Precipitation and transformation of iron species in the presence of oxygen and hydrazine in a simulated stainless steel feed water system

In the secondary water circuit of pressurised water reactors (PWRs), colloidal particles of oxidised, mostly iron-bearing corrosion products are transported with feed water to the steam generators (SG). Iron-bearing corrosion products originate from various sites in the secondary cooling system, and especially from the steam condensers. The sludge accumulated in SGs may cause a variety of problems, such as a decrease in heat-transfer capability through deposition on tubes, limitation of flow in some restricted areas (such as tube-to-tube-support plate crevices), and acceleration of corrosion in crevices, either in deep sludge piles or at blocked tube supports. The influx of oxidised corrosion products may have a particularly adverse effect on the redox environment of SG tubing, thereby increasing the probability of localised corrosion (e.g. intergranular attack and intergranular stress corrosion cracking). One way to reduce susceptibility to intergranular attack may be to minimise the ingress of oxidised species such as ferric (Fe³⁺) oxides and oxy-hydroxides, especially haematite (α-Fe₂O₃), goethite (α-FeOOH) and lepidocrocite (γ-FeOOH). Under reducing conditions, these species can be partly reduced to magnetite (Fe₃O₄), which is increasingly considered in the industry as a convenient indicator of acceptable redox conditions in feed water [1,2].