ニッケル基超合金の電解加工の研究

NaNO₃水溶液中における二次不働態化挙動

抄録

The dissolution behavior of Ni-Cr alloys from 4 % to 20% Cr has been studied in 3 M NaNO₃ solution at 30°C. The polarization measurements were carried out in static and flowing electrolytes, and the current efficiency measurements were carried out in flowing electrolyte over a range of current densities from 1 to 100 A/cm². X-ray photoelectron spectroscopy and X-ray microanalysis were then used to examine the sample surface after the dissolution tests. The results show that Ni-Cr alloys dissolve in transpassive state at the potentials above 0.8 V (vs. SCE). For the alloys containing ≤18% Cr, the secondary passivation takes place with increasing anode potential and current density. The secondary passivation film is very thin and is com posed mainly of Ni (II) and Cr (III). The dissolution current efficiency of ithe alloys containing ≤14 % Cr is only 5-30%, and a major part of the current is consumed in oxygen generation. For the 16% Cr alloy and the 18% Cr alloy, severe grain boundary attacks take place at moderately high current density. The 20% Cr alloy dissolves at 100% current efficiency at all current densities with a thick film composed mainly of Ni (II) and Cr (VI).