Effects of Hydrogen Peroxide on Corrosion of Stainless Steel, (III) Evaluation of Electric Resistance of Oxide Film by Equivalent Circuit Analysis for Frequency Dependent Complex Impedances

Abstract

Corrosive conditions in BWRs are determined mainly by hydrogen peroxide (H 2 O 2). Then, a high temperature, high-pressure H 2 O 2 water loop was fabricated to identify the effects of H 2 O 2 on corrosion and stress corrosion cracking of stainless steel. By changing concentrations of H 2 O 2 and O 2 , in situ measurements of electrochemical corrosion potential (ECP) and frequency dependent complex impedance (FDCI) of test specimens were carried out and then characteristics of oxide film on the specimens were evaluated by analyzing FDCI data based on the equivalent circuit analysis. The following points were experimentally confirmed. (1) The ECP and FDCI data of the specimens exposed to 100 ppb H 2 O 2 were not affected by co-existing O 2 with the same level oxidant concentration and they were also not affected by pre-exposure to 200 ppb O 2. From the viewpoint of ECP, this meant that corrosive conditions of hydrogen water chemistry were the same as those of normal water chemistry. (2) The low frequency semicircles of the FDCI data for the specimens exposed to 100 ppb H 2 O 2 reached a saturation value which was much smaller than saturation values for specimens exposed to 200 ppb O 2 and to 10 ppb H 2 O 2. (3) Smaller oxide dissolution resistance and larger electric resistance of the oxide film were obtained for the specimens exposed to 100 ppb H 2 O 2. This caused ECP to increase by shifting the anodic polarization curve of stainless steel to the high potential side.

Effects of Hydrogen Peroxide on Corrosion of Stainless Steel, (IV) Determination of Oxide Film Properties with Multilateral Surface Analyses

“…1,2) To determine the effects of H 2 O 2 , O 2 and their mixture on electrochemical corrosion potential (ECP) of type 304 stainless steel, the authors fabricated a high temperature, high-pressure water loop with a polytetrafluoroethylene (PTFE) inner liner which minimized H 2 O 2 decomposition and lowered possible oxygen concentration. [3][4][5][6][7] From in situ measurements, i.e., ECP and frequency dependent complex impedance (FDCI), made in previous papers, [7][8][9] the authors confirmed the following. (1) The ECP and FDCI data of the specimens exposed to 100 ppb H 2 O 2 were not affected by co-existing O 2 with the same level oxidant concentration and they were also not affected by pre-exposure to 200 ppb O 2 .…”

Effects of Hydrogen Peroxide on Corrosion of Stainless Steel, (V) Characterization of Oxide Film with Multilateral Surface Analyses

“…[5][6][7][8][9][10][11] To determine the effects of H 2 O 2 , O 2 and their mixture on ECP of type 304 stainless steel, the authors fabricated a high temperature, high-pressure water loop with a polytetrafluoroethylene (PTFE) inner liner, which minimized H 2 O 2 decomposition and lowered possible oxygen concentration. [7][8][9][10][11][12][13][14][15] From in situ measurements, i.e., ECP and frequency dependent complex impedance (FDCI), and surface characterization of oxide film on specimens made in previous papers, [12][13][14][15][16] the authors confirmed the following. (1) The ECP and FDCI data of the type 304 stainless steel specimens exposed to 100 ppb H 2 O 2 were not affected by co-existing O 2 with the same level oxidant concentration and they were also not affected by pre-exposure to 200 ppb O 2 .…”

“…(1) The ECP and FDCI data of the type 304 stainless steel specimens exposed to 100 ppb H 2 O 2 were not affected by co-existing O 2 with the same level oxidant concentration and they were also not affected by pre-exposure to 200 ppb O 2 . 13,14) The ECP measured for the specimens exposed to 100 ppb H 2 O 2 reached the saturated level in 50 h and was larger than ECP for the specimens exposed to 200 ppb O 2 ; the same ECP level was kept when the H 2 O 2 concentration was decreased to 10 ppb. 13) From the viewpoint of ECP, this meant that corrosive conditions of hydrogen water chemistry (HWC: 10 ppb H 2 O 2 ) were the same as those of normal water chemistry (NWC: 100 ppb H 2 O 2 +200 ppb O 2 ).…”

“…13. In the calculation it was assumed that growth rate of inner layer was in proportion to ECP 14) and release rates of both inner and outer layers were in proportion to the inverse of the resistances of oxide dissolution obtained from FDCI.…”

“…13) From the viewpoint of ECP, this meant that corrosive conditions of hydrogen water chemistry (HWC: 10 ppb H 2 O 2 ) were the same as those of normal water chemistry (NWC: 100 ppb H 2 O 2 +200 ppb O 2 ). 13,14) (2) The low frequency semi-circles radii of the FDCI data for the specimens exposed to 100 ppb H 2 O 2 reached a saturation value, which was much smaller than radii saturation values for specimens exposed to 200 ppb O 2 and to 10 ppb H 2 O 2 . In this paper, more precise surface characterizations of oxide film on stainless steel specimens exposed to H 2 O 2 and O 2 at elevated temperature were carried out by multilateral surface analyses following the in situ measurements, i.e., ECP and FDCI, and then contributions of surface properties to corrosion behaviors of stainless steel in BWR cooling water were considered.…”