Method for removing secondary phase particles (SPP) deposits rich in niobium from zirconium - niobium alloy components. The method involves washing and rinsing zirconium-niobium alloy parts that have just been pickled with an acidified oxalic acid or ammonium oxalate washing solution. The method of the present invention results in rapid, efficient and complete removal of surface SPP deposits without pitting from zirconium-niobium alloy components, leaving a clean, shiny surface without the use of water spray or mechanical wipe operations.
Zirconium (Zr) has many useful properties, including good physical strength and high corrosion resistance. In its hafnium purified form, zirconium is widely used as a structural material for nuclear fuel cores (cores), taking advantage of its low neutron absorption cross section. Alloys currently used for nuclear grade zirconium applications typically contain tin, iron, and sometimes nickel; More recent alloy compositions such as "zirconium" alloy also contain several hundred components of niobium (Nb) to achieve improved corrosion resistance in nuclear reactor environments.
In most metal product applications, the manufacture of nuclear reactor fuel tubes and core components leaves the metal with undesirable surface characteristics such as scratches, oxidation defects and chemical contamination from lubricants. The zirconium components are therefore pickled prior to use, and the components that make up the fuel assembly can be pickled many times during manufacturing to control surface quality and remove contaminants. A typical acid bath for zirconium contains 10-40 by weight % nitric acid and 1-5% hydrofluoric acid, which is a very invasive solution.
Before any niobium-zirconium alloy can be used in nuclear reactor applications, all deposits of "black powder" must be removed, partly to produce a bright, shiny product surface, and to prevent these particles from being later released into reactor cooling water and possibly deposited in the reactor. On accessible surfaces, removal of SPP sediment is not difficult and can be accomplished by water spraying or mechanical wiping with cloth or sponge. However, many final reactor components contain inner surfaces that are not easily accessible.
