Resistencia a la corrosión de aleaciones Ni-Cr-Mo en diferentes condiciones metalúrgicas
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Comisión Nacional de Energía Atómica. Instituto de Tecnología Sabato
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Tesis para optar al título de Magister en Ciencia y Tecnología de Materiales
Alloy C-22 (Ni-22Cr-13Mo-3W) shows an outstanding corrosion resistance in a wide variety of highly-corrosive environments. Due to its excellent corrosion resistance in oxidizing and reducing environments, Alloy C-22 has been selected as a candidate for the fabrication of the corrosion-resistant outer shell of the high-level nuclear waste containers. New Ni-Cr-Mo alloys have been developed due to the increasing demand of the industry for corrosion resistant alloys with particular properties. Alloy C-22Hs (ni-21 per cent Cr-17 per cent Mo) is a new high-strength corrosion mechanical properties. Alloy HYBRID-BC1 (Ni-22 per cent Mo-15 per cent Cr) is intended for filling the gap between Ni-Mo and Ni-Cr-Mo alloys. This novel alloy is able to withstand Hc1 and H2SO4, even in the presence of dissolved oxygen and other oxidizing species in the environment. The Mill Annealled condition (MA) of the Ni-Cr-Mo alloys is an fcc solid solution. Thermal aging of Ni-Cr-Mo alloys leads to microstructure changes depending on the temperature range and exposure time at temperature. A long Range Ordering (LRO) reaction can occur in the range of 350 C degrees to 600 C degrees. This homogeneous ordering reaction does not seem to affect the corrosion resistance and produces only a slight loss in ductility. Topollogically or Tetrahedrally Closed Packed (TCP) phases, like mu, sigma and P, may form when Ni-Cr-Mo alloys are exposed in the range of 600 to 1100 C degrees. These phases could have a detrimental effect upon corrosion resistance and cause a loss of mechanical ductility. The precipitation of TCP phases is heterogeneous and starts at grain boundaries. The sensitized alloy is prone to intergranular attack in certain environments. The aim of the present work was to compare the general corrosion rate and the crevice corrosion susceptibility of alloys C-22, C-22Hs and HYBRID-BC1 in different metallurgical conditions when exposed to 1 mol/L NaCl and 1 mol/L HCl at 90 C degrees. The effects of the alloy composition and different heat treatments were assessed. Corrosion rates of approximately 0.1 mu m/yr were found in 1 mol/L NaCl for all the tested alloys in the studied metallurgical conditions. In 1 mol/L HCl, alloys C-22 and C-22HS showed corrosion rates in the range of 1 to 3 mm/yr, while alloy HYDRID-BC1 showed corrosion rates of ca 10 mu m/yr. None of the performed thermal aging treatments affected the corrosion rate of the tested alloys in the studied conditions. Alloy HYBRID-BC1 showed lower crevice corrosion susceptibility than alloys C-22 and C-22HS in 1 mol/L NaCl at 90 C degrees. Thermal aging treatments performed to alloy C-22 did not affect its crevice corrosion resistance, while the age hardening treatment performed to alloy C-22HS slightly reduced its crevice corrosion resistance. The better performance of alloy HUBRID-BC1 in 1 mol/L HCl and its higher crevice corrosion resistance in 1 mol/L NaCl were attributed to its higher molybdenum content.
Alloy C-22 (Ni-22Cr-13Mo-3W) shows an outstanding corrosion resistance in a wide variety of highly-corrosive environments. Due to its excellent corrosion resistance in oxidizing and reducing environments, Alloy C-22 has been selected as a candidate for the fabrication of the corrosion-resistant outer shell of the high-level nuclear waste containers. New Ni-Cr-Mo alloys have been developed due to the increasing demand of the industry for corrosion resistant alloys with particular properties. Alloy C-22Hs (ni-21 per cent Cr-17 per cent Mo) is a new high-strength corrosion mechanical properties. Alloy HYBRID-BC1 (Ni-22 per cent Mo-15 per cent Cr) is intended for filling the gap between Ni-Mo and Ni-Cr-Mo alloys. This novel alloy is able to withstand Hc1 and H2SO4, even in the presence of dissolved oxygen and other oxidizing species in the environment. The Mill Annealled condition (MA) of the Ni-Cr-Mo alloys is an fcc solid solution. Thermal aging of Ni-Cr-Mo alloys leads to microstructure changes depending on the temperature range and exposure time at temperature. A long Range Ordering (LRO) reaction can occur in the range of 350 C degrees to 600 C degrees. This homogeneous ordering reaction does not seem to affect the corrosion resistance and produces only a slight loss in ductility. Topollogically or Tetrahedrally Closed Packed (TCP) phases, like mu, sigma and P, may form when Ni-Cr-Mo alloys are exposed in the range of 600 to 1100 C degrees. These phases could have a detrimental effect upon corrosion resistance and cause a loss of mechanical ductility. The precipitation of TCP phases is heterogeneous and starts at grain boundaries. The sensitized alloy is prone to intergranular attack in certain environments. The aim of the present work was to compare the general corrosion rate and the crevice corrosion susceptibility of alloys C-22, C-22Hs and HYBRID-BC1 in different metallurgical conditions when exposed to 1 mol/L NaCl and 1 mol/L HCl at 90 C degrees. The effects of the alloy composition and different heat treatments were assessed. Corrosion rates of approximately 0.1 mu m/yr were found in 1 mol/L NaCl for all the tested alloys in the studied metallurgical conditions. In 1 mol/L HCl, alloys C-22 and C-22HS showed corrosion rates in the range of 1 to 3 mm/yr, while alloy HYDRID-BC1 showed corrosion rates of ca 10 mu m/yr. None of the performed thermal aging treatments affected the corrosion rate of the tested alloys in the studied conditions. Alloy HYBRID-BC1 showed lower crevice corrosion susceptibility than alloys C-22 and C-22HS in 1 mol/L NaCl at 90 C degrees. Thermal aging treatments performed to alloy C-22 did not affect its crevice corrosion resistance, while the age hardening treatment performed to alloy C-22HS slightly reduced its crevice corrosion resistance. The better performance of alloy HUBRID-BC1 in 1 mol/L HCl and its higher crevice corrosion resistance in 1 mol/L NaCl were attributed to its higher molybdenum content.