Study of the chemical behavior of hydrofluoric, nitric and sulfuric acids mixtures applied to niobium polishing

Formation of niobium oxides by electrolysis from acidic aqueous solutions on glassy carbon

“…In the acidic mixture of HF + HNO3 + H2SO4, each of the aqueous solution constituent plays an important role: acids generally act as proton donors, NO3from HNO3 oxidizes niobium, HF is an aggressive complexing agent that dissolves niobium oxides and niobium metal, and niobium fluoride complexes are a main source for the deposition of metallic niobium [22]. Finally, SO4 2ions act as an additional complexing agent for niobium oxide species [28].…”

“…In solution C, which does not contain H2SO4, GC reversible potential moved further to more negative values (0.460 V vs. SCE). This should be attributed to an increased concentration of active niobium ions that are not engaged in complexes made with sulphate anions [28]. A niobium anode was applied as a compensating source of niobium ions in the used solutions.…”

“…7. a) and b) SEM images at lower (120×) and higher (5000×) magnification of the GC surface after being held for 180 minutes at potential E = −0.420 V vs. SCE in electrolyte C According to researchers in the field, water is an important reactant in the chemical behaviour of the niobium species. In the electrolytes containing HNO3 and HF, dissolved niobium metal in an aqueous solution is in an oxidation state +5 and participates in an intermediate compound (niobium pentoxide; Nb2O5) [28]. In the presence of water, niobium pentoxide forms a different kind of niobic acid that could precipitate in the form of more or less hydrated gels (Nb2O5 × nH2O).…”

“…HNO3, Merck, Germany) and hydrofluoric acid (p.a. HF, Acros organic, USA) [28]. A concentrated acidic solution containing niobium ions was then diluted with distilled water (electrolyte C).…”

“…Primary reason for this is the very negative niobium reduction potential, which interferes with H2 evolution in aqueous electrolytes [26,27]. There is also a predisposition of niobium to produce electrochemically inactive species when reacting with oxygen, often leading to the formation of stable clusters [24,28]. Research conducted by McCullough еt al.…”