Corrosion Behavior of Steels Composing an Atomic Power Plant and these Protective Methods

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As for an apparatus having environment similar to that in a primary cooling water system of boiling water reactor (BMR) of atomic power plant, the once-through type apparatus using at high temperature and high pressure (HT/HP) was prepared (10 ml/min, 288ﾟC and 90 kgf/cm^2) and was composed of reserver tanks, plunger pumps, pre-heater, autoclave (0.38l, Ti), cooler, backpressure valve, etc. Test specimens were prepared from commercially available, JIS SUS 304 stainless steel, JIS STPT 410 carbon steel and the stainless steel treated sensitively. The additives used were Zn [Zn(NO_3)_2, ZnSO_4 and ZnO], Al [Al(NO_3)_2 and Al_2(SO_4)_3], Zr [ZrO(NO_3)_2] and Si [Na_2SiO_3] compounds. The exposure tests were carried out in high temperature and high pressure water at 100 ppb to 1 ppm and at 1 muM of additive concentration and at < 5 to 8000 ppb of dissolved oxygen concentration with and without pre-filming treatment for 200 to 600 h. In particular, it was found that the amounts (or rate) of total corroded metals in the systems adding Zr (in stainless steel and carbon steel) and Si (in stainless steel) compounds tended to decrease in comparison with those in non-additive systems. The films of oxide compounds on steel surfaces after exposure tests were formed. And then, it was confirmed that from the depth profile data of glow discharge optical emission spectroscopy (GDS), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), etc. , the films had a double layer structure consisting of a Fe-rich outer layer and Cr-rich inner layer. From the systematic evaluation by various physicochemical measurements, it was thought that the effects of corrosion protections of steels were due to an increase of protective ability of film by Zr oxide in the system adding Zr compound and a depression of outer-diffusion of CrO_4^<2-> by adsorption of SiO_3^<2-> on steel surface in the system adding Si compound.