Niobium is well known as a chemically extremely stable material. However, the corrosion performance of 0.3 to 1.3 mum thick, unbalanced magnetron (UBM) grown niobium coatings on stainless steel substrates depends not only on the deposition parameters, but also significantly on the in vacuo etching pretreatment of the substrates prior to coating. Corrosion tests, transmission electron microscopy, and scanning transmission electron microscopy analyses have shown, that a dense fine grained, partially implanted, 5-10 nm thick niobium interface layer formed during the metal ion etching pretreatment is paramount to protect stainless steel substrates thoroughly against corrosion in chlorine containing aqueous electrolytes, whereas the pretreatment with Ar and Cr ions leads to comparably inferior corrosion results. Moreover the energy of the impinging Nb ions used during the etching process plays an important role. For 0.3 mum thick Nb coatings, maximum pitting potentials (+ 1000 mV) and minimum corrosion currents (< 3 x 10(-6) A cm(-2)) were found for bias voltages between -600 and - 1000 V. In summary, the results suggest that the UBM deposited Nb coatings are not completely dense and that the major justification for niobium as corrosion barrier depends on the existence of the thin interface layer generated by high-energy ion bombardment during the etching step. The UBM deposited coating (U-S=75 V) acts therefore as itself a chemically stable mechanical spacer protecting the thin interface layer against mechanical damage.