Boiler simulation corrosion experiments

The A and B samples removed from the auUK-lave after 2 d of exposure at 300°C were first studied by a standard SEM instrument, i.e., a system with a 

The compositions of the.se films have been probed as a function of depth using a three-dimensional (3D) SIMS imaging procedure I5. Images of the spatial distributions of secondary ions from the film arc recorded. sequentially, as a function of depth, and are stacked to form a 3D or volumetric image. The latter can be very effective in visualizing both the consistencies and variations expected in a typical corrosion film structure. 

The 3D images in Fig. 14 show that there are major qualitative differences in the A and B films. The A film contains largely oxidized nickel. Subsequent XPS measurements showed it to be a mixture of Ni(OH)2 and NiO. Whatever oxidized Cu is present is found to be distributed near the oxide/metal interface. By contrast the B film has a significant concentration of oxidized Cu. di.stributed fairly evenly through the film structure. Some measure of the differences in Ni/Cu ratio at individual points in the volume, can be gained from the intensity plots in Fig. 14, where NiO and CuO intensities are compared. Also, the distribution of oxides on the B alloy surface shows them to be less densely packed and more irregular. In addition, the crystallite size is larger. 

It appears, then, that the A alloy has an oxide that is thinner with smaller crystallites and which is enriched in nickel oxide species at the outermost surface. The thicker B oxide has an irregular network of larger crystals with oxidized Cu di.stributed throughout the film. Furthemtore. it appears that oxide 

After the A and B alloys were removed from the autoclave, they were subjected to electrochemical impedance spectroscopy (EIS) measurements under ambient conditions in pH 10 solutions. It was anticipated that such measurements might help to support the corrosion mechanisms hypothesized from the surface analysis of alloys exposed to actual boiler conditions. Analy.sis of the ambient EIS data gave film resistances of 1.2 x 10 and 1.1 x 10 Q-cm 2 for the A and B alloys, respectively, at their corrosion potential. Both are very high resistances, which suggest that a passive film can form on either alloy under these electrochemical conditions. However, in the light of their significantly different behavior under simulated boiler conditions, it can be concluded that these particular EIS tests may not be pertinent to an understanding of the corrosion problem at hand. 

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Boilers Plants always experience corrosion problems with boilers, and monitoring of corrosion has always presented a challenge relating to both access and simulation of exact conditions, e.g. condensation conditions. Applications in the monitoring of condenser tubes have been reported ... 

The model is capable of simulating multi-phase internal corrosion processes that are governed by solid-state diffusion in the bulk metal. Oxidation experiments in laboratory air and He-02-H20 mixtures revealed that internal corrosion of low-alloy steels occurs along grain boundaries as part of the inward oxide growth process. Since the low-alloy boiler steels contain small amounts of Cr, the phase composition of the iimer layer exhibits a gradual... 

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