The use of electrodes obtained by coating a valve metal substrate (for example titanium, zirconium, niobium, tantalum) with an electrocatalytic paint is known for use in different application fields. These electrodes may be useful in several electrolytic processes; for example for the evolution of chlorine from sodium chloride brine, as anodes for oxygen evolution in electrometallurgical processes or anodes for cathodic protection.
U.S. Pat. No. 3,632,498 describes a general method for the production of this type of electrodes, which consists in applying to the valve metal a precursor, that is a paint containing the electrocatalytic components in ionic form, which is converted into the catalyst by means of a thermal treatment in air (activation). The temperatures required for the conversion may be extremely high (300-800xc2x0 C.). The most common method for the industrial production of these electrodes foresees, after the application of each paint layer, heating in oven at high temperature. As these electrodes usually have a very large size, the ovens have a great thermal mass which involves high production costs and severe problems due to the need of maintaining a homogeneous temperature profile throughout the whole volume. The electrodes usually comprise a frame for anchoring to the electrochemical cells wherein they are to be used. During heating in oven it is the whole electrode structure that undergoes the thermal treatment with the consequent waste of the energy used to heat unnecessarily the frame of the electrode. However, the most severe disadvantage is represented by the distortions caused by said treatment to some particularly critical areas, such as welding and connection points
The use of electrodes obtained by coating a valve metal substrate (for example titanium, zirconium, niobium, tantalum) with an electrocatalytic paint is known for use in different application fields. These electrodes may be useful in several electrolytic processes, for example for the evolution of chlorine from sodium chloride brine, as anodes for oxygen evolution in electrometallurgical processes or anodes for cathodic protection.
U.S. Pat. No. 3,632,498 describes a general method for the production of this type of electrodes, which consists in applying to the valve metal a precursor, that is a paint containing the electrocatalytic components in ionic form, which is converted into the catalyst by means of a thermal treatment in air (activation). The temperatures required for the conversion may be extremely high (300-800xc2x0 C.). The most common method for the industrial production of these electrodes foresees, after the application of each paint layer, heating in oven at high temperature. As these electrodes usually have a very large size, the ovens have a great thermal mass which involves high production costs and severe problems due to the need of maintaining a homogeneous temperature profile throughout the whole volume. The electrodes usually comprise a frame for anchoring to the electrochemical cells wherein they are to be used. During heating in oven it is the whole electrode structure that undergoes the thermal treatment with the consequent waste. of the energy used to heat unnecessarily the frame of the electrode. However, the most severe disadvantage is represented by the distortions caused by said treatment to some particularly critical areas, such as welding and connection points among different parts. Electrodes with a thin layer of a catalyst which coats the valve metal offer the main advantage that at the end of the active lifetime there is no need for substituting the electrode but just providing for reactivation with a new catalytic paint, as described in British Patent No. 1.324.924.
The application of the coating is a simple procedure carried out by spraying, which could be made even at the plant site if it were not necessary to resort to large dimensions ovens capable of reaching the necessary high temperatures, a burden which most users cannot bear, also due to the fact that a large number of elements should be treated in order to justify the oven installation and operation costs. Therefore the exhausted electrodes are usually returned to the producers to be reactivated, with remarkable additional costs for shipping and packing of the same.
In many cases re-inserting the electrode into the production cycle requires further steps. This is the case, for example, with the anodes for oxygen evolution used in some electrometallurgical processes where it is extremely important that the whole surface operate at the same potential and where the ohmic drops of the electrode structure should be kept at very low values. For this reason a current conducting structure is welded onto the active surface of the electrode, which conductive structure consists of a metal having good conductive properties, for example, copper coated with a valve metal. In order to reactivate this type of electrodes, usually the current conductive structure must be detached, as it cannot undergo the thermal decomposition treatment at high temperature, due to the different expansion characteristics of the two metals. A high number of elements are severely damaged during the detachment and must be substituted. Further, welding of the current conductive structure to the electrode involves a strong risk of locally damaging the catalyst and must be carried out with particular care by highly qualified technicians. The application of paint onto a metal surface is not limited to the case of electrodes. A particular case is the application of catalytic paints to valve metals, as described in U.S. Pat. Nos. 4,082,900 and 4,154,897. These patents describe the application of a paints containing a first oxide of an element of the platinum group and a second oxide having special characteristics to inhibit corrosion. This type of coating is particularly useful for protecting localized areas, for example interstices and junctions where crevice corrosion could destroy the integrity of the element. As the thermal treatment is required only in these localized areas, the need to subject the whole element to a thermal treatment in oven strongly penalizes said application both under the economical and practical standpoints.
It is the main object of the present invention to overcome the prior art shortcomings by providing a method for applying an electrocatalytic or protective coating to a metal substrate comprising applying a precursor of said electrocatalytic or protective coating material to the surface of said metal substrate and subjecting the surface to a local thermal treatment by a hot air gun or blower to produce high temperature and keep it under continuous control. The control of the temperature of the metal substrate is made locally by means of surface temperature sensors or by means of infrared measuring systems.
The dimension of the surface heated by the air jet depends on the type of nozzle applied to the.blower and may vary from some square centimeters to some hundred square centimeters.
It is a particular object of the invention to provide a method for applying an electrocatalytic coating onto a substrate, which may consist of an exhausted electrode and which may be carried out at the plant site without any need for shipping the exhausted electrode structure to the producers. The method of the invention is particularly useful for reactivating anodes for oxygen evolution as it permits to avoid the risky operation of detaching the current conducting structure.
It is another object of the invention to provide a method not only for reactivating exhausted electrodes but also for treating new electrodes and elements which need a protective coating against corrosion, whereas flanges or gaskets are applied during assembling in the plant. It is a further object to provide a method for repairing a damaged area of a metal substrate, previously provided with a coating.