substrates which give a favorable grain texture and / or by using preferentially attacking etching solutions.

After application of the erfindungemäßen method, the coating of the substrates in the usual manner z. B. by

Brushing a precious metal compound and then annealing. -

Exemplary embodiments Example 1

A titanium rolled sheet is mechanically polished and chemically shined with a solution of 30% by volume HF, 40% by volume HNO ₃ (conc.) And 30% by volume H ₂ SO ₄ (conc.) For 60 to 90 seconds.

Subsequently, the sheet is rinsed with water and anodically loaded in an electrolytic cell in 5% HCL solution with a gradually increasing DC voltage to increase the passive current density and the occurrence of Stromdichteoszillationen. Thereafter, the cell voltage is switched off, rinsed the plate several times with distilled water and with a solution consisting of 48Vol .-% conc. HCl, 4% by volume conc. HNO3.48 vol.% Dist. Water at 50 ⁰ C for about 30 minutes pickled.

This treatment increases the breakdown potential by 2 to 3V.

The sample thus treated is now in the usual way with noble metal, z. As Pt, Pt-Ir or RuO ₂ -TiO ₂ coated for the Chloralkalielektrolyse.

Example 2

A rolled sheet of titanium is degreased and pickled for 2 hours in concentrated HCl at room temperature. Thereafter, the sheet is anodically loaded as in Example 1 and rinsed thoroughly with water. Subsequently, the sheet is pickled in concentrated HCl at room temperature for 30 min. This procedure achieves breakthrough potential increases of 3 to 4V.

Example 3

The Ti substrate to be coated is degreased and pickled for 2 hours in concentrated HCl at room temperature. After rinsing with water, the substrate is subjected to anodic loading in 5 M NaCl solution at room temperature until the increase in the passive current density.

Subsequently, the sheet with dist. Purified water and in a pickling solution of HCl, HNO ₃ and H ₂ O (composition s.

Example 1) at 50 ⁰ C for 20 min. Stained. The sheet is then subjected to a second anodic load in 5M NaCl solution until breakthrough. After a subsequent dressing in conc. HCI at room temperature, cleaning with water and drying in air, the substrate is coated with noble metal or Edelmetalloxid as usual.

The multiple application of the anodic loading and pickling treatment can increase the breakdown potential of the Ti sheets by 5 to 6 volts.

Example 4

For regeneration of DSA, the active layer remaining on the substrates is first removed by thermal shock treatment (cooling of high temperatures).

Thereafter, the plates are pickled for 2 hours in concentrated HCl at room temperature and further treated as in Examples 2 or 3.

Claims (2)

Invention claim: 1. A process for the preparation and regeneration of dimensionally stable anodes for the electrolysis of aqueous solutions of a pretreated support to which a eiektrokatalytisch effective cover layer is applied, characterized in that the pretreated support before coating in an aqueous solution containing the same aggressive ions contains as the electrolyte, for which the anodes are provided is anodically polarized until reaching the breakthrough at low current densities. - 2. The method according to item 1, characterized in that anodic polarization and pickling are repeated several times.