Corrosion product build-up on stainless steel and its influence on hydrazine decomposition.60Co deposition in loop tests using borated water at 300�C

Abstract

The corrosion rate of stainless steel decreases with time of operation under conditions characteristic of the primary circuit of pressurized-water reactors. Due to this the concentration of corrosion products in solution rapidly increases during heating-up and decreases after reaching the operation product levels is up to about three orders of magnitude. By a preoxidation treatment using potassium dichromate, the corrosion product release can be essentially diminished. In order to describe the influence of corrosion products in solution we investigate the thermal decomposition of hydrazine as well as the deposition behavior of60Co. the half life of the thermal N2H4 decomposition at 300°C in borated water increases from about 1 to about 60 minutes, starting with glossy loop surfaces. Without adding boric acid to the water, half lives between 1 and 120 minutes were found; with an activation energy for the decomposition being estimated in the temperature region 270–310°C. The deposition behavior of60Co is characterized by a very fast primary step, followed by retarded secondary deposition steps. The primary deposition step can be described by half lives of about 5 seconds. The influence of preoxidation treatment and subsequent decrease of corrosion product concentration on decomposition of N2H4 and deposition of60Co is discussed.