Etching of platinum-rhodium alloys in oxygen-containing atmospheres

Morphological changes introduced by chemical etching of Pt, Pt/10-30 wt.% Rh, and Rh wires in different oxygen-containing atmospheres at 900-1200 °C are studied by scanning electron microscopy and powder X-ray diffraction. The chemical etching ability of different sources for molecular oxygen is ranked as: wet O2 > N2O > dry O2 > NO. Treatment in these atmospheres gave similar surface patterns with numerous pits and small deposited crystals. Powder X-ray diffraction showed that the surfaces of the tested Rh-containing specimens are more or less covered with Rh2O3. In wet oxygen and N2O atmospheres the formation of Rh2O3 is somewhat suppressed toward lower temperatures and/or higher Rh content. None of the alloys tested at 900 °C showed any significant loss of material. Some Rh-containing specimens showed a slight weight gain due to Rh2O3 surface coverage. The occurrence of PtO2 and RhO2 in the vapor phase during the etching process was verified by in situ mass spectrometry. Factors of importance for chemical and catalytic etching are discussed.