FLOW-ASSISTED CORROSION OF STEEL AND THE INFLUENCE OF CR AND CU ADDITIONS

Flow-assisted corrosion (FAC) of steel feedwater lines occurs by dissolution of the surface oxide layer on the steel. The solubility of iron in water under FAC conditions is discussed through the use of potential-pH diagrams (Pourbaix diagrams). Alloying additions of chromium and copper both decrease FAC. An assessment is presented that Cr additions decrease FAC by forming a mixed oxide with iron instead of a pure iron oxide. The solubility of iron from the mixed oxide is smaller than for pure iron oxide and leads to a smaller FAC rate. The stable form of copper under FAC conditions is not the mixed iron-copper oxide but metallic copper, which may act in the underlying steel surface to impede FAC.