Fretting Corrosion (Logical Damage Formation Models)

Abstract

The article discusses frettingcorrosion of the material of heat-transfer tubes(HTTs) used in the steam generator (SG)of an NPP equipped with a water-cooled water-moderated power-generating reactor (VVER) as a mechanism contributing to premature incipience of cracks in the HTT material, especially taking into consideration the expected schedule of NPP operation in the power grid control mode (the maneuverable mode). From the viewpoint of a dislocation-hydrogen model (corrosiology), the SG HTT damage commences from relative sliding of HTTs along the spacer element.The passivation layer applied on the HTT surface is “scraped off” over this length, and a nodule of the future corrosion crack emerges in this area. The defect depth will grow due to the "scraping" of not so much the HTT wall metal as due to accumulating number of the erased oxide layers. The amount of released and partially absorbed hydrogen is equivalent to the mass of oxygen (in carbon mass units) added to the alloy components. The absorbed hydrogen will form a local embrittlement volume with stretched internodalbonds. A crack emerges when the limit state criterion by hydrogen is reached. A conclusion is drawn that, in order to increase the intervals of SG HTT operation between repairs, the passivation layer should be restored using inorganic film-forming inhibitors.