Studies of corrosion in power plant boiler tubes by measurement of oxygen isotopes and trace elements using secondary-ion mass spectrometry

Recent advances in secondary ionization mass spectrometry (SIMS) techniques allow the resolution of differences in the natural abundances of the isotopes of 18O and 16O, and their ratio in both conducting and insulating materials. These techniques have been used in this study to measure the oxygen isotope signatures and their spatial distribution in corrosion products formed during high-temperature oxidation of steel boiler tubes from fossil-fuel power plants. The data obtained in this study are interpreted in terms of oxygen isotope fractionation between the available oxygen reservoirs and oxides formed on or within the metal. Results are presented for three different corrosion scenarios: steamside/fireside corrosion, aqueous phosphate corrosion and corrosion due to H 2 damage. Constant, but isotopically depleted values observed in magnetites formed during steamside corrosion and H 2 damage are indicative of interaction with locally derived meteoric water (which constitutes the boiler feed water). In contrast, isotope distributions in maricite (NaFePO 4) suggest equilibrium fractionation between this phase and precursor magnetite. Oxygen isotope patterns in fireside magnetites exhibit a complex zoning that requires at least two isotopically distinct species with significantly different transport rates.