Pure Gold and Stainless-Steel Dissolution or Corrosion in Dilute Haloic Acids (HXO₃, X = Cl, Br, I) Solution containing Abundant Halide Ions

Abstract

The pure gold dissolution has been explored in (dilute) chloric, bromic, or iodic acid (HXO₃, X = Cl, Br, I) aqueous solution containing abundant halide ions at 60°C. It has been found that mixtures between sodium chlorate and HCl (NaClO₃ + HCl ⇆ HClO₃ + NaCl) in 20 mL are effective media for the dissolution of pure gold-wire (99.95 %, 0.25 mm diameter, ca. 20 mg). The gold-wire dissolution is much more enhanced by HBr than HCl in NaClO₃ solution. At 0.50 mol dm^-3 NaClO₃, for instance, log (k/s^-1) values are -5.11 and 3.40 (ca. 50-fold increase) for 1.0 mol dm^-3 HCl and HBr, respectively, where first-order dissolution rate constants (k/s^-1) have been evaluated by the changes in the mass of gold-wire. In HClO₃ solution containing abundant halide ions (X^- = Cl^-, Br^-), the Au metal should be oxidized mainly by X₂ to be AuX₄^-, where X₂ is produced by the oxidizing power from HClO₃, just as dilute HNO₃ (< 2 mol dm^-3) containing abundant NaCl. In NaBrO₃ solution, however, HBr does not acquire any superiority to HCl in the gold dissolution. In NaIO₃ solution, the effect of HX at the lower concentrations (< 0.03 mol dm^-3) of NaIO₃ increases in the order of HCl < HBr < HI. The gold dissolution in hypochlorous acid (HClO) containing halide ions has been also examined as the reference. A new diagram has been proposed in order to verify that even the conjugate anion (ClO₃^- or NO₃^-) from a strong acid (HClO₃ or HNO₃) is subject to be protonated in the “aqueous solution” modified by electrolytes of very high concentrations (> ~5 mol dm^-3). Additionally, stainless-steel corrosion has been examined in NaClO₃ solution containing aboudant HCl.