CONCENTRATING EFFECTS OF SIMULATED DEFECTS UNDER BOILING CONDITIONS IN HIGH-TEMPERATURE WATER

Concentrating effects of simulated defects (holes of 125, 250, and 500 mu m diameter) in a tube wall have been studied in high temperature water (355 C). The occurrence of concentrating effects (made evident by accelerated corrosion and compound deposition) was found to depend on defect geometry, while the pattern of corrosion attack and deposit composition were determined by the nature of the solute. Concentrating effects were only observed when the depth/diameter ratio of a defect was greater than equivalent to 2. Under these conditions, the corrosion rate was increased by up to twentyfold and deposits were formed. Of the four water chemistry situations investigated left bracket all volatile treatment (AVT), alkali fault, acid sulfate fault, and neutral chloride fault right bracket , accelerated corrosion was observed in only the alkali and acid sulfate fault cases. A cyclic boiling mechanism, with the defect acting as a bubble nucleation site, is best able to explain the observed results. A model based on this mechanism predicts that threshold levels of solute are necessary before concentrated solutions can be formed in defects. The tubes were manufactured from two chromium ferritic steels left bracket 2. 25Cr-1Mo (Nb) and 9Cr-1Mo right bracket .