Materials for High-Temperature and High-Pressure Hydrogen Peroxide Bleaching Equipment

To determine the suitability of grade 705 zirconium (UNS R60705) as a candidate material of construction for high-temperature and pressure hydrogen peroxide (P_HT) pulp-bleaching reactors, a series of electrochemical experiments were carried out in simulated P_HT environments. Because the P_HT process may be incorporated into a closed cycle mill or a mill that also uses chlorine-based chemicals to bleach pulp, the effect of chloride ion concentration on the corrosion of zirconium in P_HT environments was studied. It was found that at the potential measured in P_HT environments, zirconium was passive (–100 mV vs saturated calomel electrode [SCE]). Breakdown potentials of ≈ 500 mV_SCE were measured in chloride-containing solutions. However, when the chloride ion concentration was increased > 50 ppm, the repassivation potential of the zirconium was reduced significantly. Based on these results, grade 705 zirconium was determined to be suitable for a wide range of P_HT process conditions.