R The aim of the project is to develop a reliable method for coating steel with an impervious layer of refractory metal, such as Nb or Ta.
The aim of the present work has been to develop a reliable method for coating steel with a corrosion resistant layer of a refractory metal such as niobium or tantalum. Tantalum and niobium layers have been electrodeposited from fluoride melts and show the desired corrosion resistance towards acidic media. Process parameters can be controlled to yield a dense, adherent and smooth layer. The work has included development of process equipment, preparation and handling of salts and melts. The coating process is performed in a FLINAK melt.
In order to be able to control the important parameter of oxide content in the FLINAK melt a part of the development work also included an attempt to build an oxide sensor based on ceramics. The development of ionically conducting zirconia based ceramics to withstand corrosion in a molten FLINAK environment at 700 C was therefore studied. Conditions affecting the corrosion resistance of the ceramics were investigated. The resistance to corrosion attack by molten fluorides was investigated by employing laser Raman spectroscopy. The oxide sensor constructed from the ceramic material that gave best performance was found to be unstable with time.
A boriding process for improving corrosion and abrasion resistance of the refractory metal coatings deposited from the bath of molten salt has been developed. Quality control and characterization of the coatings has been performed. The experiments led to the definition of a new processing technique for boriding. Instead of conventional powder pack methods a hydrogen assisted paste boriding procedure was found to be recommendable.
Methods have been developed for production of chemicals for the electrolytical saltbath in the quantities necessary. An analytical method for analysing these chemicals and the saltbath, has been developed. The quality of the oxygen content influence on the saltbath and the deposited quality are very near the optimal condition for niobium (Nb). Investigations are still going on for tantalum (Ta).
Equipment for boriding experiments on Ta and Nb samples has been designed. Investigations show both hard, brittle and ductile borided layers. The quality depends on the pressure, temperature and protective gas atmosphere in the boriding furnace.
Samples, manufactured from different ceramic compositions and squeeze methods, have been corrosion tested in order to construct a ceramic oxygen sensor.
In order to increase the wear resistance of the layer, methods will be developed to cover the deposited layer with a boride layer by diffusion of B into the formed metal layer. The project will address the following problems :
-the salt bath deposition process,
-the boriding process,
-methods to measure the oxygen content of the salt bath,
-suitable methods for control of the quality of the layers.