Pure Gold Dissolution and Stainless-Steel Corrosion in Dilute Nitric or Nitrous Acid Containing Halide Salts

Abstract

The greatly enhanced oxidation ability of dilute aqueous nitric acid (0.10–2.0 mol dm-3) containing bromide and iodide salts as well as chloride salts has been examined based on the dissolution kinetics of pure gold at 30–60 °C. It has been found that bromide salts are more effective than chloride salts in gaining the ability of dissolving gold in dilute aqueous nitric acid solution. At 60 °C, a piece of gold-wire (ca. 20 mg) is dissolved in 20 mL of as low as 0.10 mol dm-3 HNO3 solution containing 1.0–5.0 mol dm-3 NaBr and the dissolution rate constant, log(k/s-1), increases linearly (from -5.78 to -4.52) with the increasing NaBr concentration. The gold dissolution ability has been examined also with nitrous acid containing chloride and bromide ions at 35 °C. The NaNO2 solution containing twice or more amounts of HX (X = Cl, Br) gives the maximum efficiency for gold dissolution, according to the log(k/s-1) values of the mixed solutions of NaNO2 (0.10–2.0 mol dm-3) and HX of various concentrations. The influence of oxidation by dilute nitric and nitrous acids on the gold dissolution is discussed from the standpoint of the redox potentials in ‘‘modified’’ aqueous solutions and not of the changes in the activity coefficients of ions. The corrosion of stainless-steel (SUS316L) in dilute nitric acid containing NaCl or NaBr has been also examined to find that the passive film on the surface is broken down by the evolved Cl2 or Br2 gas.