Effect of Boiler Water Chemistry on Corrosion and Corrosion Fatigueof Carbon Steels

摘要

Use of carbon steel for the service of high temperature water applications strongly depends upon thernformation and stability of the protective magnetite film, Fe3O4, on the waterside surface of boiler tubes.rnTests were carried out in a recirculation autoclave under industrial boiling water conditions. Boiler waterrnchemistry was controlled during a series of tests to keep the dissolved oxygen in the range of 10 ppb - 10rnppm. Initial tests were conducted to develop the magnetite film on carbon steel tube samples underrndifferent test conditions. Results have indicated that the water chemistry and ratios of anode/cathodernhave an effect on the magnetite film morphology. Film characterization by atomic force microscopyrn(AFM) and scanning electron microscopy (SEM) has shown that the magnetite film changes from a fine-grainedrnporous film with tetrahedral crystals at the surface to an irregular-grained compact and protectivernfilm when the anode to cathode area is decreases. Corrosion fatigue crack initiation and growthrnmechanisms involved in boiler water environments include magnetite film damage as an important step.rnSlow strain rate tests were carried out in simulated boiler water environments, using smooth carbon steelrnsamples, to investigate the role of temperature and water chemistry on crack initiation.