General Corrosion studies of a Titanium and Incoloy based alloys under ammoniacal medium and at 290 deg. C

For their use in future PWR applications, the general corrosion behaviors of two modified alloys of titanium and Incoloy were studied at high temperature and high pressure (290 deg. C, 7400 Kpa) under ammoniated atmosphere and compared. Coupons were exposed to solutions of varying ammonia concentrations of (10, 50 and 100 ppm ) at 290 deg. C under non-deaerated conditions in a static autoclave for 20 days. Surface characteristics of exposed coupons were studied using XRF, SEM, EDAX and XPS. The solution in the autoclave was analyzed for its specific conductivity, pH and for the elemental concentrations leached from alloy. The exposed titanium based alloy showed deposition of white crystalline material (300-1000 nm size) on the surface. Depletion of Ti and increase in the oxygen concentration on the exposed surface was observed. This indicated dissolution of Ti in solution from surface at high temperature and pressure and its reaction with oxygen in solution to form oxide and its redeposition on surface. The oxide film compositions were found to change drastically between 10 and 50 ppm ammoniated solution. Ti was found to be enriched in the oxide film when the solution contained 50 ppm of ammonia whereas the opposite effect was observed at 10 ppm of ammonia. The presence Ti⁴⁺ in oxide environment and traces of Cr³⁺ were observed but no nitrogen or Zr was detected. Specific conductivity of the exposed solution was found to increase by 30 μS/cm and pH to decrease by 1.5 units. Slight leaching of Ti was observed in solution. No Zr was found in the leached solution. Presence of other elements like Al, Cr, Ni in the exposed solution indicated leaching of autoclave construction material (hastelloy). This alloy showed good resistance for corrosion under the experimental conditions. The exposed surface of Incoloy based alloy showed Ni, Cr, Cu and Mn on the surface with deposition of crystalline particles (200-300 nm size). The exposed surface also showed a decrease in Cr (3%), Fe (5%), Ni (4%), and Mn (1%) concentration and an increase in Oxygen (18 %).This indicated initial dissolution of alloy and subsequent oxide formation of dissolved metals and their surface deposition. The film composition with respect to Cr, Fe, Ni were almost similar in both 10 and 50 ppm ammoniated solution. Presence of Ni as Ni-spinel, traces of N, and Fe as Fe³⁺, Cr in Cr³⁺ form were observed on the surface. The concentration of Cr was less and that of Ni was more relatively on the surface compared to that on unexposed surface. Specific conductivity of exposed solution increased by 25 μS/cm and pH decreased by 3.2 units which clearly showed corrosion/ dissolution of the materials followed by their hydrolysis to form hydroxide and/or oxide formation and their redeposition on the material surfaces. The results also indicated that the corrosion of Incoloy based alloy was comparable to the Titanium based alloy. (authors)