Corrosion and inhibition of aged 347 grade stainless steel boiler tubes

General and pitting corrosion of both service and laboratory aged 347 grade stainless steel superheater tubes have been investigated. SEM analysis showed the precipitate phases to have a double structure consisting of phase and carbide. EDA analysis of the precipitates indicated the Cr]/Fe] ratio to increase with exposure time. For both set of specimens, the general corrosion rate in 1m H₂SO₄ increased with aging whilst the pitting studies (in 3.5% NaCl) also indicated increased pitting susceptibility with aging. These trends are consistent with corrosion from the chromium depleted areas adjacent to the precipitates. Methoxypropylamine is effective as an inhibitor for pitting corrosion and the concentration required to prevent this form of attack increases with exposure time for both types of specimens.