The effect of zinc (Zn) and copper (Cu) additions on the catalytic behavior of noble metal alloyed 304 stainless steel (SS) in 288oC water under stoichiometric excess hydrogen was studied. It was observed that an increase in the Zn or Cu content of the water increased the electrochemical corrosion potential (ECP) of noble metal alloyed 304 SS by = 30 to 50 mV and decreased the recombination efficiency of oxygen (O2) and hydrogen (H2) by =10%. The change in the ECP and recombination rate was correlated with incorporation of zinc and copper in the oxide film, which, by covering catalytic sites, would alter the redox reaction rate.
The high temperature water in boiling water reactors (BWRs) is highly oxidizing due to radiolytically generated dissolved O2 and hydrogen peroxide (H2O2) (100 to 300 ppb) and dissolved H2 (10 to 20 ppb). In this environment, intergranular stress corrosion cracking (IGSCC) of sensitized SS components in BWRs has been a major concern [1, 2]. The IGSCC susceptibility of reactor structural materials in BWRs is known to be affected by the ECP which is controlled by the content of the total oxidant such as O2 and H2O2 Data from laboratories and reactors have shown that IGSCC can be prevented by reducing the oxidant concentration and thereby lowering the ECP below -230 mV vs. the saturated hydrogen electrode (SHE) [4-6].
H2 is being added to the feedwater of BWR to mitigate the IGSCC by reducing the dissolved oxidant concentration. This process is referred to as hydrogen water chemistry (HWC). Large amounts of H2 addition are normally required to sufficiently lower the dissolved O2 and H2O2 concentrations so that the IGSCC protection potential (-230 Mvshe) is attained. Although IGSCC can be sufficiently suppressed by the HWC process, it is not an optimum process from an operating viewpoint. The disadvantages of HWC include high H2 cost, higher Co60 buildup rate, increased N16 release to the turbine, etc. The IGSCC protection potential is also difficult to achieve in the highly oxidizing and high water flow regions.
Zinc oxide (ZnO) has been added into the BWR feedwater because of the benefical implication in reducing buildup of radiactive species in the oxide film [7, 8]. It was reported that Zn addition to high temperature water altered the oxide structure and enhanced a thin, less defect corrosion film on 304 SS [9], decreased the growth rate of IGSCC of SS and nickel-base alloys [10] and increased the oxide rupture strain of 304 SS [11]. Also, different levels of Cu had been detected in plants which mainly came from the corrosion of brass condenser tube.
Recently, noble metal technology (NMT) has been developed to achieve the thermodynamically lowest possible ECP (-500 mVshe)and lowest crack initiation/growth rates at much lower H2 addition rates and with minimal negative impact on BWR operation [12-16]. This approach invloves improving the catalytic recombination of O2 and H2O2 with H2 to from H2O on metal surfaces.
