Influence of water treatment on corrosion resistance of sg tube materials in pressurized water reactors

The influence of phosphate treatment, low-AVT and high-AVT on the corrosion resistance of the heating tubes of steam generators (SG) with U-shaped tube bundles is discussed. Phosphate acts against possible local pH-value shifts due to the ingress of salts into the SG water. This inhibits denting in steam generators with rigid tube support plates made of carbon steel. Laboratory investigations show that phosphate has an inhibiting effect on pitting. In addition, phosphate acts against the formation of copper ions, which are discussed as the oxidizing agent leading to pitting phenomena in SGs of pressurized water reactors. The oxygen content of water in the SG is influenced by phosphate only as a function of the differences in the solubility behaviour of demineralized and saline water. Because of uniform corrosion in the area of tube scales containing phosphate (so-called ''wastage corrosion''), numerous plants outside the Federal Republic of Germany have been converted for operation using AVT. This reduces the safety margin with respect to the aforementioned corrosion phenomena. Such plants must therefore be operated strictly under reducing water conditions. Hydrazine acts as a reducing agent to inhibit the oxidizing effect of O/sub 2/. At high temperatures, hydrazine also combines directly with oxygen. With high-AVT treatment, the SG water contains N/sub 2/H/sub 4/ concentrations greater than 100 μ/kg. A prerequisite for the use of high-AVT is that there are no copper materials in the secondary circuit so that copper ions as oxidants can be ruled out