Patent Number: 062597582
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been found that, by coating an alloy from the group consisting of carbon steel, alloy steel, stainless steel, nickel-based alloys, zirconium and cobalt-based alloys with a catalytically active material, or otherwise providing catalytic activity at such metal alloy surfaces, the decomposition of the hydrogen peroxide in aqueous process systems of nuclear reactors is catalysed by the catalytically active material. Such catalytic action at the surface of the alloy reduces the ECP of the alloy, thereby mitigating SCC of such alloy. Suitable coatings of catalytically active material can be deposited by methods well known in the art for depositing continuous or substantially continuous coatings on metal substrates, such as plasma spraying, chemical vapour deposition, physical vapour deposition processes such as sputtering, welding such as metal inert gas welding, electroless plating, and electrolytic plating. The catalytically active material can be a metal selected from the group consisting of manganese, molybdenum, zinc, copper, cadmium and mixtures thereof. Other suitable materials include oxides of these metals. Even further suitable materials can include chemical compounds containing these metals, where the metal in such compounds is able to dissociate and make itself available for reacting with oxygen to form an oxide. Manganese dioxide catalyzes the decomposition of hydrogen peroxide according to the following reaction mechanism: EQU H.sub.2 O.sub.2 +MnO.sub.2 +2H.sup.+.fwdarw.O.sub.2 +Mn.sup.2+ +2H.sub.2 O EQU Mn.sup.2+ +2H.sub.2 O.sub.2.fwdarw.Mn(OH).sub.2 +2H.sup.+ EQU Mn(OH).sub.2 +H.sub.2 O.sub.2.fwdarw.MnO.sub.2 +2H.sub.2 O EQU 2H.sub.2 O.sub.2.fwdarw.O.sub.2 +2H.sub.2 O It is believed that, by coating the surface of a metal alloy cooling tube of a water-cooled nuclear reactor with manganese, such component is able to maintain a lower ECP. This is because the manganese is believed to be oxidized to catalytically active manganese oxide (MnO.sub.2), which catalyses hydrogen perixode decomposition. Because very small surface concentrations are adequate to provide the necessary catalytic activity and reduce the corrosion potential of the metal, the processing as well as the physical, metallurgical or mechanical properties of the alloys and components formed therefrom are not significantly altered. Further, lower amounts of reducing species, such as hydrogen, are necessary to reduce the ECP of the metal components below the critical potential, because of the catalysed decomposition of hydrogen peroxide. As an alternative to coating the subject alloy with the catalytically active material, the catalytically active material may be injected in-situ in the process liquid for effecting decomposition of hydrogen peroxide, thereby reducing the ECP of the alloy. FIG. 4 shows the benefits of in-situ injection of manganese as Mn(NO.sub.3).sub.2.multidot.6H.sub.2 O for effecting decomposition of hydrogen peroxide. With each injection, there was a corresponding reduction in ECP of the alloy believed attributable to the decomposition of hydrogen peroxide. It is believed that the injected manganese oxidizes and precipitates out as MnO.sub.2 on the alloy surface. Once deposited on the surface, MnO.sub.2 effects the catalytic decomposition of hydrogen peroxide according to the above-described reaction mechanism. The present invention will be described in further detail with reference to the following non-limitative examples. EXAMPLE 1 A 304 SS electrode was placed in an autoclave recirculating loop, containing water at 288.degree. C. having 300 ppb hydrogen peroxide. Various concentration of dissolved Mn solution were injected directly into the autoclave where the 304 SS electrode was immersed and argon gas was continuously purged through this injection solution during the test. The ECP of the 304 SS electrode was measured over the course of 30 days using a Cu/Cu.sub.2 O/ZrO.sub.2 electrode. The measured ECP was converted to a standard hydrogen electrode (SHE) scale. FIG. 4 shows the ECP response of 304 SS electrode before, during, and after three different manganese solution injections to 288.degree. C. water containing 300 ppb hydrogen peroxide. It is evident that the addition of Mn to 300 ppb hydrogen peroxide water decreased the ECP of the 304 SS electrode. Once injections were ceased, the ECP of the 304 SS electrode remained lower than the corrosion potential observed before the injections were commenced. This indicates the possible deposition of manganese oxide on 304 SS oxide, with the concomitant catalytic decomposition of hydrogen peroxide by the deposited manganese. The presence of manganese was, in fact, confirmed by Auger electron spectroscopy, which confirmed a thin oxide layer of 2.about.4% by weight on the 304 SS surface, to a depth of 100.about.150 A. From the above test, the presence of manganese oxide on the metal surfaces enhances the decomposition of hydrogen peroxide, with a consequent decrease in ECP of the metal alloy. EXAMPLE 2 A 304 SS electrode was placed in an autoclave recirculating loop, containing water at 288.degree. C. having 100 ppb hydrogen peroxide. Zinc, as zinc oxide, was injected directly into the autoclave where the 304 SS electrode was immersed. The ECP of the 304 SS electrose was measured over the course of 25 days using a Cu/Cu.sub.2 O/ZnO.sub.2 electrode. The measured ECP was converted to a standard hydrogen electrode (SHE) scale. FIG. 5 shows the ECP response of 304 SS electrode before, during and after aqueous zinc oxide injection to 288.degree. C. water containing 100 ppb hydrogen peroxide. Clearly, once injection of the aqueous Zinc oxide began, ECP of the 304 SS became reduced. Once injection was stopped, the ECP of the 304 SS electrode remained lower than the corrosion potential observed before the injections were commenced. This indicates the possible deposition of zinc oxide on 304 SS, with the concomitant decomposition of hydrogen peroxide by the deposited zinc oxide. The present invention provides a number of important advantages. In particular, the present invention provides a metal alloy surface coated with a catalytically active material for the decomposition of hydrogen peroxide. By doing so, the ECP of such metal alloys is lowered, thereby reducing corrosion and, notably, mitigating the effects of stress corrosion cracking. This is particularly beneficially for components of water-cooled nuclear reactors, whose high temperature aqueous environment is conducive to such corrosion phenomena, and where the occurrences of such phenomena could lead to loss of coolant and consequent loss of reactor control. It will be understood, of course, that modifications can be made in the embodiments of the invention described herein without departing from the scope and purview of the invention. For a complete definition as to the scope of the invention, reference is to be made to the appended claims.