CORROSION-RESISTANT NB COATINGS GROWN BY COMBINED STEERED ARC UNBALANCED MAGNETRON DEPOSITION

Niobium is well known for its chemical stability and excellent corrosi on resistance which is based on the formation of a stable oxide layer of Nb2O5 in aqueous media. It therefore finds wide application in area s where chemical stability is important, such as in chemical engineeri ng equipment. Disadvantages of Nb are its high material costs and the fact that it is not possible to deposit Nb by the rather economic tech nique of aqueous electrodeposition. However, other deposition techniqu es such as physical vapour deposition may be toed to produce niobium c oatings. Due to its high melting point, sputtering is the preferred PV D deposition technology. Improved corrosion resistance approaching tha t of bulk Nb was found for l mu m thick Nb coatings on 304 stainless s teel substrates The corrosion performance very much depends on the met hod of ion etching prior to deposition. Microstructure, texture and mi crohardness as well are strongly influenced by the type of ion etching pre-treatment and by the temperature during deposition. Nb-ion etchin g of the 304 stainless steel substrate and consequent Nb deposition wi th unbalanced magnetron sputtering at a temperature as low as 250 degr ees C, utilising an ion to neutral ratio of 4, leads to dense films. T hese films exhibit passivation over the whole range of the applied pol arisation measurements in a 3% NaCI solution.