Coating to resist oxidation in high temperature steam turbines
Publication Date: 2002-Aug-19
The IP.com Prior Art Database
Components for steam turbines operated at high temperatures and fabricated of conventional steels are provided with a nickel coating. The coating is applied by an electroless coating method. The coating thickness ranges from 2.5 to 100 microns. In a a subsequent thermal treatment of the coated component predominantly the nickel from the coating and elements from the parent steel interdiffuse which yields a stronger diffusion bond between base material and coating. At the same time the hardness of the coating can be increased. Componenets of any size and shape may be coated by this method including components that are typically too large for an immersion bath. For example, a valve chest may be coated by this method in that the valve chest iteself is used as the bath and reservoir for the coating solution.
In steam turbines designed to operate at high temperatures the base materials used for key components may suffer from increased oxidation. This oxidation can lead to, among other problems, the potential loss of surface due to oxidation and spalling and to an associated loss of turbine efficiency or damage to certain components due to erosion by spalled oxide particles.
In order to limit the extent of such damage to the component surfaces, materials with higher oxidation resistance, such as austenitic steels and nickel base alloys, can be used. However, these materials and their manufacturing are high in cost.
The design of steam turbines operated at high temperatures should allow the use of materials that are both reasonable in cost as well as resistant to high temperature oxidation.
Electrolytic nickel plating has been effectively used in the past to correct machining errors on moving blades for high temperature service. The same type of nickel plating has also been applied to protect rotor surfaces against high temperature oxidation.
The solution presented here involves the use of conventional steels that are manufactured by established processes for turbines normally operated at conventional steam temperatures. The surface of the machined steel component is either coated with a thin electroless nickel deposit. For this, a machined component is immersed in a bath or becomes a bath of a solution of nickel sulphate mixed with sodium hypophosphite at about 80°C. The hypophosphite acts as a catalyst for the auto-catalytic depostion of phosphorous bearing nickel plating. No electric field is applied.
The coating method allows the production of a...