Anticorrosive coating and method for obtaining same

Abstract

The invention relates to a coating for providing protection against interstitial corrosion for titanium surfaces such as flanges or other equipment used in highly aggressive electrolytic environments, for example hydrochloric acid electrolysis cells. The coating according to the invention comprises a passivating layer, on which a film of water-repellent material is applied. The invention further relates to a method for providing anticorrosive protection for flanges of electrochemical cells.

Claims

1. An anticorrosive coating for valve metal substrates or alloys thereof comprising at least one passivating layer, and at least one layer of a hydrophobic polymer applied on said at least one passivating layer, said at least one passivating layer comprising a mixture of at least one oxide of at least one platinum group metal and at least one valve metal oxide, the valve metal oxide comprising tantalum oxide, titanium oxide, niobium oxide or zirconium oxide, said at least one passivating layer having a porosity partially occluded by said at least one layer of hydrophobic polymer applied on said at least one passivating layer, and wherein said hydrophobic polymer is a fluorinated polymer.

2. The coating according to claim 1 comprising 2-8 g/m.sup.2 of said hydrophobic polymer.

3. The coating according to claim 1 wherein said at least one layer of hydrophobic polymer has a contact angle with water greater than 97.

4. The coating according to claim 1 wherein said fluorinated polymer is polytetrafluoroethylene.

5. The coating according to claim 1 wherein said at least one oxide of at least one platinum group metal is a ruthenium oxide and said at least one valve metal oxide is a tantalum oxide.

6. The coating according to claim 5 wherein said passivating layer contains a loading of ruthenium expressed as metal in a range from 1 to 4 g/m.sup.2.

7. The coating according to claim 1, wherein said valve metal substrate is titanium or titanium alloy.

8. The coating according to claim 7 wherein said substrate of titanium or titanium alloy is a flange of an electrolyser.

9. A method for preparing the anticorrosive coating according to claim 1 comprising: applying a solution of ruthenium and tantalum compounds to the valve metal substrate in one or more coats with drying and thermal decomposition after each coat at a temperature of from 450 to 550 C. for a time between 5 and 20 minutes and a final thermal treatment for 1-3 hours at a temperature from 450 to 500 C. to obtain said passivating layer; and applying an aqueous suspension of a hydrophobic polymer in one or more coats with drying at a temperature from 40 to 80 C. for a time between 5 and 20 minutes after each coat and final thermal treatment at a temperature from 300 to 360 C. for a time between 5 and 15 minutes.

10. A method of retrofitting of a flange of an electrochemical cell comprising depositing the anticorrosive coating according to claim 1 to the flange of the electrochemical cell.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a scanning electron microscope (SEM/EDAX) image of a sample prepared according to example 1, together with indication of the concentration profiles of the elements present. PTFE indicates the concentration profile of fluorine entirely attributable to the presence of PTFE.

(2) The following examples are provided to demonstrate particular embodiments of the invention, the feasibility of which has been abundantly verified within the range of values claimed. A person skilled in the art will appreciate that the compositions and the techniques described in the following examples represent compositions and techniques for which the inventors found good functionality in practical application of the invention; however, in light of the present description, a person skilled in the art will appreciate that many changes may be made to the specific embodiments disclosed, while still obtaining a similar or analogous result, yet remaining within the scope of the invention.

EXAMPLE 1

(3) A plate of AKOT titanium alloy of dimensions 50 mm50 mm was degreased with acetone in an ultrasonic bath and pickled in boiling 20% HCl for 15 minutes.

(4) A passivating solution was obtained by mixing RuCl.sub.3 at 20 wt % and TaCl.sub.5 at 50 g/l in 10% hydrochloric acid.

(5) The passivating solution was applied by brush in 4-5 coats with drying at 50 C. for 10 minutes and thermal treatment of decomposition at 500 C. for 5 minutes after each coat, until a deposit of oxides of tantalum and ruthenium was obtained with a total ruthenium loading of about 3 g/m.sup.2. At the end of the thermal decomposition process, the plate underwent a further thermal cycle of 2 hours at 500 C.

(6) A suspension of PTFE-31JR (60 wt %) from the Du Pont-Mitsui Fluorochemicals Company, Ltd., was diluted with deionized water and applied in three cycles of brushing and subsequent drying at 60 C. for 8 minutes after each coat with final thermal treatment of 15 minutes at 370 C. for a total loading of PTFE of 6 g/m.sup.2. The sample thus produced, identified as 1, was characterized in the electron microscope; the composition of the passivating layer is indicated in Table 1, referring to FIG. 1.

(7) TABLE-US-00001 TABLE 1 Compounds wt % PTFE 6.24 Ru 43.61 Ti 0.62 Ta 49.53

(8) In particular, the fluorine profile identifies the presence of PTFE and demonstrates that PTFE has penetrated into the passivating layer. This penetration, permitted by the physiological porosity of the passivating layer, greatly increases the adhesion between the polymer layer and the passivating layer.

COUNTER-EXAMPLE 1

(9) A plate of AKOT titanium alloy of dimensions 50 mm50 mm was degreased with acetone in an ultrasonic bath and pickled in boiling 20% HCl for 15 minutes.

(10) A passivating solution was obtained by mixing RuCl.sub.3 at 20 wt % and TaCl.sub.5 at 50 g/l in 10% hydrochloric acid.

(11) The passivating solution was applied by brush in 4-5 coats with drying at 50 C. for 10 minutes and thermal treatment of decomposition at 500 C. for 5 minutes after each coat, until a deposit of oxides of tantalum and ruthenium was obtained with a total ruthenium loading of about 3 g/m.sup.2. At the end of the thermal decomposition process, the plate underwent a further thermal cycle of 2 hours at 500 C. The sample thus produced was identified as C1.

EXAMPLE 2

(12) The two samples 1 and C1 were submitted to a corrosion test that simulates the conditions of interstitial corrosion that may occur on the flanges of electrolysers for the production of chlorine or in other shielded zones. The first series of samples was immersed in a known volume of 20 wt % HCl at 45 C. under a nitrogen stream, to simulate conditions of electrolyte stagnation. At the end of the 80-hour test, the solution was analysed to determine the concentration of chromium and nickel released from the substrate. The results are given in Table 2.

(13) TABLE-US-00002 TABLE 2 sample # Type Chromium Nickel 1 RuTa + PTFE 0.024 0.05 C1 RuTa 0.1 0.25

(14) The foregoing description is not intended to limit the invention, which may be used according to various embodiments without deviating from the stated aims, the scope of which is defined unambiguously by the appended claims.

(15) In the description and in the claims of the present application, the word comprise and its variations such as comprising and comprises do not rule out the presence of other additional elements, components or process steps.

(16) Discussion of documents, deeds, materials, apparatus, articles and the like is included in the text for the sole purpose of supplying a context for the present invention; however, it is not intended that this material or part thereof should constitute general knowledge in the field relating to the invention before the priority date of each of the claims appended to the present application.